The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross-sections. Results are based on 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.
The missing transverse momentum ETmiss in the multi-jet validation region VR1 for the two-tau MVA analysis.
The effective mass meff in the multi-jet validation region VR2 for the two-tau MVA analysis.
The stransverse mass mT2 in the W+jets validation region VR1 for the two-tau MVA analysis.
A search is presented for pair production of a new heavy quark ($Q$) that decays into a $W$ boson and a light quark ($q$) in the final state where one $W$ boson decays leptonically (to an electron or muon plus a neutrino) and the other $W$ boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of $Q\bar{Q}$ production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming BR$(Q\to Wq)=1$. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR$(Q\to Wq)$ versus BR$(Q\to Hq)$.
The expected and observed 95% CL upper limits on the cross section as a function of $m_Q$ when setting BR$(Q\to Wq) = 1$, which would be the case for a new chiral quark. Also shown are the $\pm 1\sigma$ and $\pm 2\sigma$ intervals on the distribution of expected results for the chiral model if no signal exists.
The upper and lower bounds on the range of heavy quark masses expected and observed to be excluded at 95% CL, as a function of the branching ratio of the heavy quark to $Wq$ versus $Hq$, with the branching ratio to $Zq$ fixed by the requirement BR$(Q\!\to\!Zq) = 1 - \text{BR}(Q\!\to\!Wq) - \text{BR}(Q\!\to\!Hq)$.
A search is presented for photonic signatures motivated by generalised models of gauge-mediated supersymmetry breaking. This search makes use of $20.3{\rm fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV recorded by the ATLAS detector at the LHC, and explores models dominated by both strong and electroweak production of supersymmetric partner states. Four experimental signatures incorporating an isolated photon and significant missing transverse momentum are explored. These signatures include events with an additional photon, lepton, $b$-quark jet, or jet activity not associated with any specific underlying quark flavor. No significant excess of events is observed above the Standard Model prediction and model-dependent 95% confidence-level exclusion limits are set.
Observed and expected exclusion limits in the gluino-bino mass plane, using the $\rm{SR}^{\gamma\gamma}_{S-H}$ analysis for $m_{\tilde{\chi}_1^0}\geq 800 {\rm GeV}$ and $\rm{SR}^{\gamma\gamma}_{S-L}$ analyses for $m_{\tilde{\chi}_1^0} < 800 {\rm GeV}$.
Observed and expected exclusion limits in the wino-bino mass plane, using the $\rm{SR}^{\gamma\gamma}_{W-H}$ analysis for $m_{\tilde{\chi}_1^0}\geq 350 {\rm GeV}$ and $\rm{SR}^{\gamma\gamma}_{W-L}$ analyses for $m_{\tilde{\chi}_1^0} < 350 {\rm GeV}$.
Observed exclusion limits in the gluino-neutralino mass plane, for the higgsino-bino GGM model with $\mu < 0$, using the merged $\rm{SR}^{\gamma b}_{L}$ and $\rm{SR}^{\gamma b}_{H}$ analyses.
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 $\sqrt{s}$ = 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.
Vertex-level efficiency as a function of the vertex radial position for an RPV SUSY model of squark production with $\tilde{q}\to q[\tilde{\chi}_1^0\to\mu qq]$, where $m(\tilde{q}) = 700$ GeV, $m(\tilde{\chi}_1^0) = 494$ GeV and $c\tau(\tilde{\chi}_1^0)$ = 175 mm. This result is also represented in Figure 3b and Auxiliary Figure 1.
Vertex-level efficiency as a function of the vertex radial position for an RPV SUSY model of squark production with $\tilde{q}\to q[\tilde{\chi}_1^0\to\mu qq]$, where $m(\tilde{q}) = 700$ GeV, $m(\tilde{\chi}_1^0) = 108$ GeV and $c\tau(\tilde{\chi}_1^0)$ = 101 mm.
Vertex-level efficiency as a function of the vertex radial position for an RPV SUSY model of squark production with $\tilde{q}\to q[\tilde{\chi}_1^0\to\mu qb]$, where $m(\tilde{q}) = 700$ GeV, $m(\tilde{\chi}_1^0) = 494$ GeV and $c\tau(\tilde{\chi}_1^0)$ = 175 mm. The other SUSY model point in the figure is tabulated in http://hepdata.cedar.ac.uk/view/ins1362183/d1.
A search for physics beyond the standard model in events with at least three leptons is presented. The data sample, corresponding to an integrated luminosity of 19.5 inverse femtobarns of proton-proton collisions with center-of-mass energy sqrt(s) = 8 TeV, was collected by the CMS experiment at the LHC during 2012. The data are divided into exclusive categories based on the number of leptons and their flavor, the presence or absence of an opposite-sign, same-flavor lepton pair (OSSF), the invariant mass of the OSSF pair, the presence or absence of a tagged bottom-quark jet, the number of identified hadronically decaying tau leptons, and the magnitude of the missing transverse energy and of the scalar sum of jet transverse momenta. The numbers of observed events are found to be consistent with the expected numbers from standard model processes, and limits are placed on new-physics scenarios that yield multilepton final states. In particular, scenarios that predict Higgs boson production in the context of supersymmetric decay chains are examined. We also place a 95% confidence level upper limit of 1.3% on the branching fraction for the decay of a top quark to a charm quark and a Higgs boson (t to c H), which translates to a bound on the left- and right-handed top-charm flavor-violating Higgs Yukawa couplings, lambda[H, tc] and lambda[H, ct], respectively, of sqrt(abs(lambda[H, tc])^2 + abs(lambda[H, ct])^2) < 0.21.
Observed and expected numbers of events with four or more leptons with the scalar sum of jet transverse momentum values HT > 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.
Observed and expected numbers of events with four or more leptons with the scalar sum of jet transverse momentum values HT < 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.
Observed and expected numbers of events with exactly three leptons with the scalar sum of jet transverse momentum values HT > 200 GeV. "On-Z" refers to events with at least one E+ E- or MU+ MU- (OSSF) pair with dilepton mass between 75 and 105 GeV, while "Off-Z" refers to events with one or two OSSF pairs, none of which fall in this mass range. The OSSFN designation refers to the number of E+ E- and MU+ MU- pairs in the event. Search channels binned in ET have been combined into coarse ET bins for the purposes of presentation.