A micro-pattern gas detector based muon system for the CMS experiment at the high-luminosity LHC
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- Tez No: 523388
- Danışmanlar: Dr. MICHAEL TYTGAT
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2018
- Dil: İngilizce
- Üniversite: Ghent University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 218
Özet
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Özet (Çeviri)
CERN, the European laboratory for nuclear research, was created in 1954 by 12 European countries with the idea of bringing together technical, nancial and human resources in order to build the most sophisticated particle accelerator complex, essential for the research in particle physics. In the early nineties, after a long history of successful projects, the green light was given for the present experiments at the Large Hadron Collider, which was to be constructed inside the LEP tunnel at CERN. Four large detector systems were placed at the collision points in the next years: ATLAS (A Toroidal Lhc ApparatuS) in point 1, CMS (Compact Muon Solenoid) in point 5, LHCb (LHC beauty) in point 8 and ALICE (A Large Ion Collider Experiment) in point 2. ATLAS and CMS are multi-purpose experiments that are meant to search for new physics through precise measurements of the elementary particles, the reconstruction of hadron jets and the identi cation of the missing energy corresponding to weakly interacting particles. During my PhD studies, I worked for the CMS experiment. The scienti c goals are precision measurements of the Standard Model, the under- standing of the mass of the elementary particles and the search for the physics beyond the Standard Model, which is the theory describing the electromagnetic, weak, and strong interactions, and also managing the dynamics of the known elementary particles. The CERN Large Hadron Collider and the associated experiments have already produced ex- cellent scienti c results so far, with the primary example being the Higgs boson discovery. However, to improve the LHC experiments' discovery potential, which is the basis of the high-luminosity LHC upgrade, a general upgrade of the detectors and their components is required. Muons play a crucial role for precision measurements and discoveries in a hadron collider environment, as they form characteristic signatures on the huge hadronic back- ground, and these particles must be identi ed already at trigger level to achieve the goal. Their directions and momentum must be measured accurately to reconstruct the processes producing the muons. The Higgs decomposition into two Z bosons, both decaying into muon pairs is a good example of a channel that played a major role in the discovery of the Higgs particle. With my PhD thesis, I focused on the Muon System upgrade of the CMS experiment; speci cally, on the muon endcap stations that in the view of the HL-LHC upgrade will be extended with Gas Electron Multiplier (GEM) detectors. In the rst part of my PhD studies, I worked on the GEM endcap station 1 ring 1, namely GE1/1, i.e. triple-GEM detectors that will be installed into CMS during LHC Long Shutdown 2. By now, the production of these GEM chambers has been started at various production sites around the world, including also Ghent University. In the second part of my PhD, I studied another station in the muon part, ME0, which is in the very forward jj region of the CMS Muon System. For triggering, muon identi cation and momentum measurement, this forward region is the most challenging one given the high track density and huge background rate. The GE1/1 project to equip the rst CMS muon endcap station with GEM chambers was started in 2009, while I joined to the GEM group in 2012. Over the years, many generations of chamber prototypes were produced and studied before we arrived at the nal version suitable for installation inside CMS. We organized test beams to study the performance of the each version. My contributions included the test beam preparations, taking shifts and responsibilities during these tests, data collection and analysis to obtain the eciency, spatial resolution, cluster size and time resolution of the prototypes. The rst test beam I joined was performed in November 2012. The GEM detectors were readout with digital VFAT/TURBO electronics. After this successful beam test period, I took care of the analysis with a dedicated package for data collected with TURBO software. An eciency of 98% was achieved when the detector operated with high voltage that corresponds to a gain of about 104 during this test beam. The measured spatial resolution of 267 m was in agreement with the value expected for the 0.88 mm strip pitch using digital VFAT readout. Afterwards, we organized another test beam in December 2014, during which the performance of a GE1/1 chamber was evaluated with Ar/CO2/CF4 45/15/40 and Ar/CO2 70=30% gas mixtures. Very good eciency of almost 98% was achieved in all cases. In the region of the eciency bigger than 95%, the time resolution was measured between 6 ns and 8 ns with both gas mixtures. These results showed that Ar/CO2 70=30% as a non greenhouse gas mixture satis es the CMS requirements. Overall, the measured GE1/1 performance is in agreement with the requirements for CMS in terms of space and time resolution, high detection eciency, high-rate capability and resilience against aging e ects. Moreoever, advances in GEM foil production and assembly techniques developed in the course of this project, allowed the construction of large-area GEM detectors as required for the CMS muon system. The experience gained during the R&D phase helped to identify the critical character- istics of the large detectors and to precisely de ne the quality control (QC) procedures. After the agreement on the di erent quality control steps within the collaboration, all production sites implemented identical setups and adopted the same QC protocol, i.e. every GEM QC laboratory now contains a foil leakage current test setup, a high voltage and gas leak stand and an X-ray irradiation station. The rst step of the acceptance test consists of applying voltage to the GEM foils and to measure the leakage current between the top and the bottom electrodes. The second step of the acceptance test consists of measuring the HV long-term stability of the GEM foils in a dry gas environment. The so-called QC2 long test is initially performed at CERN before the shipment of the foils to the production sites. It consists of monitoring the leakage current and the possible sparks when the GEM foil is subject to HV, typically up to 600 V during a period of 30 minutes to 1 hour. The QC3 gas leak test aims to identify the gas leak rate of a GE1/1 detector by monitoring the drop of the internal over-pressure as a function of the time. This part is divided into two steps: the calibration of the system and the leak measure- ment of the detector and also with the system. A gas leak test is necessary to ensure that there is no pollution or air molecules can penetrate the ampli cation region, degrading the performances of the detector. The QC4 test aims to determine the voltage vs. current curve of a GE1/1 detector and identify possible malfunctions, defects in the HV circuit and spurious signals. Furthermore, a new technique was developped in order to measure simultaneously the e ective gain at every readout strip with a SRS/APV data-acquisition system. At the beginning of my PhD studies, Ghent university was involved the CMS GEM collaboration, but the GEM laboratory in our university had not been constructed yet. With my thesis project, I started to test one of the very rst GEM prototypes at Ghent University with the newly constructed setup. Moreover, we constructed a full size GE1/1 prototype for the rst time in Ghent in December 2013. For the Ghent production site, I started to prepare the test setups for QC3 (gas leak test), QC4 (HV test) and QC5 (gain uniformity test with APV+SRS DAQ) for the rst time. Today, the GEM laboratory at Ghent University is fully operational and certi ed as an ocial CMS GE1/1 assembly site, where the construction of the laboratory was initially built up with my help during this PhD study. Ghent University will produce triple-GEM long chambers for the GE1/1 superchambers to be installed into CMS. In the second part of my PhD work, I focused on the ME0 station upgrade of the CMS Muon System. This station is in the very forward region of CMS, and extends the current muon coverage into the region 2.4 < jj < 2.8, thereby reducing the uninstrumented area behind HGCAL. The primary challenge of designing ME0 is to device a system to e- ciently identify muons at low transverse momentum, while maintaining a low background rate of misidenti ed muons in this harsh environment during the HL-LHC period. Several di erent detector technologies were considered, and I worked on two candidates for ME0 station, i.e. the back-to-back or stacked GEM (double triple-GEM) and the Fast Timing Micro-pattern gas detector (FTM). Firstly, I worked on the FTM prototype. In order to reach the 1 ns or better time resolution, segmenting the drift gap into multiple smaller stages is the main principle behind the FTM, i.e. the single thick drift region is replaced by many thinner regions, each coupled to its own ampli cation stage. Each ampli cation region is based on a pair of kapton foils stacked due to the electrostatic force induced by the polarization of the foils. The very rst FTM chamber with two layers was tested in the laboratory at CERN to understand its feasibility. The response of the detector was linear for both layers. In addition, the two data sets collected from these two electrodes were comparable, which was giving an indication of the electrical transparency of the layers. Afterwards, the prototype was taken to the test beam for the time resolution measurement. The time resolution was measured of the order of 2.4 ns with muon beam, which is already a signi cant improvement with respect to the standard GEM timing performance. In the past, the baseline solution considered for the ME0 station was the back-to-back, i.e. a double triple-GEM chamber, which was also the very rst prototype assembled for this purpose. The space is limited at the CMS experiment for ME0 station, and the detector gap con guration is a very important parameter to keep the total thickness of the detectors suitable for this region. Therefore, to optimize the thickness of the chamber even more, the con guration was modi ed, i.e. instead of a 3 mm gap between two drift electrodes, a new 0.5 mm thick PCB with a drift electrode copper cladded on both sides was produced, and I assembled the stacked-GEM prototype with new con guration. While measurements with the rst back-to-back prototype showed non-uniform results for the two layers of the chamber, this new con guration yielded much better results. After the characterization tests of the stacked-GEM prototype at the laboratory, the gain reached at the eciency plateau was the order 104. Furthermore, the prototype was taken to the test beam for the eciency and time resolution measurements. Two test beam campaigns were performed to access the stacked-GEM prototype performance, including also the timing characteristics with Ar/CO2/CF4 and Ar/CO2 70=30 gas mixtures. In the end, we demonstrated that this more compact device reaches a similar performance level as standard triple-GEMs. At the time of writing, the selected baseline solution for the ME0 station is still regular triple-GEMs. Although the stacked-GEM technique was proven to work, the space constraints for ME0 were relaxed in the nal design of the endcap region. The FTM technology was also not selected since much more development is required to arrive at a mature detector. This innovative technology will be taken forward for use in future experiments or for non particle physics applications where improved timing is important. Overall, the development of the GEM technology speci cally for CMS has spanned many years that have seen continuous improvements in the design and performance of the detector modules. These e orts have led to a nal detector technology and design for the CMS GEM modules that is shared between the GE1/1, GE2/1, and ME0 systems. As a summary, the CMS detector has worked excellently since the start of data taking in 2009. The CMS apparatus must now be upgraded to handle the aging and the much higher particle rates at HL-LHC, so that its physics performance remains as strong as in the current Phase 1 running.
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