UNIVERSITA` DEGLI STUDI DI ROMA TOR VERGATA FACOLTA` DI SCIENZE MATEMATICHE, FISICHE E NATURALI CORSO DI LAUREA IN FISICA

Measurement of the Z0 Forward-Backward in pairs with the ATLAS experiment at LHC

Tesi di laurea magistrale in Fisica Nucleare e Subnucleare

Professoressa Presentata da

Anna Di Ciaccio Giulio Cornelio Grossi

Anno Accademico 2010/2011 Vale per tutti quelli che vivono in tempi come questi, ma non spetta a loro decidere, possiamo solamente decidere cosa fare con il tempo che ci viene concesso. (J.R.R. Tolkien) Contents

1 Introduction 6

2 The of Particle Physics 9 2.1 QED and QCD ...... 10 2.2 The Electroweak Theory ...... 12 2.3 The Higgs Mechanism ...... 14

3 The ATLAS Experiment at LHC 17 3.1 The LHC ...... 17 3.2 Physics goals ...... 19 3.3 Detector overview ...... 23 3.4 Tracking ...... 25 3.5 Calorimetry ...... 28 3.5.1 LAr electromagnetic calorimeter ...... 29 3.5.2 Hadronic calorimeters ...... 29 3.6 Muon system ...... 30 3.6.1 The toroid magnets ...... 31 3.6.2 Muon chamber types ...... 32 3.7 Trigger, readout, data acquisition, and control systems ...... 35

3 List of Figures

3.1 Overview of the CERN area. The LHC ring and its four experi- ments are schematically indicated...... 18

3.2 Higgs Cross section as a function of mH for different decay channels. 20 √ 3.3 LHC cross sections as a function of s...... 22 3.4 Cut-away view of the ATLAS detector. The dimensions of the detector are 25 m in height and 44 m in length. The overall weight of the detector is approximately 7000 tonnes...... 24 3.5 Cut-away view of the ATLAS inner detector...... 26 3.6 Cut-away view of the ATLAS calorimeter system...... 28 3.7 Cut-away view of the ATLAS Muon system...... 31 3.8 Scetch of a monitored drift tube chamber, with six layers of tubes, ordered in two multi-layers...... 33 3.9 Cross section view of an MDT tube...... 34

4 List of Tables

2.1 Generations of elementary particles ...... 10 2.2 Fundamental forces and gauge bosons ...... 11

3.1 General performance goals of the ATLAS detector. The units for

E and pT are in GeV...... 25 3.2 Main parameters of the inner-detector system...... 27 3.3 Main parameters of the muon spectrometer...... 32

5 Chapter 1

Introduction

During 1960s, S.L. Glashow, A. Salam and S. Weinberg unified weak and electromagnetic forces, creating a single theory that describes the two interac- tions, known as the electroweak theory. The theory predicted the existence of neutral currents and W and Z gauge bosons. In 1973 Weak Neutral Currents (WNC) were discovered simultaneously by two neutrino experiments at CERN [1] and Fermilab [2]. In 1983 the discovery of the W and Z gauge bosons by the UA1 and UA2 experiments [3][4] proved possible to measure their masses with great precision, which has allowed to start stringent comparisons of the elec- troweak theory with the experiments. A fundamental parameter of the model is 2 the Weinberg angle, θW , and in particular the value of sin θW . This parameter has already been measurement by LEP and Tevatron with high precision [5][6]. The electroweak theory represents a fundamental component of what is be- lieved to be the theory that describes all the particles and their interactions, the Standard Model (SM). Despite the great success of the Standard Model, it is not believed to be the final theory that describes particle interactions. Many open questions remain, i.e. the verification of mass generation by spontaneous symmetry breaking and the origin of the so called dark matter, which is a major component of the universe and cannot be explained in terms of know particles. The Large Collider (LHC) at CERN, as the largest -proton collider in the history of particle physics, is designed to answer these questions. Since its first physics run on 20 November 2009 at 900 KeV, it has reached a center of mass of 7 TeV in 2010 and by September 2011 an integrated luminosity of about 3.5 fb−1. Four main experiments have been built along the

6 ring at the four interaction points: Alice, ATLAS, CMS and LHC-b. Alice stud- ies heavy ions collisios (Pb-Pb), ATLAS and CMS are general purpose detectors and LHC-b studies mainly the properties of beauty . The discovery of the Higgs boson is presently one of the most important physics goals of the ATLAS and CMS experiments. At the same time, the integrated luminosity recorded at LHC has already allowed to perform several tests of the Standard Model. In ATLAS, the process pp → l+l− + X, with two in the final state, is currently produced with an high rate. This process is mediated primarily by

virtual at low values of di- invariant mass (Mll)[7], primarily by

the Z at Mll ∼ MZ , and by a combination of photons and Z bosons outside these regions. The presence of both vector and axial-vector couplings of electroweak bosons to fermions in the process qq¯ → Z/γ∗ → l+l− gives rise to a forward-

backward asymmetry (AFB) in the polar angle of the lepton relative to the incoming momentum in the rest frame of the lepton pair.

Moreover, a measurement of AFB versus the invariant mass of the lepton pair can constrain the properties of any additional non-standard model amplitudes contributing to qq¯ → l+l−, and is complementary to the direct searches for non- standard model amplitudes that look for an excess in the total cross section. In this thesis a measurement of the forward-backward asymmetry with the √ ATLAS experiment, using pp → Z/γ∗ → µ+µ− events at s = 7 TeV, is pre- sented. The analysis focuses on the decay of the Z boson into two , since this provides a relative clean signature which can be clearly discriminated from other processes. In Chapter 2 a brief overview of the actual theoretical under- standing of elementary particles and their interactions is given. This is followed by a short introduction to the and the ATLAS experi- ment in Chapter 3. Chapter 4 focuses on the definition of the forward-backward 2 eff asymmetry and the effective weak mixing angle sin θW . In Chapter 5, Chap- ter 6 and Chapter 7, I describe the physics results on both measurements that I have obtained analyzing 1827.34 pb−1 of collected data with ATLAS. In Chap- ter 5 I present the analysis to select the Z/γ∗ candidates and show the features ∗ of the selected Z/γ in data sample. Chapter 6 focuses on the AFB measure- ment as a function of the di-muon invariant mass. First, a measurement of raw forward-backward asymmetry distribution is presented, I have then applied the

corrections to the raw AFB distribution to obtain the final results. In Chapter 7, I have performed the extraction of the weak mixing angle using two methods based on a fit on the asymmetry distribution. The results obtained on data

7 are compared with the official PDG value showing a quite good agreement with previous experimental results [8].

