Professor Chris Hays

Abstract:

The first observation of a new meson state decaying into $B_c^{+}$ and a photon is presented using a dataset of $pp$ collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . The $B_c^{+}$ mesons are reconstructed from a three-muon final state from $B_c^{+}\to J/\psi ({\mu }^{+}{\mu }^{-}){\mu }^{+}{\nu }_{\mu }X$ decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state $B_c^{+}$ meson is measured to be $64.5\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\mathrm{MeV}$ . This corresponds to a mass for the observed state of $6339.0\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\pm 0.3(m_{B_c^{+}})\mathrm{MeV}$ , where the last uncertainty is due to the precision of the best knowledge of the $B_c^{+}$ meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the $\overline{b}c$ system, the $B_c^{*+}$ meson.

Abstract:

This Letter presents a combination of searches for Higgs boson pair ( $HH$ ) production performed by the ATLAS and CMS Collaborations using proton-proton collision datasets recorded at $\sqrt{s}=13\mathrm{TeV}$ during the Large Hadron Collider Run 2, corresponding to integrated luminosities ranging between 126 and $140{\mathrm{fb}}^{-1}$ . The upper limit at the 95% confidence level on the total $HH$ signal strength, defined as the ratio of the measured cross section to the standard model (SM) prediction, corresponds to 2.5, with an expected value of 1.7 (2.8) assuming the absence (presence) of the SM $HH$ signal. The strength of the $HH$ signal is measured to be ${0.8}_{-0.7}^{+0.9}$ relative to the SM prediction. The observed significance is found to be 1.1 standard deviations whereas 1.3 are expected for the SM $HH$ signal. Constraints are set on the Higgs boson trilinear self-coupling and on the couplings of two Higgs bosons to two vector bosons, both normalized to the SM predictions and denoted as ${\kappa }_{\lambda }$ and ${\kappa }_{2V}$ , respectively. The observed individual constraints at the 95% confidence level are $-0.71<{\kappa }_{\lambda }<6.1$ and $0.73<{\kappa }_{2V}<1.3$ , while the expected constraints assuming the presence of the SM $HH$ signal are $-1.3<{\kappa }_{\lambda }<6.7$ and $0.66<{\kappa }_{2V}<1.4$ .

Abstract:

This Letter presents a measurement of prompt charged-particle production in proton-oxygen interactions at $\sqrt{s_{\mathrm{NN}}}=9.62\mathrm{TeV}$ center-of-mass energy with the ATLAS detector, corresponding to $634\mu {\mathrm{b}}^{-1}$ of integrated luminosity. A total of $246\times {10}^6$ selected events have at least one track with transverse momentum $p_{\mathrm{T}}>500\mathrm{MeV}$ and pseudorapidity $|\eta |<2.5$ . The measured fiducial proton-oxygen cross section is ${\sigma }_{\mathrm{fid}.}^{p\mathrm{O}}=396\pm 6(\exp .)\pm 9(\mathrm{lumi}.)\mathrm{mb}$ and the extrapolated inelastic proton-air cross section is ${\sigma }_{\mathrm{inel}.}^{p+\mathrm{air}}=406\pm 6(\exp .)\pm 9(\mathrm{lumi}.)\pm 28(\mathrm{th}.)\mathrm{mb}$ . Measurements of charged-particle multiplicity, $p_{\mathrm{T}}$ , and $\eta$ distributions are an order-of-magnitude more precise than differences between hadronic-interaction models. These results enable improved modeling of cosmic-ray air showers, which is important for astroparticle physics.

Abstract:

This article details two studies of Higgs boson properties using the vector-boson fusion production mode and the γγjj final state. Both efforts are based on a data sample corresponding to 164 fb−1 of s=13.6$$ \sqrt{s}=13.6 $$ TeV proton-proton collisions recorded by the ATLAS experiment at the Large Hadron Collider. The first study employs matrix-element-based optimal observables to constrain CP-odd couplings beyond the Standard Model within the Standard Model Effective Field Theory framework, expressed in the Warsaw basis. The second study exploits angular distributions to probe the Higgs boson’s couplings to longitudinally and transversely polarised W and Z bosons in the production of the Higgs boson. To maximise the sensitivity, the constraints of the CP-odd couplings are combined with those from a previous analysis performed in γγjj events in a data sample of proton-proton collisions at s=13$$ \sqrt{s}=13 $$ TeV, corresponding to an integrated luminosity of 140 fb−1. A significant improvement with respect to the previous analysis is achieved through the implementation of a new neural-network-based classification algorithm. All measurements are in agreement with the Standard Model prediction of a CP-even Higgs boson with the expected relative coupling strengths to longitudinally and transversely polarised vector bosons.

Abstract:

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of $Z$ bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to $Z{Z}^{*}$ spin-density-matrix elements in the $H\to Z{Z}^{*}\to {\ell }^{+}{\ell }^{-}{\ell }^{+}{\ell }^{-}$ process yield coefficients $C_{2,1,2,-1}=-0.71\pm 0.45$ and $C_{2,2,2,-2}=0.08\pm 0.44$ , consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected $4.9\sigma$ ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.