Dr William Balunas

Abstract:

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 three 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.

Abstract:

Abstract 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 $$\sqrt{s} = 13$$ s = 13 TeV collected with the ATLAS detector during Run 2 at the Large Hadron Collider. The jet calibration uncertainty for anti- $$k_T$$ k T jets with a jet radius parameter of R $$_\textrm{jet} = 0.4$$ jet = 0.4 and in the central jet rapidity region is about 2.5% for transverse momenta ( $$p_{\text {T}}$$ p T ) of 20 $$\text {GeV}$$ GeV , about $$0.5\%$$ 0.5 % for $$p_{\text {T}} =300$$ p T = 300 $$\text {GeV}$$ GeV and $$0.7\%$$ 0.7 % for $$p_{\text {T}} =4$$ p T = 4 $$\text {TeV}$$ TeV . Excellent agreement is found with earlier determinations obtained from $$p_\textrm{T}$$ p T -balance based in situ methods ( $$Z/\gamma $$ Z / γ +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 $$0.3\%$$ 0.3 % at $$p_\textrm{T}=300$$ p T = 300 GeV and $$0.6\%$$ 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 $$_\textrm{jet}$$ 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.

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.