8 Chapter 2

The Standard Model of Particle Physics

The Standard Model of particle physics describes matter and its interactions in terms of elementary particles. The first class of elementary particles are point- like spin 1/2 fermions, which describe the matter part of the theory. The second class are spin 1 bosons, also called gauge bosons, which mediate the fundamental interactions. Fermions, which can interact via the strong force, are called quarks; all other fermions are called leptons. The fermions of the Standard Model are classified into six quark and six leptons plus the corresponding anti-particles. They can be grouped in three generations, with same quantum numbers but different masses, as shown in Table 2.1. The particles of the second and third generation decay via the in particles of the first generation. Both electroweak and QCD theories are gauge field theories, meaning that they model the forces between fermions by coupling them to bosons which medi- ate the forces. The Lagrangian of each set of mediating bosons is invariant under gauge transformation, so these mediating bosons are referred to as gauge bosons.

The eight massless gluons, gα, mediate strong interactions among quarks. The massless , γ, is the exchange particle in electromagnetic interactions, and the three massive weak bosons, W  and Z, are the corresponding inter- mediate bosons that mediate the weak interaction. Table 2.2 summarizes the fundamental forces and the properties of their gauge bosons [9]. The Standard Model has been extensively tested in many experiments [10]. Nevertheless is missing the discovery of one important component: the Higgs

9 2.1. QED and QCD

1 Leptons (spin= 2 ) Flavors Charge (|e|) Mass (MeV/c2)

e -1 0.511 −6 νe 0 < 3×10

µ -1 105.66

νµ 0 <0.19

τ -1 1776.90.29

ντ 0 <18.2

1 Quarks (spin= 2 ) Flavors Charge (|e|) Mass (MeV/c2)

u +2/3 1.5-5 d -1/3 3-9

c +2/3 (1.0-1.4)×103 s -1/3 60-170

t +2/3 (178.04.3)×103 b -1/3 (4.0-4.5)×103

Table 2.1: Generations of elementary particles boson. In addition the Standard Model cannot explain many experimental facts like: the precence of dark matter in the universe, the gravitational force, the absence of antimatter and the neutrino’s masses.

2.1 QED and QCD

The mathematical concept, which describes the behavior of elementary parti- cles, must obviously be a quantum theory, but it is also clear that the Schrodinger Equation, which describes atomic physics at a very high accuracy, is not suffi- cient for elementary particle physics, since for example the decay or the creation of particles cannot be explained. A first step in the right direction was done by Paul Dirac [11] by formulating a Lorentz-invariant version of the Schrodinger

10 2.1. QED and QCD

Force Boson Name Symbol Charge (|e|) Spin Mass (GeV/c2)

Strong Gluon g 0 1 0

Electromagnetic Photons γ 0 1 0

W-boson W 1 1 80.423  0.039 Weak Z-boson Z0 0 1 91.1876  0.0021

Gravitational Graviton G 0 2 0

Table 2.2: Fundamental forces and gauge bosons

Equation. It can be expressed by the Lagrange density

¯ µ ¯ L = iψγµ∂ ψ − mψψ (2.1)

The application of the Euler Lagrange formalism on 2.1 to the free Dirac Equation µ (γ pµ − m) ψ(x) = 0 (2.2)

where ψ are four dimensional vectors, called spinors, γµ are the Dirac matrices

and pµ is the momentum operator i∂µ [12]. The Dirac equation has two essential properties: Firstly it allows describing relativistic spin 1 particles naturally, secondly it predicts antimatter. In order to allow the creation and annihilation of particles, a quantization of the field ψ is needed. This leads to a first quantum field theory of free fermions. The last ingredient for a meaningful physical theory is the inclusion of in- teractions. It is believed nowadays that the so called gauge-theories build the basis of the connection between particles and their interactions. Requiring, that the Lagrange density in equation 2.1 is invariant under the local Gauge transformation ψ(x) → eiα(x)ψ(x) (2.3)

leads to 1 L = ψ¯ (iγµ∂ − m) ψ + eψγ¯ Aµψ − F F µν (2.4) µ µ 4 µν

with Fµν = ∂µAν − ∂ν Aµ. A vector field Aµ had to be introduced to achieve ¯ µ this invariance. The term eψγµA ψ represents the interaction of the fermion µν field ψ with the vector field Aµ. The term Fµν F is the kinetic energy of the vector field and has the same structure as in Maxwell’s equations. The requirement of 2.3 corresponds to a local U(1) group symmetry and hence the

11 2.2. The Electroweak Theory

Lagrange density in equation 2.4 is called to be locally U(1)-gauge invariant.

The second quantization of the fields ψ and Aµ leads to a theory, called Quantum Electrodynamics, which describes the interaction of fermions via the exchange of the quanta of the electromagnetic field Aµ. These quanta are known as 2 µ photons. Since no mass term m AµA appears, the photon must be a massless gauge boson [13]. The theory of strong interaction, called Quantum Chromo Dynamics (QCD) is based on an local SU(3)-gauge invariant Lagrange density. Hence, each quark is a triplet of the QCD gauge group, which implies three kinds of charges, called red (r), blue (b) and green (g), corresponding to the three primitive colors. The gauge bosons of QCD are called gluons and form the octet representation and hence carry color charges themselves, since the SU(3) is a non-Abelian group. As a consequence, the gluons do not only interact with quarks, but also among themselves. It is believed nowadays, that the self-interaction can explain, what is commonly known as confinement. Confinement describes the fact, that color charged objects cannot be observed individually but only in combinations, which are color-neutral. Colorless particles, which consist of one quark and one anti-quark are called , particles, which consist of three quarks are called .

2.2 The Electroweak Theory

In order to formulate the electroweak theory in an elegant way, a new quan- tum number is introduced, the weak isospin, I . Each generation of left-handed fermions builds a doublet with I = 1/2 and I3 = 1/2 , which reflects the coupling to the charged current. The charged current does not interact with right-handed fermions, thus they build an isospin singlet, I = I3 = 0. The terms “left handed” and “right handed”, which denote the chirality of a mas- sive particle, are essential to understand the weak force: it is defined by the eigenvectors of (1  γ5), with γ5 = iγ0γ1γ2γ3 and γi the Dirac matrices. In case of the leptons of the first generation, the projection of the left-handed doublet onto its left-handed component is [14]:   1 e L = (1  γ5) (2.5) 2 νe

12 2.2. The Electroweak Theory

For the right-handed singlet one has:

