Professor Chris Hays

Abstract:

A combination of measurements of the $CP$ properties of Higgs boson interactions with electroweak gauge bosons is presented, using $140{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ recorded by the ATLAS detector. Results from vector boson fusion $H\to \tau \tau /W{W}^{*}/\gamma \gamma$ , inclusive $H\to Z{Z}^{*}$ , and $WH,H\to b\overline{b}$ channels are combined. No evidence of $CP$ violation is observed, and constrains on the $CP$ -violating operators in the Standard Model effective field theory framework (SMEFT) are set in the Warsaw basis. The results from the combination improve by over 40% on previous individual limits on $c_{H\tilde{W}}$ and, for the first time, simultaneous constraints on three coefficients $c_{H\tilde{W}}$ , $c_{H\tilde{B}}$ , and $c_{H\tilde{W}B}$ are set. These limits are the most stringent constraints to date on the relevant Wilson coefficients in the SMEFT framework with minimum model dependence.

Abstract:

A search is presented for the electroweak production of a top-quark pair in association with a $W$ boson and at least one additional jet known as the $t\overline{t}W{j}_{\mathrm{EW}}$ process. This process has embedded within it a $tW$ -scattering vertex, which is probed directly for the first time. The collision data were collected with the ATLAS detector during Run 2 of the LHC and correspond to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ at $\sqrt{s}=13\mathrm{TeV}$ . The search uses same-charge pairs of electrons and muons together with jets, of which at least one is $b$ -tagged. The properties of the most forward jet relative to the rest of the event are used to discriminate the electroweak $t\overline{t}W{j}_{\mathrm{EW}}$ production process from its strong $t\overline{t}W$ production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at ${\sigma }_{t\overline{t}W{j}_{\mathrm{EW}}}<251\mathrm{fb}$ (230 fb), to be compared with the expected Standard Model (SM) cross section of 47.7 fb. Limits are set on the SM effective field theory (EFT) operators $O_{Ht}$ and $O_{HQ}^{(1)}$ , which modify the electroweak couplings of the top quark through contributions to the $tW$ -scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multiprocess, and multioperator EFT contributions.

Abstract:

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb−1, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.

Abstract:

This paper presents two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb−1 of s=13 TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events are required to feature an energetic jet, large missing transverse momentum, and at least one low-momentum charged particle that serves as a candidate higgsino decay product. In the first search, targeting higgsino mass splittings in the range of 0.3–1 GeV, the higgsinos are expected to predominantly decay into pions that are identified as low-momentum charged particles with large transverse impact parameters due to the long higgsino lifetime (cτ ≈ 𝒪(0.1–10 mm)), and neural networks are used to discriminate between signal and background processes. The second search targets larger mass splittings in the range of 1–3 GeV, where the higgsinos are expected to decay promptly into low-momentum leptons, one of which is identified by dedicated low-momentum electron or muon taggers based on neural networks utilising tracking and calorimeter information. No significant excess above the Standard Model prediction is observed in either search and the results are interpreted within simplified models, to set lower limits on the masses of the higgsino-like charginos and neutralinos. Together, these searches exclude chargino masses below 126 GeV at 95% confidence level for mass splittings between the chargino and lightest neutralino in the range of 0.3–2 GeV. This represents the first ATLAS constraints in a portion of this parameter space and surpasses the limits previously set by other experiments.

Abstract:

A search for resonant structures in the $J/\psi +\psi (2\mathrm{S})$ mass spectrum is performed using proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ , corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ , recorded by the ATLAS experiment at the LHC. The decay channels of $J/\psi +\psi (2\mathrm{S})\to 4\mu$ and $J/\psi +\psi (2\mathrm{S})\to 4\mu +2\pi$ are analyzed. An excess near 6.9 GeV is observed in both channels with a combined significance of $8.9\sigma$ . No significant signal is observed near 7.2 GeV, and an upper limit on its yield relative to $X(6900)$ is provided. A simultaneous fit with the di- $J/\psi$ channel is carried out under assumptions regarding the resonance interferences, yielding a ratio of the partial decay widths between the $J/\psi +\psi (2\mathrm{S})$ and di- $J/\psi$ channels of $1.08\pm 0.20_{-0.17}^{+0.40}$ for the resonance near 6.9 GeV.