Studies of the spin and parity quantum numbers of the Higgs boson are presented, based on proton–proton collision data collected by the ATLAS experiment at the LHC. The Standard Model spin–parity JP=0+JP=0+ hypothesis is compared with alternative hypotheses using the Higgs boson decays H→γγH→γγ, H→ZZ⁎→4ℓH→ZZ⁎→4ℓ and H→WW⁎→ℓνℓνH→WW⁎→ℓνℓν, as well as the combination of these channels. The analysed dataset corresponds to an integrated luminosity of 20.7 fb−1 collected at a centre-of-mass energy of √s=8TeV. For the H→ZZ⁎→4ℓH→ZZ⁎→4ℓ decay mode the dataset corresponding to an integrated luminosity of 4.6 fb−1 collected at √s=7TeV is included. The data are compatible with the Standard Model JP=0+JP=0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this Letter, namely some specific JP=0−,1+,1−,2+JP=0−,1+,1−,2+ models, are excluded at confidence levels above 97.8%. This exclusion holds independently of the assumptions on the coupling strengths to the Standard Model particles and in the case of the JP=2+JP=2+ model, of the relative fractions of gluon-fusion and quark–antiquark production of the spin-2 particle. The data thus provide evidence for the spin-0 nature of the Higgs boson, with positive parity being strongly preferre

A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 4.5 fb −1 of proton-proton collisions data at ffiffi ffi s p ¼ 7 TeV and 20.3 fb −1 at ffiffi ffi s p ¼ 8 TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be μ ¼ 1.17 AE 0.27 at the value of the Higgs boson mass measured by ATLAS, m H ¼ 125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of m H . They are found to be μ ggF ¼ 1.32 AE 0.38, μ VBF ¼ 0.8 AE 0.7, μ WH ¼ 1.0 AE 1.6, μ ZH ¼ 0.1 þ3.7 −0.1 , and μ ttH ¼ 1.6 þ2.7 −1.8 , for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a W or Z boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.

This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated luminosity of 36.1 fb −1 of pp collisions at √ s = 13 TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns. Keywords: Hadron-Hadron scattering (experiments)ArXiv ePrint: 1712.02118Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP 3 .https://doi.org/10.1007/JHEP06 (2018) [7], SUSY particles are produced in pairs and decay such that their final products consist only of SM particles and the stable lightest supersymmetric particle (LSP). In many supersymmetric models, the supersymmetric partners of the SM W boson fields, the wino fermions, are the lightest gaugino states. In this case, the lightest of the charged mass eigenstates, a chargino, and the lightest of the neutral mass eigenstates, a neutralino, are both almost pure wino and nearly mass-degenerate. As a result, the lightest chargino can have a lifetime long enough that it can reach the AT-LAS detector before decaying. For example, anomaly-mediated supersymmetry breaking (AMSB) scenarios [8,9] naturally predict a pure wino LSP, which is a dark-matter candidate. The mass-splitting between the charged and neutral wino (∆mχ 1 ) in such models is suppressed at tree level by the approximate custodial symmetry; it has been calculated at the two-loop level to be around 160 MeV [10], corresponding to a chargino lifetime of about 0.2 ns [11]. This prediction for the value of the lifetime is actually a general feature of models with a wino LSP: within the generated models of the ATLAS phenomenological Minimal Supersymmetric Standard Model (pMSSM) scan [12] that have a wino-like LSP, about 70% have a charged-wino lifetime between 0.15 ns and 0.25 ns. Most of the models in the other 30% have a larger mass-splitting (and therefore the charged wino has a shorter lifetime) due to a non-decoupled higgsino mass. The search presented here is sensitive to a wide range of lifetimes, from 10 ps to 10 ns, and reaches maximum sensitivity for lifetimes around 1 ns.The decay products of SUSY particles that are strongly mass-degenerate with the lightest neutralino leave little visible energy in the detector. T...

Study of the rare decays of B 0 s and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector The ATLAS Collaboration A study of the decays B 0 s → µ + µ − and B 0 → µ + µ − has been performed using 26.3 fb −1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in µ + µ − invariant mass is comparable to the B 0 s -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B 0 s → µ + µ − ) = 3.2 +1.1 −1.0 × 10 −9 and an upper limit B(B 0 → µ + µ − ) < 4.3 × 10 −10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B 0 s → µ + µ − ) = 2.8 +0.8 −0.7 ×10 −9 and B(B 0 → µ + µ − ) < 2.1×10 −10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B 0 → µ + µ − )-B(B 0 s → µ + µ − ) plane.

A search for scalar particles decaying via narrow resonances into two photons in the mass range 65-600 GeV is performed using 20.3  fb(-1) of √s 8 TeV pp collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95% confidence level on the production cross section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.