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

Topical Workshop on Electronics for Particle Physics

The purpose of the workshop was to present results and original concepts for electronics research and development relevant to particle physics experiments as well as accelerator and beam instrumentation at future facilities; to review the status of electronics for the LHC experiments; to identify and encourage common efforts for the development of electronics; and to promote information exchange and collaboration in the relevant engineering and physics communities

Proof-of-concept of a single-point Time-of-Flight LiDAR system and guidelines towards integrated high-accuracy timing, advanced polarization sensing and scanning with a MEMS micromirror

Dissertação de mestrado integrado em Engenharia Física (área de especialização em Dispositivos, Microssistemas e Nanotecnologias)The core focus of the work reported herein is the fulfillment of a functional Light Detection and Ranging (LiDAR) sensor to validate the direct Time-of-Flight (ToF) ranging concept and the acquisition of critical knowledge regarding pivotal aspects jeopardizing the sensor’s performance, for forthcoming improvements aiming a realistic sensor targeted towards automotive applications. Hereupon, the ToF LiDAR system is implemented through an architecture encompassing both optical and electronical functions and is subsequently characterized under a sequence of test procedures usually applied in benchmarking of LiDAR sensors. The design employs a hybrid edge-emitting laser diode (pulsed at 6kHz, 46ns temporal FWHM, 7ns rise-time; 919nm wavelength with 5nm FWHM), a PIN photodiode to detect the back-reflected radiation, a transamplification stage and two Time-to-Digital Converters (TDCs), with leading-edge discrimination electronics to mark the transit time between emission and detection events. Furthermore, a flexible modular design is adopted using two separate Printed Circuit Boards (PCBs), comprising the transmitter (TX) and the receiver (RX), i.e. detection and signal processing. The overall output beam divergence is 0.4º×1º and an optical peak power of 60W (87% overall throughput) is realized. The sensor is tested indoors from 0.56 to 4.42 meters, and the distance is directly estimated from the pulses transit time. The precision within these working distances ranges from 4cm to 7cm, reflected in a Signal-to-Noise Ratio (SNR) between 12dB and 18dB. The design requires a calibration procedure to correct systematic errors in the range measurements, induced by two sources: the timing offset due to architecture-inherent differences in the optoelectronic paths and a supplementary bias resulting from the design, which renders an intensity dependence and is denoted time-walk. The calibrated system achieves a mean accuracy of 1cm. Two distinct target materials are used for characterization and performance evaluation: a metallic automotive paint and a diffuse material. This selection is representative of two extremes of actual LiDAR applications. The optical and electronic characterization is thoroughly detailed, including the recognition of a good agreement between empirical observations and simulations in ZEMAX, for optical design, and in a SPICE software, for the electrical subsystem. The foremost meaningful limitation of the implemented design is identified as an outcome of the leading-edge discrimination. A proposal for a Constant Fraction Discriminator addressing sub-millimetric accuracy is provided to replace the previous signal processing element. This modification is mandatory to virtually eliminate the aforementioned systematic bias in range sensing due to the intensity dependency. A further crucial addition is a scanning mechanism to supply the required Field-of-View (FOV) for automotive usage. The opto-electromechanical guidelines to interface a MEMS micromirror scanner, achieving a 46º×17º FOV, with the LiDAR sensor are furnished. Ultimately, a proof-of-principle to the use of polarization in material classification for advanced processing is carried out, aiming to complement the ToF measurements. The original design is modified to include a variable wave retarder, allowing the simultaneous detection of orthogonal linear polarization states using a single detector. The material classification with polarization sensing is tested with the previously referred materials culminating in an 87% and 11% degree of linear polarization retention from the metallic paint and the diffuse material, respectively, computed by Stokes parameters calculus. The procedure was independently validated under the same conditions with a micro-polarizer camera (92% and 13% polarization retention).O intuito primordial do trabalho reportado no presente documento é o desenvolvimento de um sensor LiDAR funcional, que permita validar o conceito de medição direta do tempo de voo de pulsos óticos para a estimativa de distância, e a aquisição de conhecimento crítico respeitante a aspetos fundamentais que prejudicam a performance do sensor, ambicionando melhorias futuras para um sensor endereçado para aplicações automóveis. Destarte, o sistema LiDAR é implementado através de uma arquitetura que engloba tanto funções óticas como eletrónicas, sendo posteriormente caracterizado através de uma sequência de testes experimentais comumente aplicáveis em benchmarking de sensores LiDAR. O design tira partido de um díodo de laser híbrido (pulsado a 6kHz, largura temporal de 46ns; comprimento de onda de pico de 919nm e largura espetral de 5nm), um fotodíodo PIN para detetar a radiação refletida, um andar de transamplificação e dois conversores tempo-digital, com discriminação temporal com threshold constante para marcar o tempo de trânsito entre emissão e receção. Ademais, um design modular flexível é adotado através de duas PCBs independentes, compondo o transmissor e o recetor (deteção e processamento de sinal). A divergência global do feixe emitido para o ambiente circundante é 0.4º×1º, apresentando uma potência ótica de pico de 60W (eficiência de 87% na transmissão). O sensor é testado em ambiente fechado, entre 0.56 e 4.42 metros. A precisão dentro das distâncias de trabalho varia entre 4cm e 7cm, o que se reflete numa razão sinal-ruído entre 12dB e 18dB. O design requer calibração para corrigir erros sistemáticos nas distâncias adquiridas devido a duas fontes: o desvio no ToF devido a diferenças nos percursos optoeletrónicos, inerentes à arquitetura, e uma dependência adicional da intensidade do sinal refletido, induzida pela técnica de discriminação implementada e denotada time-walk. A exatidão do sistema pós-calibração perfaz um valor médio de 1cm. Dois alvos distintos são utilizados durante a fase de caraterização e avaliação performativa: uma tinta metálica aplicada em revestimentos de automóveis e um material difusor. Esta seleção é representativa de dois cenários extremos em aplicações reais do LiDAR. A caraterização dos subsistemas ótico e eletrónico é minuciosamente detalhada, incluindo a constatação de uma boa concordância entre observações empíricas e simulações óticas em ZEMAX e elétricas num software SPICE. O principal elemento limitante do design implementado é identificado como sendo a técnica de discriminação adotada. Por conseguinte, é proposta a substituição do anterior bloco por uma técnica de discriminação a uma fração constante do pulso de retorno, com exatidões da ordem sub-milimétrica. Esta modificação é imperativa para eliminar o offset sistemático nas medidas de distância, decorrente da dependência da intensidade do sinal. Uma outra inclusão de extrema relevância é um mecanismo de varrimento que assegura o cumprimento dos requisitos de campo de visão para aplicações automóveis. As diretrizes para a integração de um micro-espelho no sensor concebido são providenciadas, permitindo atingir um campo de visão de 46º×17º. Conclusivamente, é feita uma prova de princípio para a utilização da polarização como complemento das medições do tempo de voo, de modo a suportar a classificação de materiais em processamento avançado. A arquitetura original é modificada para incluir uma lâmina de atraso variável, permitindo a deteção de estados de polarização ortogonais com um único fotodetetor. A classificação de materiais através da aferição do estado de polarização da luz refletida é testada para os materiais supramencionados, culminando numa retenção de polarização de 87% (tinta metálica) e 11% (difusor), calculados através dos parâmetros de Stokes. O procedimento é independentemente validado com uma câmara polarimétrica nas mesmas condições (retenção de 92% e 13%)

Calibration, bias and monitoring system for the VFAT3 ASIC of the CMS GEM detector

VFAT3 is the last version of a family of multichannel trigger and tracking ASICs designed for the upgrade of the CMS experiment in the LHC. The chip has been developed to provide fast trigger information from the readout of gas particle detectors improving the resolution of the time measurement. The VFAT3 architecture comprises 128 analog channels, each one composed by a low noise and low power charge sensitive amplifier, shaper and constant fraction discriminator. The comparator output is synchronized with the LHC clock and sent both to a fixed latency path for trigger signal generation and to a variable latency path for storage and readout. The front-end amplifier is programmable in terms of gain and pulse shaping time, in order to adapt it to a wide range of gaseous detectors as well as silicon detectors. The chip also comprises a programmable calibration system that can provide both voltage and current pulses. There are also two internal 10 bit ADCs for the monitoring of the internal bias references. The digital logic provides trigger generation, digital data tagging and storage, data formatting and data packet transmission with error protection on 320Mbps e-link. The digital design is triplicated in order to improve the radiation hardness of the system. A first run of the chip of 9.1×6.1mm2 in 130nm technology node has been submitted and produced. Chip architecture, measurements and characterization of the calibration, bias and monitoring system will be shown