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

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

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

Modeling the triple-GEM detector response to background particles for the CMS Experiment

An estimate of environmental background hit rate on triple-GEM chambers is performed using Monte Carlo (MC) simulation and compared to data taken by test chambers installed in the CMS experiment (GE1/1) during Run-2 at the Large Hadron Collider (LHC). The hit rate is measured using data collected with proton-proton collisions at 13 TeV and a luminosity of 1.5$\times10^{34}$ cm$^{-2}$ s$^{-1}$. The simulation framework uses a combination of the FLUKA and Geant4 packages to obtain the hit rate. FLUKA provides the radiation environment around the GE1/1 chambers, which is comprised of the particle flux with momentum direction and energy spectra ranging from $10^{-11}$ to $10^{4}$ MeV for neutrons, $10^{-3}$ to $10^{4}$ MeV for $\gamma$'s, $10^{-2}$ to $10^{4}$ MeV for $e^{\pm}$, and $10^{-1}$ to $10^{4}$ MeV for charged hadrons. Geant4 provides an estimate of detector response (sensitivity) based on an accurate description of detector geometry, material composition and interaction of particles with the various detector layers. The MC simulated hit rate is estimated as a function of the perpendicular distance from the beam line and agrees with data within the assigned uncertainties of 10-14.5%. This simulation framework can be used to obtain a reliable estimate of background rates expected at the High Luminosity LHC.Comment: 16 pages, 9 figures, 6 table

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

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV