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2008 BEAM TEST ANALYSIS OF CASTOR CALORIMETER AND PEDESTAL STABILITY OF HCAL DURING GLOBAL RUNS *

CASTOR Kalorimetresinin 2008 Hüzme Testi Analizleri ve HCAL’İN Genel Veri Alımı Sırasındaki Pedestal Kararlılığı

Emine GÜRPINAR Gülsen ÖNENGÜT Fizik Anabilim Dalı Fizik Anabilim Dalı

ABSTRACT Centauro and Strange Object Research (CASTOR) which is a tungsten/quartz Cerenkov sampling calorimeter, is installed in the very forward region of the (CMS) experiment at the (LHC). It will cover the pseudo rapidity range 5.1

ÖZET Centauro ve Acayip Cisim Arastırmaları detektörü (CASTOR), Büyük Hadron Carpıştırıcısı (LHC)’deki Compact Muon Solenoid (CMS) deneyinin ileri bölgesine yerleştirilecek olan Çerenkov ışıması ilkesine dayanan bir tungsten-kuvartz örnekleme kalorimetresidir. Etkileşme noktasından 14.38 m uzaklığa konulacaktır ve 5.1<ŋ<6.6 pseudorapidite aralığını kaplayacaktır. CASTOR kalorimetresinin performansını test etmek amacıyla 2008 yılında CASTOR’un IV. prototipinin CERN/SPS H2 deney alanında hüzme testi yapılmıştır. LHC’de CMS deneyinin alt sistemi olan Hadronik Kalorimetre (HCAL) Hadronik fıçı (HB), Hadronik kapak (HE), dış kısım (HO), ve ileri kalorimetre (HF) gibi 4 alt dedektör içermektedir. HCAL’de pedestal, müon enerjisini ve kalibrasyonun kalitesini belirlediği için önemlidir. Analizimde ayrıca Cosmic Run At Four Tesla (CRAFT) sırasında alınan veriler kullanarak HCAL’in tüm altdedektörlerinin pedestal kararlılığı araştırılmıştır. Anahtar Kelimeler: CASTOR, HCAL, CMS, LHC.

* Yüksek Lisans Tezi-MSc. Tehsis 138

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Introduction High energy physics searches the elementary constituents of matter and the interactions between them. It concentrates on subatomic particles. These contain atomic constituents like electrons, , and neutrons. Protons and neutrons are really combined particles which are made up of quarks. All the particles and their interactions observed until now can almost be described entirely by a quantum field theory called Standard Model (SM). The Standard Model is the common theory of quarks and leptons and their electromagnetic, weak and strong interactions. But it is not a complete theory because it has many important unanswered questions. Because of this, beyond the Standard model physics research is needed. Beyond the SM physics will be studied of the experiments A Torodial LHC Apparatus (ATLAS), Compact Muon Solenoid (CMS), A Large Ion Collider Experiment (ALICE) and A Large Hadron Collider Beauty (LHC-B) on the Large Hadron Collider (LHC) ring at European Nuclear Research Laboratory (CERN). . The Large Hadron Collider (LHC) The Large Hadron Collider (LHC) which is the world’s highest-energy , was built by the European Organization for Nuclear Research (CERN). LHC aims to collide opposing particle beams, protons at a center of mass energy of 14 TeV. Experiments on the LHC are believed strongly to help scientist to answer the existence of mysterious questions like what gives mass to a particle?, what is the nature of dark matter?, do extra dimensions exist? etc. LHC has four big experiments. They are the Compact Muon Solenoid (CMS), A Large Torodial LHC Apparatus (ATLAS), Large Hadron Collider b-quark experiment (LHC-b) and A Large Ion Collider Experiment (ALICE). The CMS and ATLAS are multipurpose experiments. They have the same scientific aims but the technical solution and design of detector magnet system are different. The LHC-b is a specialized experiment which will be investigating the differences between matter and antimatter by studying a type of particle called the ’beauty quark’. The ALICE will study the quark-gluon plasma in heavy ion collisions.

CMS Deneyi The CMS experiment is a general-purpose detector. CMS experiment will investigate new physics at TeV scale, discover the Higgs boson and look for evidence of physics beyond the SM, SUSY or extra dimensions. The CMS detector consists of subdetectors which are a silicon tracker, an electromagnetic calorimeter and a hadron calorimeter, surrounded by a solenoid which generates a strong magnetic field of 4 T, in order to measure the tracks, energy and momentum of photons, electrons, muons and the other particles over a large energy range and at high luminosity. An overall picture of the CMS can be seen in Figure 1.

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Figure 1. The CMS detector. (The Collaboration, 2007)

Hadronic Calorimeter (HCAL) HCAL which will measure quark, gluon and neutrino directions and energies by means of measuring the energy and direction of particle jets and of the missing transverse energy flow, is subsystem of the CMS detector. The HCAL consists of four subdetectors which are Hadronic Barrel (HB), Hadronic Endcap (HE), Hadronic Outer (HO) and Hadronic Forward (HF). HB covers the ŋ range -1.4< |ŋ| < 1.4 and the HCAL endcaps (HE) cover the pseudorapidity range 1.3< |ŋ| <3.0. They are the sampling calorimeters which consist of plastic scintillators as active material inserted between copper absorber plates, which are placed between the ECAL and the magnet. Light collected from the scintillators are read out by the HybridPhoto Diodes (HPD). The HB is not deep enough to contain a hadronic shower fully. Thus, the HO comes in to play to catch the tails of a hadronic shower. The HO contains scintillators with a thickness of 10 mm, is physically located inside the barrel muon system. It covers the region - 1.26< |ŋ| <1.26. It is divided into 5 sections along ŋ, called rings -2, -1, 0, 1, and 2. The HF calorimeters, the last subdetector of HCAL, are placed 11 m away from the interaction point. The HF calorimeter is located at 3.0< |ŋ| <5.0. It uses the quartz fibers as the active medium.

The CASTOR Calorimeter The Centauro and Strange Object Research (CASTOR) calorimeter which will search the Centauro-type events in heavy-ion collisions, is one of the forward detectors of CMS. The CASTOR calorimeter (see Figure 2.) has been a part of the CMS detector since June 2009. It will search the electromagnetic and hadronic contents of the interactions by measuring the energies of the particles.

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Figure 2. The CASTOR Calorimeter

It is a tungsten/quartz Cerenkov electromagnetic and hadronic sampling calorimeter, an octagonal cylinder in shape. Castor will cover the region 5.2 ≤ |ŋ| ≤ 6.4. It is divided into 16 sectors in azimuth. Also it is divided longitudinally into 14 sections, 2 sections for the EM part and 12 sections for the HAD parts in depth. The electromagnetic section consists of 2x16 channels. The hadronic section has 12x16 channels. CASTOR calorimeter consists of successive layers of tungsten plates (W) as absorber and fused silica quartz (Q) plates as active medium. Thicknesses of W plates and Q-plates are 5mm and 2mm respectively for hadronic section the W and Q plates have thicknesses of 10mm and 4mm larger, than the W plates and Q plates of EM, tilted at 450 with respect to the direction of the impinging particles due to capture maximum of Cerenkov light in the quartz. Cerenkov light is produced by the passage of particles through the medium and is collected in sections of 5 W/Q then focused by air-core light guides onto the PMTs. The CASTOR Calorimeter has 224 (16x14) subdivisions in total. The Cerenkov light produced in each one is collected and focused by air-core light guides onto the corresponding PMTs. There are 5 tungsten/quartz layers called Sampling Units (SU) in both the EM and HAD sections, each read by a Readout Unit (RU) (CASTOR EDR, 2007). This calorimeter design and components are shown in Figure 3.

Figure 3. The details of the CASTOR Calorimeter

Analysis And Results Introduction In this chapter, I present the analysis results of the CASTOR calorimeter test beam of prototype IV data collected at CERN in the summer of 2008. My analysis

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consists of two parts:I studied the X-surface scan by using E=80 GeV pion and E=100 GeV electrons

Beam Test of CASTOR Prototype-IV The beam test of prototype IV was performed in the H2 line at CERN Super Synchrotron (SPS). The energy linearity, resolution and uniformity, as well as the surface scan were studied for electrons, pions and muons of various energies. The prototype IV was a full-length octant which consisted of EM and HAD sections with a total of 28 readout-units (RUs). W plates, as absorber, and Q plates as active medium were installed in one octant of Castor prototype-IV. Light is produced by the passage of relativistic particles via Q medium and collected by 5 W/Q layers. Then it is focused by air-core light guides onto the PMTs. Schematic drawing of the beam test with 28 RUs indicated are shown in Figure 4. The beam comes from the left impinging on the EM sections.

Figure 4. Schematic drawing of Castor prototype-IV (Aslanoglou et al., 2008).

X-Surface Scan Analysis X-Surface Scan with electron runs Electron beams at 100 GeV energy with various X-positions were used to study X-surface scan for electron runs. I have analyzed 16 electron runs. A beam cut was applied to the beam profile for all runs, the beam for each point was subdivided into a number of smaller parts, each of diameter 2 cm, so more impact points could finally be used. Also some spatial cuts were applied for all runs. These are scintillator cuts, muon cuts, electromagnetic fraction cut. Scintillator cuts (SC1, SC2, SC4) were applied to tag the single particle events. Muons were rejected using the muon veto counter (MVB) placed behind the CASTOR prototype. For rejection of pions, FEM cut which requires that the ratio of the mean value of total electromagnetic channels (EM) to the mean value of total channels (EM+HAD) has to be larger than 0.95 used. Figure 5. exhibits the signal distribution for the electron beam of E = 100 GeV before and after all the cuts. The results of the X-scan analysis are shown in Figure 6. It shows the response of the EM two semi-octants (Saleve and Jura side) as the beam impact point moves across the front face of the calorimeter. The sigmoid nature of the response curve is evident. The X-derivative of the response is calculated, giving the width of the electromagnetic shower. We observe that one standard deviation amounts to σEM =1.903 mm for the (Saleve side) EM shower and σEM =1.601 for the (Jura side) EM shower.

