Test of an ALFA Prototype Detector at the CERN SPS

Test of an ALFA prototype detector at the CERN SPS

MASTERARBEIT

zur Erlangung des akademischen Grades Master of Science (M.Sc. Phys.)im Fach Physik

eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät I Institut für Physik Humboldt-Universität zu Berlin

von Yohany Rodríguez García geboren am 14.09.1973 in Bogotá - Kolumbien

Präsident der Humboldt-Universität zu Berlin: Prof. Dr. Jan-Hendrik Olbertz Gutachter: 1. Herr Prof. Dr. Thomas Lohse 2. Herr Dr. Ulrich Husemann

eingereicht am: 05. April 2011 Abstract

ALFA (Absolute Luminosity for ATLAS) is a high-precision scintillating fibre detector which has been designed to determine the LHC absolute luminosity at the ATLAS interaction point. This detector consists of eight Roman Pots arranged in four pairs of detector modules on each side of ATLAS at a distance of 240 m. Each detector module is made of ten layers of two times 64 scintillating fibres each (U and V planes). The fibres are coupled to 64 channels Multi-Anodes PhotoMultipliers Tubes read out by compact front-end electronics. The detector modules approach the LHC beam axis up to 1.5 mm to determine the luminosity from the differential spectrum of elastically scattered protons using the optical theorem. This Master thesis deals with the study of hit multiplicity, layer efficiency and spatial resolution of an ALFA prototype using the data obtained during the test beam performed in 2009 at Super Proton Synchrotron (SPS) of CERN. Zusammenfassung

ALFA (Absolute Luminosity for ATLAS) ist ein hochpräziser Spurdetektor aus szintillierenden Fasern, der konzipiert wurde, um die absoluten Luminosität von LHC an der ATLAS Interaktionspunkt zu messen. Dieser Detektor besteht aus acht Roman Pots angeordnet in vier Paaren von Detektor-Module auf jeder Seite von ATLAS in einer Entfernung von 240 m. Jedes Detektor-Modul besteht aus szintillierenden Fasern, aufgeteilt auf 10 doppeltseitige Ebenen in U- und V-Geometrie. Die Fasern werden zu 64 Kanälen Multi-Anoden Photomultiplier Röhren gekoppelt und durch kompakte Front-End-Elektronik ausgelesen. Die Detektor-Module nähern sich zur Strahlachse von LHC bis 1.5 mm um aus dem differentiellen t-Spektrum der elastisch gestreuten Protonen mit der Benutzung des optischen Theorem, die Luminosität zu bestimmen. Diese Masterarbeit beschäftigt sich mit dem Studium der Hit Multipli- city, Effizienz der Ebenen und räumlichen Auflösung von einem Alfa-Prototyp. Die Analyse wurde mit den Daten des Test-Strahls in 2009 am Super Proton Synchrotron (SPS) des CERN durchgeführt. A mis padres, de quienes aprendí que las dificultades de la vida siempre conducen a felices logros ...

i Contents

1 The LHC and its experiments 2 1.1 CERN ...... 2 1.2 Large Hadron Collider (LHC) ...... 2 1.2.1 Experiments at LHC ...... 2 1.3 ATLAS Experiment ...... 4 1.4 Forward Detectors for ATLAS ...... 8

2 Physics at LHC 12 2.1 Standard Model ...... 12 2.2 Soft Diﬀraction studies in ATLAS ...... 13

3 Luminosity 15 3.1 Methods to measure the luminosity ...... 15 3.2 Absolute luminosity determination ...... 17 3.2.1 Elastic scattering at small angles ...... 17 3.2.2 Luminosity determination from Coulomb nuclear interference . 18 3.3 Required beam optics properties ...... 19 3.3.1 An overview of the optics ...... 19

4 The ALFA method 23 4.1 Roman Pot stations ...... 23 4.1.1 ALFA scintillating ﬁbre detector ...... 25 4.1.2 Overlap detectors ...... 27 4.2 ALFA readout electronics ...... 28

5 Set-up of test beam 32 5.1 Test beam conditions ...... 32 5.2 EUDET silicon pixel detector ...... 33 5.3 Conditions of data taking ...... 34 5.3.1 Data collection ...... 37

6 Data analysis 39 6.1 Hit multiplicity ...... 39 6.2 Layer eﬃciency ...... 43 6.3 Spatial Resolution ...... 46

7 Conclusions and outlook 49

ii List of Figures

1.1 Overall view of the LHC experiments ...... 3 1.2 Computer generated image of the whole ATLAS detector ...... 4 1.3 A schematic view of inner detector...... 5 1.4 Computer Generated image of the ATLAS calorimeter...... 7 1.5 Relative position of forward detectors for ATLAS...... 8 1.6 LUCID detector...... 9 1.7 Relative position of ZDC detectors to the Interaction Point ...... 10 1.8 Side view of the EM and hadronic ZDC modules ...... 11

2.1 Soft Diﬀractive Processes...... 14

3.1 Lepton pair production in pp collisions: pp → p + `+`− + p ...... 16 3.2 Diﬀerential elastic cross section in dependence on momentum transfer t. 19 3.3 Quadrupole magnet in an accelerator...... 20 3.4 Geometry of the parallel-to-point optics...... 21

4.1 Schematic view of the positioning of the Roman Pots on each side of the ATLAS IP ...... 23 4.2 Schematic view of Roman Pot concept ...... 24 4.3 Roman Pot unit used for the test beam...... 25 4.4 Scintillating ﬁbres of ALFA detector ...... 26 4.5 Multi Anode Photomultiplier Tube (MAPMT) ...... 27 4.6 Front view of the arrangement of the ﬁbres in main and overlap detector 28 4.7 Picture of Photo Multiplier Front-end (PMF) ...... 30 4.8 Readout system of PMF ...... 31 4.9 Block diagram of MAROC ...... 31

5.1 Geometry layout of the test beam setup...... 32 5.2 EUDET telescope used in the test beam...... 33 5.3 Charge spectrum from a MAMPT channel...... 35 5.4 Plots for the gain equalization calibration...... 35 5.5 The S-curve of the trigger eﬃciency for a MAPMT channel...... 36 5.6 The S-curve of all channels of a PMF tube...... 37 5.7 Tracks on the active areas of the lower and overlap detectors . . . . . 38

6.1 Hit multiplicity plots...... 39 6.2 Mean hit multiplicity for each layer...... 40 6.3 Simulated mean hit multiplicity...... 41 6.4 Mean hit multiplicity as a function of the discriminator threshold. . . 41 6.5 Mean hit multiplicity plots as a function of gain factor and HV. . . . 42

iii 6.6 Plots of track hits occupancy for U and V projections in ALFA 1 . . 43 6.7 A reconstructed track for an event...... 44 6.8 Layer efficiency of all layers in ALFA 1 ...... 44 6.9 Plots of track hits occupancy for U and V projections in ALFA 2 . . 45 6.10 Layer efficiency of all layers in ALFA 2 ...... 45 6.11 Mean layer efficiency as a function of HV and gain factor...... 46 6.12 Resolution determination using the two halves of ALFA...... 47 6.13 Spatial resolution as a function of threshold signal...... 48 6.14 Spatial resolution as a function of overlap cut ...... 48

iv List of Tables

1.1 Table of resolution and η coverage values for each ATLAS detector component...... 5

2.1 Table of elementary particles with some of their main properties. . . . 13

1 Chapter 1

The LHC and its experiments

1.1 CERN

CERN, the European Organization for Nuclear Research, is the world’s largest centre for scientiﬁc research to study the basic constituents of matter. It is located in Geneva on the Swiss-French border and was created in 1954 by 12 Member States. Today, CERN has 20 Members States, 6 additional countries with observer status, Non-Member States as well as international organizations.

1.2 Large Hadron Collider (LHC)

The Large Hadron Collider (LHC) is a two-ring superconducting hadron accelerator and collider built around 50 to 170 m underground in an existing circular tunnel of 27 km length at CERN so that it straddles the Swiss and French borders in the outskirts of Geneva. The tunnel was constructed between 1984 and 1989 for the CERN Large Electron-Positron Collider (LEP) machine, which operated until 2000 when it was removed to make way for the construction of LHC. The LHC√ is able to collide two counter rotating beams of protons at center of mass energy s = 14 TeV and has a design luminosity of 1034 cm−2s−1. The LHC will also collide heavy ions as lead nuclei, at 5.5 TeV per nucleon pair, but a design luminosity of 1027 cm−2s−1. The first collision at an energy of 3.5 TeV per beam took place in March 2010 with a luminosity of 1027 cm−2s−1. The LHC is composed of 1232 superconducting main dipoles of 15 m lenght used to guide the beams and 392 main quadrupole magnets of 7 m length used to focus the beam. They produce a magnetic field of 8.3 Tesla, the highest ever used in an accelerator, needed to reach 14 TeV energy collision. To achieve this, the winding cables inside thes magnets must be cooled to a temperature of 1.8 K to get the most efficient superconducting state without loss of energy.

1.2.1 Experiments at LHC There are six experiments at the LHC that are run by international collaborations, which are distinguished from each other by the characteristics of particle detector (Fig. 1.1). According to their size can be ordered as follows: ATLAS, CMS, ALICE, LHCb, TOTEM and LHCf. In what follows only the ﬁrst four will be introduced.

2 3

Figure 1.1: The four main LHC experiments at CERN [1].

1. CMS: The CMS (Compact Muon Solenoid) is, together with ATLAS, one of the largest LHC experiments. It has been built as a general-purpose detector to investigate a wide range of physics from Standard Model, such as searching the Higgs boson, to the physics beyond Standard Model, such as searching for extra dimensions and particles that could make up dark matter. CMS has its technical design based on a compact detector magnets capable to produce a magnetic ﬁeld of 4 T.

2. ALICE: The heavy-ion detector ALICE (A Large Ion Collider Experiment) was designed to study the physics of strongly interacting matter at the TeV scale, where the formation of a new state of matter, the gluon plasma, is expected [2]. For this purpose, ALICE will study the hadrons, protons, muons, photons and electrons produced in the collision of the heavy nuclei such as Pb-Pb as well as in pp colisions.

3. LHCb: The main goal of LHCb (Large Hadron Collider beauty) experiment is to detect the decay of particles containing b and anti-b quarks, known as B mesons. In contrast to the other experiments that enclose completely the point of collision, LHCb detector uses a series of sub-detectors to detect mainly forward particles.

ATLAS will be discussed in detail in the next section. 4

Figure 1.2: Overview of the ATLAS detector. Some parts have been removed to show the inner of the detector and to indicate the various subsystems [1].

1.3 ATLAS Experiment

At the interaction point (IP) 1 of the LHC the ATLAS (A Toroidal LHC Appa- ratuS) detector is placed with a length of 44 m, a diameter of 25 m and a weight about 7000 tons. ATLAS is a multi-purpose detector which has been built to detect particles in the high-energy regime. The cylindrical geometry of the ATLAS detector allows to describe its performance in terms of cylindrical coordinates. The z axis points along the beam pipe, θ and φ are the polar angle with axis and azimuthal angle in the x − y plane, respectively. The radial distance from the z−axis is R. There is a spatial coordinate known as the pseudorapidity η which is used instead of the polar angle θ and deﬁned as

η = −log(tan(θ/2)) (1.1) and it describes the angle of a particle relative to the beam axis. The ATLAS detector includes an inner tracking detector inside a 2 T solenoid, electromagnetic (ECAL) and hadronic (HCAL) calorimeters outside the solenoid, and in the external regions, barrel and end-cap muon chamber inside three superconducting toroidal magnets (Fig. 1.2), which will be described in detail as follow. The required resolution and η coverage of each detector component are indicated in the Table 1.1.

• The Inner Detector (ID) is the part closest to the IP in ATLAS and its goal is to reconstruct the trajectories (tracks) of the charged particles that are produced in the proton collisions. The ID consists of three concentric subsystems sorted from the inside out: the pixel detector, the Silicon Tracker 5

(SCT) and the Transition Radiation Straw Tracker (TRT) and each of them is composed of a barrel and two end-caps (see Fig. 1.3). These subsystems are located inside the 2 T magnetic ﬁeld generated by the solenoid magnet (Fig. 1.2). The precision tracking detectors (pixel and SCT) cover the region |η| < 2.5, with full coverage in φ. The ID has been designed to provide a transverse momentum resolution, in the plane perpendicular to the z−axis, of σpT /pT = 0.005%pT GeV ⊕ 1% [3].

Figure 1.3: A schematic view of inner detector. The relevant components can be identiﬁed : pixel detector, TRT and SCT [1].

Detector component Required Resolution η coverage Tracking σ /p = 0.05%p ⊕ 1% |η| < 2.5 pT T √ T EM calorimeter σE/E = 11.5%/ E ⊕ 0.5% Central Barrel 0 < |η| < 1.475 Extended barrels √ 1.375 < |η| < 3.2 Hadronic calorimeter σE/E = 50%/ E ⊕ 3% Central barrel 0 < |η| < 1 Extended barrels 0.8 < |η| < 1.7 Forward calorimeter 3.1 < |η| < 4.9

Table 1.1: Required resolution and η coverage values for each ATLAS detector component [3].

The pixel detector sensitive elements cover radial distances between 50.5 mm and 150 mm. They consist of 1744 silicon pixel modules arranged in three concentric barrel layers and two endcaps of three disks each, covering |η| < 2.5 6

and measuring 3 precise space points on each track (σR−φ ∼ 12 µm, σz ∼ 60 µm) [4]. The layers and disks are outﬁtted with silicon pixel that are segmented into small rectangles, the pixels. There are 47232 pixels on each module, most of size 50 µm×400 µm. Thus the total number of pixels is more than 82.4×106.

The SemiConductor Tracker (SCT) was designed to provide four measurements per track in the intermediate radial range for particles originating in the beam-interaction region. These measurements correspond to the determination of momentum, impact parameter and vertex position, as well as a good pattern recognition using high granularity. The SCT detector extends over radial distances from 299 mm to 560 mm and consists of 4088 modules of silicon-trip detectors arranged in four concentric barrels and two endcaps of nine disks each and it provides eight precision strip measurements (four space- points). Each silicon detector is 6.36 × 6.40 cm2 with 768 readout strips each with 80 µm pitch [5]. The strips in the barrel are approximately parallel to the solenoid ﬁeld and beam axis, while in the endcaps the strip direction is radial and of variable pitch. The endcaps modules are similiar in construction but use tapered strips, with one set aligned radially. The Transition Radiation Tracker (TRT) covers radial distances from 563 mm to 1066 mm and it consists of 370 000 proportional drift tubes (straws) of 4 mm in diameter, made of Kapton with a conductive coating. Each straw acts as a cathode and is kept at high voltage of negative polarity. In the centre of the straw there is a 30 µm diameter gold-plated tungsten sense wire that acts as an anode. The drif tubes are read out by 350 848 channels of electronics and each channel provides a drift time measurement, giving a spatial resolution of 170 µm per straw, and two independent thresholds. The straws in the central barrel are arranged in three cylindrical layers and 32 φ sectors and the straws in the endcap regions are radially oriented and arranged in 80 wheel modular structures. The TRT was designed to detect charged particles with transverse momentum pT > 0.5 GeV and with pseudorapidity |η| < 2.0 that cross more than 30 straws. Electron identiﬁcation is possible through the detection of the transition radiation photons created in a radiator between the straws [6]. 7

Figure 1.4: Cut-away view of the ATLAS calorimeter system [1].

• The Calorimeter is divided into an electromagnetic and hadronic calorimeter. This distinction is made due to diﬀerent types of interaction between the calorimeter and electrons/photons on one side and hadrons on other side. The calorimeter consists of a number of sampling detectors with full φ−symmetry and coverage around the beam axis. The calorimeters closest to the beam-axis consist of three cryostats: one barrel and two end-caps. The barrel cryostat is composed of the electromagnetic barrel calorimeter, whereas the two end-caps cryostats is divided into an electromagnetic end-cap calorimeter (EMEC), a hadronic end-cap calorimeter (HEC), housed behind the EMEC, and a forward calorimeter (FCal) to cover the region closets to the beam axis (Fig. 1.4). Li- quid argon is used as the active medium in all of these calorimeters due to its intrinsic linear behaviour, its stability of response over time and its intrinsic radiation-hardness [3]. The outer hadronic calorimeter consists of scintillator tiles as sampling medium and steel as absorber medium. The tile calorimeter is divided into three parts, one central barrel and two extended barrels.

The coverage values given by |η| for each subsystem of the calorimeter is shown in Table 1.1. The electromagnetic calorimeters are lead-argon detectors with accordion-shape absorbers and electrodes due to this geometry allows, in addition to having several active layers in depth, to have three layers in the precision-measurement region (0 < |η| < 2.5) and two layers in the higher |η| region (2.5 < |η| < 3.2) and in the overlap region between the barrel and the EMEC. The FCal is a copper-tungsten calorimeter and it provides also coverage at higher |η| values, covering the region 3.1 < |η| < 4.9. It is split longitudinally into an electromagnetic compartment and two hadronic 8

compartments. The copper and tungsten have a regular grid of holes that hold the tube- and rod-shaped electrodes. The space between the tubes and rods is ﬁlled with liquid argon. The FCAL is integrated in the same cryostat as the electromagnetic and hadronic endcap calorimeters.

• The Muon Spectometer is based on the magnetic deflection of muon tracks in the large superconducting air-core toroid magnets, instrumented with sepa- rate trigger and high-precision tracking chambers. Over the range |η| < 1.4, magnetic bending is provided by a large barrel toroid. For 1.6 < |η| < 2.7, muon tracks are bent by two smaller end-cap magnets inserted into both ends of the barrel toroid. Over 1.4 < |η| < 1.6, usually referred to as the transition region, magnetic deflection is provided by a combination of barrel and end-cap fields. This magnet configuration provides a field which is mostly orthogonal to the muon trajectories, while minimising the degradation of resolution due to multiple scattering. In the barrel region, tracks are measured in chambers arranged in three cylindrical layers around the beam axis. In the transition and end-cap regions, the chambers are installed in planes perpendicular to the beam, also in three layers [3].

1.4 Forward Detectors for ATLAS

In addition to the ATLAS subsystems mentioned above, three dedicated systems are also built to cover the forward region as shown in Figure 1.4 with the positions relative to the interaction point (IP). These systems are the Luminosity Cerenkov Integrating Detector (LUCID), the Zero Degree Calorimeter (ZDC) and the Absolute Luminosity For ATLAS detector (ALFA).

Figure 1.5: Relative position of LUCID, ZDC and ALFA detectors on one side of IP [7].

1. LUCID, the only detector primarily dedicated to real time luminosity monitoring, is a relative luminosity detector. Its main purpose is to detect inelastic pp scatering in the forward direction in order to both measure the integrated luminosity and to provide online monitoring of the instantaneous luminosity and beam conditions. Potentially LUCID could also be used for diﬀractive studies (studies about forward protons with momenta close to the beam momentum which arise either from elastic scattering or from single or double 9

diﬀraction), for example as a rapidity-gap veto or as a tag for a diﬀractive signal [8]. The luminosity monitoring is based on the fact that the inelastic pp rate (Rpp) seen by LUCID is proportional to the luminosity,

Rpp = µLUCID · fBC = σinel · εLUCID · L (1.2)

where µLUCID is the mean number of inelastic pp interactions per bunch - crossing (BC) seen by LUCID and it is related to the luminosity L by the inelastic cross section σinel and the LUCID detection eﬃciency εLUCID. fBC represents the bunch crossing rate. The value of µLUCID can be measured by LUCID in several ways [9],

• Zero Counting: µLUCID = −ln(NZeroBC /NT otBC )

• Hit Counting: µLUCID = hNHits/BC i/hNHits/ppi

• Particle Counting: µLUCID = hNP articles/BC i/hNP articles/ppi

The ﬁrst method determines µLUCID from the ratio between the number of non-colliding BCs and the total number of BCs. The two following methods in principle determine µLUCID from the ratio of the mean number of particles per BC and the mean number of particles per inelastic interaction, both seen by LUCID. Hit counting normally refers to particle counting, but where the counting capability of the detector is limited by its granularity.