1 R = (1  γ )e (2.6) 2 5

The theory is invariant under rotations in the space of the weak isospin, i.e.,

invariant under SU(2)L transformations. L denotes the acting on left-handed

fermions only. Another invariance of the arises for U(1)Y phase-transitions of the weak hypercharge Y . Like the Gell-Mann-Nishijima formula in the theory of the strong force, the hypercharge in the electroweak theory links the electrical charge Q to the third component of the weak isospin

I3 : 1 Y = I + Q (2.7) 3 2 By requiring not only global but also local gauge invariance for the com-

bined group SU(2)L×U(1)Y , four additional vector fields have to be introduced, µ µ namely Wi with i =1, 2, 3 and B . In order to guarantee gauge invariance, the fields have to transform correctly:

Wµ → Wµ + ∂µθ(x) + gθ(x) × Wµ (2.8)

Bµ → Bµ + ∂µλ(x)

where θ(x) are arbitrary functions of space-time. The Lagrangian of the elec- troweak interactions then reads as follows:

1 1 Y L = − W Wµν − B Bµν +iψ¯0γ ∂µψ0−gψ¯0γ ∂µI · Wµψ0−g0 ψ¯0γ ∂µBµψ0 ew 4 µν 4 µν µ µ 2 µ (2.9) Here, the vector fields are composed of gauge-invariant field-tensors, which are

given by Wµν = ∂µWν − ∂ν Wµ − gWµ × Wν and Bµν = ∂µBν − ∂ν Bµ, respectively. These fields are not to be confused with the particles or fields mediating the actual interaction. In Equation 2.9, ψ represents the Dirac spinor of all fermion fields, hence left-handed and right-handed quarks and leptons.

The SU(2)L×U(1)Y structure is represented by their operators I and Y , and the associated coupling constants g and g0.

The actual weak interactions are mediated by superpositions of the Wµ and  Bµ fields. The “charged current” W is composed of linear combinations of W1 µ and W2, whereas the “neutral current” Z and the “photon field” Aµ of the

13 2.3. The Higgs Mechanism

µ electromagnetic interaction consists of superpositions of W3 and B :

1 W µ = √ (W µ  iW µ) (2.10) 2 1 2       µ µ Z cos θW − sin θW W   =    3  (2.11) µ µ A sin θW cos θW B

In Equation 2.11, the “weak mixing angle” or “Weinberg angle” θW has been introduced. It can be expressed as:

g0 e sin θW = p = (2.12) g2 + g02 g

The coupling strengths of the physical particles W  , Z and γ can be derived by combining Equations 2.11 2.10 and Equation 2.9. With the W  only coupling to left-handed fermion-fields, the coupling is given by

2 G√F g = 2 (2.13) 2 8MW

Here, GF denotes the Fermi constant. The coupling of the photon to fermions does not distinguish between left-handed and right-handed fields. It is given by

0 e = g sin θW = g cos θW (2.14)

Although the Z boson, like the photon, couples to both fields, it discriminates axial and axial-vector couplings, known as the “V − A theory”. Its couplings are denoted as gV and gA. Looking back to Equation 2.9, it becomes obvious that a mechanism is miss- ing that allows particles to acquire mass. This mechanism is the subject of the following section.

2.3 The Higgs Mechanism

In order to allow for mass-generating terms within the electroweak La- µν grangian (Equation 2.9), quadratical terms like MW Wµν W , which violate the invariance under gauge transformations, would have to be added. P. Higgs et al. restored the invariance of the Lagrangian by introducing a scalar doublet

14 2.3. The Higgs Mechanism

φ [15], [16]   φ0 φ =   (2.15) φ−

whose non-vanishing expectation-value will spontaneously break the

given SU(2)L×U(1)Y symmetry. With this scalar field φ and its coupling to the vector fields Wµ, Equation 2.9 can be extended by the contribution from the Higgs field: h  i µ † 2 † † 2 LHiggs = D φ Dµφ − µ φ φ + λ φ φ (2.16)

µ µ − · µ − 0 Y µ with the covariant derivative D = ∂ igI W ig 2 B . Since the minimum † − µ2 2 at φ φ = 2λ of the potential V (φ) in Equation 2.16 is non-zero for µ > 0 and λ < 0, the ground state is degenerated. With the vacuum expectation   1 | h i | − µ2 2 value, v = φ = 2λ , the mass of the charged spin-1 boson can now be generated, 1 M = vg (2.17) W 2 whereas the masses of the neutral vector-fields read as p MW 1 2 02 MZ = = v g + g (2.18) cos θW 2

MA = 0 (2.19)

Obviously, Aµ can be identified as the photon field with zero mass. With the same considerations the masses of the fermions can be generated, too. The interaction of the fermion field with the gauge field and therefore the Higgs field, can be expressed as follows:

¯0 µ 0 ¯0 0 ψ γµD ψ + Gψψ φψ (2.20)

In this Langrangian, the parameter Gψ denotes the Yukawa couplings of the fermions to the Higgs field. Via the non-vanishing vacuum expectation value of the Higgs field after the spontaneous symmetry breaking, the second term allows fermions to acquire their masses:       u0 d0 e0       ¯0 ¯0 ¯0  0  ¯0 ¯0 ¯0  0 ¯0 ¯0 ¯0  0 Lmass = −(u , c , t )RMu c  −(d , s , b )RMd s  −(e , µ , τ )RMl µ  +h.c. t0 b0 τ 0 L L L (2.21)

15 2.3. The Higgs Mechanism

The mass matrices Mu and Md for the quarks and Ml for the leptons are com- posed of Gψ and v. With the Higgs mechanism for the gauge bosons of the weak interaction and the Yukawa coupling to the Higgs field, the mass generation within the Standard Model is explained. But the theory not only manifests itself by the predictions of particle masses and interaction strengths, but the excitation of the Higgs field itself give rise to a new, yet undiscovered, particle of the Standard Model: the Higgs boson. Its detection is one of the main goals of the LHC and would complete the Standard Model of Particle Physics.

16 Chapter 3

The ATLAS Experiment at LHC

3.1 The LHC

The LHC is a proton-proton collider with a designed center-of-mass energy of 14 TeV. It is built in the tunnel of the former LEP collider, which has a circumference of 26.7 km. Super conducting magnets are the basic technology of the LHC and are used for bending and focusing the counter rotating proton beams. A designed luminosity 1×1034 cm−2s−1 will be reached hopefully in 2014, by accelerating of 2835 proton bunches per direction, consisting of 1×1011 particles, with a bunch length of 7.5 cm and a time between the collisions of 25 ns. The beam-pipe of the LHC contains two separate beam-lines for the oppo- site direction of the two proton beams, which also makes an opposite magnetic field for both beam-lines necessary. The solution to this technical problem are so-called twin-bore magnets, which consist of a set of coils. The advantage of this approach is that the whole structure can use the same cooling infrastruc- ture within the same beam-pipe. The cooling of 1232 magnets with a field strength of 8.33 Tesla and 392 quadrupoles is achieved by super-fluid Helium at a temperature of 2◦K. Before injecting the proton beams into the LHC, they traverse several other acceleration steps. The are extracted from hydrogen gas and accelerated in bunches of ∼ 1011 protons by the Linac-accelerator to 50 MeV. These bunches

17 3.1. The LHC

Figure 3.1: Overview of the CERN area. The LHC ring and its four experiments are schematically indicated. are further accelerated by the PS booster to 1.4 GeV, followed by the Proton Synchroton (PS) and the Super Proton Synchroton (SPS) which accelerate the proton bunches to 26 GeV and finally up to an injection energy of 450 GeV, respectively. It is planed also to inject nuclei and accelerate them to an energy of 1150 TeV with a luminosity of L = 1027cm−2s−1. Four main particle detectors are currently installed at the LHC: ATLAS (A Toroidal LHC ApparatuS) and CMS () are general pur- pose detectors, which cover a broad field of experimental studies. The ALICE (A Large Ion Collider Experiment) experiment is designed to study the quark gluon plasma, i.e. a state of matter in which the quarks and gluons can be considered as free particles. The LHCb experiment is dedicated to B- physics and will study CP-violation to high precision. LHC started operating on 10 September 2008, when first proton beams were successfully circulated in the main ring of the LHC for the first time, but 9 days later operations were halted due to a serious fault. On 20 November 2009 they were successfully circulated again, with the first recorded proton–proton collisions occurring 3 days later at the injection energy

18 3.2. Physics goals of 450 GeV per beam. After the 2009 winter shutdown, the LHC was restarted and the beam was ramped up to 3.5 TeV per beam. On 30 March 2010, the first planned collisions took place between two 3.5 TeV beams, a new world record for the highest-energy man-made particle colli- sions. After another shutdown on 6 December 2010 new runs with proton beams begun on 13 March 2011. Actually LHC is still running at an energy of 3.5 GeV per beam. An integrated luminosity of about 2.5 fb−1 has been recorded un- til now. On 21 Apr 2011 LHC becomes the world’s highest-luminosity hadron accelerator achieving a peak luminosity of 4.67×1032 cm−2 s−1, beating the Tevatron’s previous record of 4×1032 cm−2 s−1 held for one year. The LHC will continue to operate at half energy until the end of 2012; it will not run at full energy until 2014.

3.2 Physics goals

The Large Hadron Collider will provide a rich physics potential, ranging from more precise measurements of Standard Model parameters to the search for new physics phenomena. Furthermore, nucleus-nucleus collisions at the LHC provide an unprecedented opportunity to study the properties of strongly interacting matter at extreme energy density, including the possible phase transition to a color-deconfined state: the quark- gluon plasma. Requirements for the ATLAS detector system [17] have been defined using a set of processes covering much of the new phenomena which one can hope to observe at the TeV scale. The high luminosity and increased cross-sections at the LHC enable further high precision tests of QCD, electroweak interactions, and flavor physics. The will be produced at the LHC at a rate of a few tens of Hz, providing the opportunity to test its couplings and spin. The search for the Standard Model Higgs boson has been used as a bench- mark to establish the performance of important sub-systems of ATLAS. It is a particularly important process since there is a range of production and decay mechanisms, depending on the mass of the Higgs boson, H. At low masses

(mH < 2mZ ), the natural width would only be a few MeV, and so the observed width would be defined by the instrumental resolution. The predominant decay mode into would be difficult to detect due to QCD backgrounds, and the two-photon decay channel would be an important one. Other promising channels could be, for example, associated production of H such as ttH¯ , WH,

19 3.2. Physics goals

Figure 3.2: Higgs Cross section as a function of mH for different decay channels. and ZH, with H → b¯b, using a lepton from the decay of one of the top quarks or of the vector boson for triggering and background rejection. For masses above 130 GeV, Higgs-boson decays, H → ZZ , where each Z decays to a pair of oppositely charged leptons, would provide the experimentally cleanest channel to study the properties of the Higgs boson. For masses above approximately 600 GeV, WW and ZZ decays into jets or involving neutrinos would be needed to extract a signal. The tagging of forward jets from the WW or ZZ fusion production mechanism has also been shown to be important for the discovery of the Higgs. Studying the H → lνlν decay mode an Higgs boson with a mass in the range from 154 GeV to 186 GeV was excluded at 95% confidence level[18]. An excess of events in data, corresponding to more than 2σ significance, was observed for the Higgs boson low mass range [19]. Combination of measurements in different decay channels set important limits on the Higgs mass. The Higgs boson mass ranges from 146 GeV to 232 GeV, 256 GeV to 282 GeV and 296 GeV to 466 GeV are excluded at the 95% CL [19]. Bosons beyond the Standard Model, such as the A and H of the min- imal supersymmetric extension of the Standard Model, require sensitivity to processes involving τ-leptons and good b- tagging performance. A search for neutral Higgs bosons decaying to pairs of τ leptons has been done. The limit on the production cross section times branching ratio into a pair

20 3.2. Physics goals

of τ leptons for a generic Higgs boson φ is in the range between approximately 300 pb for a Higgs boson mass of 90 GeV and approximately 10 pb for a Higgs boson mass of 300 GeV [20]. Should the Higgs boson be discovered, it would need to be studied in several modes, regardless of its mass, in order to fully disentangle its properties and establish its credentials as belonging to the Standard Model or an extension thereof. New heavy gauge bosons W 0 and Z0 could be accessible for masses up to ∼ 6 TeV. To study their leptonic decays, high-resolution lepton measurements and

charge identification are needed in the pT range of a few TeV. Another class of

signatures of new physics may be provided by very high pT jet measurements. As a benchmark process, quark compositeness has been used, where the signature would be a deviation in the jet cross-sections from the QCD expectations. The ATLAS detector has been used to search for new high-mass states decay- 0 0 ing to a lepton plus missing ET , such as W . Recent results excluded W s with masses up to 2.15 TeV at 95 % of C.L. [21]. Searches on di-lepton resonances have set a lower limit on the Z0 mass at 1.83 TeV [22]. The decays of supersymmetric particles, such as squarks and gluinos, would involve cascades which, if R-parity is conserved, always contain a lightest stable supersymmetric particle (LSP). As the LSP would interact very weakly with the detector, the experiment would measure a significant missing transverse energy, miss ET , in the final state. The rest of the cascade would result in a number of leptons and jets. In schemes where the LSP decays into a photon and a gravitino, an increased number of hard isolated photons is expected. Latest results about search for supersymmetric particles have set some limits on the masses of squarks and gluinos. searches for supersymmetry in final states containing one isolated or muon, jets, and missing transverse momentum set limits on the cross section for these processes up to 41 fb for electron channel and 53 fb for muon channel [23]. For R-parity conserving models in which sbottoms are the only squarks to appear in the gluino decay cascade, gluino masses below 720 GeV are excluded at the 95 % C.L. for sbottom masses up to 600 GeV [24]. For R-parity conserving models in which stops are the only squarks to appear in the gluino decay cascade, gluino masses below 500-520 GeV, depending on the stop mass, are excluded at the 95% C.L. [25]. For R-parity violating supersymmetric models a search for a high mass neu- tral particle that decays directly to the eµ final state has been done. For this

21 3.2. Physics goals channel the limit on the cross section times branching ratio was set to 130  11 fb [26]. Several new models propose the existence of extra dimensions lead-

√ Figure 3.3: LHC cross sections as a function of s. ing to a characteristic energy scale of quantum gravity in the TeV region. In terms of experimental signatures, this could lead to the emission of gravitons miss which escape into extra dimensions and therefore generate ET , or of Kaluza- Klein excitations which manifest themselves as Z-like resonances with ∼ TeV separations in mass. Other experimental signatures could be anomalous high- mass di-jet production, and miniature black-hole production with spectacular decays involving democratic production of fundamental final states such as jets, leptons, photons, neutrinos, W ’s, and Z’s. For the production of the graviton mass modes, studies on phenomena in- volving the production of a monojet plus a missing transverse energy excluded a region between 2.3 and 1.8 TeV [27]. Invariant mass distributions of jet pairs

22 3.3. Detector overview

have been studied, in order to set limits on the presence of new physical hy- potheses: excited quarks are excluded for masses below 2.91 TeV, axigluons are excluded for masses below 3.21 TeV and color octet scalar resonances are excluded for masses below 1.91 TeV [28]. The formidable LHC luminosity and resulting interaction rate are needed because of the small cross-sections expected for many of the processes mentioned above. However, with an inelastic- proton-proton cross-section of 80 mb, the LHC will produce a total rate of 109 inelastic events/s at design luminosity. This presents a serious experimental difficulty as it implies that every candidate event for new physics will on the average be accompanied by 23 inelastic events per bunch- crossing. The nature of proton-proton collisions imposes another difficulty. QCD jet production cross- sections dominate over the rare processes mentioned above, re- quiring the identification of experimental signatures characteristic of the physics miss processes in question, such as ET or secondary vertices. Identifying such fi- nal states for these rare processes imposes further demands on the integrated luminosity needed, and on the particle-identification capabilities of the detector.

3.3 Detector overview

The overall ATLAS detector layout is shown in figure 3.4 and its main per- formance goals are listed in table 3.1. The coordinate system and nomenclature used to describe the ATLAS de- tector and the particles emerging from the p-p collisions are briefly summarized here. The nominal interaction point is defined as the origin of the coordinate system, while the beam direction defines the z-axis and the x-y plane is trans- verse to the beam direction. The positive x-axis is defined as pointing from the interaction point to the center of the LHC ring and the positive y-axis is de- fined as pointing upwards. The azimuthal angle φ is measured as usual around the beam axis, and the polar angle θ is the angle from the beam axis. The − θ pseudorapidity is defined as η = lntan 2 (in the case of massive objects such 1 E+pz as jets, the rapidity y = ln is used). The transverse momentum pT , the 2 E−pz miss transverse energy ET , and the missing transverse energy ET are defined in the x-y plane. The distance ∆R in the pseudorapidity-azimuthal angle space is p defined as ∆R = ∆η2 + ∆φ2. The ATLAS detector is nominally forward-backward symmetric with respect

23 3.3. Detector overview

Figure 3.4: Cut-away view of the ATLAS detector. The dimensions of the detec- tor are 25 m in height and 44 m in length. The overall weight of the detector is approximately 7000 tonnes. to the interaction point. The magnet configuration comprises a thin supercon- ducting solenoid surrounding the inner-detector cavity, and three large super- conducting toroids (one barrel and two end-caps) arranged with an eight-fold azimuthal symmetry around the calorimeters. This fundamental choice has driven the design of the rest of the detector. The inner detector is immersed in a 2 T solenoidal field. Pattern recognition, momentum and vertex measurements, and electron identification are achieved with a combination of discrete, high-resolution semiconductor pixel and strip detectors in the inner part of the tracking volume, and straw-tube tracking detectors with the capability to generate and detect transition radiation in its outer part. High granularity liquid-argon (LAr) electromagnetic sampling calorimeters, with excellent performance in terms of energy and position resolution, cover the pseudorapidity range |η| < 3.2. The hadronic calorimetry in the range |η| < 1.7 is provided by a scintillator-tile calorimeter, which is separated into a large barrel and two smaller extended barrel cylinders, one on either side of the central barrel. In the end-caps (|η| > 1.5), LAr technology is also used for the

24 3.4. Tracking

hadronic calorimeters, matching the outer |η| limits of end-cap electromagnetic calorimeters. The LAr forward calorimeters provide both electromagnetic and hadronic energy measurements, and extend the pseudorapidity coverage to |η| = 4.9. The calorimeter is surrounded by the muon spectrometer. The air-core toroid system, with a long barrel and two inserted end-cap magnets, gener- ates strong bending power in a large volume within a light and open structure. Multiple-scattering effects are thereby minimized, and excellent muon momen- tum resolution is achieved with three layers of high precision tracking chambers.

Detector component Required resolution η coverage Measurement Trigger ⊕  Tracking σpT /pT = 0.05%√ pT 1% 2.5 EM calorimetry σE /E = 10%/ E ⊕ 0.7% 2.5 2.5 Hadronic calorimetry √ ⊕   barrel and endcap σE /E = 50%/√E 3% 3.2 3.2 forward σE /E = 100%/ E ⊕ 10% 3.1 < |η| < 4.9 3.1 < |η| < 4.9   Muon spectrometer σpT /pT = 10% at pT = 1 TeV 2.7 2.4

Table 3.1: General performance goals of the ATLAS detector. The units for E and pT are in GeV.

The muon instrumentation includes, as a key component, trigger chambers with timing resolution of the order of 1.5-4 ns. The muon spectrometer defines the overall dimensions of the ATLAS detector. The proton-proton interaction rate at the design luminosity of 1034 cm−2 s−1 is approximately 1 GHz, while the event data recording, based on technology and resource limitations, is limited to about 200 Hz. This requires an overall rejection factor of 5×106 against minimum-bias processes while maintaining maximum efficiency for the new physics. The Level-1 (L1) trigger system uses a subset of the total detector information to make a decision on whether or not to continue processing an event, reducing the data rate to approximately 75 kHz. The subsequent two levels, collectively known as the high-level trigger, are the Level-2 (L2) trigger and the event filter. They provide the reduction to a final data-taking rate of approximately 200 Hz.

3.4 Tracking

Approximately 1000 particles will emerge from the collision point every 25 ns within |η| < 2.5, creating a very large track density in the detector. To achieve the momentum and vertex resolution requirements imposed by the benchmark

25 3.4. Tracking physics processes, high-precision measurements must be made with fine detector granularity. Pixel and silicon microstrip (SCT) trackers, used in conjunction with the straw tubes of the Transition Radiation Tracker (TRT), offer these features.

Figure 3.5: Cut-away view of the ATLAS inner detector.

The layout of the Inner Detector (ID) is illustrated in figure 3.5. Its basic parameters are summarized in table 3.2.The ID is immersed in a 2 T magnetic field generated by the central solenoid, which extends over a length of 5.3 m with a diameter of 2.5 m. The precision tracking detectors (pixels and SCT) cover the region |η| < 2.5. In the barrel region, they are arranged on concentric cylinders around the beam axis while in the end-cap regions they are located on disks perpendicular to the beam axis. The pixel layers are segmented in R − φ and z with typically three pixel layers crossed by each track. All pixel sensors are identical and have a minimum pixel size in R − φ × z of 50 × 400 µm2 . The intrinsic accuracies in the barrel are 10 µm (R − φ) and 115 µm (z) and in the disks are 10 µm (R − φ) and 115 µm (R). The pixel detector has approximately 80.4 million readout channels. For the SCT, eight strip layers (four space points) are crossed by each track. In the barrel region, this detector uses small-angle (40 mrad) stereo strips to measure both coordinates, with one set of strips in each layer parallel to the beam direction, measuring R−φ . They consist of two 6.4 cm long daisy-chained sensors with a strip pitch of 80 µm. In the end-cap region, the detectors have a set of strips running radially and a set of stereo strips at an angle of 40 mrad. The mean pitch of the strips is also approximately 80 µm. The intrinsic accuracies per module in the barrel are 17

26 3.4. Tracking

µm (R − φ) and 580 µm (z) and in the disks are 17 µm (R − φ) and 580 µm (R). The total number of readout channels in the SCT is approximately 6.3 million. A large number of hits (typically 36 per track) is provided by the 4 mm diameter straw tubes of the TRT, which enables track-following up to |η| = 2.0. The TRT only provides R−φ information, for which it has an intrinsic accuracy of 130 µm per straw. In the barrel region, the straws are parallel to the beam axis and are 144 cm long, with their wires divided into two halves, approximately at |η| = 0. In the end-cap region, the 37 cm long straws are arranged radially in wheels. The total number of TRT readout channels is approximately 351000.

Item Radial extension (mm) Length (mm) Overall ID envelope 0 < R < 1150 0 < |z| < 3512 Beam-pipe 29 < R < 36 Pixel Overall envelope 45.5 < R < 242 0 < |z| < 3092 3 cylindrical layers Sensitive barrel 50.5 < R < 122.5 0 < |z| < 400.5 2 × 3 disks Sensitive endcap 88.8 < R < 149.6 495 < |z| < 650

SCT Overall envelope 255 < R < 549 (barrel) 0 < |z| < 805 251 < R < 610 (endcap) 810 < |z| < 2797 4 cylindrical layers Sensitive barrel 299 < R < 514 0 < |z| < 749 2 × 9 disks Sensitive endcap 275 < R < 560 839 < |z| < 2735

TRT Overall envelope 554 < R < 1082 (barrel) 0 < |z| < 780 617 < R < 1106 (endcap) 827 < |z| < 2744 73 straw planes Sensitive barrel 563 < R < 1066 0 < |z| < 712 160 straw planes Sensitive endcap 644 < R < 1004 848 < |z| < 2710

Table 3.2: Main parameters of the inner-detector system.

The combination of precision trackers at small radii with the TRT at a larger radius gives very robust pattern recognition and high precision in both R − φ and z coordinates. The straw hits at the outer radius contribute significantly to the momentum measurement, since the lower precision per point compared to the silicon is compensated by the large number of measurements and longer measured track length. The electron identification capabilities are enhanced by the detection of transition-radiation photons in the xenon-based gas mixture of the straw tubes. The semiconductor trackers also allow impact parameter measurements and vertexing for heavy-flavor and τ-lepton tagging. The secondary vertex mea- surement performance is enhanced by the innermost layer of pixels, at a radius of about 5 cm.

27 3.5. Calorimetry

3.5 Calorimetry

A view of the sampling calorimeters is presented in figure 3.6. These calorime- ters cover the range |η| < 4.9, using different techniques suited to the widely varying requirements of the physics processes of interest and of the radiation environment over this large η-range. Over the η region matched to the inner detector, the fine granularity of the EM calorimeter is ideally suited for pre- cision measurements of and photons. The coarser granularity of the rest of the calorimeter is sufficient to satisfy the physics requirements for jet miss reconstruction and ET measurements.

Figure 3.6: Cut-away view of the ATLAS calorimeter system.

Calorimeters must provide good containment for electromagnetic and hadronic showers, and must also limit punch-through into the muon system. Hence, calorimeter depth is an important design consideration. The total thickness of the EM calorimeter is > 22 radiation lengths (X0) in the barrel and > 24X0 in the end-caps. The approximate 9.7 interaction lengths (λ) of active calorimeter in the barrel (10λ in the end-caps) are adequate to provide good resolution for high- energy jets. The total thickness is 11λ at η = 0 and has been shown both by measurements and simulations to be sufficient to reduce punch-through. To- miss gether with the large η-coverage, this thickness will also ensure a good ET measurement, which is important for many physics signatures and in particular for SUSY particle searches.

28 3.5. Calorimetry

3.5.1 LAr electromagnetic calorimeter

The EM calorimeter is divided into a barrel part |η| < 1.475 and two end- cap components 1.375 < |η| < 3.2, each housed in their own cryostat.The barrel calorimeter consists of two identical half-barrels, separated by a small gap (4 mm) at z = 0. Each end-cap calorimeter is mechanically divided into two coaxial wheels: an outer wheel covering the region 1.375 < |η| < 2.5, and an inner wheel covering the region 2.5 < |η| < 3.2. The EM calorimeter is a lead- LAr detector with accordion-shaped kapton electrodes and lead absorber plates over its full coverage. The accordion geometry provides complete φ symmetry without azimuthal cracks. The lead thickness in the absorber plates has been optimized as a function of η in terms of EM calorimeter performance in energy resolution. Over the region devoted to precision physics |η| < 2.5, the EM calorimeter is segmented in three sections in depth. For the end-cap inner wheel, the calorimeter is segmented in two sections in depth and has a coarser lateral granularity than for the rest of the acceptance. In the region of |η| < 1.8, a presampler detector is used to correct for the energy lost by electrons and photons upstream of the calorimeter. The presam- pler consists of an active LAr layer of thickness 1.1 cm (0.5 cm) in the barrel (end-cap) region.

3.5.2 Hadronic calorimeters

Tile calorimeter. The tile calorimeter is placed directly outside the EM calorimeter envelope. Its barrel covers the region |η| < 1.0, and its two extended barrels the range 0.8 < |η| < 1.7. It is a sampling calorimeter using steel as the absorber and scintillating tiles as the active material. The barrel and extended barrels are divided azimuthally into 64 modules. Radially, the tile calorimeter extends from an inner radius of 2.28 m to an outer radius of 4.25 m. It is segmented in depth in three layers, approximately 1.5, 4.1 and 1.8 interaction lengths (λ) thick for the barrel and 1.5, 2.6, and 3.3λ for the extended barrel. The total detector thickness at the outer edge of the tile-instrumented region is 9.7λ at η = 0. Two sides of the scintillating tiles are read out by wavelength shifting fibers into two separate photomultiplier tubes.

LAr hadronic end-cap calorimeter. The Hadronic End-cap Calorimeter (HEC) consists of two independent wheels per end-cap, located directly behind

29 3.6. Muon system the end-cap electromagnetic calorimeter and sharing the same LAr cryostats. To reduce the drop in material density at the transition between the end-cap and the forward calorimeter (around |η| = 3.1), the HEC extends out to |η| = 3.2, thereby overlapping with the forward calorimeter. Similarly, the HEC η range also slightly overlaps that of the tile calorimeter (|η| < 1.7) by extending to |η| = 1.5. Each wheel is built from 32 identical wedge-shaped modules and divided into two segments in depth, for a total of four layers per end-cap. The wheels closest to the interaction point are built from 25 mm parallel copper plates, while those further away use 50 mm copper plates. The outer radius of the copper plates is 2.03 m, while the inner radius is 0.475 m. The copper plates are interleaved with 8.5 mm LAr gaps, providing the active medium for this sampling calorimeter.

LAr forward calorimeter. The Forward Calorimeter (FCal) is integrated into the end-cap cryostats, as this provides clear benefits in terms of uniformity of the calorimetric coverage as well as reduced radiation background levels in the muon spectrometer. The FCal is approximately 10 interaction lengths deep, and consists of three modules in each end-cap: the first, made of copper, is opti- mized for electromagnetic measurements, while the other two, made of tungsten, measure predominantly the energy of hadronic interactions. Each module con- sists of a metal matrix, with regularly spaced longitudinal channels filled with the electrode structure consisting of concentric rods and tubes parallel to the beam axis. The LAr in the gap between the rod and the tube is the sensitive medium.

3.6 Muon system

The conceptual layout of the muon spectrometer is shown in figure 3.7 and the main parameters of the muon chambers are listed in table 3.3. It is based on the magnetic deflection of muon tracks in the large superconducting air-core toroid magnets, instrumented with separate trigger and high-precision tracking chambers. Over the range |η| < 1.4, magnetic bending is provided by the 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

30 3.6. Muon system

Figure 3.7: Cut-away view of the ATLAS Muon system. a field which is mostly orthogonal to the muon trajectories, while minimiz- ing the degradation of resolution due to multiple scattering. The high level of particle flux has had a major impact on the choice and design of the spectrom- eter instrumentation, affecting performance parameters such as rate capability, granularity, ageing properties, and radiation hardness. 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.

3.6.1 The toroid magnets

A system of three large air-core toroids generates the magnetic field for the muon spectrometer. The two end-cap toroids are inserted in the barrel toroid at each end and line up with the central solenoid. Each of the three toroids consists of eight coils assembled radially and symmetrically around the beam axis. The end-cap toroid coil system is rotated by 22.5◦ with respect to the barrel toroid coil system in order to provide radial overlap and to optimize the

31 3.6. Muon system

Monitored drift tubes MDT Coverage |η| < 2.7 (innermost layer:|η| < 2.0) Number of chambers 1150 Number of channels 354000 Function Precision tracking Cathod strip chambers CSC Coverage 2.0 < |η| < 2.7 Number of chambers 32 Number of channels 31000 Function Precision tracking Resistive plate chambers RPC Coverage |η| < 1.05 Number of chambers 606 Number of channels 373000 Function Triggering, second coordinate Thin gap chambers TGC Coverage 1.05 < |η| < 2.7 Number of chambers 3588 Number of channels 318000 Function Triggering, second coordinate

Table 3.3: Main parameters of the muon spectrometer.

bending power at the interface between the two coil systems. The barrel toroid coils are housed in eight individual cryostats, with the linking elements between them providing the overall mechanical stability. Each end-cap toroid consists of eight racetrack-like coils in an aluminum alloy housing. They are cold-linked and assembled as a single cold mass, housed in one large cryostat. The performance in terms of bending power is characterized by the field R integral Bdl, where B is the field component normal to the muon direction and the integral is computed along an infinite-momentum muon trajectory, between the innermost and outermost muon-chamber planes. The barrel toroid provides 1.5 to 5.5 Tm of bending power in the pseudorapidity range 0 < |η| < 1.4, and the end-cap toroids approximately 1 to 7.5 Tm in the region 1.6 < |η| < 2.7. The bending power is lower in the transition regions where the two magnets overlap (1.4 < |η| < 1.6).

3.6.2 Muon chamber types

The design of the ATLAS Muon Spectrometer is to reach a momentum resolution of 10 % for 1 TeV muons. Assuming a magnetic field strength of 0.5 T , which is roughly the average of the ATLAS toroidal magnetic field and an average trajectory length of 5 m, this leads to a required precision of 50 µm of the sagitta measurement. This required precision is achieved by four chamber technologies:

32 3.6. Muon system

• Monitored Drift Tube (MDT) chambers: Precise muon tracking.

• Cathode Strip Chambers (CSCs): Precise muon tracking in the very for- ward region.

• Resistive Plate Chambers (RPCs): Trigger chamber is the barrel region.

