Differential Cross Section Measurement of the pp→WH→WWWpp\rightarrow WH\rightarrow WWW Process With the ATLAS Experiment

In this thesis, a Simplified Template Cross Section (STXS) analysis for the $pp\rightarrow WH\rightarrow WWW\rightarrow l\nu l\nu l\nu$ process is presented, using final states with no lepton pairs of the same flavor and opposite charge. The analysis uses proton-proton collisions at a center of mass energy of $13~TeV$ with an integrated luminosity of $139~fb^{-1}$, which were measured by the ATLAS detector at the Large Hadron Collider. Neural networks are used to reconstruct the transverse momentum of the associated $W$ boson and to separate signal and background. The expected signal strengths of the STXS bins were measured to be $\mu(0~GeV\leq p_T^W<75~GeV)=1\pm 1.24$, $\mu(75~GeV\leq p_T^W<150~GeV)=1\pm 1.52$, $\mu(150~GeV\leq p_T^W<250~GeV)=1\pm 2.20$ and $\mu(p_T^W\geq 150~GeV)=1\pm 2.22$

Charged-hadron and identified-hadron (KS0,Λ,Ξ−)(K_S^0, Λ, Ξ^−) yield measurements in photonuclear Pb+Pb and p+Pb collisions at sNN\sqrt{{s}_{NN}} = 5.02 TeV with ATLAS

This paper presents the measurement of charged-hadron and identified-hadron $(K_S^0, Λ, Ξ^−)$ yields in photonuclear collisions using 1.7 nb$^{−1}$ of $\sqrt{{s}_{NN}}$ = 5.02TeV Pb+Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photonuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photonuclear collisions as a function of charged-particle multiplicity. These photonuclear results are compared with 0.1nb$^{−1}$ of $\sqrt{{s}_{NN}}$ = 5.02 TeV p+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photonuclear measurements shed light on potential quark-gluon plasma formation in photonuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo dpmjet-iii generator and hydrodynamic calculations to test whether such photonuclear collisions may produce small droplets of quark-gluon plasma that flow collectively

Search for long-lived charged particles using large specific ionisation loss and time of flight in 140 fb−1^{−1} of pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 fb$^{−1}$ of proton-proton collisions at $\sqrt{s}$ = 13 TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.[graphic not available: see fulltext

Search for light neutral particles decaying promptly into collimated pairs of electrons or muons in pp collisions at s\sqrt{s} = 13 TeV\text {T}\text {e}\hspace{-1.00006pt}\text {V} with the ATLAS detector

A search for a dark photon, a new light neutral particle, which decays promptly into collimated pairs of electrons or muons is presented. The search targets dark photons resulting from the exotic decay of the Standard Model Higgs boson, assuming its production via the dominant gluon-gluon fusion mode. The analysis is based on $140~\hbox {fb}^{-1}$ of data collected with the ATLAS detector at the Large Hadron Collider from proton-proton collisions at a center-of-mass energy of 13 $\text {T}\text {e}\hspace{-1.00006pt}\text {V}$. Events with collimated pairs of electrons or muons are analysed and background contributions are estimated using data-driven techniques. No significant excess in the data above the Standard Model background is observed. Upper limits are set at 95% confidence level on the branching ratio of the Higgs boson decay into dark photons between 0.001% and 5%, depending on the assumed dark photon mass and signal model

Measurements of the production cross-sections of a Higgs boson in association with a vector boson and decaying into WW∗^{*} with the ATLAS detector at s \sqrt{s} = 13 TeV

Measurements of the total and differential Higgs boson production cross-sections, via WH and ZH associated production using H → WW$^{*}$ → ℓνℓν and H → WW$^{*}$ → ℓνjj decays, are presented. The analysis uses proton-proton events delivered by the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector between 2015 and 2018. The data correspond to an integrated luminosity of 140 fb$^{−1}$. The sum of the WH and ZH cross-sections times the H → WW$^{*}$ branching fraction is measured to be ${0.44}_{-0.09}^{+0.10}{\left(\textrm{stat}.\right)}_{-0.05}^{+0.06}\left(\textrm{syst}.\right)$ pb, in agreement with the Standard Model prediction. Higgs boson production is further characterised through measurements of the differential cross-section as a function of the transverse momentum of the vector boson and in the framework of Simplified Template Cross-Sections.[graphic not available: see fulltext

Search for heavy neutral leptons in decays of W bosons using leptonic and semi-leptonic displaced vertices in s \sqrt{s} = 13 TeV pp collisions with the ATLAS detector

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 fb$^{−1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as 10$^{−7}$ are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.[graphic not available: see fulltext

Configuration, Performance, and Commissioning of the ATLAS b-jet Triggers for the 2022 and 2023 LHC data-taking periods

In 2022 and 2023, the Large Hadron Collider producedapproximately two billion hadronic interactions each second frombunches of protons that collide at a rate of 40 MHz. The ATLAStrigger system is used to reduce this rate to a few kHz forrecording. Selections based on hadronic jets, their energy, andevent topology reduce the rate to (10) kHz whilemaintaining high efficiencies for important signatures resulting in b-quarks, but to reach the desired recording rate of hundreds ofHz, additional real-time selections based on the identification ofjets containing b-hadrons (b-jets) are employed to achieve lowthresholds on the jet transverse momentum at the High-Level Trigger.The configuration, commissioning, and performance of the real-timeATLAS b-jet identification algorithms for the early LHC Run 3collision data are presented. These recent developments providesubstantial gains in signal efficiency for critical signatures; forthe Standard Model production of Higgs boson pairs, a 50%improvement in selection efficiency is observed in final states withfour b-quarks or two b-quarks and two hadronically decaying τ-leptons

Searches for direct slepton production in the compressed-mass corridor in s \sqrt{\textrm{s}} = 13 TeV pp collisions with the ATLAS detector

This paper presents searches for the direct pair production of charged light-flavour sleptons, each decaying into a stable neutralino and an associated Standard Model lepton. The analyses focus on the challenging “corridor” region, where the mass difference, ∆m, between the slepton ($\overset{\sim }{e}$ or $\overset{\sim }{\mu }$) and the lightest neutralino (${\overset{\sim }{\chi}}_1^0$) is less or similar to the mass of the W boson, m(W), with the aim to close a persistent gap in sensitivity to models with ∆m ≲ m(W). Events are required to contain a high-energy jet, significant missing transverse momentum, and two same-flavour opposite-sign leptons (e or μ). The analysis uses pp collision data at $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 140 fb$^{−1}$. Several kinematic selections are applied, including a set of boosted decision trees. These are each optimised for different ∆m to provide expected sensitivity for the first time across the full ∆m corridor. The results are generally consistent with the Standard Model, with the most significant deviations observed with a local significance of 2.0 σ in the selectron search, and 2.4 σ in the smuon search. While these deviations weaken the observed exclusion reach in some parts of the signal parameter space, the previously present sensitivity gap to this corridor is largely reduced. Constraints at the 95% confidence level are set on simplified models of selectron and smuon pair production, where selectrons (smuons) with masses up to 300 (350) GeV can be excluded for ∆m between 2 GeV and 100 GeV.[graphic not available: see fulltext

Search for vector-like leptons coupling to first- and second-generation Standard Model leptons in pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

A search for pair production of vector-like leptons coupling to first- and second-generation Standard Model leptons is presented. The search is based on a dataset of proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{−1}$. Events are categorised depending on the flavour and multiplicity of leptons (electrons or muons), as well as on the scores of a deep neural network targeting particular signal topologies according to the decay modes of the vector-like leptons. In each of the signal regions, the scalar sum of the transverse momentum of the leptons and the missing transverse momentum is analysed. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No significant excess above the Standard Model background expectation is observed and limits are set at 95% confidence level on the production cross-sections of vector-like electrons and muons as a function of the vector-like lepton mass, separately for SU(2) doublet and singlet scenarios. The resulting mass lower limits are 1220 GeV (1270 GeV) and 320 GeV (400 GeV) for vector-like electrons (muons) in the doublet and singlet scenarios, respectively.[graphic not available: see fulltext

Search for single-production of vector-like quarks decaying into Wb in the fully hadronic final state in pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

A search for T and Y vector-like quarks produced in proton-proton collisions at a centre-of-mass energy of 13 TeV and decaying into Wb in the fully hadronic final state is presented. The search uses 139 fb$^{−1}$ of data collected by the ATLAS detector at the LHC from 2015 to 2018. The final state is characterised by a hadronically decaying W boson with large Lorentz boost and a b-tagged jet, which are used to reconstruct the invariant mass of the vector-like quark candidate. The main background is QCD multijet production, which is estimated using a data-driven method. Upon finding no significant excess in data, mass limits at 95% confidence level are obtained as a function of the global coupling parameter, κ. The observed lower limits on the masses of Y quarks with κ = 0.5 and κ = 0.7 are 2.0 TeV and 2.4 TeV, respectively. For T quarks, the observed mass limits are 1.4 TeV for κ = 0.5 and 1.9 TeV for κ = 0.7.[graphic not available: see fulltext