Figure 1.6: LUCID detector with its Cerenkov tubes (left) and read-out module with photomultipliers and ﬁbers (right) [10].

LUCID is composed of two modules located at ±17 m from the interaction point that provide a coverage 5.5 < |η| < 5.9 for charged particles. Each LUCID detector is a symmetric array of 1.5 m long polished aluminium tubes that surrounds the beam pipe and points toward the ATLAS interaction point (see Fig. 1.6) . This results in a maximum of Cerenkov emision from charged particles from the IP that transverse the full length of the tube. Each tube is 15 mm in diameter and ﬁlled with C4F10 gas maintained at a pressure of 1.2 − 1.4 bar giving a Cerenkov light emitted by the particle traversing the tube has a half-angle of 3o and is reﬂected an average 3 − 4 times before the 10

light is measured by photomultiplier tubes which match the size of Cerenkov tubes [11].

2. ZDCs are compact calorimeters that are located at approximately zero degrees to the incident beams on either side of IP1, 140 m downstream from IP (see Fig. 1.4 and 1.7). Thus ZDCs observe forward going neutral particles that are produced in heavy ion, pA, or pp collisions. ZDCs provide coverage in the region |η| > 8.3 and reside in a slot in the TAN (Target Absorber Neutral) absorber, which would otherwise contain copper shielding. The ZDC is located at ±140 m from the interaction point, at a place where the straight section of the beam-pipe divides into independent beam pipes as shown in Figure 1.7.

Figure 1.7: Position of ZDC detectors near to the IP (left). Schematic view of one arm of ZDC in the TAN (right) [10].

There are four ZDC modules installed per arm: one electromagnetic module (EM) and three hadronic modules (see Fig. 1.8). Each EM module consists of 11 tungstens plates, with their faces perpendicular to the beam direction. The height of these plates is extended in the vertical direction with 290 mm long steel plates. Two types of quartz radiator are used: vertical quartz strips for energy measurement and horizontal quartz rods which provide position information [11]. 11

Figure 1.8: Side view of the EM and hadronic ZDC modules [10].

3. The ALFA detector has been designed to measure the absolute luminosity via elastic scattering at small angles in the Coulomb-nulcear interference region. More details about ALFA will be shown in chapter 4. Chapter 2

Physics at LHC

The main goal of the LHC experiment is to explore for ﬁrst time a new scale of energies and distances, to TeV scales and beyond. Among the speciﬁc goals of this exploration are:

• Higgs boson discovery in standard electroweak Weinberg-Salam model

• Physics beyond the Standard Model (SM), such as Supersymmetry discovery.

However, there are some others lines for research such as B-physics, heavy ion physics, top quark physics, Standard Model physics (QCD, electroweak interactions), among others [12].

2.1 Standard Model

The laws of Nature are sumarized in Standard Model of particle physics and provides a valid framework for the description of Nature, tested from microscopic scales of order 10−16 cm up to cosmological distances of order 1028 cm. The SM is divided into three components and they are described as follow.

1 1. The basics constituents of matter are fundamental spin 2 fermions: six leptons and six quarks which are organized in three families with the same structure as described in table 2.1. In the SM the matter field are the quarks which are the constituents of protons, neutrons, and all hadrons, endowed with color and electro-weak charges, and − − − the leptons (electron e , muon µ , tau τ and their associated neutrinos νe, νµ, ντ .) without color charge but electroweak charge. Although there is not explanation yet for this repetition of fermion families, the entire set of these constituents has been identified experimentally. However, the profile of the top quark, the mixing between the lepton states and the quark states, and in particular, the structure of the neutrino sector, are at present the properties least known of the basic constituents [14]. The electromagnetic and weak forces are unified in the Standard (Glashow- Weinberg-Salam) Model. The fields associated with these forces and the fields associated with the strong force, are spin-1 fields, describing the photon γ, the electroweak gauge bosons W ± and Z0, and the gluons g. There are also three strong charges, called color charges and three electro-weak charges (which

12 13

Name Symbol Mass Q/|e| J Leptons Electron e 0.511 MeV -1 Muon µ 105.7 MeV -1 Tau τ 1776.82 ± 0.16 MeV -1 Electron neutrino νe < 2 eV 0 1/2 Muon neutrino νµ < 0.19 MeV 0 Tau neutrino ντ < 18.2 MeV 0 2 Quarks up u 1.5 to 3.0 MeV + 3 2 charm c 1.27 ± 0.09 GeV + 3 2 top t 172.7±0.9 GeV + 3 1 down d 3 to 7 MeV − 3 1/2 1 strange s 101 ± 29 MeV − 3 1 botom b 4.20 ±0.07 GeV − 3 Gauge and Photon γ < 1 × 10−18 eV 0 bosons W ± bosons W 80.40 ± 0.03 GeV ± 1 1 Z± boson Z 91.1876 ± 0.0021 GeV 0

Table 2.1: Table of elementary particles of the four fundamental interactions with their measured masses and some of their quantum numbers [13].

include the electric charge). The force carriers, all of spin-1, are the photon γ, the gauge bosons W +,W − and Z0 and the eight strong interaction gluons g. On the other hand, the photons and gluons have zero masses due to the exact conservation of corresponding symmetry generators, the electric charge and the eight color charges.

2. The third component of the Standard Model is the Higgs Mechanism, which predicts the presence in the physical spectrum of one spin 0 particle, the Higgs boson, which is expected to be observed and this is one of the main goals of LHC experiments. The Higgs production mechanism will be described in detail in the next section.

Even if all above constituents of the Standard Model will be established experimentally in the near future, the model cannot be considered as the last truth about the matter and forces. Neither the fundamental parameters, masses and couplings, nor the symmetry pattern can be derived. These elements are merely built into the model by hand. Additionaly, the gravity with a structure quite diﬀerent from the electroweak and strong forces, is not coherently incorporated in the theory [12].

2.2 Soft Diﬀraction studies in ATLAS

Diffractive processes (elastic and inelastic) constitute a substantial part (about 1/2) of the total interaction cross sections of hadrons at high energies. Investigation of these processes provides important information on the mechanisms of high-energy interactions. The majority of the physics studies in the ATLAS experiment are made using its central detectors. The forward detectors can be used as either a trigger or to measure the properties of the diffractive events in early ATLAS data. The Minimum Bias Trigger Scintillator (MBTS) is used to trigger both minimum 14 bias and diffractive events and can be used to impose a rapidity gap (i.e. particle veto) in the regions 2.1 < |η| < 3.8. In addition to MBTS, LUCID and the ZDC will be used to trigger events. In principle, boths LUCID and ZDC can also be used to define rapidity gaps in the very forward region in order to select central exclusive events and single diffractive events [15].

Soft single diffraction is a low t-process (t being standard Mandelstam variable), where a color singlet is exchanged in t channel between two protons and one of the protons break up into a dissociative system. A similar process is soft double diffraction, but in this process both protons breaks up into dissociative system (see Fig. 2.1). As the exchanged object is color singlet, there is a large rapidity gap between intact proton and dissociative system (or between dissociative systems in case of double diffraction). Expected cross sections at LHC are about 12 mb in case of soft single diffraction and about 7 mb in case of soft double diffraction.

Figure 2.1: Soft processes with the elastic and the single diﬀraction [16].

Soft single diffraction can be measured by ALFA by tagging the outgoing proton, but it can be measured also with the more central detectors by imposing a pseudorapidity gap in the forward regions. With such high cross section, only about two weeks of data taking at lowest luminosity (1031 cm−2s−1) is required by this analysis (it is expected to collect sample of about one million events in two weeks at luminosity of 1031 cm−2s−1). The measurement of soft single diffraction at ATLAS will be focused on the diffractive mass distribution, MX , and the fractional energy loss of M√X proton, ξ = s . ATLAS will cover several orders of magnitude in ξ. MBTS, LUCID and ZDC will be used as triggers. When installed, ALFA will be able to measure directly proton momentum loss ξ during the special runs with high-β∗ optics and luminosity of 1027 cm−2s−1 [15]. Chapter 3

Luminosity

To describe precisely the properties of a collider accelerator, various parameters are used, and among the most important is the luminosity. The luminosity L is a general concept that can be understood as the number of particles per unit area per unit time generated in the collision of the two beams, being the factor of proportionality between event rate R and the interaction cross section σint [13]:

R = L σint, (3.1) −2 −1 thus the unit of the luminosity is cm s . If two particle bunches, containing n1 and n2 particles, collide head-on with frequency f, the luminosity is given by n n L = f 1 2 (3.2) A where A denotes the beam cross section at the collision point. Numerous methods of the LHC luminosity measurements have been proposed, which can be consulted in [17–19]. The next section will discuss the generalities of some of them.

3.1 Methods to measure the luminosity

In general, the methods for the luminosity measurements are based on : 1. the machine parameters and measured properties of the colliding beams: The basic idea is to measure the absolute luminosity under much simplified and carefully controlled conditions and calibrate any relative luminosity monitor of the machine or of the experiments under such optimal conditions [20]. A simple case is illustrated by the expression (3.2) , where the revolution frequency f in a collider is accurately known. The number of particles circulating can be continuosly measured with beam current transformers, but the determination of particles n1 and n2 contributing to the luminosity is non trivial because must be sure that there is not a significant number of particles outside the nominal bunches. The effective transverse area A, in which the collisions take place, can be known depending on the type of the distribution that characterizes the bunches. For a uniform transverse particle distribution, A would be directly equal to the transverse beam cross section. For more general distributions, A can be calculated from the overlap integral of the two transverse beam distributions g1(x, y) and g2(x, y) according to [21] 1 Z = g (x, y)g (x, y)dxdy. (3.3) A 1 2 15 16

If the transverse distributions are equal and with Gaussian shape, then A is given by A = 4πσxσy. (3.4) This method oﬀers, in the worst case when the beams do not collide head-on, an accuracy of the order of 10% for a 2σ separation between both beams [20].

2. the measurement of the rates of the electromagnetic processes: There is a possibility to measure the luminosity at the LHC observing exclusive lepton production via photon-photon fusion

pp → p + `+`− + p, (3.5)

where ` = e or µ [22]. The figure 3.1a shows the Born amplitude which may be calculated within QED frame, and there are not strong interactions involving the leptons in the final state [18]. Figure 3.1b shows a correction arising from a inelastic proton-proton rescattering, but it can be ignored because the main part of the Born amplitude (fig. 3.1a) comes from large impact parameters b, whereas the rescattering occurs at small b. The proton dissociation contributions (fig. 3.1c) vanish for small transverse momentum pt and they may be also ignored due to the event selection of leading protons with pt . 30 MeV [22].

Figure 3.1: (a) Lepton pair production in pp collisions, (b) one of the rescattering corrections, and (c) a possible contamination coming from proton dissociation into X,Y systems [17].

The detection of µ+µ− and e+e− processes differ and therefore their use as a luminosity monitor is different. For muons processes, the applied cuts to the transverse energy (Et & 5 GeV) and the transverse momentum (pt . 30 MeV) of muons, which are necessary for their identification, could reduce the cross section significantly. However, the muons have the advantage that is possible to trace the tracks back to the interaction vertex, and hence isolate the interaction in pile-up events. Thus, µ+µ− production can act as a luminometer in high luminosity LHC runs. An accuracy of ±2% is claimed provided the muon trigger is good enough [18].

+ − For e e events production is not necessary to select events with large pt, and + − so pt domain can be considered small where e e production cross section is much larger, and where the rescattering corrections becomes totally negligible 17

[18]. For low luminosities L . 1032 cm−2s−1, it is claimed that an absolute luminosity measurement down ±1% is possible, but for high luminosity the e+e− method may be limited by pile-up eﬀects.

3. W and Z production: The W and Z production in high energy pp collisions have clean signatures through their leptonic decay modes, W → `ν and Z → `+`−, and so may be considered as potential luminosity monitors. A vital ingredient is the accuracy to which the cross section for W and Z production can be theoretically calculated. The cross sections depend on parton distributions, especially quark densities, in a kinematic region where they are believed to be reliably known [17]. The cross sections for W and Z production can be predicted to next-to-next-leading order corrections, which at ﬁrst sight would seem to provide an LHC luminometer with ±1% accuracy. However, the uncertainties in the input to the global parton indicate that the error could be as larger as ±4% [17].

4. elastic scattering of protons at small angles: An extrapolation to zero scattering angles in combination with a measurement of the total inelastic rate can be used to determine the luminosity via the optical theorem [20]. This method will be disscused in detail in the next section.

3.2 Absolute luminosity determination

3.2.1 Elastic scattering at small angles The rate of elastic scattering is linked to the total interaction rate through of the optical theorem, which states that the total cross section is directly proportional to the imaginary part of the forward elastic scattering amplitude extrapolated to zero momentum transfer [23]: σtot = 4π · Im[fel(t = 0)] (3.6) where at small values of t, −t = (pθ)2. (3.7) Here p is the beam momentum and θ the forward scattering angle. Equation (3.7) is a valid approximation for the small values of θ. The optical theorem implies that a measurement of elastic scattering in the forward direction will always provide additional information on the luminosity. This fact can be used as it will be described below.

The luminosity can be determined using elastic scattering and the inelastic rate.

By measuring the total interaction rate Rtot and the elastic rate dRel/dt in the t=0 forward direction simultaneosly, both the luminosity and the total cross section can be determined. The expressions, wich can be directly derived from the optical theorem and the deﬁnition of Luminosity L = R/σ, are given by

1 R2 (1 + ρ2) tot L = (3.8) 16π dRel/dt t=0 18 and

dRel/dt 16π t=0 σtot = 2 (3.9) (1 + ρ ) Rtot where ρ is deﬁned as:

Refel(t) ρ = . (3.10)

Imfel(t) t=0 The ρ-parameter is suﬃciently well known not to contribute signiﬁcantly to the systematic error. Recent predictions of the ρ-parameter at LHC energies is in the range 0.13 − 0.14 [23]. Assuming an error of ±0.02 implies an uncertainty of less than 0.5% in the luminosity from the uncertainty of ρ.

Using 3.8 is the standard way of determining the luminosity from elastic scattering. This method requires a precise measurement of the inelastic rate with good coverage in rapidity |η|. The |η| range of ATLAS is somewhat limited in this context. The accuracy of the luminosity determination will be dominated by the uncertainty of the Monte-Carlo based extrapolations of inelastic rate to the highest |η| values.

3.2.2 Luminosity determination from Coulomb nuclear interference The rate of elastic scattering at small t-values can be written as ! 2 dNel 2α σtot = L π(f + f )2 ' L π − + (i + ρ)e−b|t|/2 (3.11) C N dt t=0 |t| 4π where the first term corresponds to the Coulomb and the second to the strong interaction amplitude, α is the fine-structure constant and b is the slope of the forward differential cross section with respect to t. In this equation the proton form factor has been excluded [23]. The formula is oversimplified and there are also other corrections that should be included in the final analiysis. Since in practice the data are fitted to the equation 3.1 the ρ-parameter, the total cross section (σtot) and b can be determined as well [24]. Figure 3.2 shows the different contributions to the elastic cross section as given by 3.11. The Coulomb region, the strong interaction region and the interference region are clearly seen. 19

1000 ]

mb -2 [ b=18 GeV , σ =100 mb 800 tot

/dt ρ=0.15 σ

d ρ=0 ρ=0, α=0 600

400

200

0 0 0.005 0.01 0.015 0.02 0.025 0.03 -t [GeV2]

Figure 3.2: Diﬀerential total cross section as a function of t around the Coulomb Nuclear Interference region (CNI). The data have been plotted for ρ = 0 and ρ = 0.15 to illustrate the CNI region. The strong interaction contribution alone is also shown for α = 0 [23].

At the nominal energy of the LHC (7 TeV), the strong amplitude is expected to equal the electromagnetic amplitude for |t| = 0.00065 GeV 2. This corresponds to a scattering angle of 3.5µrad. To indicate the scale of the dificulty: at SPS collider the Coulomb region was reached at scattering angles of 120µrad. This large difference is due to the energy difference but also because the total cross section increases with energy. Such small scattering angles can be readed using special optics and running conditions of the LHC machine, accompanied with very performant edges- less detectors and Roman Pot (RP) system. The optics conditions will be explained in the next section.

3.3 Required beam optics properties

3.3.1 An overview of the optics Modern accelerators employ alternating gradient focussing provided by quadrupole magnetic field. Using quadrupoles is one of the most efficient way to keep the beam focused throughout the pipe and to control the twiss parameters of the beam. If the quadrupole poles have a hyperbolic shape (see Fig. 3.3), then it can generate an approximately constant magnetic field gradient B0, which acts as a focusing lens in x and defocusing in y, or vice versa. 20

Figure 3.3: Quadrupole magnet in an accelerator. The beam axis goes into the page [25].

Particle motion around a closed orbit is called betatron motion and it is characterized by its small amplitude. The linearized betatron equation that describes betatron motion is known as Hill’s equation

00 00 x + Kx(s)x = 0, y + Ky(s)y = 0 (3.12)

0 −2 0 where B is the magnetic ﬁel amplitude, Kx(s) = B /(Bρ)+ρ , Ky(s) = −B (s)/(Bρ), 0 and B (s) = ∂Bz/∂x evaluated at the longitudinal coordinate s. ρ is known as the bending radius. The focusing functions are periodic with Kx,y(s+L) = Kxy(s), with L the length of a periodic structure [26]. The solutions of Hill’s equation 3.12 has the form q x(s) = A (β(s))cos(ψ(s) + δ) (3.13) A x0(s) = −q [α(s)cos(ψ(s) + δ) + sin(ψ(s) + δ)], β(s) where x0(s) = dx(s)/ds, A and δ are constants of integration, β(s) is the betatron amplitude function and α(s) = −β0(s)/2. ψ(s) is the advance phase and is related to beta function β(s) through dψ 1 = . (3.14) ds β The general solution of Hill’s equation can be also expressed using the matrix nota- tion as follows     u(s) u(0)      u0(s)  = M  u0(0)  , (3.15) (∆p)/p (∆p(0))/p with M given by

 q √  β/β0(cosψ + α0sinψ) ββ0sinψ Du  √ q   0  (3.16)  ((α0 − α)cosψ − (1 + αα0)sinψ)/ ββ0 β/β0(cosψ − αsinψ) Du  0 0 1 21 where u denotes either the transversal space coordinate x or y and u0 the trajectory slope, D is the dispersion acting on particles with a momentum loss ∆p/p and β0 α0 are β-function and α evaluated at s = 0, espectively. For elastic scattering, the momentum loss ∆p/p is irrelevant and can be neglected [23]. Thus, from 3.15 the observed displacement u oﬀ the nominal orbit in the detector plane is related to the vertex and trajectory slope at the IP by q q 0 u = β/β0(cosψ + α0sinψ)u(0) + ββ0sinψ · u (0). (3.17) 0 Here, u (0) = θu(0) is the u-component of the scattering angle. Since there are detectors on both sides of the IP to measure the elastic scattering, one should take into account the diﬀerence of the left (L) and right (R) arm measurement, so that

uL − uR = 2Leff (3.18) √ where the effective lever arm L =eff ββ0θ(0) determines the precision of the scattering angle measurement. The t-values is then determined by 2 2 2 −t = p (θx(0) + θy(0)). (3.19) The LHC has two beam pipes in the horizontal plane separated by 194 mm and it represents a considerable technical advantage to approach the beam from above and below the beam axis compared to approach from the sides. Then, can be used what is known as parallel-to-point optics from the interaction region to the detector location as shown in figure 3.4. In such an optics, the betatron oscillation between the interaction point of the elastic collision and the detector position has a 90 degree phase difference in the measuring vertical plane such that all particles scattered at the same angle are focused at the same locus at the detector, independent on their vertex position [24].

Figure 3.4: Concept of the parallel-to-point optics [27].

The general formula 3.17 reduces to q y = ββ0θy(0) (3.20) when the phase advance is π/2 and α(0) ∼= 0 in the vertical plane. The minimum t-value reachable tmin will then given by particles unscattered in the horizontal plane and with the minimum scattering angle in the vertical plane: 2 2 2 p ydet −tmin = (pθmin) = (3.21) ββ0 22

wihere ydet is the smallest distance possible between the center of the beam and the edge of the detector. The relevant variable for beam halo considerations is the distance from the beam center expressed in terms of the multiple of the rms size of teh beam spot at the detector. The beam spot is given by q σ = εβ (3.22) so that ydet can be written as q ydet = nd εβ (3.23) where nd is the smallest possible distance to the beam center expressed as a multiple of the beam spot rms size. Combining 3.21 and 3.23 −tmin can be deﬁned as

2 2 ε −tmin = p nd (3.24) β0 with ε = εN /γ, where ε denotes the emmitance [23]. If a normalized emmitance εN of 1 µm rad and a minimum distance to the detector corresponding to nd = 15, a 2 tmin = 0.0006 GeV can be reached for a β0 of 2600 m as larger. At the value of tmin the acceptance vanishes and to get for example an acceptance of at least 50% at −t = 0.0006 GeV 2 the detector has to be placed at a closer distance of about nd = 12. Chapter 4

The ALFA method

As mentioned in the previous chapter the machine parameters of the LHC can be used to determine the luminosity with an accuracy of . 10%. Other methods such as measurement of the rates of the lepton or W/Z production oﬀer a precision about 5%. The measurement of elastic pp-scattering is a way to reach a better precision approaching the beam to milimeter range. These measurements can only be performed with special conditions, which are also used for calibration of the LUCID detector. High-beta (β0) optics in combination with reduced beam emittance are required, as has been discussed in section 3.3. The solution is given by the ALFA (Absolute Luminosity For ATLAS) detector project. This system is composed by a set of 4 Roman Pot (RP) units. More details will be given in the next section.

Figure 4.1: Schematic view of the positioning of the Roman Pots on each side of the ATLAS IP [28].

4.1 Roman Pot stations

The Roman Pot (RP) technique has been successfully used in the past for measurements very close to circulating beams and therefore been adopted for ATLAS. The RPs will be located ±240 m away from the IP, and on each side there will be two RP stations separated by 4 m (see Fig. 4.1). The RP have been designed to move the detectors as close as 1 mm (10σ, where σ is given by Eq. 3.22) to the beam, but only from above and below, due to the mechanical constraints imposed by the two

23 24 horizontal beam-pipes of the LHC, as shown in ﬁgure 4.2, in which the working and retracted positions can be seen. The working position will bring the bottom surface of a pot to a minimal distance of 10σ from the beam [23].

Figure 4.2: The RP concept: the retracted position is shown on the left, where the Pot is placed out from beam. On the right the working position is shown, where the Pot is approached up to 10σ to the beam [23].

The design of the RPs obeys strict requirements to ensure the positioning precision and the operating conditions of the LHC accelerator. Such requirements are the use of Ultra High Vacuum (UVH) equipments to guarantee long beam life and the physical separation of the detectors and read-out electronics from the LHC primary vacuum, but ensuring the closest possible approach of the detector to the beam. The device that allow the independent movement of the top and bottoms pots is known as Roman Pot unit. Each unit is composed of a main body ensuring the needed stiﬀness of the system, and two sets of movable arms, each able to ensure the precise movement of the two pots. Each Roman Pot unit uses a high precision roller screw moved by a step motor to allow the independent moving of the top and bottom pot to the nominal position. Figure 4.3(a) shows the prototype of the Roman Pot unit used in the test beam. Figure 4.3(b) shows a 3D view of a Roman Pot unit installed in the tunnel [23]. 25

(a) (b)

Figure 4.3: A picture of the Roman Pot unit is shown in (a) before test beam [29]. In (b) is shown a 3D view of a Roman Pot unit installed in the tunnel [23].

4.1.1 ALFA scintillating fibre detector The operation of standard scintillating fibres is based on the principle that the scintillation light produced in the fibre by the passage of a ionizing particle or radiation is trapped by internal reflections on the fibre interface. ALFA uses scintillating fibers of square cross section of 0.5 × 0.5 mm2 (SCSF-78, S-type from Kuraray Co LTD) with a cladding thickness of 10 µm, which gives a sensitive area of 480 × 480 µm2 per fiber. Ten titanium plates support two layers of 64 fibres arranged in UV o geometry under an angle of ±√45 relative to the vertical axis. The planes, staggered by multiple of (500µm/10)× 2 = 70.7 µm, are assembled on precision pins to obtain an effective fibre pitch of 50µm. Its ultimate√ spatial resolution, ignoring any geometrical√ imperfection, is σx,y = (500µm/10)/ 12 = 14.4µm, where the factor 1/ 12 comes from the fact that the width of the fibers are uniformly distributed. 26

(a) (b)

Figure 4.4: (a). Arrangement of the scintillating fibres using UV-geometry. The overlap region of the fibres layers determines the active area. (b). ALFA detector with the scintillating fibers [29].

The ﬁbres are routed in groups of 64 to the photodetectors, so that the scintillation light is read by Multi Anode Photo Multiplier Tubes (MAPMTs). The MAMPTs fulﬁll some requirements, among these [23]:

• High quantum efficiency at the wavelength of maximum scintillation and capability to detect single photons. The quantum efficiency of the bi- or multialkali photocathode is of order of 20% in the blue range of the optical spectrum. Sin- gle photoelectrons can be detected with an efficiency of typically 70%, leading to an effective detection efficiency of 14%.

• High gain in order to amplify the tiny signals feed into the read-out electronics.

• Acceptable low cost per readout channel.

The above requirements are achieved by Hamamatsu R7600-00-M64 MAPMTs, which have been used for the ALFA test beam [30]. The quantum efficiency of the standard Bialkali photocathode is the order of 25% in the peak. However, a second type of the MAPMTs, the Ultra Bialkali (UBA), has been used for the test beam. This offers a quantum efficiency 10% higher than the Bialkali photocathode at wavelength of 450nm, where the scintillating fibre’s emission spectrum has its maximum approximately, as shown the figure 4.5. 27

(a) (b)

Figure 4.5: (a) MAPMT from the top. (b) Quantum eﬃciency for current Bialkali and Ultra Bialkali (UBA) MAPMTs from Hamamatsu [31].

4.1.2 Overlap detectors Since the exact position of the LHC beam spot at the location of the ALFA tracking station may vary from fill to fill by a couple of mm, it is necessary to position the upper and lower ALFA detectors adequately. The Beam Position Monitor located near the Roman Pot units can be used for a first rough positioning, but the precision with which the distance between the two detector halves is known has a direct consequence on the uncertainty of the luminosity. Considering the error contributions from positioning and angular determination [23], it can be conclude that the vertical distance between the two ALFA half detectors must be known with a precision of about 10µm. Thus, each RP has overlap detectors (ODs) to determine the relative position of the upper and the lower pot. The overlap detectors are detectors which measure only the vertical coordinate. Two ODs are mounted below (and above) the actual detector planes. They move with the detector planes and their relative position to those is fixed and well known. The ODs detect particles in the beam halo region. The active areas of the ODs begin to overlap when the detector halves approach each to other. The distance of the detector halves can be calculated from measurements of particles which traverse boths ODs [23]: 1 X d = (y1,i − y2,i) (4.1) N i The required measurement precision is obtained by recording a sufficiently large number of tracks and calculating their average deviation in the two ODs. The achievable precision depends on three factors: (1) the intrinsic spatial resolution of 28 the OD, (2) the statistics of particles detected with the OD, and (3) the alignment uncertainty between the ODs and the detector halves.

The overlap detectors consist of scintillating fibres of the same type and size as the main detector. They cover an active area of 6 × 15 mm. An OD comprises 3 planes of 30 fibres. The three planes are vertically staggered by 166 and 333 µm, respectively, such that a detector with an effective pitch of 166 µm is formed. The horizontal fibres are bent by 90o and routed upwards to the MAPMTs. In order to maximize the bending radii of the fibres, the 30 fibres are split into two layers of 15 fibres each which are mounted on the front and the back side, respectively, of a ceramic support plate. A fourth ceramic plate supports plastic scintillator tiles which act as trigger counters for the overlap detectors. Figure 4.6 shows a front view of the overlap detectors integrated with the main detectors.

Figure 4.6: Front view of the full detector assembly with the overlap detectors (in blue). The spot and the circle (in red) in the centre represent the beam axis and the beam tube of diameter 50 mm [23].

4.2 ALFA readout electronics

A complete read-out and control system is needed for the ALFA detector. Since the reconstruction of the tracks in the ﬁbre detector is possible through knowledge of the ﬁbre hit then a binary readout system is suitable. Therefore there are some requirements which can be summarized as follows [23]:

1. Channel-by-channel adjustable ampliﬁer gain to compensate for the MAPMT 29

gain spread.

2. High speed: it must be possible to associate signals unambiguosly with a LHC bunch crossing.

3. Adjustable threshold with a minimum setting of < 0.5 photo-electrons(p.e.) in order to guarantee high detection eﬃciency. A common threshold for 64 channels is acceptable if the gains can be adjusted.

4. Negligible cross-talk between channels: < 3%.

5. Compatible with standard ATLAS read-out scheme.

6. Reliability and robustness: the Roman Pot detectors are located in the LHC tunnel, about 240 m from the ATLAS cavern, making interventions extremely diﬃcult. The electronics moves together with the Roman Pots between beam and retracted position. Although these motions are foreseen to be smooth and slow, the mechanics and connectivity of the read-out system must be designed such that a reliable and robust operation can be guaranteed over extended periods.

7. Radiation tolerance: the radiation environment during the specific luminosity runs is not expected to pose serious problems for the electronics. In normal physics runs, once the LHC machine is operated at close to nominal luminosity, the scintillating fibres would soon suffer from radiation damage, and, to a lower degree, the electronics could be degraded as well. The connectivity of the system must allow for a removal and installation of the system.

The readout system is composed by 23 Photo Multiplier Front-end (PMF) arranged in a 5 × 5 matrix, which are linked to the mother board via a kapton cable. Each PMF is composed of a MAPMT (see section 4.1.1) and three printed circuit boards (3cm × 3cm) as shown in Figure 4.7. The ﬁrst one is the HV board, which delivers the high voltage to the 10 dynodes of the MAMPT, the second one is the passive (or intermediate) board which routes the signal to connectors located on the edges, and the last one is an active board which has a Multi-Anode Read-Out Chip (MAROC) directly wire bonded on the printed circuit boards on one side and a FPGA type readout chip on the other [23,32]. 30

Figure 4.7: Picture of MAPMT together with the three printed circuit boards making a PMF unit [23].

Figure 4.8 shows a schematic description of the readout system for the PMF. Inside a PMF, each PMF anode is connected to the input of one channel of the 64 channel readout chip MAROC. The MAROC receives the analog signals from the MAMPT. The block diagram shown in Figure 4.9 explains the functions of MAROC. The signals are then amplified individually. At this stage the signals are splits up to a charge readout patch and a digital readout patch. The signal amplitude is compared to a threshold. If the amplitude is higher than the threshold, the output of the channel is set to a logical "one", otherwise the output stays to logical "zero". The threshold is set by an internal 12 bit DAC composed by a 4 bit thermometer DAC for coarse tuning and a 8 bits mirror for the fine tuning. A trigger output is produced. As the discriminating value is the same for all channels it is important that the gain amplifiers have counteracted the different MAPMT channel gains so the discrimination is similar on all channels. The calibration of the MAMPT gain before the assignment of the threshold will be discussed in Section 5.3. 31

Figure 4.8: General descripton of the readout system of the PMF [23].

Figure 4.9: Block diagram of MAROC [23]. Chapter 5

Set-up of test beam

5.1 Test beam conditions

In October 2009 two complete ALFA Roman Pots have been tested in one of the secondary beam lines (H6) of the Super Proton Synchrotron (SPS), in the North Experimental Area located on the Prévessin (France) site of CERN, using a 120 GeV pion beam. One of the main goals was to perform the test of the complete ALFA station similar to those that will be ﬁnally installed in the LHC tunnel. The ALFA station is composed of the upper and lower pots containing in ALFA1 and ALFA2 prototypes, respectively. A sketch of the setup for the test beam can be seen in ﬁgure 5.1. The set is composed by the EUDET pixel telescope, as external tracking reference, which will be discussed in the next section, and the ALFA station with detectors in the upper and lower RPs. The main detector and main triggers have been indicated as well as the overlap triggers and overlap detectors.

Figure 5.1: Geometry layout of the test beam setup including ALFA and the EUDET pixel telescope as an external reference. (a) Main ﬁber detector. (b) Main trigger tile. (c) Overlap ﬁber detector. (d) Overlap trigger tile.

32 33

5.2 EUDET silicon pixel detector

An external tracking reference is necessary to know very well the position of the incident beam particles. To achieve this a silicon pixel telescope has been used, which is developed by the EUDET project, supported by the European Union in the 6th Framework Programme to provide infrastructures for R&D of detector technologies. The EUDET pixel telescope has a high position resolution (σ < 3.0 µm) and a readout rate of about 1 kHz [33,34]. It consists of the followings components: • Support mechanics to position the sensor boards in the particle beam. • Pixel sensors with a single point resolution of 2 − 3 µm. • Two boxes with 3 silicon pixel sensors each are placed before and after the Device Under Test (DUT) to form a 2-arm telescope1. Within the boxes the planes are separated by 10 cm. • A DAQ system including hardware, trigger and software. • An analysis software for reconstruction and alignment.

Figure 5.2: EUDET telescope installed for the test beam along the beam line [29].

The sensors had just been upgrade to Mimosa 26 with an active area of 21.2×10.6 mm2 and 1152 × 572 pixels. More details about the pixel telescope can be found in [33]. For the test beam only 5 sensors were working and ALFA was placed after EUDET because the space for DUT was insuﬃcient. Thus the tracks have been extrapolated, resulting a lower resolution compared to placing ALFA as a DUT between the two EUDET arms. 1For the test beam there was not the device under test between both arms. 34

5.3 Conditions of data taking

About 3×107 events were recorded during the whole test beam period. The following conditions of data taking were changed:

• high voltage (HV) of the MAMPTs

• threshold for the digital readout

• gain of the MAMPTs before digital readout

• position scan of the active areas of ALFA detector with respect to the EUDET telescope

Three parameter scans were used during the data taking. They are denoted as GAIN, Thermo and HV. Each of them will be explained in what follows.

1. GAIN : GAIN is the overall gain factor for all MAPMT channels. This is used to amplify or reduce the original analogue signal. Each PMF has 64 charge amplifiers and each amplifier has a 6 bits which resolves in 64 levels of amplification. Then the GAIN parameter has to be divided by 16 to obtain the amplified factor. This means that GAIN=16 corresponds to amplification factor equal to 1, GAIN=32 corresponds to 2 and so on. But the gain of the different MAMPT channels can vary up to a factor 2-3. Thus, the overall gain of the MAPMT varies also about a factor 2-3. To compensate these gain variations uniformity corrections (Gain equalization) of individual channels are required. To obtain uniform signals from all MAPMT channels the gain factor is calculated from W anted Q (in ADC counts) Gain factor = 1PE × 16. (5.1) F itted Q1PE (in ADC counts)

where Q1PE is the mean charge related to 1 photoelectron for a MAPMT gain of 106 2. Figure 5.3 shows the ﬁt of the MAMPT charge spectrum to determine the position of the 1 photoelectron (pe). It is possible to take the distance between pedestal and mean of 1 pe contribution as correction factor for each channel. The pedestal represents triggers with a charge in the MAPMT channel. 2The gain of the MAMPT is the order 106 with a BIAS voltage of −1000 V. 35

Figure 5.3: Charge spectrum from a MAMPT channel. The position of the 1 pe is indicated. A small 2 pe (red) contribution is also shown. [30]

The 1 pe position of the MAPMT channels of the MAPMTs in ALFA 1 as they were before and after the calibration can be seen in Figures 5.4(a) and 5.4(b), respectively. The eﬀect of the gain equalization calibration is clearly visible from these plots.

(a) (b)

Figure 5.4: 1 PE position of the MAMPT channels of ALFA 1. In (a) before the calibration and in (b) after the calibration. [30]

The gain factors were scanned at values: 12, 16, 24, equalized and 1.5×equalized. 36

2. Thermo: As mentioned in section 4.2 the threshold is set thanks a DAC register composed by a 4 bit thermometer DAC and a 8 bit mirror DAC. The value of the threshold is given by the formula

DAC = T hermo ∗ 256 + mirror (5.2)

where T hermo = [1, 15] and mirror = [0, 255]. This allows to understand another consequence of the gain equalization on the digital output. To verify the performance of electronic and MAPMTs the S-curves are used, which scan the threshold applied to the PMF and its effect on the trigger efficiency. As an example a S-curve is shown in Figure 5.5 for one MAPMT channel. A large plateau of the trigger efficiency is observed going from DAC ∼ 1000 to 2300, where 1 pe will be related to approximately DAC = 1000. If the S-curves are evaluated for GAIN = 16 and equalized gain then the gain equalization is only visible on the homogeneity of the final decrease as shown in Figure 5.6.

Figure 5.5: This S-curve which represents the trigger eﬃciency vs. the threshold has been obtained with a LED calibrated to 1 pe. One can observe a large plateau at light level adjusted to 0.2 pe. [30] 37

(a) (b)

Figure 5.6: S-curves of the channels of a PMF tube. The plot in (a) was obtained for GAIN= 16 while the plot in (b) was obtained for equalized gains. [30]

The threshold was scanned at values thermo: 6, 7, 8, 9, 10, 11 and 12.

3. HV : The high voltage (HV) of the MAPMTs was scanned at values -800 V, -850 V, -900 V and -950 V.

5.3.1 Data collection Most of the test beam was dedicated to optimizing the gain, high voltage of the MAPMTs and the threshold of the readout of the MAPMTs. First GAIN was ﬁxed at gain factors equalized (or gain factor 16) and the threshold at T hermo = 7. Then the HV was scanned at the mentioned values.

Similarly, GAIN was ﬁxed to equalized gain and the HV at -900 V while the threshold was scanned at values of Thermo mentioned above. Additional runs were carried out setting the threshold at Thermo=7 and the HV at -900 V. Then GAIN was scanned at diﬀerent values.

In adition to the above measurements also some runs were performed with the alignment of the detector perpendicular to the beam to get best resolution by best stagger. Other runs were performed to:

• test the entire ALFA detector with the EUDET telescope in front moving the ALFA station and to determine the precise position of the main detectors relative to overlap detectors.

• test diﬀerent conditions of losing the vacuum to investigate the eﬀect on the ALFA detector.

Figure 5.7(a) is an example of the tracks obtained on the active area of the lower detector whit the beam proﬁle clearly visible. In (b) the inside of the vacuum chamber can be seen with of the Roman Pots in working position close to the beam. 38

Hit map for full pot HitMap Entries 90867

40 Mean x 0.04586

Mean y -6.794 450 RMS x 7.974 30 RMS y 3.641 400

20 350

10 300

0 250

200 -10 150 -20 100

-30 50

-40 0 -30 -20 -10 0 10 20 30

(a) (b)

Figure 5.7: (a). An example of the tracks on the active areas of the lower (ALFA 2) and overlap detectors. (b). Internal view of the vacuum chamber with the RPs in working position [29].

