Search for long-lived particles in events with a displaced vertex using the ATLAS detector with the full Run 2 dataset

A large number of physics models beyond the Standard Model predict the existence of new, massive, long-lived particles. Searches for these particles rely on the detection of the decay products at a significant distance from the collision point. This signature provides interesting technical challenges due to their special reconstruction requirements as well as their unusual backgrounds. Recent results in the search for long-lived SUSY particles using a displaced vertex in the Inner Detector in association with jets with the ATLAS full Run 2 data are presented

Search for long-lived supersymmetric particles using displaced vertices and multiple jets in pppp collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

The Standard Model (SM) of particle physics describes all fundamental forces of electromagnetic, weak, and strong interactions, except for gravity in the universe. The behaviour of elementary particles predicted by the SM is in good agreement with the results of the experiments obtained so far. However, there are some problems that cannot be solved by the SM alone; the Higgs boson mass 125 GeV cannot be naturally derived and the dark matter cannot be formed from the particles in the SM. Supersymmetry (SUSY) could solve these problems. Also, SUSY possibly unifies the electroweak and strong interactions at a high energy scale. SUSY particles have been searched for up to $O(1)$ TeV at the Large Hadron Collider (LHC), but no SUSY particles have been found. A large parameter space for SUSY particles, which provides all above benefits, has been excluded. Therefore, I focused on the SUSY model, which solves the mass problem of the Higgs boson at least. I focused on the $R$-parity violating model. In this model, the lightest SUSY particles can decay to the SM particles. The coupling constants between SM and SUSY particles are predicted to be small due to proton decay constraints. Since regions with very small coupling constant have already been searched for in other analyses at the ATLAS experiment, this thesis focuses on the case where the coupling constant is $O(10^{-4})$-$O(10^{-3})$. This case does not allow SUSY particles to form dark matter candidates, but the other two benefits can be kept. This thesis presents a search for long-lived SUSY particles in events with displaced vertices and multiple jets using the 139 $\rm fb^{-1}$ of $pp$ collision data collected at $\sqrt{s}$ = 13 TeV by the ATLAS detector at the LHC. Two signal processes are searched for, which are $pp \rightarrow \tilde{g}\tilde{g}$, $\tilde{g} \rightarrow qq\tilde{\chi}_{1}^{0}(\rightarrow qqq)$ and $pp \rightarrow \tilde{\chi}_{1}^{0}\tilde{\chi}_{1}^{\pm}, \tilde{\chi}_{1} \rightarrow qqq$. The lightest electrowikinos $\tilde{\chi}_{1}$ have a long lifetime due to the small $R$-parity violating coupling. I focus on the models characterized by multiple jets and the decay vertex of the long-lived electrowikino being $O(1)$-$O(100)$ mm away from the interaction point, which is called displaced vertex. We established the analysis method using dedicated techniques to reconstruct the tracks derived from the long-lived electrowikino decay and the decay vertex of the electrowikino. A signal region (SR) is defined for each signal process. In SRs, at least one displaced vertex and multiple high momentum jets are required. The displaced vertex must be reconstructed from five or more tracks and the invariant mass calculated from the four-momenta of these tracks, obtained by assuming the pion mass for each track, must be larger than 10 GeV. The dominant background events are due to accidental reconstruction of displaced vertices by some sources. I established a new background estimation method and achieved accurate estimation. The new method estimates the background events inclusively using the relation between the number of accidentally reconstructed displaced vertices and the number of jets. The number of observed events did not significantly exceed the background estimations in each SR. At 95% confidence level, the neutralino mass up to 1.58 TeV for lifetime of 0.1 ns is excluded. This analysis allows the search for the models in which the lightest electrowikino with lifetimes of 0.01 - 1 ns decays into light-flavour quarks, which has not been searched for to date

Projection of inclusive Search for Gluinos and Squarks at the HL-LHC ATLAS experiment

Supersymmetry is one of the most promising theories beyond the Standard Model and can solve the Hierarchy problem. For more sensitive searches of such New Physics and more precise measurements of Higgs couplings, the High Luminosity LHC (HL-LHC) experiment is planned to start in 2026, which it is expected to acquire 3000 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of 14 TeV. The ATLAS detector will be upgraded in order to handle higher luminosities. In 2018, groups in the ATLAS and CMS experiments study projections of main physics analysis channels toward the HL-LHC experiment. In particular, we focus on strongly-produced supersymmetric particles -- gluinos and squarks

The ATLAS Muon Trigger Design and Performance

Muon triggers are essential for studying a variety of physics processes in the ATLAS experiment, including both standard model measurements and searches for new physics. The ATLAS muon trigger consists of a hardware based system (Level 1), as well as a software based reconstruction (High Level Trigger). The muon triggers have been optimised during Run 2 to provide a high efficiency while keeping the trigger rate low. We present how we trigger on muons, recent improvements, the performance of the muon trigger in Run 2 data, and the improvements and the readiness for Run 3

Search for long-lived particles in events with a displaced vertex using the ATLAS detector with the full Run2 dataset

A large number of physics models that extend the Standard Model predict the existence of new, massive, long-lived particles. Searches for these processes may target its decay products at a significant distance from the collision point. This signature provides interesting technical challenges due to their special reconstruction requirements as well as their unusual backgrounds. This poster will present recent results in long-lived SUSY searches using a displaced vertex in the Inner Detector in association to jets with the ATLAS full Run 2 data

The ATLAS Muon Trigger Design and Performance

Muon triggers are essential for studying a variety of physics processes in the ATLAS experiment, including both standard model measurements and searches for new physics. The ATLAS muon trigger consists of a hardware based system (Level 1), as well as a software based reconstruction (High Level Trigger). The muon triggers have been optimised during Run 2 to provide a high efficiency while keeping the trigger rate low. We will present an overview of how we trigger on muons, recent improvements, the performance of the muon trigger in Run 2 data, and the improvements and the readiness for Run 3