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Search for SUSY Electroweak production at LHC Run 2 and study of CMOS sensor for ITk replacement in second-half of HL-LHC

Abstract : The Standard Model (SM) has been a remarkably successful theory so far. However, it is still far from being a complete description of nature. The discovery of the SM Higgs boson by ATLAS and CMS experiments in 2012 has confirmed the electroweak symmetry breaking mechanism and also highlighted the hierarchy problem due to the unnatural fine-tuning in the Higgs boson mass. Supersymmetry (SUSY), a theoretical extension to the SM, directly resolves the hierarchy problem by postulating the existence of a new supersymmetric partner for each particle in the SM. In addition, SUSY also provides an explanation for many other open questions such as the Dark Matter origin and the unification of fundamental interactions at high energy scale. This thesis concentrates on an analysis targeting SUSY searches for electroweak production of chargino-chargino ($\Conepm\Conemp$) or chargino-neutralino ($\Cone\Ntwo$) pairs in the context of $R$-parity conserving simplified models in which the lightest chargino ($\Cone$) decays into a $W$ boson and the lightest neutralino ($\None$), while the next-to-lightest neutralino ($\Ntwo$) into a $Z$ boson and the lightest neutralino. The analysis focuses on final states characterized by the presence of one isolated lepton (either electron or muon) accompanied by jets and missing transverse momentum. It is performed for the first time at the LHC and targets an integrated luminosity of 139 fb$^{-1}$ corresponding to the full Run 2 $pp$ collision data recorded by the ATLAS detector. This analysis exploits large-R jets to enhance the search sensitivities. Due to the fact that the analysis is not approved at the time of writing, the analysis results presented in this thesis are obtained with observed data still being blind in Signal Regions and with both the experimental and theoretical systematic uncertainties included. A compatibility of the observed and expected event yields can be observed in Control Regions and Validation Regions. The expected exclusion limit at 95\% CL extends up to 680 (720) GeV in the $\Cone /\Ntwo$ mass for the massless $\None$ in the $\Conepm\Conemp$ ($\Cone\Ntwo$) search. In the High Luminosity LHC phase starting from 2027, the current ATLAS Inner Detector will fully be replaced by a new all-silicon detector, the Inner Tracker (ITk). During the entire HL-LHC lifetime, the innermost ITk layers will be exposed to an expected integrated radiation fluence of up to $2 \times 10^{16}$ $\mathrm{n_{eq}/cm^2}$. However, the ITk pixel sensors can only withstand the maximum fluence of roughly $10^{16}$ $\mathrm{n_{eq}/cm^2}$, implying that these innermost layers should be replaced by the half-life of the HL-LHC. For this replacement, if the CMOS sensor technology is ready to withstand a fluence of up to $1.6 \times 10^{16}$ $\mathrm{n_{eq}/cm^2}$ by then, the CMOS sensors could become the most promising candidates to replace the tradition planar and 3D sensors. In this thesis, a new ITk Pixel detector layout with the CMOS sensors of 50 $\mathrm{\mu m}$ thick and $25 \times 25$ $\mathrm{\mu m^2}$ pixel size used in the innermost layers is simulated. The simulation has shown the advantages achieved using the CMOS sensors with a reduction in the cluster Time-over-Threshold (ToT), width and an improved precision in the track transverse parameter reconstruction for a sample of $\pt = 100$ GeV muons. Measuring leakage current is one of the most characterised methods to monitor the silicon radiation damage. While the leakage current at the module granularity level can be measured precisely using the power supply sub-system, it is difficult to achieve the precise measurements for the pixel-level leakage current. In this thesis, the MonLeak (monitoring of the leakage current) mechanism is used to measure the pixel-level leakage current of modules in the IBL, the Pixel layers (B-Layer, Layer 1, Layer 2) and disks. Measurements obtained from the MonLeak scans in 2019 show discrepancies with the power supply measurements recorded by the Detector Control System (DCS), being higher by a factor of about 2 in the Pixel layers and disks, while extremely higher by a factor of around one order of magnitude in the IBL. At the high luminosities of the LHC, there are several inefficiency sources contributing to the loss of hit data during the readout mechanism. The loss rates of these sources have not been simulated yet, but only an ad-hoc fraction of 0.9\% of Monte-Carlo (MC) hits was discarded randomly to account for these inefficiencies in simulation. This assumption was fine for Run 1 but no longer suitable for Run 2. In this thesis, new methods are developed, using real data to estimate more precisely the Double-hit (DH) loss rate due to the loss of a hit arriving at a pixel while this pixel is still busy with the previous hit and the Late copying (LC) loss rate corresponding the loss of hits arriving at Double-Columns buffers later than the trigger latency. The preliminary DH loss rate of 0.96\% for B-Layer is obtained from the new method and can be considered as a lower limit for the loss rate in this layer.
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Submitted on : Tuesday, February 1, 2022 - 2:59:33 PM
Last modification on : Wednesday, February 2, 2022 - 3:38:37 AM
Long-term archiving on: : Tuesday, May 3, 2022 - 8:48:21 AM


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Ngoc Khanh Vu. Search for SUSY Electroweak production at LHC Run 2 and study of CMOS sensor for ITk replacement in second-half of HL-LHC. Physics [physics]. AMU - Aix Marseille Université, 2021. English. ⟨NNT : ⟩. ⟨tel-03551191⟩



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