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

Studies of a neutral Higgs boson produced in gluon-gluon fusion and vector boson fusion

This paper presents an outline of the generation of mass for the massive Standard Model particles (fermions, $W^\pm$, $Z^0$) through electroweak symmetry breaking via the Higgs mechanism, and how the Higgs boson emerges from this framework. A Monte Carlo study was done on the decay $H\rightarrow\tau\tau$, with one leptonically and one hadronically decaying tau, with two different production channels for the $H$, gluon-gluon fusion (gg) and vector boson fusion (VBF), at \sqrt s = 7\tev with a Higgs mass m_H = 120\gev. The kinematics of these two production channels were compared and it was found that the transverse momentum of muons produced in VBF were higher on average than those produced in gg. This differance was greater in muons originating from the leptonically decaying tau in the Higgs decay, than those produced by other processes in the underlying event. In the latter, however, the difference was still noticable. Jets were slightly more abundant in VBF than in gg, and were in VBF more distributed along the beam axis. The separation in pseudorapidity between the two jets with highest transverse momentum was found to be greater in VBF than in gg. An attempt to reconstruct the Higgs mass using Monte Carlo data run through a simulation of the ATLAS detector was done. The estimator used was the transverse mass of the system consisting of the visible part of the hadronically decaying tau, the lepton from the leptonically decaying tau and the total missing transvese energy. In gg the mean of the transvese mass distribution was found to be 89.26\gev with a root mean square uncertainty (RMS) of 23.86\gev. In VBF the mean was found to be 85.57\gev with RMS 27.08\gev

Exploration of extended Higgs sectors, development of a displaced track trigger, and improvements in GRID middleware

Two possible solutions to the Standard Model (SM) fine-tuning problem are presented in this thesis, MSSM and Compositeness. The Higgs sector of the MSSM can be tested by searching for charged Higgs bosons. Two direct searches for charged Higgs bosons decaying into a top- and bottom-quark are performed using 13.2 and 36.1 fb–¹ respectively of pp collision data collected at √s = 13 TeV with the ATLAS detector. No significant excess over the SM background is observed and upper limits on σ(pp→tbH+)×B(H+→tb) between 0.07 and 2.9 pb are set at a 95% confidence level in the charged Higgs boson mass range 200–2000 GeV using the CLs method. The phenomenology of a vector-like top-partner decaying into a BSM (pseudo-) scalar is explored and interpreted in 2HDM and Compositeness models. The experimental reach is evaluated using γγ and Zγ final states, and the LHC is determined to be sensitive to top-partner and (pseudo-) scalar masses in the 1–2 TeV range both in the Run 2 and expected Run 3 datasets. Two hardware based hit collection methods targeting long-lived particles are studied for a regional track trigger in anticipation of the HL-LHC upgrade. The efficiency is estimated to be ~20% for a wide range of track parameters for tracks originating from a displaced vertex. The prospect of using GPU-computing on the LHC GRID is considered. Extensions of the ARC middleware Information Providers integrating local GPU information are demonstrated

Comparison of dilepton events in simulation and pppp collision data at \sqrt s = 8\tev gathered by the ATLAS detector at the LHC

This thesis presents the results of a comparison between collision data and simulations based on Monte Carlo methods. The experimental dataset consists of $20.3\,\mathrm{fb}^{-1}$ of proton-proton collision data at \sqrt s = 8\tev collected during 2012 by the ATLAS experiment located at the Large Hadron Collider. The final state used is $e\mu + \mathrm{jets}$. Four regions are defined, pretag ($\geq0$ jets, $\geq0$ $b$-jets), $\geq1$-tag ($\geq1$ jets, $\geq1$ $b$-jets), $\geq2$-jet ($\geq2$ jets, $\geq0$ $b$-jets), and 2-tag ($\geq2$ jets, 2 $b$-jets). Data and simulations are consistent in all regions considered

Comparison of dilepton events in simulation and pppp collision data at \sqrt s = 8\tev gathered by the ATLAS detector at the LHC

This thesis presents the results of a comparison between collision data and simulations based on Monte Carlo methods. The experimental dataset consists of $20.3\,\mathrm{fb}^{-1}$ of proton-proton collision data at \sqrt s = 8\tev collected during 2012 by the ATLAS experiment located at the Large Hadron Collider. The final state used is $e\mu + \mathrm{jets}$. Four regions are defined, pretag ($\geq0$ jets, $\geq0$ $b$-jets), $\geq1$-tag ($\geq1$ jets, $\geq1$ $b$-jets), $\geq2$-jet ($\geq2$ jets, $\geq0$ $b$-jets), and 2-tag ($\geq2$ jets, 2 $b$-jets). Data and simulations are consistent in all regions considered

Studies of a neutral Higgs boson produced in gluon-gluon fusion and vector boson fusion

This paper presents an outline of the generation of mass for the massive Standard Model particles (fermions, $W^\pm$, $Z^0$) through electroweak symmetry breaking via the Higgs mechanism, and how the Higgs boson emerges from this framework. A Monte Carlo study was done on the decay $H\rightarrow\tau\tau$, with one leptonically and one hadronically decaying tau, with two different production channels for the $H$, gluon-gluon fusion (gg) and vector boson fusion (VBF), at \sqrt s = 7\tev with a Higgs mass m_H = 120\gev. The kinematics of these two production channels were compared and it was found that the transverse momentum of muons produced in VBF were higher on average than those produced in gg. This differance was greater in muons originating from the leptonically decaying tau in the Higgs decay, than those produced by other processes in the underlying event. In the latter, however, the difference was still noticable. Jets were slightly more abundant in VBF than in gg, and were in VBF more distributed along the beam axis. The separation in pseudorapidity between the two jets with highest transverse momentum was found to be greater in VBF than in gg. An attempt to reconstruct the Higgs mass using Monte Carlo data run through a simulation of the ATLAS detector was done. The estimator used was the transverse mass of the system consisting of the visible part of the hadronically decaying tau, the lepton from the leptonically decaying tau and the total missing transvese energy. In gg the mean of the transvese mass distribution was found to be 89.26\gev with a root mean square uncertainty (RMS) of 23.86\gev. In VBF the mean was found to be 85.57\gev with RMS 27.08\gev

Exploration of extended Higgs sectors, development of a displaced track trigger, and improvements in GRID middleware

Two possible solutions to the Standard Model (SM) fine-tuning problem are presented in this thesis, MSSM and Compositeness. The Higgs sector of the MSSM can be tested by searching for charged Higgs bosons. Two direct searches for charged Higgs bosons decaying into a top- and bottom-quark are performed using 13.2 and 36.1 fb–¹ respectively of pp collision data collected at √s = 13 TeV with the ATLAS detector. No significant excess over the SM background is observed and upper limits on σ(pp→tbH+)×B(H+→tb) between 0.07 and 2.9 pb are set at a 95% confidence level in the charged Higgs boson mass range 200–2000 GeV using the CLs method. The phenomenology of a vector-like top-partner decaying into a BSM (pseudo-) scalar is explored and interpreted in 2HDM and Compositeness models. The experimental reach is evaluated using γγ and Zγ final states, and the LHC is determined to be sensitive to top-partner and (pseudo-) scalar masses in the 1–2 TeV range both in the Run 2 and expected Run 3 datasets. