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Abstract:

Measuring the Higgs trilinear self-coupling λhhh is experimentally demanding but fundamental for understanding the shape of the Higgs potential. We present a comprehensive analysis strategy for the HL-LHC using di-Higgs events in the four b-quark channel (hh → 4b), extending current methods in several directions. We perform deep learning to suppress the formidable multijet background with dedicated optimisation for BSM λhhh scenarios. We compare the λhhh constraining power of events using different multiplicities of large radius jets with a two-prong structure that reconstruct boosted h → bb decays. We show that current uncertainties in the SM top Yukawa coupling yt can modify λhhh constraints by ∼ 20%. For SM yt, we find prospects of −0.8 < 𝜆ℎℎℎ/𝜆SMℎℎℎ < 6.6 at 68% CL under simplified assumptions for 3000 fb−1 of HL-LHC data. Our results provide a careful assessment of di-Higgs identification and machine learning techniques for all-hadronic measurements of the Higgs self-coupling and sharpens the requirements for future improvement.

Abstract:

Constraints on selected mediator-based dark matter models and a scalar dark energy model using up to 37 fb−1s√ = 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015-2016 are summarised in this paper. The results of experimental searches in a variety of final states are interpreted in terms of a set of spin-1 and spin-0 single-mediator dark matter simplified models and a second set of models involving an extended Higgs sector plus an additional vector or pseudo-scalar mediator. The searches considered in this paper constrain spin-1 leptophobic and leptophilic mediators, spin-0 colour-neutral and colour-charged mediators and vector or pseudo-scalar mediators embedded in extended Higgs sector models. In this case, also s√ = 8 TeV pp collision data are used for the interpretation of the results. The results are also interpreted for the first time in terms of light scalar particles that could contribute to the accelerating expansion of the universe (dark energy).

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 $0.66<{\kappa }_{2V}<1.4$ .