• Thin Gap Chambers (TGCs): Trigger chambers in the endcap region.

The MDT chambers are placed in three layers in the barrel region at radii of about 5 m, 7.5 m and 10 m. In the barrel, particles are measured near the inner and the outer magnetic field boundaries, and inside the field volume, in order to determine the momentum from the sagitta of the trajectory. There are also three layers of MDT-chambers in the endcap region, concen- tric around the beam axis at 7 m, 10 m, 14 m and 21 m from the interaction point.A relatively large background rate is expected in the very forward region of the Muon Spectrometer. Hence, CSCs are used instead of the MDT cham- bers in the inner-most ring of the inner-most endcap layer, because of their finer granularity and less occupancy. the MDT chambers are the core element of the Muon Spectrometer and, therefore, it is justified to discuss them in more de- tail. A schematic sketch of an MDT chamber is shown in Figure 3.8. An MDT chamber consists of six to eight drift tube layers, which are arranged in two so called multi layers with a spacing of 200 mm. The aluminum drift tubes have diameter of 30 mm and are filled with Ar : CO2 gas mixture 97 : 3 at 3 bar absolute pressure. A central wire is positioned in the middle of the tube.

Figure 3.8: Scetch of a monitored drift tube chamber, with six layers of tubes, ordered in two multi-layers.

33 3.6. Muon system

A high energetic muon, which passes through a tube, ionizes the gas. An applied potential difference between wire and tube of 3080 V leads to an electric field, which lets the electrons drift towards the wire, while the positive ions drift towards the tube wall. When the drifting electrons reach some critical velocity they can ionize further gas molecules around them. This creates an avalanche of further electrons and leads to a so called Townsend avalanche, which consists of electrons and positive charge ions.The ions drift through the whole potential difference to the tube wall and induce a measurable signal in the electrodes [29], as sketched in figure 3.9.

Figure 3.9: Cross section view of an MDT tube.

By measuring the drift time (the time which is needed for the ionization clus- ter to reach the wire) one can determine the drift radius (the minimal distance of the muon trajectory to the central wire). Having measured the drift radii for all tubes which have been hit, one can fit a tangential line which approximates the muon trajectory within one MDT chamber. These fitted straight lines are called segments. The CSCs are multiwire proportional chambers which are used in the very forward region of the Muon Spectrometer instead of the MDT chambers. They have an expected single track resolution of < 60 µm. This good resolution is achieved by a cathode strip readout which measures the charge induced on the segmented cathode by the electron avalanche formed on the anode wires. The transverse coordinate can be calculated via the measurement of the orthogo- nal strips on the second cathode of the chamber. The chambers have a small sensitivity to photons (∼ 1%) and also a small neutron sensitivity (< 10−4 ). The small neutron sensitivity is achieved by the small gas volume used and the absence of hydrogen in the operating gas, which is a Ar/CO2 /CF4 mixture.

34 3.7. Trigger, readout, data acquisition, and control systems

The RPCs are the trigger elements for the barrel region, which provide a fast momentum estimation of muons for the hardware based trigger and also the necessary timing information for the drift time measurement of the MDT chambers. They have a spatial resolution of 1 cm and timing resolution less than 2 ns. The RPCs are made of two bakelite plates which form a narrow gap. The gap is filled with 94.7% of C2H2F4, 5% of C4H10 and 0.3% of SF6. Incident muons lead to ionizes of the and the high electric field between the bakelite plates produce a charge avalance that is collected by the electrode planes. The two bakelite plates are overlapped with two readout strip panels on the outside, which are orthogonal with respect to each other. This allows an η and φ measurement of the muon track. The TGCs are the trigger elements for the endcap region. They consist of two cathode plates with a distance of 1.4 mm. The gap between the plates is filled with a gas mixture of C5H10 and CO2. Evenly spaced anode wires (1.8 mm spacing distance) are placed in between the plates and a high voltage of 3.1 kV is applied across the wires. Each wire collects a certain number of ionization electrons caused by an incident muon. The measured ionization electron distribution across all wires is used to identify the path of an incident muon. These chambers are combined to two or three layers to provide also a spatial coordinate measurement. The muon spectrometer is designed for a transverse momentum resolution of about 2 − 3% for muons with a pT = 50 GeV and 10% for muons with pT = 1 TeV . The contribution of multiple scattering to the resolution is relatively small for low and high energetic muons due to the choice of air-core magnetic field configuration, which minimizes the use of material. The resolution is dominated by energy loss fluctuations on the calorimeters for low energetic muons (< 20 GeV ) and by the precision of the drift radii measurement for high energetic muons (> 300 GeV ).

3.7 Trigger, readout, data acquisition, and con- trol systems

The Trigger and Data Acquisition (collectively TDAQ) systems, the timing- and trigger-control logic, and the Detector Control System (DCS) are parti- tioned into sub-systems, typically associated with sub-detectors, which have the

35 3.7. Trigger, readout, data acquisition, and control systems same logical components and building blocks. The trigger system has three dis- tinct levels: L1, L2, and the event filter. Each trigger level refines the decisions made at the previous level and, where necessary, applies additional selection criteria. The data acquisition system receives and buffers the event data from the detector-specific readout electronics, at the L1 trigger accept rate, over 1600 point-to-point readout links. The first level uses a limited amount of the total detector information to make a decision in less than 2.5 µs, reducing the rate to about 75 kHz. The two higher levels access more detector information for a final rate of up to 200 Hz with an event size of approximately 1.3 Mbyte.

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39 Ringraziamenti

Un ringraziamento particolare va alla professoressa Anna Di Ciaccio per avermi dato la possibilit`adi realizzare un sogno che coltivo da sette anni. Vor- rei anche ringraziare i dottori Andrea Di Simone, Giordano Cattani e Luca Mazzaferro per aver seguito i miei primi passi in ATLAS ed avermi insegnato molte cose. I miei amici di corso Lorenzo Paolozzi, Damiano Lupetto, Matteo Cremonesi e Vincenzo Stomaci con cui ho percorso cinque anni di ansie uni- versitarie ma anche di momenti divertenti. Ringrazio anche Ilaria Migliaccio, Cristina Papaleo e Francesco Guescini per i loro preziosi consigli. Un grande ringraziamento va alla mia famiglia: mio padre che coltiva sempre i miei interessi, mia madre e mia sorella che mi sopportano e soprattutto mia nonna che mi ascolta quando le spiego come funziona il mondo delle particelle. Infine grazie ai miei veri amici Ciava, Cimotto, Skiaffo, Sancio, Cinese, Zilletto e Casty che sono la colla che tiene unito il mio cervello.

40