With these and more combinations, the raw data were converted into an appropiate format to do the analysis based on the ROOT analysis software [30]. Although a basic analysis was performed during the test beam to monitor the data during data collection, the main analysis was carried out after the test beam period. Chapter 6

Data analysis

In this chapter some important parameters of the ALFA ﬁber detector will be analy- sed: hit multiplicity, layer eﬃciency and spatial resolution. These parameters allow the tracking performance of the detector.

6.1 Hit multiplicity

The hit multiplicity is a simple monitor of the data quality. The hit multiplicity gives information about the number of fibers per layer that have detected a particle passing the ALFA detector. Although in the ideal case the hit multiplicity should be exactly equal to one in each layer, there is also a certain noise contribution that may increase it. Moreover, there is an effect that may decrease the mean hit multiplicity. The cladding thickness of the scintillating fiber will reduce the active area by about 20µm/500µm = 4%. It means that if a proton is passing through the cladding area there will be no scintillating light. The creation of showers from the interaction of particles with other parts of the detector can also cause an increase in the average multiplicity.

105 106 Mean 1.497 Mean 1.309 RMS 1.422 RMS 0.7897 4

Ocurrence 5 Ocurrence 10 10

104 103

3 10 102

102 10

10 1 0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 40 45 Hit multiplicity of all layers Hit multiplicity of layer 2 (a) (b)

Figure 6.1: Hit multiplicity of ALFA 1 (HV=900 V, Thermo=7, GAIN=Eq.). In (a) the hit multiplicity of all layers is shown. The plot in (b) corresponds to the hit multiplicity of layer 2.

Figure 6.1(a) show a histogram of hit multiplicity of all 20 layers of ALFA 1

39 40

(upper detector) where the mean value is about 1.5 hits per layer. In ﬁgure 6.1(b) the hit multiplicity only for layer 2 is shown with a mean value of 1.3 hits. The contribution of the those hits that extend to the maximum multiplicity of 64 results from hadronic particle showers.

As mentioned in section 3 three Ultra Bialkali MAMPTs were used in the readout system of the lower detector (ALFA 2). The MAMPTs with Ultra Bialkali are characterized by higher eﬃciency which will result in higher hit multiplicity. A comparison of the mean hit multiplicity of all layers in the upper and lower detector can be seen in the ﬁgure 6.2.

2.5 Gain=Eq, HV=900, Thermo=7 2.5 Gain=Eq, HV=900, Thermo=7

ALFA 1 ALFA 2 2 2

1.5 Mean hit multiplicity 1.5 Mean hit multiplicity

1 1

0.5 0.5

0 0 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Layer Layer (a) (b)

Figure 6.2: Mean hit multiplicity for each layer in (a) ALFA 1 and (b) ALFA2. For both detectors the mean hit multiplicity is about 1.5 hits per layer. In (b) the layers 5, 10 and 17 are read by the UBA MAMPTs which explains the increase in the hit multiplicity.

In Figure 6.2(b) the dashed line of the mean value was computed without taking into account layers 5, 10 and 17. The contribution of the UBA MAMPTs is higher by ∼ 50% of the mean value of the hit multiplicity for layers read by Bialkali MAMPTs. These plots also show a slight slope upward and downward to the right in (a) and (b) respectively. For the upper detector ALFA1 the multiplicity increases with the layer index in the direction of the particles traversing the detector and generating in a fraction of events hadronic showers. The lower detector ALFA2 is rotated by 180o and the order of fibres is inverted, thus a negative slope in the multiplicity evolution across the layers is observed. This is confirmed with a simulation made of the effect of hadronic showers on the mean hit multiplicity [35]. Two plots from this study are shown in Figure 6.3. They show the simulated mean hit multiplicity for different layers of different RPs with and without hadronic interactions. 41

Figure 6.3: Simulated mean hit multiplicity for all layers of diﬀerent RPs. The plot on left shows the mean hit multiplicity only with the electromagnetic contribution. Electromagnetic and hadronic contribution are shown on the right [35].

For ALFA 2 the calculation of mean multiplicity is made without taking into account the contributions of the Ultra Bialkali MAMPTs of the layers 5, 10 and 17. The dependence of the mean hit multiplicity on the readout parameters shown in Figures 6.4 and 6.5 for ALFA 1 and ALFA 2. These plots were obtained without the track reconstruction with EUDET telescope and therefore they include the contribution of the hadronic showers.

HV=900 V 1.8 ALFA 1 ; Gain = Eq. ALFA 2 ; Gain = Eq. 1.7 ALFA 2 ; Gain = 16 1.6

1.5 Mean hit multiplicity 1.4

1.3

1.2

1.1

1 6 6.5 7 7.5 8 8.5 9 9.5 10 Thermo

Figure 6.4: Mean hit multiplicity as a function of the discriminator threshold.

The mean hit multiplicity decreases with increasing signal threshold as seen in Figure 6.4. The curves are very similar if ALFA 1 and ALFA 2 have both equalized GAIN settings. This is due to the overlap of the good performance of both detectors under the same conditions. A range of threshold values lower than T hermo = 9 would be suitable for tracking. 42

ALFA 2: Thermo=7,HV=900 V 2

1.9

1.8

1.7

1.6 Mean hit multiplicity 1.5

1.4

1.3

1.2

1.1

1 12 14 16 18 20 22 24 Gain (a) ALFA 1: Gain=16 2.2

2 Thermo=6 Thermo=7 Thermo=8 1.8 Thermo=9 Thermo=10 Mean hit multiplicity 1.6

1.4

1.2

1 840 860 880 900 920 940 960 HV (V)

(b)

Figure 6.5: Dependence of the mean hit multiplicity on (a) gain factor and (b) high voltage HV.

During the beam test a few runs were performed by varying the GAIN factor in ALFA 2 with fixed values of threshold. The result of such a variation on the mean hit multiplicity is shown in Fig. 6.5(a). An increase in mean hit multiplicity can be seen which does not exceed 1.5 hits. On the other hand a study were also performed on the dependence between mean hit multiplicity and HV but this time using the ALFA 1 detector. In the Figure 6.5(b) the mean hit multiplicity exhibits an increase up to 1.55 hits with increasing of high voltage between 850 and 950 V for five different threshold values. 43

6.2 Layer eﬃciency

Another parameter of interest is the efficiency of single fiber layers. The layer efficiency indicates the probability to record a signal of a traversing ionizing particle in a particular detector layer. A good indicator of the layer efficiency is the track hit multiplicity. The best way to analyse the layer efficiency is to use the track prediction by the EUDET telescop. Since the EUDET data processing was not ready this was done using the occupancy of all 20 layers. To define a track a minimum number of track hits has been used in each of U and V-projections as tracking criterion. All hits per event in each of U/V-projections were counted to determine the minimum number of hits that constitute a track. The layer occupancy distribution for ALFA 1 in both projections is shown in Figure 6.6. On the average 9 track hits are observed in 10 fiber layers. A negligible fraction of projections has less than 6 hits. This follows from the fact the tracking algorithm, with the request of at least 5 hits, generates no losses of tracking efficiency [36].

The binomial distribution was used because in each track a ﬁber has only two possibilities: to be or not to be hit. Since each U/V projection is composed of 10 layers there are n = 10 possible hits per event as the example in Figure 6.7 illustrates. The mean number of hits is determined by the µ = np with p the probability obtained from the ﬁt.

U-Projection track1 V-Projection track2 Entries 66128 Entries 66128

30000 Mean 8.878 Mean 9.215 µ= 8.9± 0.01 RMS 0.973 µ= 9.14± 0.01 RMS 0.8512

30000 25000

25000

20000

15000 15000

10000 10000

5000 5000

0 0 2 4 6 8 10 12 2 4 6 8 10 12 Number of hits Number of hits

Figure 6.6: Track hits occupancy of U and V projections for ALFA 1. The curve is a binomial distribution ﬁtted to occupancy distribution. The mean number of hits obtained by the ﬁt is both cases about 9. 44

Figure 6.7: The plot on right shows a reconstructed track wich is composed by 8 hits on ﬁber 22 of each the layers of the U projection. On the left the multiplicity in each layer for U projection can be seen for the same event on the left side.

Using as cut the 6 hits per track can be obtained the plot of the Figure 6.8 where a mean eﬃciency of 91.7% is observed according with the mean values obtained from the binomial ﬁt. The contribution of the layer 4 is reduced due to a readout problem.

100 Mean ≈ 91.7% 95

80 Layer Efficiency (%) 75

50 0 2 4 6 8 10 12 14 16 18 20 Layer number

Figure 6.8: Layer eﬃciency of all layers in ALFA 1.

Similar plots for ALFA 2 are shown in Figures 6.9 and 6.10. The contribution of Ultra Bialkali MAMPTs is visible again for the layers 5, 10 and 17 with an increase about 5% of layer efficiency. In general the efficiency of layers read by Bi-alkali MAMPTs is as expected ∼ 90%. A better estimate of the layer efficiency can be obtained using the external reference of EUDET telescope. However, the method presented here provides a good approximation of the efficiency of the layers. 45

U-Projection track1 V-Projection track2 Entries 50906 Entries 50906

25000 Mean 9.04 25000 Mean 8.986 µ= 9.09± 0.01 RMS 0.9106 µ= 9.02± 0.01 RMS 0.939

20000 20000

15000 15000

10000 10000

5000 5000

0 0 2 4 6 8 10 12 2 4 6 8 10 12 Number of hits Number of hits

Figure 6.9: Track hits occupancy of U and V projections for ALFA 2. The mean number of track hits obtained by binomial ﬁt (in blue) is slightly higher than in the case of ALFA 1 detector due to the contribution of UBA MAMPTs.

