281 Search Results

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

The CP properties of the Higgs boson are studied in the vector-boson fusion production mode. The analysis exploits the decay mode of the Higgs boson into two τ-leptons using 140 fb⁻¹ of proton-proton collision data at $$\sqrt{s}$$ = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Results are obtained using the Optimal Observable method. CP-violating interactions between the Higgs boson and electroweak gauge bosons are considered in the effective field theory framework, with the interaction strength described in the HISZ basis by $$\tilde{d}$$, and in the Warsaw basis by c_{H$$\tilde{W}$$}, c_{H$$\tilde{B}$$}, and c_{H$$\tilde{W}$$B}. No deviations relativemore » to the Standard Model are observed, and limits are obtained on the strength parameters. The $$\tilde{d}$$ parameter is constrained to the interval [−0.012, 0.044] at the 95% confidence level while c_{H$$\tilde{W}$$} is constrained to [−0.24, 0.83], when considering both linear and quadratic effects of physics beyond the Standard Model.« less

A search for the production of a Higgs boson in association with a single top quark, tH, is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb⁻¹ at a centre-of-mass energy of 13 TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into $$b\bar{b}$$, WW^*, ZZ^*, and ττ, accompanied by an isolated lepton (electron or muon) from the top-quark decay. Multivariate techniques are employed to enhance the separation between signal and background processes. The observed signal strength, μ_tH, defined as the ratio between the measured cross-section and the predictedmore » Standard Model value, is μ_tH = 8.1 ± 2.6 (stat.) ± 2.0 (syst.). The significance of the observed (expected) signal above the background-only expectation is 2.8 (0.4) standard deviations. The corresponding observed (expected) upper limit at the 95% confidence level on the tH cross-section is found to be 13.9 (6.1) times the value predicted by the Standard Model. An interpretation with an inverted sign of the top-quark Yukawa coupling is performed, and the signal strength and corresponding limit are reported.« less

The production cross-section of high-mass τ-lepton pairs is measured as a function of the dilepton visible invariant mass, using 140 fb⁻¹ of $$\sqrt{s}=13$$ TeV proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider. The measurement agrees with the predictions of the Standard Model. A fit to the invariant mass distribution is performed as a function of b-jet multiplicity, to constrain the non-resonant production of new particles described by an effective field theory or in models containing leptoquarks or Z′ bosons that couple preferentially to third-generation fermions. The constraints on new particles improve on previous results, andmore » the constraints on effective operators include those affecting the anomalous magnetic moment of the τ-lepton.« less

A measurement of off-shell Higgs boson production is performed in the H* → WW channel. The measurement uses a proton–proton collision dataset with an integrated luminosity of 140 fb^-1 collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both W bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95 %more » symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95 % confidence level on the Higgs boson total width of 13.1 (17.3) MeV.« less

This letter reports the measurement of double parton scattering in same-sign W boson pair production with the ATLAS detector. The data set used corresponds to an integrated luminosity of 140 fb^-1 of proton–proton collisions at a center-of-mass energy of 13 TeV, collected during Run 2 of the Large Hadron Collider. The study is performed in final states including two same-charge leptons, electron or muon, missing transverse momentum, and up to one jet. An excess of events is observed over the expected background contributions with a significance of 8.8 standard deviations. The measured fiducial cross section times leptonic branching fraction ismore » 4.59 ± 0.64 fb. The measurement corresponds to a double parton scattering effective cross section of 10.6 ± 1.8 mb.« 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 performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a Z′ mediator in which the dark Higgs boson s decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least threemore » b-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and Z′ mediators up to 2.3 TeV.« less

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into τ-leptons, H → aa → τ⁺τ^–τ⁺τ^–, is presented. In models with Yukawa-like couplings, the decay to τ-leptons is favoured for light a-bosons, with mass in the range of 2m_τ < m_a < 2m_b. Results are presented in the range of 4 GeV < m_a < 15 GeV using the 140 fb^–1 of proton–proton collisions at $$\sqrt{s}$$ = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. This search focuses on the scenario where, for both di-τ pairs, one of the τ-leptons decays to hadrons and neutrinos,more » while the other decays to a muon and neutrinos. In this mass range, the a → τ⁺τ^– is Lorentz-boosted and a dedicated muon removal technique is used to reconstruct the di-τ pairs.« less

The jet energy calibration and its uncertainties are derived from measurements of the calorimeter response to single particles in both data and Monte Carlo simulation using proton–proton collisions at $$\sqrt{s} = 13$$ TeV collected with the ATLAS detector during Run 2 at the Large Hadron Collider. The jet calibration uncertainty for anti-$$k_T$$ jets with a jet radius parameter of R$$_\textrm{jet} = 0.4$$ and in the central jet rapidity region is about 2.5% for transverse momenta ($$p_{\text {T}}$$) of 20 $$\text {GeV}$$ , about 0.5% for $$p_{\text {T}} = 300$$ GeV and 0.7% for $$p_{\text {T}} = 4$$ TeV . Excellentmore » agreement is found with earlier determinations obtained from -balance based in situ methods ($$Z/\gamma$$ +jets). The combination of these two independent methods results in the most precise jet energy measurement achieved so far with the ATLAS detector with a relative uncertainty of 0.3% at $$p_\textrm{T} = 300$$ GeV and 0.6% at 4 TeV. The jet energy calibration is also derived with the single-particle calorimeter response measurements separately for quark- and gluon-induced jets and furthermore for jets with R_jet varying from 0.2 to 1.0 retaining the correlations between these measurements. Differences between inclusive jets and jets from boosted top-quark decays, with and without grooming the soft jet constituents, are also studied.« less