Study of the operation and trigger performance of GEM detectors in the CMS experiment

At the end of 2018, the CERN Large Hadron Collider (LHC) accelerator started, in concert with its experiments, an upgrade campaign to reach the goal of the High-Luminosity LHC project: proton-proton collisions at a center-of-mass energy √s = 14TeV with an instantaneous luminosity around 5 − 7 · 10^34 cm−2s−1, aiming to deliver a 3000 fb−1 integrated luminosity. To cope with these new experimental conditions, the CMS experiment started, among others, an upgrading campaign of its muon system, programming the installation of three new stations using the Gas Electron Multiplier (GEM) technology: GE1/1, GE2/1 and ME0. The motivations of the installation of these stations are to increase the redundancy in the CMS muon system, to keep the trigger rate under control, to have radiation hard detectors in the CMS forward region, and to improve the detection of physics channels at high pseudorapidity (η), extending the angular coverage of the muon system to |η| < 2.8. This thesis was developed in the framework of the GEM upgrade, focusing, in particular, on the production and commissioning of the GE1/1 station and on the development of trigger algorithms for the Phase 2 upgrade, exploiting the possibilities offered by the foreseen GEM detectors. The first chapter presents the LHC accelerator and the CMS experiment, describing its foreseen upgrades. The second chapter introduces the GEM upgrade, then it moves to a detailed description of the GE1/1 station, and finally presents the GE2/1 and ME0 stations, whose installation is foreseen between 2023 and 2026. The third chapter describes the activities I have performed in the production and the validation process of GE1/1 chambers, before their installation in the CMS experiment. The second part of this chapter describes the activities I carried out during the commissioning in the experimental site, focusing on the monitoring of the power systems and on the study of HV trips in different experimental conditions, for example during the commissioning of the CMS magnet and in the early LHC collisions performed at the end of the Long Shutdown 2 period. The fourth chapter illustrates a trigger study I have carried out, dedicated to the τ → 3μ decay channel, a Lepton Flavour Violating decay with a branching ratio heavily suppressed in the Standard Model and brought in a statistically significant region by some Beyond the Standard Model models. This decay is characterised by a multi-muon final state, with low transverse momentum muons and collimated in the forward region. Due to these features, the research carried out on the τ → 3μ channel would turn out to be useful also for other channels with similar characteristics. In the discussion some trigger paths of interest, that exploit the possibilities introduced by the installation of the GEM stations and by other CMS Phase 2 upgrades, are presented.At the end of 2018, the CERN Large Hadron Collider (LHC) accelerator started, in concert with its experiments, an upgrade campaign to reach the goal of the High-Luminosity LHC project: proton-proton collisions at a center-of-mass energy √s = 14TeV with an instantaneous luminosity around 5 − 7 · 10^34 cm−2s−1, aiming to deliver a 3000 fb−1 integrated luminosity. To cope with these new experimental conditions, the CMS experiment started, among others, an upgrading campaign of its muon system, programming the installation of three new stations using the Gas Electron Multiplier (GEM) technology: GE1/1, GE2/1 and ME0. The motivations of the installation of these stations are to increase the redundancy in the CMS muon system, to keep the trigger rate under control, to have radiation hard detectors in the CMS forward region, and to improve the detection of physics channels at high pseudorapidity (η), extending the angular coverage of the muon system to |η| < 2.8. This thesis was developed in the framework of the GEM upgrade, focusing, in particular, on the production and commissioning of the GE1/1 station and on the development of trigger algorithms for the Phase 2 upgrade, exploiting the possibilities offered by the foreseen GEM detectors. The first chapter presents the LHC accelerator and the CMS experiment, describing its foreseen upgrades. The second chapter introduces the GEM upgrade, then it moves to a detailed description of the GE1/1 station, and finally presents the GE2/1 and ME0 stations, whose installation is foreseen between 2023 and 2026. The third chapter describes the activities I have performed in the production and the validation process of GE1/1 chambers, before their installation in the CMS experiment. The second part of this chapter describes the activities I carried out during the commissioning in the experimental site, focusing on the monitoring of the power systems and on the study of HV trips in different experimental conditions, for example during the commissioning of the CMS magnet and in the early LHC collisions performed at the end of the Long Shutdown 2 period. The fourth chapter illustrates a trigger study I have carried out, dedicated to the τ → 3μ decay channel, a Lepton Flavour Violating decay with a branching ratio heavily suppressed in the Standard Model and brought in a statistically significant region by some Beyond the Standard Model models. This decay is characterised by a multi-muon final state, with low transverse momentum muons and collimated in the forward region. Due to these features, the research carried out on the τ → 3μ channel would turn out to be useful also for other channels with similar characteristics. In the discussion some trigger paths of interest, that exploit the possibilities introduced by the installation of the GEM stations and by other CMS Phase 2 upgrades, are presented

Towards a muon collider

A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work

Towards a muon collider

A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work

Erratum:Towards a muon collider

LPT

Towards a Muon Collider

A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.Comment: 118 pages, 103 figure

Triple-GEM discharge probability studies at CHARM: Simulations and experimental results

The CMS muon system in the region with 2.03<|η|<2.82 is characterized by a very harsh radiation environment which can generate hit rates up to 144 kHz/cm$^{2}$ and an integrated charge of 8 C/cm$^{2}$ over ten years of operation. In order to increase the detector performance and acceptance for physics events including muons, a new muon station (ME0) has been proposed for installation in that region. The technology proposed is Triple—Gas Electron Multiplier (Triple-GEM), which has already been qualified for the operation in the CMS muon system. However, an additional set of studies focused on the discharge probability is necessary for the ME0 station, because of the large radiation environment mentioned above. A test was carried out in 2017 at the Cern High energy AcceleRator Mixed (CHARM) facility, with the aim of giving an estimation of the discharge probability of Triple-GEM detectors in a very intense radiation field environment, similar to the one of the CMS muon system. A dedicated standalone Geant4 simulation was performed simultaneously, to evaluate the behavior expected in the detector exposed to the CHARM field. The geometry of the detector has been carefully reproduced, as well as the background field present in the facility. This paper presents the results obtained from the Geant4 simulation, in terms of sensitivity of the detector to the CHARM environment, together with the analysis of the energy deposited in the gaps and of the processes developed inside the detector. The discharge probability test performed at CHARM will be presented, with a complete discussion of the results obtained, which turn out to be consistent with measurements performed by other groups

Detector Control System for the GE1/1 slice test

Gas Electron Multiplier (GEM) technology, in particular triple-GEM, was selected for the upgrade of the CMS endcap muon system following several years of intense effort on R&D. The triple-GEM chambers (GE1/1) are being installed at station 1 during the second long shutdown with the goal of reducing the Level-1 muon trigger rate and improving the tracking performance in the harsh radiation environment foreseen in the future LHC operation [1]. A first installation of a demonstrator system started at the beginning of 2017: 10 triple-GEM detectors were installed in the CMS muon system with the aim of gaining operational experience and demonstrating the integration of the GE1/1 system into the trigger. In this context, a dedicated Detector Control System (DCS) has been developed, to control and monitor the detectors installed and integrating them into the CMS operation. This paper presents the slice test DCS, describing in detail the different parts of the system and their implementation

Impact of magnetic field on the stability of the CMS GE1/1 GEM detector operation

The Gas Electron Multiplier (GEM) detectors of the GE1/1 station of the CMS experiment have been operated in the CMS magnetic field for the first time on the 7$^{th}$ of October 2021. During the magnetic field ramps, several discharge phenomena were observed, leading to instability in the GEM High Voltage (HV) power system. In order to reproduce the behavior, it was decided to conduct a dedicated test at the CERN North Area with the Goliath magnet, using four GE1/1 spare chambers. The test consisted in studying the characteristics of discharge events that occurred in different detector configurations and external conditions. Multiple magnetic field ramps were performed in sequence: patterns in the evolution of the discharge rates were observed with these data. The goal of this test is the understanding of the experimental conditions inducing discharges and short circuits in a GEM foil. The results of this test lead to the development of procedure for the optimal operation and performance of GEM detectors in the CMS experiment during the magnet ramps. Another important result is the estimation of the probability of short circuit generation, at 68 % confidence level, p$_{short}$$^{HV}$ $^{OFF}$ = 0.42$^{-0.35+0.94}$% with detector HV OFF and p$_{short}$$^{HV}$ $^{OFF}$ < 0.49% with the HV ON. These numbers are specific for the detectors used during this test, but they provide a first quantitative indication on the phenomenon, and a point of comparison for future studies adopting the same procedure

CAD-based computer vision: the automatic generation of recognition stragtegies

Journal ArticleThree-dimensional model-based computer vision uses geometric models of objects and sensed data to recognize objects in a scene. Likewise, Computer Aided Design (CAD) systems are used to interactively generate three-dimensional models during these fields. Recently, the unification of CAD and vision systems has become the focus of research in the context of manufacturing automation. This paper explores the connection between CAD and computer vision. A method for the automatic generation of recognition strategies based on the geometric properties of shape has been devised and implemented. This uses a novel technique developed for quantifying the following properties of features which compose models used in computer vision: robustness, completeness, consistency, cost, and uniqueness. By utilizing this information, the automatic synthesis of a specialized recognition scheme, called a Strategy Tree, is accomplished. Strategy Trees describe, in a systematic and robust manner. the search process used for recognition and localization of particular objects in the given scene. They consist of selected features which satisfy system constraints and Corroborating Evidence Subtrees which are used in the formation of hypotheses. Verification techniques, used to substantiate or refute these hypotheses, are explored. Experiments utilizing 3-D data are presented