Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Search for single production of vector-like quarks decaying into Wb in pp collisions at √s = 13 TeV with the ATLAS detector

A search for singly-produced vector-like quarks Q, where Q can be either a T quark with charge 2/3 or a Y quark with charge 4/3, is performed in 3.2 fb−1 of proton–proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC. This poster will detail the results of this analysis

Search for single production of vector-like quarks decaying into a <em>Wb</em> final state at &radic;s = 13 TeV with the ATLAS detector

In this thesis, a search for single-production of vector-like quarks was performed using data collected during Run 2 of the Large Hadron Collider (LHC) by the ATLAS detector. The primary focus of this thesis was on the single production of vector-like quarks (T/Y) decaying into a Wb final state. This work can be divided into two major analyses, each targeting orthogonal phase spaces. The first analysis concentrated on the T/Y vector-like quarks decaying via a leptonic channel, where the W-boson decays into either an electron or a muon along with a neutrino. This analysis utilised pp collision data from 2015-16. The strategy involved identifying high transverse momentum (pT) b-jets with significant angular separation from the leptons. This analysis also considered interference effects between the signal and background processes, leading to the development of novel statistical methods for calculating coupling limits. Ultimately, no significant excess over the Standard Model background was observed, and mass and coupling limits were set for different multiplet representations of the vector-like quarks. The second analysis explored the same final state through the hadronic decay mode, where the W-boson decays into a quark-anti-quark pair. This analysis, which is first of its kind, uses ATLAS data collected from 2015 to 2018. Due to the high masses of the T/Y quarks under investigation, their decays resulted in Lorentz-boosted W-bosons. The reconstruction of the W-jet was accomplished using advanced jet-taggers. To account for the substantial mismodelling in the background simulation, data-driven ABCD methods were introduced to accurately estimate the multijet background. Like the previous analysis, no significant excess was observed, and mass and mass-coupling limits were established for the T/Y quarks, thus introducing new physics constraints. This thesis also underscores the development of novel statistical methods to handle interference effects and presents an improved ABCD method, which could benefit future hadronic searches. By tackling both leptonic and hadronic decay modes, this work explores the full phase space for T/Y → Wb decays during Run 2 of the LHC. The research lays the foundation for further investigations by the ATLAS collaboration and underscores the potential of advanced jet-taggers in exploring hadronic final states for future LHC runs

Evidence for the Higgs Boson Decay to a <math display="inline"><mi>Z</mi></math> Boson and a Photon at the LHC

The first evidence for the Higgs boson decay to a Z boson and a photon is presented, with a statistical significance of 3.4 standard deviations. The result is derived from a combined analysis of the searches performed by the ATLAS and CMS Collaborations with proton-proton collision datasets collected at the CERN Large Hadron Collider (LHC) from 2015 to 2018. These correspond to integrated luminosities of around 140  fb-1 for each experiment, at a center-of-mass energy of 13 TeV. The measured signal yield is 2.2±0.7 times the standard model prediction, and agrees with the theoretical expectation within 1.9 standard deviations

Measurement of coherent exclusive J/ψ→μ+μ−J/ψ\toμ^+μ^- production in ultraperipheral Pb+Pb collisions at sNN=5.36\sqrt{s_{\textrm{NN}}}=5.36 TeV with the ATLAS detector

International audienceThe ATLAS experiment has performed a measurement of coherent exclusive $J/ψ\toμ^+μ^-$ production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}=5.36$ TeV. The data was recorded at the Large Hadron Collider (LHC) during 2023, and corresponds to an integrated luminosity of 78 $μ$b$^{-1}$. Exclusive $J/ψ$ candidates were selected with a dedicated track-sensitive trigger based on the ATLAS transition radiation tracker. The analysis involves reconstruction of the dimuon invariant mass based on muon tracks from the inner detector, as the muon transverse momentum range of interest precludes the use of the standard muon reconstruction and identification algorithms. Differential cross sections are measured as a function of $J/ψ$ rapidity and are compared with theoretical predictions. After extrapolation to $\sqrt{s_{\textrm{NN}}}=5.02$ TeV, they are also compared with previous measurements performed by other experiments using data from LHC Run 2. While the results agree reasonably well with theoretical predictions, they are in tension with previous Run-2 results for the central rapidity region

A search for electroweak ttˉWjt\bar{t}Wj production in multileptonic final states at s=13\sqrt{s} = 13 TeV with the ATLAS detector and bounds on effective field theory operators

International audienceA search is presented for the electroweak production of a top quark pair in association with a $W$ boson and at least one additional jet, known as the ${t\bar{t}W\kern-0.2em j_{\mathrm{EW}}}$ process. This process has embedded within it a $tW$-scattering vertex, which is probed directly for the first time. The collision data were collected with the ATLAS detector during Run 2 of the LHC and correspond to an integrated luminosity of 140 $\mathrm{fb^{-1}}$ at $\sqrt{s}=13$~TeV. The search uses same-charge pairs of electrons and muons together with jets, of which at least one is $b$-tagged. The properties of the most forward jet relative to the rest of the event are used to discriminate the electroweak ${t\bar{t}W\kern-0.2em j_{\text{EW}}}$ production process from its strong $t\bar{t}W$ production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at $σ_{{t\bar{t}W\kern-0.2em j_{\mathrm{EW}}}} < 251$~fb (230 fb), to be compared with the expected SM cross section of 47.7 fb. Limits are set on the SM Effective Field Theory (EFT) operators $O_{Ht}$ and $O^{(1)}_{HQ}$, which modify the electroweak couplings of the top quark through contributions to the $tW$-scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multi-process and multi-operator EFT contributions

Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector

International audienceAnisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system's radial expansion, has received less attention. Notably, experimental evidence for the global and collective nature of radial flow has been lacking. This Letter presents the first measurement of transverse momentum ($p_{\mathrm{T}}$) dependence of radial flow fluctuations ($v_0(p_{\mathrm{T}})$) over $0.5<p_{\mathrm{T}}<10$ GeV, using a two-particle correlation method in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in $p_{\mathrm{T}}$, and centrality-independent shape in $p_{\mathrm{T}}$. The comparison with a hydrodynamic model demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP