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

Study of Higgs boson pair production in the HH→bb‾γγHH \rightarrow b \overline{b} \gamma\gamma final state with 308 fb−1^{-1} of data collected at s=13\sqrt{s} = 13 TeV and 13.6 TeV by the ATLAS experiment

International audienceA search for Higgs boson pair production in the $b \overline{b} γγ$ final state is performed. The proton-proton collision dataset in this analysis corresponds to an integrated luminosity of 308 fb$^{-1}$, consisting of two samples, 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV and 168 fb$^{-1}$ at 13.6 TeV, recorded between 2015 and 2024 by the ATLAS detector at the CERN Large Hadron Collider. In addition to a larger dataset, this analysis improves upon the previous search in the same final state through several methodological and technical developments. The Higgs boson pair production cross section divided by the Standard Model prediction is found to be $μ_{HH} = 0.9^{+1.4}_{-1.1}$ ($μ_{HH} = 1^{+1.3}_{-1.0}$ expected), which translates into a 95% confidence-level upper limit of $μ_{HH}<3.8$. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range $-1.7 < κ_λ< 6.6$ ($-1.8 < κ_λ< 6.9$ expected)

Study of Higgs boson pair production in the HH→bb‾γγHH \rightarrow b \overline{b} \gamma\gamma final state with 308 fb−1^{-1} of data collected at s=13\sqrt{s} = 13 TeV and 13.6 TeV by the ATLAS experiment

International audienceA search for Higgs boson pair production in the $b \overline{b} γγ$ final state is performed. The proton-proton collision dataset in this analysis corresponds to an integrated luminosity of 308 fb$^{-1}$, consisting of two samples, 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV and 168 fb$^{-1}$ at 13.6 TeV, recorded between 2015 and 2024 by the ATLAS detector at the CERN Large Hadron Collider. In addition to a larger dataset, this analysis improves upon the previous search in the same final state through several methodological and technical developments. The Higgs boson pair production cross section divided by the Standard Model prediction is found to be $μ_{HH} = 0.9^{+1.4}_{-1.1}$ ($μ_{HH} = 1^{+1.3}_{-1.0}$ expected), which translates into a 95% confidence-level upper limit of $μ_{HH}<3.8$. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range $-1.7 < κ_λ< 6.6$ ($-1.8 < κ_λ< 6.9$ expected)

Measurements and interpretations of W±ZW^{\pm}Z production cross-sections in pppp collisions at s=\sqrt{s} = 13 TeV with the ATLAS detector

International audienceMeasurements of integrated and differential cross-sections for $W^{\pm}Z$ production in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of $\sqrt{s} =$ 13 TeV are used, corresponding to an integrated luminosity of 140 fb$^{-1}$. The $W^{\pm}Z$ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons or muons. The integrated cross-section per lepton flavour for the production of $W^{\pm}Z$ is measured in the detector fiducial region with a relative precision of 4%. The measured value is compared with the Standard Model prediction at a precision of up to next-to-next-to-leading-order in QCD and next-to-leading-order in electroweak. Cross-sections for $W^+Z$ and $W^-Z$ production and their ratio are presented. The $W^{\pm}Z$ production is also measured differentially as functions of various kinematic variables, including new observables sensitive to CP-violation effects. All measurements are compared with state-of-the-art Standard Model predictions from fixed-order calculations or Monte Carlo generators based on next-to-leading-order matrix elements interfaced with parton showers. An effective field theory interpretation of the measurements is performed, considering both CP-conserving and CP-violating dimension-6 operators modifying the $W^{\pm}Z$ production. In the absence of observed deviations from the Standard Model, limits on CP-conserving Wilson coefficients are extracted using the transverse mass of the $W^{\pm}Z$ system. For CP-violating coefficients a machine learning approach is used to construct an observable with enhanced sensitivity to CP-violation effects

Search for emerging jets in pppp collisions at s=13\sqrt{s} = 13 TeV with the ATLAS experiment

International audienceA search is presented for emerging jets using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the ATLAS experiment between 2015 and 2018. The search looks for the existence of a dark sector with symmetries similar to those in quantum chromodynamics. This dark sector is populated with dark quarks, which undergo showering similar to quarks in the Standard Model, leading to a high multiplicity of long-lived dark hadrons within a dark jet. These dark hadrons subsequently decay to Standard Model particles via a new heavy scalar mediating particle $ϕ$. This results in jets which contain multiple displaced vertices, known as emerging jets. This analysis targets four-jet topologies, with two emerging jets and two Standard Model jets, resulting from the decay of pair-produced scalar mediators. No significant excess above the Standard Model background is observed. For dark pion proper decay lengths of 20 mm, mediator masses are excluded between 1 TeV and 2 TeV assuming a dark pion mass of 20 GeV

Evidence for the dimuon decay of the Higgs boson in pp collisions with the ATLAS detector

International audienceA search for the dimuon decay of the Higgs boson is presented based on pp collision data recorded by ATLAS during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 165 fb$^{-1}$ at $\sqrt{s} = 13.6$ TeV. To enhance the sensitivity, the results are combined with those from Run 2. An excess of events over the background is observed with a significance of $3.4 σ$ ($2.5 σ$ expected). The best-fit signal strength is $μ= 1.4\pm0.4$. This result provides evidence for the $H \to μμ$ decay with ATLAS data and offers a direct probe of the Higgs-boson Yukawa coupling to second-generation fermions

Search for Higgs bosons produced in association with a high-energy photon via vector-boson fusion and decaying to a pair of bb-quarks in the ATLAS detector

International audienceA search for Standard Model Higgs bosons produced in association with a high-energy photon and decaying to $b\bar{b}$ is performed using 133 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data collected with the ATLAS detector at the Large Hadron Collider at CERN. The photon requirement reduces the multijet background, and the $H \rightarrow b\bar b$ decay is the dominant decay mode. Event selection requirements target events produced by vector-boson fusion, the dominant production mode in this channel. Several improvements enhance the search sensitivity compared to previous measurements. These improvements include better background modelling and characterization, the use of a neural-network classifier, and an updated signal extraction strategy adopting a direct binned-likelihood fit to the classifier output. With these improvements, the Higgs boson signal strength is measured to be $0.2 \pm 0.7$ relative to the Standard Model prediction. This corresponds to an observed significance of $0.3$ standard deviations, compared to an expectation of $1.5$ standard deviations assuming the Standard Model

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

Search for Beyond the Standard Model physics with anomaly detection in multilepton final states in pppp collisions at s=13\sqrt{s}=13 TeV with the ATLAS detector

International audienceA model-agnostic search for Beyond the Standard Model physics is presented, targeting final states with at least four light leptons (electrons or muons). The search regions are separated by event topology and unsupervised machine learning is used to identify anomalous events in the full 140 fb$^{-1}$ of proton-proton collision data collected with the ATLAS detector during Run 2. No significant excess above the Standard Model background expectation is observed. Model-agnostic limits are presented in each topology, along with limits on several benchmark models including vector-like leptons, wino-like charginos and neutralinos, or smuons. Limits are set on the flavourful vector-like lepton model for the first time