Proceeding for LHCP2014 Poster: Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of $20.1 \rm{fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm} \rightarrow b W^{\left(\ast\right)} \tilde{\chi}_{1}^{0}$, where $\tilde{\chi}_{1}^{0}$ ($\tilde{\chi}_{1}^{\pm}$) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 30 GeV. For a branching fraction of 50% to either $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm}$, and assuming the $\tilde{\chi}_{1}^{\pm}$ mass to be twice the $\tilde{\chi}_{1}^{0}$ mass, top squark masses in the range 250-550 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 60 GeV.Comment: 5 pages, 6 figues. Proceeding for the poster present at LHCP 201

Revisiting Theories with Enhanced Higgs Couplings to Weak Gauge Bosons

Based on recent LHC Higgs analyses and in anticipation of future results we revisit theories where Higgs bosons can couple to weak gauge bosons with enhanced strength relative to the Standard Model value. Specifically, we look at the Georgi-Machacek model and its generalizations where higher "spin" representations of SU(2)_L break electroweak symmetry while maintaining custodial SU(2). In these theories, there is not only a Higgs-like boson but partner Higgs scalars transforming under representations of custodial SU(2), leading to a rich phenomenology. These theories serve as a consistent theoretical and experimental framework to explain enhanced couplings to gauge bosons, including fermiophobic Higgses. We focus on the phenomenology of a neutral scalar partner to the Higgs, which is determined once the Higgs couplings are specified. Depending on the parameter space, this partner could have i) enhanced fermion and gauge boson couplings and should be searched for at high mass (> 600 GeV), ii) have suppressed couplings and could be searched for at lower masses, where the Standard Model Higgs has already been ruled out, and iii) have fermiophilic couplings, where it can be searched for in heavy Higgs and top resonance searches. In the first two regions, the partner also has substantial decay rates into a pair of Higgs bosons. We touch briefly on the more model-dependent effects of the nontrivial SU(2)_C multiplets, which have exotic signals, such as a doubly-charged Higgs. We also discuss how the loop induced effects of these scalars tend to reduce the Higgs decay rate to photons, adding an additional uncertainty when extracting the couplings for the Higgs boson.Comment: 9 pages, 9 figures, revtex4; v2, references adde

Jets from Jets: Re-clustering as a tool for large radius jet reconstruction and grooming at the LHC

Jets with a large radius $R\gtrsim 1$ and grooming algorithms are widely used to fully capture the decay products of boosted heavy particles at the Large Hadron Collider (LHC). Unlike most discriminating variables used in such studies, the jet radius is usually not optimized for specific physics scenarios. This is because every jet configuration must be calibrated, insitu, to account for detector response and other experimental effects. One solution to enhance the availability of large-$R$ jet configurations used by the LHC experiments is {\it jet re-clustering}. Jet re-clustering introduces an intermediate scale $r<R$ at which jets are calibrated and used as the inputs to reconstruct large radius jets. In this paper we systematically study and propose new jet re-clustering configurations and show that re-clustered large radius jets have essentially the same jet mass performance as large radius groomed jets. Jet re-clustering has the benefit that no additional large-R calibration is necessary, allowing the re-clustered large radius parameter to be optimized in the context of specific precision measurements or searches for new physics

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements