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Développement du détecteur PLUME et désintégrations de charmonia en baryons Lambda* anti-Lambda* avec l'expérience LHCb

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Auteur / Autrice : Vsevolod Yeroshenko
Direction : Sergey Barsuk
Type : Projet de thèse
Discipline(s) : Physique des particules
Date : Inscription en doctorat le 01/10/2021
Etablissement(s) : université Paris-Saclay
Ecole(s) doctorale(s) : École doctorale Particules, hadrons, énergie et noyau : instrumentation, imagerie, cosmos et simulation (Orsay, Essonne ; 2015-....)
Partenaire(s) de recherche : Laboratoire : Laboratoire de Physique des deux Infinis Irène Joliot-Curie
Référent : Faculté des sciences d'Orsay
graduate school : Université Paris-Saclay. Graduate School Physique (2020-....)

Résumé

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The proposed thesis focuses on the development, characterization and commissioning of the new detector aiming at precise determination of luminosity for the LHCb experiment at CERN. During the operation in Run 1 and 2, from 2010 to 2018, the LHC beam was composed of a maximum of 2556 proton bunches with a population of about 10^11 protons. Such bunches collided at four beam interaction points (IPs) every 50 ns until 2016, and every 25 ns since then. Starting in 2022, LHCb in Run 3 will see up to a 5-fold increase of luminosity and of the number of visible interactions per bunch crossing denoted Mu. As the production of heavy quarks takes predominantly place at low angle from the beam axis, LHCb has been designed as a single arm forward spectrometer. High occupancy can degrade the performance of forward detectors like LHCb, which was designed to work at a lower luminosity with respect to the general purpose detectors ATLAS and CMS. Hence, the instantaneous luminosity at the LHCb interaction point is reduced by increasing the transversal distance between the two beams. While collisions decrease the beam intensity during an LHC fill, the transverse distance between the beams in LHCb is reduced, keeping the instantaneous luminosity stable within about 5% range. This technique, named “luminosity levelling', minimises the effects of luminosity decay, allowing to maintain the same trigger configuration during a fill and to reduce systematic uncertainties due to changes in the detector occupancy. Strong variations of luminosity, beam size, machine-induced background, ghost charge, satellites and beams crossing angle are expected during the LHC operation in Run 3. Measuring the instantaneous luminosity in real time is essential for the operation of the LHC machine, in particular to guide the levelling procedure at LHCb. Three LHCb luminosity detectors operating in Runs 1 and 2 should be replaced or significantly upgraded in order to sustain the new challenging radiation conditions. One important parameter determining the running conditions is the instantaneous luminosity at the LHCb IP. In order to maintain the instantaneous luminosity averaged over beam crossings constant over an LHC fill and to measure the instantaneous luminosity for individual bunch crossings, a new dedicated detector has to be built. This will allow operating the LHCb detector in stable conditions while measuring the luminosity in real-time. It has been estimated that for the online operation the resolution of about 5% is required on the particle multiplicity. This is even more crucial for the upgrade of LHCb fixed-target program with the new SMOG2 detector installed upstream of the LHCb Vertex Locator (VELO). From Run 3 onward it will rely on external multiplicity determination in order to separate pp/pN background from NN collisions and to determine the centrality of the collisions. The detector also aims at contributing to veto and centrality determination in the heavy ion program with beams and a new SMOG2 system. During Run 3, the monitoring of the radiation level and of the beam induced backgrounds will become even more crucial for maintaining reliable data quality and for extending the detector lifetime. Understanding of the background structure, the online monitoring and feeding back this information to the LHC are, therefore, essential features of the proposed new detector. Beam-induced background conditions should receive more careful monitoring at higher luminosity, with the goal of monitoring precisely the occurrence of bad quality vacuum and of undefined falling objects inside the beam pipe. Unstable machine conditions as well as issues during beam injection, should trigger real-time response from both LHCb and the LHC itself. The new detector should deliver the online luminosity and Mu measurements for the luminosity levelling; perform these measurements per bunch; measure the radiation background induced by the accelerator or bad vacuum, produce alarms, measure the level of the ghost charges; cross-check the LHC filling scheme in real time; contribute to the centrality determination in the fixed-target program; provide accurate offline luminosity determination. The readout system should be designed with a dual setup able to deliver the real-time data both in the standalone mode and within the common LHCb DAQ system. The new dedicated luminometer will be based on photomultipliers placed around the beam pipe and detecting Cherenkov light emitted by particles traversing fused silica radiators. The PhD student will play a leading role in the development, installation, characterization and commissioning of this new detector, detector simulation, will contribute to the detector calibration, data analysis and luminosity determination. The student will evaluate a contribution of the new detector to the upstream LHCb tracking. A bonus part of the thesis will be an implementation of a new approach to determine the beam crossing timing using the amplitude measurements in several elementary detectors. This approach inherits from the position measurement in the calorimeters relying on the comparison of amplitudes in the neighbor cells. The time measurement from comparing amplitudes will be implemented for the first time. The student will take part in the LHCb experiment shifts in the control room for detector operation and as on-call luminometer expert.