322 Search Results

The ATLAS experiment at the Large Hadron Collider explores the use of modern neural networks for a multi-dimensional calibration of its calorimeter signal defined by clusters of topologically connected cells (topo-clusters). The Bayesian neural network (BNN) approach not only yields a continuous and smooth calibration function that improves performance relative to the standard calibration but also provides uncertainties on the calibrated energies for each topo-cluster. The results obtained by using a trained BNN are compared to the standard local hadronic calibration and to a calibration provided by training a deep neural network. The uncertainties predicted by the BNN are interpretedmore » in the context of a fractional contribution to the systematic uncertainties of the trained calibration. They are also compared to uncertainty predictions obtained from an alternative estimator employing repulsive ensembles.« less

A calibration of the ATLAS flavour-tagging algorithms using a new calibration procedure based on optimal transportation maps is presented. Simultaneous, continuous corrections to the b-jet, c-jet, and light-flavour jet classification probabilities from jet-tagging algorithms in simulation are derived for b-jets using $$t\bar{t} \rightarrow e\mu \nu \nu bb$$ data. After application of the derived calibration maps, closure between simulation and observation is achieved for jet flavour observables used in ATLAS analyses of Large Hadron Collider (LHC) Run 2 proton-proton collision data. This continuous calibration opens up new possibilities for the future use of jet flavour information in LHC analyses and alsomore » serves as a guide for deriving high-dimensional corrections to simulation via transportation maps, an important development for a broad range of inference tasks.« 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

A sample of 𝑝⁢𝑝 collision data, corresponding to an integrated luminosity of 5.4 fb⁻¹ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the $$Ξ^0_b$$ baryon to that of the $$Λ^0_b$$ baryon, 𝑟_𝜏 $$≡$$ $$𝜏_{Ξ^0_b}$$$$/𝜏_{Λ^0_b}$$. The value 𝑟$$^{\textrm{Run 2}}_𝜏$$ =1.004 ± 0.009 ± 0.006 is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain 𝑟_𝜏 =1.004 ± 0.008 ± 0.005. Multiplying by the known value of the $$Λ^0_b$$ lifetime yields $$𝜏_{Ξ^0_b}$$ = 1.475 ±more » 0.012 ± 0.008 ± 0.009 ps, where the last uncertainty is due to the limited knowledge of the $$Λ^0_b$$ lifetime. This measurement improves the precision of the current world average of the $$Ξ^0_b$$ lifetime by about a factor of two, and is in good agreement with the most recent theoretical predictions.« 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

A measurement is presented of elliptic (𝑣₂) and triangular (𝑣₃) azimuthal anisotropy coefficients for charged particles produced in Pb + Pb collisions at $$\sqrt{s_{\textrm{NN}}}$$ = 5.02 TeV using a dataset corresponding to an integrated luminosity of 0.44nb⁻¹ collected with the ATLAS detector at the LHC in 2018. The values of 𝑣₂ and 𝑣₃ are measured for charged particles over a wide range of transverse momentum (𝑝_T), 1–400 GeV, and Pb + Pb collision centrality, 0–60%, using the scalar-product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive valuesmore » of 𝑣₂ are observed up to a 𝑝_T of approximately 100 GeV from both methods across all centrality intervals. Positive values of 𝑣₃ are observed up to approximately 25 GeV using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest 𝑝_T. At high 𝑝_T (𝑝_T ⪆ 10 GeV), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in Pb + Pb collisions.« less

Measurements of W⁺W⁻ → e^±νμ^∓ν production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics and the electroweak theory. The measurements are based on data from pp collisions at $$\sqrt{s}$$ = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015–2018, corresponding to an integrated luminosity of 140 fb⁻¹. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with b-hadron decays. An improved methodology for estimating the remaining top-quark background enables a precise measurement of W⁺W⁻ cross-sections with no additional requirements onmore » jets. The fiducial W⁺W⁻ cross-section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total W⁺W⁻ cross-section of 127 ± 4 pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of W⁺W⁻ events. The measurements are compared with state-of-the-art theory calculations and excellent agreement with predictions is observed. A charge asymmetry in the lepton rapidity is observed as a function of the dilepton invariant mass, in agreement with the Standard Model expectation. A CP-odd observable is measured to be consistent with no CP violation. Limits on Standard Model effective field theory Wilson coefficients in the Warsaw basis are obtained from the differential cross-sections.« less