100 Mean ≈ 89.2% Mean UBA ≈ 95.2% 95

80 Layer Efficiency (%) 75

50 0 2 4 6 8 10 12 14 16 18 20 Layer number

Figure 6.10: The layer eﬃciency of all layers for ALFA 2. The contribution of the UBA MAMPTs increases the eﬃciency (in red) by about 5% with respect to those layers read by the standard BA MAMPTs.

The dependence of the mean layer efficiency on the readout parameters can be seen in Figure 6.11. In (a) the mean layer efficiency as a function of Thermo for two different HV values is shown. A plateau of the efficiency at level of 90% is observed with an expected decrease beyond a thermo value of 9. The efficiency is found to be rather more stable with respect to gain variations, as shown in Fig. 6.11(b). 46

ALFA 1, Gain=16 HV=900, Thres.= 7 100 94

90 92

80 90

88 70 Mean layer efficiency (%) Mean layer efficiency (%) HV=950 86 60 HV=900 84

50 82 ALFA 2

40 80 5 6 7 8 9 10 12 14 16 18 20 22 24 Thermo Gain (a) (b)

Figure 6.11: (a) Mean layer efficiency in ALFA 1 as a function of threshold for two different HV values. Layer efficiency as a function of gain factor for ALFA 2.

6.3 Spatial Resolution

A main goal of the test beam is to determine the intrinsic spatial resolution of the ALFA detector. As mentioned in chapter√ 4 the planes of the main detector are staggered by multiples of (500µm/10) × 2 = 70.7µm which leads to an effective fiber pitch of 50µm. Since the beam spot size at the ALFA position is σd = 130µm, the spatial resolution of detector should be much smaller. The effective fiber overlap of ten fibers is 50µm so that the protons should be distributed uniformly along√ those 50µm. Thus the spatial resolution of an ideal detector is given by 50µm/ 12 ∼ 14.4µm. However, this value ignores the 10µm of plastic cladding. From the last test beams and simulations a spatial resolution of the order of 30µm which is appropiate in comparison with the size of the beam spot.

Although the best way to determine the spatial resolution is through the use of EUDET tracking reference, in this section the resolution will be determined using the method known as stand-alone [37]. This consists in the reconstruction of tracks (in x and y coordinates) by ALFA as two independent track segments, using the first five planes of the detector as a telescope. So each reconstructed position is the reference value. The analysis is based on the difference of track coordinates in both half detectors. As an example, Figure 6.12 shows this distribution for a run with ALFA 1. A Gaussian fit gives σ12 = 63.6µm. Since both half detectors are approximately√ identical the spatial resolution of each of them will be σH1 = σH2 = 64/ 2 ∼ 45µm. This value is√ considerably higher than the theoretical half detector resolution given by 100µm/ 12 ∼ 28.9µm. 47

X first half - X second half x_diff

Entries 94294

12000 2 χ / ndf 1369 / 79

Constant 1.165e+04 ± 5.008e+01 10000 Mean 0.0007575 ± 0.0002106

Sigma 0.06355 ± 0.00018 8000

6000

4000

2000

0 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 X_H1-X_H2 (mm)

Figure 6.12: The residual distribution xH1 − xH2 (mm) reconstructed by the two ALFA 1 (upper detector) halves.

To obtain the full detector resolution from the determined half detector resolution it is assumed that each have twice the resolution of the full detector. Therefore the above considerations can be summarized as q √ 2 2 σ12 = σH1 + σH2 = 2 2σF ull. (6.1)

Of course this assumption ignores other effects that should be taken into account in a more detailed analysis. A comparison between the simulated and measured residual resolutions√ with runs taken in the test beam has been performed in [35], where the factor 2 2 of the equation 6.1 has been replaced by a factor obtained dividing the simulated residual σ12 resolution by the simulated full σF ull detector resolution. The factors were 2.05±0.15 and 1.60±0.13 for upper and lower detectors, respectively. With these factors the dependence of the full spatial resolution on some variables has been analyzed. For example, Figure 6.13 shows the spatial resolution in dependence on the threshold for ALFA 2 with the expected behavior in both spatial coordinates. Figure 6.14 shows the dependence of the full detector resolution on the overlap width cut and mean layer efficiency. The overlap region is determined by the hits that are projected on an imaginary line in the direction of the U and V projections. The overlap region gives an estimate of the uncertanity of the track reconstruction. Thus, as shows Figure 6.14(a) the spatial resolution improves when the width of the overlap region decreases. However the bad effect is that the tracking efficiency decreases strongly. 48

Gain=Eq,HV=900 V

40 ALFA Resolution

35 resolution in X 30 resolution in Y

6 6.5 7 7.5 8 8.5 9 Thermo

Figure 6.13: Spatial resolution in dependence on the threshold in ALFA 2, for HV=900 V and Gain=16.

Resolution (µm) Resolution (µm) 50 34

32 45 30

28 40

26 35 24

22 30 20 in x in x in y in y 18 25

16 20 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 90 90.5 91 91.5 92 92.5 93 93.5 94 overlap width cut Layer Eficiency % (a) (b)

Figure 6.14: Spatial resolution for ALFA 2 in x and y coordinates as a function of (a) the overlap region and (b) mean layer eﬃciency. Chapter 7

Conclusions and outlook

In this master thesis an introduction to ALFA system detector has been presented, designed to measure the absolute luminosity at ATLAS experiment. Alternative methods to determine the luminosity at LHC has been also exposed, emphasizing the precision that can be achieved with each. Among these methods is of special interest based on the elastic scattering at small angles, which is the task of ALFA. This detector consists of four stations located symetrically at each side of the interaction point 1, where each ALFA stations is composed by two Roman Pots, which can be moved to the beam from top and bottom, respectively. Each Roman Pot uses scintillating ﬁbres arranged in UV-geometry layers to deﬁne the sensitive active areas of the detector.

The test beam was performed in the SPS of CERN where were taken about 3 × 107 events. The runs were dedicated to investigate the response of the detector to changes of readout parameters. In addition, other properties were investigated such as the alignment of the detector and the position scan on the active areas of the main and overlap detectors. The data analysis was focused on the study of hit multiplicities, the layer efficiency and the spatial resolution as well as their dependence on the readout parameters. The contribution of the Ultra Bialkali Multianode Photo Multipliers (MAMPTs) to the hit multiplicity and the layer efficiency of the layers was considered to compare with the those layers read by standard Bialkali MAMPTs. In this case, the hit multiplicity was found a mean hit multiplicity no larger than 1.5 hits per layer in contrast with ∼ 2.3 hits on average for layers read by UBA MAMPTs. The determination of layer efficiency using the track hit occupancy has yielded a mean value of 90% per layer as expected. In general, the behavior of the mean hit multiplicity and the layer efficiency as a function of the readout parameters are as expected.

The spatial resolution of the ALFA detector was computed without of tracking reference of EUDET telescope. The reconstruction of tracks using the five first planes of the detector as telescope allowed an estimate of full detector resolution. A value about 30µm has been determined for the full resolution in both detectors. The spatial resolution in dependence on THERMO parameter and layer efficiency was studied and the behavior of this dependence was as expected.

49 Bibliography

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[40] W.N. Cottingham and D.A. Greenwood. An introduction to the standard model of particle physics. Cambridge University Press, 2008. Danksagung

Besonders möchte ich mich bei Herrn Professor Dr. Thomas Lohse bedanken. Er hat mich mir grossem Einfühlungsvermögen und Umsicht bei der Herstellung von wichti- gen Verbindungen, die ich für die Ausarbeitung der Thematik unbedingt benötigte geholfen. Er war es auch, der es immer wieder verstanden hat mich in schwierigen Situationen aufzubauen, damit ich die vielfältigen Probleme mit denen ich oftmals zu kämpfen hatte überwinden konnte.

In gleicher Weise möchte ich Herrn Dr. Karl-Heinz Hiller meinen Dank aussprechen. Er hat mich bei der praktischen Arbeit sehr unterstüzt. So etwa bei der Begrenzung und Konkretisierung meines Arbeitsthemas und der sehr intensiven Betreung meiner Praxiseinsätze im CERN. Zu nennen ist weiter seine grosse Hilfe bei der Anfertigung meiner Masterarbeit. Hierbei hat er sehr viel Geduld und Umsicht walten lassen.

Für die Herausarbeitung meines zubearbeitenden Forchungsthemas bedanke ich mich auch bei Felix Pfeiﬀer, Matthieuh Heller und schließlich bei den Mittarbeiten, die mich im CERN unterstützt haben.

Ich möchte mich bei Herrn Manfred Spranger und Frau Christine Spanehl für ihre wertvolle Freundschaft und Hilfe bedanken. Auch meine Dankbarkeit für Kati Künne, Gerhard Prestel, Bärbel Schulze, Peter Prignitz und Herr Werner Mette.

Quiero manifestar de forma especial mi gratitud a la Universidad Anonio Nariño quien a través del acuerdo ALECOL con el DAAD y Colciencias facilitó y ﬁnanció mi estancia en Alemania durante el desarrollo de mi maestria.

A mi señora madre Rosalba por haber sido siempre la coautora de mis exitos y por su inﬁnito apoyo y amor en todas las circunstancias. A la memoria de mi padre Ramón quien con sus enseñanzas inspiró en mi el placer de aprender y trabajar. A mis hermanos Yamile y Yeinzon quienes además de ser los mejores hermanos han sido siempre también mi mejor respaldo. A mi querida abuelita por su cariño y su gran espiritu de unión. A mis adorados sobrinos Jineth Tatiana, Juan David y Michellle quienes son parte de mi motivacion e inspiracion en mi trabajo. A mis tios, primos y amigos mi reconocimiento.

53 Erklärung

Hiermit erkläre ich, dass ich die vorliegende Arbeit ohne unerlaubte fremde Hilfs- mittel und nur mit der angegebenen Literatur verfasst habe.

Mit der Auslage meiner Masterarbeit in der Bibliothek der Humbold-Universität zu Berlin bin ich einverstanden.