69 Search Results

Anisotropic 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, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (𝑝_T) dependence of radial flow fluctuations (𝑣₀⁡(𝑝_T)) over 0.5 < 𝑝_T < 10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at $$\sqrt{𝑠_{NN}}$$ = 5.02 TeV. The data revealmore » three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in 𝑝_T, and centrality-independent shape in 𝑝_T. The comparison with a hydrodynamic model demonstrates the sensitivity of 𝑣₀⁡(𝑝_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.« less

This article reports on a search for dijet resonances using 132  fb⁻¹ of 𝑝⁢𝑝 collision data recorded at $$\sqrt{𝑠}$$ = 13  TeV by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of 𝑝_T ≥ 85  GeV and jet rapidity separation of |𝑦*| <0.6 are analysed for dijet resonances with invariant masses from 375 to 1800  GeV. A data-driven background estimate is used tomore » model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at 95% confidence level on coupling values for a benchmark leptophobic axial-vector 𝑍′ model and on the production cross section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.« less

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using 140 fb⁻¹ of proton-proton collision data at $$\sqrt{𝑠}$$ =13 TeV recorded in 2015–2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicatedmore » analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than 𝒪⁡(10) cm.« less

An anomaly detection search for narrow-width resonances beyond the Standard Model that decay into a pair of jets is presented. The search is based on 139 fb⁻¹ of proton-proton collisions at $$\sqrt{s}$$ = 13 TeV recorded during 2015–2018 with the ATLAS detector at the Large Hadron Collider. The analysis is optimized without a particular signal model and aims to be sensitive to a broad range of new physics. It uses two different machine learning strategies to estimate the background in different signal regions. In each region, a weakly supervised classifier is trained to distinguish this background model from data. Themore » analysis focuses on events with high transverse momentum jets reconstructed as large-radius jets. The mass and substructure of these jets are used as inputs to the classifiers. After a classifier-based selection, the distribution of the invariant mass of the two jets is used to search for potential local excesses. The model-independent results of both the anomaly detection methods show no signs of significant local excesses. In addition to model-independent results, a representative set of signal models is injected into the data, and the sensitivity of the methods to these scenarios is reported.« less

A search for exotic decays of the Higgs boson 𝐻 into new scalar or pseudoscalar particles that subsequently decay into 𝑏-quarks is presented. The search considers 𝑍⁢𝐻 production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, 𝐻 → 2⁢𝑎 → 4⁢𝑏, second to a pair of scalars with different masses (𝑚_𝑎⁢1< 𝑚_𝑎⁢2), either directly, 𝐻 → 𝑎₁⁢𝑎₂ → 4⁢𝑏, or via a longer decay chain, 𝐻 → 𝑎₁⁢𝑎₂ → 3⁢𝑎₁ → 6⁢𝑏. The analysis uses proton-proton collision data at $$\sqrt{𝑠}$$ = 13 TeV collected with the ATLAS detector at the Large Hadron Collider,more » corresponding to an integrated luminosity of 140 fb⁻¹. No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross section to the SM prediction times the branching ratio of Higgs bosons decaying into 4⁢𝑏 or 6⁢𝑏, between 4% and 25% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻) × ℬ⁡(𝐻→ 2⁢𝑎→ 4⁢𝑏), between 24% and 38% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻) × ℬ⁡(𝐻 → 𝑎₁⁢𝑎₂ → 4⁢𝑏), and between 10% and 20% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻) × ℬ⁡(𝐻→𝑎_1⁢𝑎₂ → 3⁢𝑎₁ → 6⁢𝑏), depending on the masses of the scalar particles.« less

This Letter reports the first evidence of electroweak production of same-sign 𝑊 boson pairs where at least one of the 𝑊 bosons is longitudinally polarized and the most stringent constraint to date for the production of two longitudinally polarized same-sign 𝑊 bosons. The dataset used corresponds to an integrated luminosity of 140 fb⁻¹ of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the ATLAS detector during run 2 of the Large Hadron Collider. The study is performed in final states including two same-sign leptons (electrons or muons), missing transverse momentum, and at least two jets with amore » large invariant mass and a large rapidity difference. Two independent fits are performed targeting the production of same-sign 𝑊 bosons with at least one, or two longitudinally polarized 𝑊 bosons. The observed (expected) significance of the production with at least one longitudinally polarized 𝑊 boson is 3.3 (4.0) standard deviations. An observed (expected) 95% confidence level upper limit of 0.45 (0.70) fb is reported on the fiducial production cross section of two longitudinally polarized same-sign 𝑊 bosons.« less

A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with an emphasis on signatures of a strongly coupled hidden dark sector accessed via resonant production of a 𝑍′ mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 is used, consisting of proton-proton collisions at $$\sqrt{𝑠}$$ = 13 TeV and corresponding to an integrated luminosity of 140 fb⁻¹. The 𝑍′ mediator is considered to decay to two dark quarks, which each hadronize and decay to showers containing both dark and Standard Model particles, producing a topology of interacting and noninteractingmore » particles within a jet known as “semivisible.” Machine learning methods are used to select these dark showers and reject the dominant background of mismeasured multijet events, including an anomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonance search is performed by fitting the transverse mass spectrum based on a functional form background estimation. No significant excess over the expected background is observed. Results are presented as limits on the production cross section of semivisible jet signals, parametrized by the fraction of invisible particles in the decay and the 𝑍′ mass, and by quantifying the significance of any generic Gaussian-shaped mass peak in the anomaly region.« less

A search for leptons displaced from the primary vertex is performed with the ATLAS detector at the Large Hadron Collider. The search includes the full proton-proton collision dataset collected during Run 2 at $$\sqrt{𝑠}$$ = 13 TeV and a partial dataset collected during Run 3 in 2022–2023 at $$\sqrt{𝑠}$$ = 13.6 TeV, corresponding to integrated luminosities of 140 fb⁻¹ and 56.3 fb⁻¹, respectively. Final states with displaced electrons or muons are considered, and novel triggers introduced in Run 3 are employed that use large impact parameter tracking to reconstruct displaced tracks with low momentum. In addition, photon reconstruction and multivariatemore » techniques are employed to broaden the sensitivity to channels with large background rates or highly displaced electrons, respectively. The results are consistent with the Standard Model background expectations and are used to set model-independent limits on the production of displaced electrons and muons. The analysis is also interpreted in the context of a gauge-mediated supersymmetry breaking model with pair-produced long-lived sleptons and a dark sector model with pair-produced chargino-like states. The results include 95% confidence level exclusions of selectrons with lifetimes from 4 ps to 60 ns and a mass of 150 GeV, and exclusions of selectrons, smuons, and staus with a lifetime of 0.3 ns for masses up to 740, 830, and 440 GeV, respectively. Dark charginos with masses up to 380 GeV are excluded for a mass difference with the neutral state of 40 GeV, and mass differences down to 17 GeV are excluded for dark charginos with a 100 GeV mass.« less

A search for cascade decays of charged sleptons and sneutrinos using final states characterized by three leptons (electrons or muons) and missing transverse momentum is presented. The analysis is based on a dataset with 140 fb⁻¹ of proton-proton (pp) collisions at a center-of-mass energy of $$\sqrt{𝑠} = 13$$ TeV recorded by the ATLAS detector at the Large Hadron Collider. This paper focuses on a supersymmetric scenario that is motivated by the muon anomalous magnetic moment observation, dark-mattter relic density abundance, and electroweak naturalness. A mass spectrum involving light Higgsinos and heavier sleptons with a bino at intermediate mass is targeted.more » No significant deviation from the Standard Model expectation is observed. This search enables us to place stringent constraints on this model, excluding at the 95% confidence level charged slepton and sneutrino masses up to 450 GeV when assuming a lightest neutralino mass of 100 GeV and mass-degenerate selectrons, smuons and sneutrinos.« less

This paper presents the measurement of charged-hadron and identified-hadron (𝐾$$^{0}_{𝑆}$$, Λ, $$Ξ$$⁻) yields in photonuclear collisions using 1.7nb⁻¹ of $$\sqrt{s_{NN}}$$ = 5.02 TeV 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 0.1 nb⁻¹ of $$\sqrt{s_{NN}}$$ = 5.02 TeV p + Pb data collected in 2016 by ATLASmore » 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 generator and hydrodynamic calculations to test whether such photonuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.« less