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Abstract:

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 is 4.59 ± 0.64 fb. The measurement corresponds to a double parton scattering effective cross section of 10.6 ± 1.8 mb.

Abstract:

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into τ -leptons, H → a a → 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 2 m τ < m a < 2 m b . Results are presented in the range of 4 GeV < m a < 15 GeV using the 140 fb − 1 of proton–proton collisions at 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, 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. No significant excess above the Standard Model background prediction is observed. Upper limits on ( σ ( H ) / σ SM ( H ) ) × B ( H → a a → 4 τ ) at 95 % confidence level are provided, ranging from 0.03 to 0.10 depending on the a -boson mass.

Abstract:

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 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 on 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.

Abstract:

Abstract A search for a pseudoscalar a produced in association with a top-quark pair, or in association with a single top quark plus a W boson, with the pseudoscalar decaying into b -quarks ( $$a\rightarrow b\bar{b}$$ a → b b ¯ ), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12 and 100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard b -tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio $$\text {BR}(a\rightarrow b\bar{b})=100\% $$ BR ( a → b b ¯ ) = 100 % , the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95% confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95% confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.

Abstract:

This paper presents searches for the direct pair production of charged light-flavour sleptons, each decaying into a stable neutralino and an associated Standard Model lepton. The analyses focus on the challenging “corridor” region, where the mass difference, Δ𝑚, between the slepton (𝑒˜ or 𝜇˜) and the lightest neutralino (𝜒˜ 0 1 ) is less or similar to the mass of the 𝑊 boson, 𝑚(𝑊), with the aim to close a persistent gap in sensitivity to models with Δ𝑚 ≲ 𝑚(𝑊). Events are required to contain a high-energy jet, significant missing transverse momentum, and two same-flavour opposite-sign leptons (𝑒 or 𝜇). The analysis uses 𝑝 𝑝 collision data at √ 𝑠 = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 140 fb−1 . Several kinematic selections are applied, including a set of boosted decision trees. These are each optimised for different Δ𝑚 to provide expected sensitivity for the first time across the full Δ𝑚 corridor. The results are generally consistent with the Standard Model, with the most significant deviations observed with a local significance of 2.0 𝜎 in the selectron search, and 2.4 𝜎 in the smuon search. While these deviations weaken the observed exclusion reach in some parts of the signal parameter space, the previously present sensitivity gap to this corridor is largely reduced. Constraints at the 95% confidence level are set on simplified models of selectron and smuon pair production, where selectrons (smuons) with masses up to 300 (350) GeV can be excluded for Δ𝑚 between 2 GeV and 100 GeV.