A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb−1 collected at √s=7 TeV in 2011 and 5.8 fb−1 at √s=8 TeV in 2012. Individual searches in the channels H→ZZ(⁎)→4ℓ, H→γγ and H→WW(⁎)→eνμν in the 8 TeV data are combined with previously published results of searches for H→ZZ(⁎), WW(⁎), bb and τ+τ− in the 7 TeV data and results from improved analyses of the H→ZZ(⁎)→4ℓ and H→γγ channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson
Many extensions of the Standard Model posit the existence of heavy particles with long lifetimes. This article presents the results of a search for events containing at least one long-lived particle that decays at a significant distance from its production point into two leptons or into five or more charged particles. This analysis uses a data sample of proton-proton collisions at ffiffi ffi s p ¼ 8 TeV corresponding to an integrated luminosity of 20.3 fb −1 collected in 2012 by the ATLAS detector operating at the Large Hadron Collider. No events are observed in any of the signal regions, and limits are set on model parameters within supersymmetric scenarios involving R-parity violation, split supersymmetry, and gauge mediation. In some of the search channels, the trigger and search strategy are based only on the decay products of individual long-lived particles, irrespective of the rest of the event. In these cases, the provided limits can easily be reinterpreted in different scenarios.
Dijet events are studied in the proton-proton collision data set recorded at ffiffi ffi s p ¼ 13 TeV with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to integrated luminosities of 3.5 fb −1 and 33.5 fb −1 respectively. Invariant mass and angular distributions are compared to background predictions and no significant deviation is observed. For resonance searches, a new method for fitting the background component of the invariant mass distribution is employed. The data set is then used to set upper limits at a 95% confidence level on a range of new physics scenarios. Excited quarks with masses below 6.0 TeV are excluded, and limits are set on quantum black holes, heavy W 0 bosons, W Ã bosons, and a range of masses and couplings in a Z 0 dark matter mediator model. Model-independent limits on signals with a Gaussian shape are also set, using a new approach allowing factorization of physics and detector effects. From the angular distributions, a scale of new physics in contact interaction models is excluded for scenarios with either constructive or destructive interference. These results represent a substantial improvement over those obtained previously with lower integrated luminosity.
A search is performed for narrow resonances decaying into W W , W Z, or ZZ boson pairs using 20.3 fb −1 of proton-proton collision data at a centre-of-mass energy of √ s = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted W or Z boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the W Z channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the W Z final state of a new heavy gauge boson, W , and for the W W and ZZ final states of Kaluza-Klein excitations of the graviton in a bulk Randall-Sundrum model, as a function of the resonance mass. W bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level. The ATLAS collaboration 22 IntroductionThe substantial dataset of Large Hadron Collider (LHC) proton-proton (pp) collisions at √ s = 8 TeV collected by the ATLAS experiment provides a distinct opportunity to search for new heavy resonances at the TeV mass scale. This paper presents a search for narrow diboson resonances (W W , W Z and ZZ) decaying to fully hadronic final states. The fully hadronic mode has a higher branching fraction than leptonic and semileptonic decay modes, and is therefore used to extend the reach of the search to the highest possible resonance masses.W and Z bosons resulting from the decay of very massive resonances are highly boosted, so that each boson's hadronic decay products are reconstructed as a single jet. The signature of the heavy resonance decay is thus a resonance structure in the dijet invariant mass spectrum. The dominant background for this search is due to dijet events from QCD processes, which produce a smoothly falling spectrum without resonance structures. To cope with this large background, jets are selected using a boson tagging procedure based on -1 - JHEP12(2015)055a reclustering-mass-drop filter (BDRS-A, similar to the method introduced in ref.[1]), jet mass, and further substructure properties. The tagging procedure strongly suppresses the dijet background, although these QCD processes still overwhelm the expected backgrounds from single boson production with one or more jets, Standard Model (SM) diboson production, single-top and top-pair production. As all of these background sources produce dijet invariant mass distributions without resonance peaks, the expected background in the search is modelled by a fit to a smoothly falling distribution.Diboson resonances are predicted in several extensions to the SM, such as technicolour [2][3][4], warped extra dimensions [5][6][7], and Grand Unified Theories [...
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