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Figure 5. Signal distribution of the sum of the signals in EM1, EM3 and HAD1 channels after applied all cuts (a), Signal distribution of the sum of the signals in EM1, EM3 and HAD1 channels after applied all cuts (b).

Figure 6. Response of the semi-octant of the EM section (Saleve side, Jura side) as the beam scans the front face of the calorimeter (a). The derivative of the response with respect to x, indicating the width (σ=1.903 mm for Saleve side, σ=1.601 mm for Jura side) of the EM shower (b).

X-Surface Scan with pion runs Pion beams at 80 GeV energy were used to study X-surface scan with pion runs. The main goals of X-surface scan with pion runs are to determine the width of the HAD shower profile. To study the X-surface scan of the hadronic section of the calorimeter, a central point in the Saleve side sector was exposed to beams of various positions at 80 GeV. I have analyzed 16 pion runs. The beam for each point was subdivided into a number of smaller parts, each of diameter is 2 cm. In X-surface scan with pion runs analysis, some cuts were used to select the pion events such as the scintillator cut, muon cut, FEM cut. As can be seen in Figure 7., the quality of the spectra was significantly improved after applying all the cuts, although a significant fraction of the events was finally filtered out reducing the available statistics. The surface response of the prototype calorimeter to pions was obtained from hadronic semi-octant sectors, by moving the beam along the X-

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direction. Figure 8. (a) (b) show the X-scan for pions of 80 GeV energy (right) and the derivative of this response with respect to x (left).

Figure 7 Figure 8 Figure 7. Energy spectrum for an pion beam of E=80, before and after applying all cuts (a), Signal distribution after applied all cuts (b). Figure 8. X-scan along the face of prototype for 80 GeV pions (a). The derivative of the response with respect to X, the width of the HAD shower is given by (σ=6.081 mm) (b).

HCAL PEDESTAL STABILITY STUDIES IN CMS Introduction In HCAL, pedestal subtraction is important to determine the muon energy deposits and for calorimetry based muon isolation. Precision of pedestal determination has also a direct impact on the quality of calibration of HCAL. In this method, pedestal is defined on an event-by-event basis. During September and November 2008 (Cosmic Run At Four Tesla (CRAFT) period) we have studied the stability of HCAL pedestals. We have analyzed over 50 CRAFT global runs In general, during CRAFT individual HCAL channels were stable. Run to run variation (RMS) of pedestals for most of the channels was in the range of 0.001 to 0.002 ADC per Time Slice (TS). Assuming 8 time slices were used to reconstruct the HCAL energy, this variation is equivalent to RMS of 2 to 4 MeV per readout channel. For few individual channels, pedestals were not stable and exhibited not just a shift, but short or long term drifts. Pedestal shifts of individual channels were not correlated (Barbaro, 2009).

Stability of HCAL Pedestals during CRAFT Pedestal Definition Cosmic ray muons permeate only few HCAL towers (out of a total of over ten thousand HCAL readout channels). So reliable source of data to monitor the stability of HCAL pedestals is provided by events triggered by cosmic ray muons (Barbaro, 2009). HCAL uses Charge Integrator and Encoder (QIE) cards which are 7 bit ADC. HCAL ADCs rotate via four independent capacitors (CapIDs) for each readout channel to measure the charge. Pedestal values averaged over four 144

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CapIDs are more stable than the values calculated for individual CaIDs (Barbaro, 2009). We can calculate the average pedestal value of four CapIDs:

1 ped(channel,run) =  ADCTS 8 Ntrig where NTRIG is number of trigger and ADCTS is ADC per time slice.

Pedestal Calculation Pedestal mean is given by,

(ped)  RMSped / 8 Ntrig RMS(ped) is RMS of pedestal for a single channel in a single run, is defined as:

1 2 RMS(ped)= ADC(channel,TS)  ped(channel,run) NS where ADC(channel,TS) is ADC of a channel per time slice and ped(channel,run) is pedestal of a channel per run.

