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

Searching for Supersymmetry in Boosted Fully-Hadronic Final States with B-Jets and Calibrating B-Jet Identification Using Spatially Matched Muons at the ATLAS Detector

This thesis presents two analyses that involve the identification of hadronic jets produced by the fragmentation of $b$-quarks or ``$b$-jets''. The identification exploits the hard fragmentation of $b$-quarks and relatively long lifetime of $B$ hadrons. The first analysis is the calibration of the efficiency of the $MV2c10$ $b$-jet identification algorithm using a sample of $b$-jets with muons from $B$ hadron decays and a kinematic property of these jets knows as ``$p_{\text{T}}^{\text{rel}}$'', which is constructed from the muons. This analysis was performed using 68 $fb^{-1}$ of data collected at $\sqrt{s} = 13$ $Te\kern -0.1em V$ with the ATLAS detector during Run 2 of the Large Hadron Collider. Improvements in the methods of this calibration have led to a reduction by more than an order of magnitude in the uncertainty of earlier measurements of $b$-jet tagging efficiency using this approach. This work included substantial contributions to the software and computing framework used to the study the identification of jets produced by heavy flavor decay; these contributions are described herein. The second analysis is a search for electroweakly-produced supersymmetric partners of the gauge bosons or ``gauginos''. This search uses 140 $fb^{-1}$ of data collected at $\sqrt{s} = 13$ $Te\kern -0.1em V$ with the ATLAS detector during Run 2 of the Large Hadron Collider. My work focused on developing a new signal region that targeted final states specifically containing two $b$-jets and two light-quark jets. As part of this optimization, I examined different supersymmetric scenarios and explored alternative techniques for estimating the Standard Model backgrounds. Based on preliminary results, in the wino-bino scenario, this search is expected to provide sensivity to charginos with masses up to $\sim1$ $Te\kern -0.1em V$. For a scenario in the general-gauge-mediation model (where higgsino becomes the next-lightest SUSY particle), this search will provide sensitivity for higgsinos with masses up to 600 $Ge\kern -0.1em V$ (discovery) or 800 $Ge\kern -0.1em V$ (exclusion) for most of the possible $Z/h$ branching ratios. Finally, the sensitivity to the scenarios where higgsino is next-lightest and bino is the lightest SUSY particle has been studied, on which no explicit exclusion has been reported yet by the searches in LHC

Searching for Supersymmetry in Boosted Fully-Hadronic Final States with B-Jets and Calibrating B-Jet Identification Using Spatially Matched Muons at the Atlas Detector

This thesis presents two analyses that involve the identification of hadronic jets produced by the fragmentation of b-quarks or b-jets\u27\u27. The identification exploits the hard fragmentation of b-quarks and relatively long lifetime of B hadrons. The first analysis is the calibration of the efficiency of the MV2c10 b-jet identification algorithm using a sample of b-jets with muons from B hadron decays and a kinematic property of these jets knows as pTrel\u27\u27, which is constructed from the muons. This analysis was performed using 68 fb-1 of data collected at √s = 13$ TeV with the ATLAS detector during Run 2 of the Large Hadron Collider. Improvements in the methods of this calibration have led to a reduction by more than an order of magnitude in the uncertainty of earlier measurements of b-jet tagging efficiency using this approach. This work included substantial contributions to the software and computing framework used to the study the identification of jets produced by heavy flavor decay; these contributions are described herein. The second analysis is a search for electroweakly-produced supersymmetric partners of the gauge bosons or gauginos\u27\u27. This search uses 140 \ifb\ of data collected at √s = 13 TeV with the ATLAS detector during Run 2 of the Large Hadron Collider. My work focused on developing a new signal region that targeted final states specifically containing two b-jets and two light-quark jets. As part of this optimization, I examined different supersymmetric scenarios and explored alternative techniques for estimating the Standard Model backgrounds. Based on preliminary results, in the wino-bino scenario, this search is expected to provide sensivity to charginos with masses up to ~1 TeV. For a scenario in the general-gauge-mediation model (where higgsino becomes the next-lightest SUSY particle), this search will provide sensitivity for higgsinos with masses up to 600 GeV (discovery) or 800 GeV (exclusion) for most of the possible Z/h branching ratios. Finally, the sensitivity to the scenarios where higgsino is next-lightest and bino is the lightest SUSY particle has been studied, on which no explicit exclusion has been reported yet by the searches in LHC

Searching for Supersymmetry in Boosted Fully-Hadronic Final States with B-Jets and Calibrating B-Jet Identification Using Spatially Matched Muons at the Atlas Detector

This thesis presents two analyses that involve the identification of hadronic jets produced by the fragmentation of b-quarks or b-jets\u27\u27. The identification exploits the hard fragmentation of b-quarks and relatively long lifetime of B hadrons. The first analysis is the calibration of the efficiency of the MV2c10 b-jet identification algorithm using a sample of b-jets with muons from B hadron decays and a kinematic property of these jets knows as pTrel\u27\u27, which is constructed from the muons. This analysis was performed using 68 fb-1 of data collected at √s = 13$ TeV with the ATLAS detector during Run 2 of the Large Hadron Collider. Improvements in the methods of this calibration have led to a reduction by more than an order of magnitude in the uncertainty of earlier measurements of b-jet tagging efficiency using this approach. This work included substantial contributions to the software and computing framework used to the study the identification of jets produced by heavy flavor decay; these contributions are described herein. The second analysis is a search for electroweakly-produced supersymmetric partners of the gauge bosons or gauginos\u27\u27. This search uses 140 \ifb\ of data collected at √s = 13 TeV with the ATLAS detector during Run 2 of the Large Hadron Collider. My work focused on developing a new signal region that targeted final states specifically containing two b-jets and two light-quark jets. As part of this optimization, I examined different supersymmetric scenarios and explored alternative techniques for estimating the Standard Model backgrounds. Based on preliminary results, in the wino-bino scenario, this search is expected to provide sensivity to charginos with masses up to ~1 TeV. For a scenario in the general-gauge-mediation model (where higgsino becomes the next-lightest SUSY particle), this search will provide sensitivity for higgsinos with masses up to 600 GeV (discovery) or 800 GeV (exclusion) for most of the possible Z/h branching ratios. Finally, the sensitivity to the scenarios where higgsino is next-lightest and bino is the lightest SUSY particle has been studied, on which no explicit exclusion has been reported yet by the searches in LHC

Searches for the ZγZ\gamma decay mode of the Higgs boson and for new high-mass resonances in pppp collisions at s=13\sqrt{s} = 13 TeV with the ATLAS detector

International audienceThis article presents searches for the Zγ decay of the Higgs boson and for narrow high-mass resonances decaying to Zγ, exploiting Z boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb$^{−1}$ of pp collisions at $\sqrt{s}=13$ recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected — assuming Standard Model pp → H → Zγ production and decay) upper limit on the production cross section times the branching ratio for pp → H → Zγ is 6.6. (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level

Measurements of ttˉt\bar{t} differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pppp collisions at s=13 \sqrt{s}=13\, TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and $t\bar{t}$ system kinematic observables using proton--proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a $b$-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated $\chi^2$ values. The cross-section for $t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \ \rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical prediction of $384 \pm 36$ fb