Università degli Studi di Pavia Dipartimento di Fisica DOTTORATO DI RICERCA IN FISICA – XXXI CICLO Micropattern Gas Detectors for the CMS Experiment’s Muon System Upgrade: Performance Studies and Commissioning of the first GEM Detectors Martina Ressegotti Submitted to the Graduate School of Physics in partial fulfillment of the requirements for the degree of DOTTORE DI RICERCA IN FISICA DOCTOR OF PHILOSOPHY IN PHYSICS at the University of Pavia Supervisor: Prof. Paolo Vitulo Cover: The CMS detector. Micropattern Gas Detectors for the CMS Experiment’s Muon System Upgrade: Performance Studies and Commissioning of the first GEM Detectors Martina Ressegotti PhD thesis - University of Pavia Pavia, Italy, November 2018 The work I carried out during the PhD is focused on the upgrade of the muon system of the Compact Muon Solenoid (CMS) experiment with Mi- cropattern Gas Detectors (MPGDs). The discovery potential of the experiments at the Large Hadron Collider (LHC) is strictly related to the delivered luminosity, hence the LHC has an upgrade program in order to increase the instantaneous luminosity in the next years. It plans to enhance it up to about five times the nominal value by 2025 and to deliver about 3000 fb−1 of integrated luminosity in the next ten years of operation, an order of magnitude higher than the one collected since the first physics run in 2011 until now. On the other hand, the LHC upgrade represents a major challenge for the experiments, as the high radiation dose and event pileup would degrade their performance if they don't undergo an upgrade program involving several as- pects as well. One example is the necessity to improve the CMS muon system in the re- gion close to the beam direction to allow maintaining a trigger performance at least as high as during previous LHC runs, but also to compensate for possible performance losses of the existing muon detectors due to aging effects after years of operation. The upgrade of the muon system with Gas Electron Multi- plier (GEM) detectors, involving three new muon stations (GE1/1, GE2/1 and ME0), serves this goal. In addition, it will also extend the muon system closer to the beam direction, increasing its total acceptance and hence the discovery potential of several physics channels. Throughout the PhD I first took part in the R&D on MPGDs for the upgrade of CMS muon system, in particular in the development and char- acterization of the first prototypes of Back to Back GEM detector and Fast Timing Micropattern detector for the ME0 station, presented in chapter3. They have been considered to cope with the challenges characteristic of this station, represented by the necessity to instrument a small available space with six detector layers and the capability to handle the high background rate present in that forward position. Then I continued my work participating in the commissioning and integration of the first GEM detectors { the GE1/1 slice test { into the CMS experiment, described in chapter4. The installation of the full GE1/1 station is scheduled in 2019-20, but a demonstrator made of ten detectors was installed already at the beginning of 2017 and was operated in 2017-2018 with the goal to acquire installation and commissioning expertise, to develop the integration into the CMS online system and to prove the oper- ability of the system in the CMS environment. The GE1/1 slice test represents a peerless test bench to get ready for the installation of the complete GEM subsystem. Finally, currently I am also carrying out some trigger studies to understand how to use the ME0 station, which will extend the muon system to higher pseudorapidity, to trigger specific physics channels in the extended region where no other muon station is present. The first preliminary results are presented in chapter5. Contents List of Figures vi List of Tables xxvii 1 Introduction1 1.1 The CMS experiment at the LHC.................1 1.2 Physics at the CMS experiment..................2 1.2.1 The Higgs Boson......................3 1.2.2 Physics Beyond the Standard Model...........5 1.3 Structure of the CMS detector................... 11 1.4 The Muon System......................... 18 1.4.1 Drift Tubes......................... 19 1.4.2 Cathode Strip Chambers.................. 20 1.4.3 Resistive Plate Chambers................. 23 1.5 The CMS trigger system...................... 26 1.5.1 The Level-1 Trigger..................... 26 1.5.2 The Muon Trigger..................... 27 1.6 The Upgrades of LHC and CMS.................. 29 1.6.1 Overview.......................... 29 1.6.2 CMS operation at the HL-LHC.............. 31 1.6.3 The Phase-II Upgrade of the CMS detector....... 33 2 The GEM Upgrade of the CMS Muon System 37 2.1 Motivation.............................. 37 2.2 The GEM Technology....................... 40 2.2.1 Micropattern Gas Detectors................ 40 2.2.2 The Gas Electron Multiplier................ 42 2.2.3 Detector Performance................... 44 2.3 The GE1/1 station......................... 49 2.3.1 GE1/1 chambers minimum requirements......... 54 2.3.2 The Slice Test........................ 55 2.4 The GE2/1 station......................... 55 iii CONTENTS 2.5 The ME0 station.......................... 58 2.5.1 Impact on physics in the forward region......... 59 2.5.2 Effect of the time resolution................ 62 2.5.3 Choice of the detector technology............. 66 3 Development and Performance of New Micropattern Gas De- tectors 71 3.1 The Back to Back GEM detector................. 71 3.2 Characterization of the B2B detector in laboratory....... 75 3.2.1 Detector's Rate and Gain................. 77 3.2.2 Summary and discussion.................. 83 3.3 Test Beam studies on the B2B detector.............. 85 3.3.1 Test beam setup...................... 85 3.3.2 Detector Efficiency with tracking............. 88 3.3.3 Timing performance.................... 97 3.3.4 Detector efficiency without tracking............ 100 3.3.5 Summary and discussion.................. 104 3.3.5.1 Detector efficiency................ 104 3.3.5.2 Timing performance............... 109 3.4 The Fast Timing Micropattern detector.............. 114 3.5 Characterization of the FTMv1 in laboratory........... 117 3.5.1 Linearity and transparency................. 119 3.5.2 Scan in electric field.................... 119 3.5.3 Detector's Gain....................... 122 3.5.4 Detector's Efficiency.................... 125 3.6 Test beam studies on the FTMv1 detector............ 127 3.6.1 Timing performance.................... 127 3.6.2 Summary and discussion.................. 131 3.7 Next FTM detector prototypes.................. 131 3.7.1 Preliminary considerations................. 131 3.7.2 Second FTM prototype................... 134 3.8 Recent updates on the FTM detector............... 146 3.9 Conclusions............................. 148 4 The GE1/1 Slice Test 151 4.1 The GE1/1 Slice Test........................ 151 4.2 System overview.......................... 151 4.2.1 High Voltage........................ 152 4.2.2 Readout system and low voltage.............. 155 4.2.3 Gas system......................... 157 4.3 Operating the multi-channel power supply............ 157 4.4 The Detector Control System................... 159 4.5 The CMS and LHC automation.................. 170 4.5.1 LHC operation....................... 173 4.5.2 The automation matrix................... 174 iv CONTENTS 4.6 The GEM Finite State Machine.................. 174 4.6.1 The Gas System FSM sub-tree.............. 177 4.6.2 The High Voltage FSM sub-tree.............. 177 4.6.3 The Low Voltage FSM sub-tree.............. 181 4.7 The FSM of the multi-channel supply............... 182 4.7.1 The GEM Configurator Device Unit........... 183 4.7.2 The GEM Configurator Control Manager......... 185 4.8 The GEM Detector Protection system.............. 188 4.8.1 The Detector Protection for the single-channel power supply............................ 190 4.8.2 The Detector Protection for the multi-channel power supply............................ 191 4.9 Inclusion into the central CMS DCS................ 197 4.10 The DAQ system.......................... 198 4.10.1 Calibration......................... 199 4.11 System performance........................ 204 4.11.1 Stability........................... 204 4.11.2 Detector Performance................... 206 4.12 Conclusions and future perspectives................ 210 4.12.1 The Detector Control System............... 210 4.12.2 The DAQ system and Detector Performance....... 211 5 A Trigger Study for the ME0 Station 213 5.1 Introduction............................. 213 5.1.1 The ME0 station...................... 213 5.1.2 L1 trigger for specific analyses............... 213 5.1.3 The τ ! 3µ decay..................... 214 5.2 Tau production at the LHC.................... 215 5.2.1 Signal simulation...................... 216 5.3 Muon reconstruction in the ME0 station............. 219 5.3.1 Evaluation of the bending thresholds........... 226 5.4 Evaluation of the trigger efficiency................. 226 5.5 Evaluation of the trigger rates................... 233 5.6 Some example cases......................... 234 5.7 Conclusions and future prospects................. 239 6 Conclusions 241 A Trigger efficiencies 245 Bibliography 249 v List of Figures 1.1 Candidate events for the Higgs decays H ! γγ (top) and H ! ZZ∗ ! eeµµ (bottom). In the top panel the green lines represent two reconstructed photons, in the bottom panel the green and red lines pointing towards the center of the detec- tor represent two reconstructed electrons and