Stability of the HE and HB pedestals during CRAFT We have analyzed the stability of the pedestals of HE and HB channels. During CRAFT the average pedestal of all HB-HE channels is very stable. Run to run RMS of the average pedestal is equal to 0.9 MeV for HB and 0.5 MeV for HE. 23 out of a total of 5184 channels in HB and HE, less 0.5 percent of the total showed run-to-run shifts of pedestals above the level of 0.010 ADC per TS. For few individual channels, pedestals were not stable. There were not just a shift, but short or long term drifts. Pedestal shifts of individual channels were not correlated. (Barbaro, 2009).

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Figure 9. Pedestal mean versus run number for individual HCAL channels, HB (eta=2, phi=23,depth=1) and HE (eta=18, phi=8, depth=1). For these two channels, pedestal shifts were apparent throughout several runs (long term drifts, as opposed to shift) (Barbaro, 2009).

Stability of HF pedestals during CRAFT We have analyzed the stability of pedestals of HF channels. Run-to-run RMS of average HF pedestal is 0.00029 ADC counts (Barbaro, 2009). In general most of HF channels are stable, with an average RMS of 0.0016 ADC counts. However, few channels exhibit instabilities. There are ten channels (out of a total of 1728) with run-to-run RMS above 0.0050 ADC counts. In some cases, large RMS is the result of a single shift, in other cases, pedestal is unstable throughout the entire CRAFT. In particular, a single with largest RMS has a shift of 0.5 ADC counts. In HF (eta=-31, phi=7, depth=1) channel there was a systematic shift of about 0.030 ADC counts/TS . HF (eta=29, phi=71, depth=1) channel was unstable, with long term drift (down and up) 0.100 ADC counts/TS took place during CRAFT.

Figure 10. Pedestal average for HF (eta=-31, phi=7, depth=1) channel versus run number (a), Pedestal average for HF (eta=29, phi=71, depth=1) channel versus run number (b).

Stability of HO pedestal during CRAFT We have checked the stability of pedestal in HO channels during CRAFT. Run-to-run RMS average HO pedestal is 0.00044 ADC counts. In general, 99.9% of HO channels are stable. Average RMS is 0.0012 ADC counts. But few channels (0.1%) exhibit instabilities. There are seventeen (out of total of 2160 channels) with run-to-run RMS above 0.0050 ADC counts. In some cases, large RMS is result of a single shift, in other cases, pedestal is unstable throughout entire CRAFT. In particular, there is a single channel with largest RMS.

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Figure 11. HO channel (eta=-4, phi=69, depth=4) with unstable pedestal (a). HO channel (eta=15, phi=24, phi=4) with unstable pedestal. Pedestal value changes by up to 2 ADC counts (b).

CONCLUSION My thesis contains two major studies. The first one is the beam test of the final prototype IV of CASTOR calorimeter. In the second study, I present my analysis of stability of HCAL pedestals by using data taken during CRAFT. In the first part of the thesis, surface scan were studied with an electron beam at 100 GeV, pion beam at 80 GeV. The purposes of the area scanning are to check the uniformity of the EM calorimeter response to electrons hitting at different points on the sector area, to estimate the width of the EM shower profile and to assess the amount of the effects and lateral leakage from the calorimeter which could lead to cross-talk between neighboring sectors. The derivative of the response is calculated and the electromagnetic shower width is found to be 1.903 mm for Saleve side and 1.601 mm for Jura side, hadronic shower width is found to be 6.081 mm for Saleve side, As expected, the pion shower was larger than the corresponding electromagnetic shower. In the second part of the thesis HCAL pedestal stability have been studied. I checked HCAL pedestal stability using CRAFT global runs and looked at over 50 runs. Most (99.9%) of HCAL readout channels have very stable pedestal. So 0.1% of HCAL channels are unstable. Typical RMS is 0.001 to 0.002 ADC counts. RMS is 0.001-0.002 ADC/TS. This value is equivalent to RMS of 2-3 MeV per channel. Some channels (20 out of 5k, <0:5% of total) showed pedestal variation above the level of 0.010 ADC/TS (equivalent to 15 MeV/channel).

REFERENCES ASLONLOGLOU et al., 2007, Performance Studies of Prototype II for the CASTOR forward Calorimeter at the CMS Experiment, arXiv:0706.2641v2 [physics.ins-det]. ASLANOGLOU et al., 2008, Performance studies of the final prototype for the CASTOR forward calorimeter at the CMS experiment CMS NOTE 2008/-(in preparation) CASTOR Engineering Design Report, 2007, CERN, CMS Collaboration, The Hadron Calorimeter Technical Design Report, CERN/LHCC 1997-031 (1997) 147

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DE BARBARO et al., 2009, Stability of HCAL hardware pedestals during CRAFT and Proposal for definition of pedestal tags and Intervals of Validity (IOVs) for HCAL energy reconstruction, CMS IN-2009/005 GUMUS, K., 2008. Search for new Physics in the Compact Muon Solenoid (CMS) experiment and the response of the CMS Calorimeters to particles and jets, Texas Tech University, 113s.

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