Ph.d. degree
Department of Physics and Astrophysics, University of Delhi, India, 2021

Mohit Gola

Supervisor: Ashok Kumar

A new look at triple-GEM detector and dark matter search at Large Hadron Collider

Keywords:

• Gaseous detectors
• Electron multipliers (gas)
• Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MICROPIC, MICROMEGAS, InGrid, etc)
• Data analysis

Abstract:

The research in this thesis is mainly based on the physics analysis and the upgrade of the muon system of Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). In physics analysis, it consists of the associated production of the dark matter with Higgs boson decaying to pair of bottom quarks. On the detector contribution, it includes the production and testing of Gas Electron Multiplier (GEM) detectors for the CMS upgrade. In addition to this, it includes the extensive R&D on the India made GEM foils and advanced tests performed on these foils.
There is a necessity of evolution in detectors to progress in Elementary Particle Physics. Starting from the Multiwire Proportional Counter (MWPC) invented by G. Charpak in 1968 which consists of thin anode wire sandwiched between the two cathodes. Despite their long term use in nuclear and particle experiments, it has some limitations, mainly shows the reduction in gain for the incoming flux of ∼ 10^4 mm^2. To get rid of this limitation in rate handling capacity, in 1968 the Micro Strip Gas Chamber (MSGC) is developed by A. Oed, consists of adjacent cathode and anode strips. It gives two orders of magnitude better rate handling capacity with respect to MWPCs but it faces the destructive discharges which is leading to the irreversible damages. To overcome this issue a concept of multi-level amplification was introduced by F. Sauli, in which the amplification layers are operated at the gain far below the discharge limit. Using the same concept, a Gas Electron Multiplier (GEM), consists of polymer (∼ 50 Μm) coated both sides with metallic surface (∼ 5 Μm) has been invented.
The LHC is the most powerful and world’s largest particle accelerator to date having four collision points, out of which, CMS is one of the general-purpose detectors.
To extract the new physics at the LHC, it requires the upgrade of the detector elements to cope up with the harsh radiation environment. The LHC will be upgraded in several phases that will allow significant expansion of its physics program. The final luminosity of the accelerator is expected to exceed 5 X 10^34cm^-2s^-1, five times more than the original design value. The forward region of the CMS muon system consists of Cathode Strip Chambers (CSCs) and the degradation in the performance of these chambers with time will ruin the Level-1 (L1) trigger efficiency. To cope with the corresponding increase in background rates and trigger requirements, the installation of additional sets of muon detectors based on GEM technology, referred to as GE1/1, GE2/1 and ME0, has been planned. The installation and commissioning of the GE1/1 chambers is ongoing, while the GE2/1 and ME0 detectors are expected to be installed between the years 2022 and 2024. Before the installation of these chambers, a detailed quality controls (QCs) procedure has been set up and described in great detail in this thesis. Also, the description of the GE2/1 and ME0 upgrade is included for the sake of completeness.
Out of several QCs designed to validate the detector for CMS muon chamber upgrade, gain uniformity of the detector is one of the major tests since it is an important parameter of any gaseous detector. To overcome the limitation of doing gain uniformity sector-by-sector, a new readout system is designed by RD51 known as a Scalable Readout System (SRS), and later opted by CMS GEM collaboration for the quality assurance of the GEM detectors. Which consists of APV25 front-end chip with 128 readout channels connected to the readout board of the detector. It is important to quantify the non-uniformity present in the channels of ASIC in order to disentangle with the non-uniformity of the detector. Also, due to big size (∼1 m) and no spacers present in the middle of the detector, there is a finite probability of the bending in the readout PCB which may results in the non-uniformity in the induction gap.
A novel technique has been developed to observe the possible bending in the readout board/induction gap.
Furthermore, the increasing demand for GEM foils has been driven by their application in many current and proposed High Energy Physics (HEP) experiments.
Keeping in mind the demanding GEM foil production process, the commercialization of GEM foils has been realized and established for the first time in India by Micropack Pvt. Ltd., a Bengaluru based company. However, it’s a long and laborious effort to validate the foils delivered by these companies to claim that the GEM detectors made from them are compatible with high scientific standards. An extensive R&D has been performed on the different set of foils including single and double mask samples produced by the company.
An important part of the Ph.D. work includes the search for the dark matter candidate at LHC. The search was based on the assumption that if non-gravitational interaction occurs between dark matter and standard model particle, the search of dark matter like a candidate is feasible at the LHC energy scale. Such topologies are known as mono-X searches, where X (=g, q, γ, Z, W, or Higgs boson) is the standard model particle. The production of standard model particle is either due to Initial State Radiation (ISR) or due to new vertex couplings. For Higgs boson, ISR is highly suppressed hence the mono-h channel is only due to the direct coupling of dark matter with standard model particle. The data collected using proton-proton collisions at center of mass energy (sqrt(s)) of 13 TeV in the year 2015, corresponds to an integrated luminosity of 2.3 fb^-1. The Higgs boson decaying to a pair of bottom quarks with missing transverse energy in the final state has been studied. Finally, the results were interpreted using two-Higgs Doublet Model (2HDM).