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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:

The first measurement of the magnitude of the CKM quark mixing matrix element |Vcb| using on-shell W-boson decays is presented. The measurement is made with tt¯$$ t\overline{t} $$ events from pp collision data corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. A value of:Vcb=50−14+11×10−3$$ \left|{V}_{cb}\right|=\left({50}_{-14}^{+11}\right)\times {10}^{-3} $$is measured, where the uncertainties have roughly equal contributions from the limited size of the data sample and from systematic effects. This is consistent with existing measurements made at much lower energy scales in B hadron decays. While the sensitivity does not yet rival that of previous determinations, this novel approach probes a very different physical situation, namely the rare hadronic t→bb¯c$$ t\to b\overline{b}c $$ decay at high momentum-transfer, rather than semileptonic B decays at low momentum-transfer.

Abstract:

This paper presents differential cross-section measurements of events containing a charged lepton, missing transverse momentum, two b-jets, and two light jets, consistent with the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state. The analysis is based on 140 fb−1 of proton-proton collision data at s=13$$ \sqrt{s}=13 $$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). Differential cross-sections are unfolded at particle level for a range of kinematic observables and in three kinematic regions. Both regions and observables are optimised for sensitivity to the interference between W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ produced with singly and doubly resonant top quarks. Results are compared with various theoretical predictions and deviations are observed in regions sensitive to the interference between singly and doubly resonant top-quark production. The unfolded results provide new information about the modelling of interference and off-shell effects of the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state, improving the understanding of top-quark production mechanisms and providing new inputs for future Monte Carlo generator development.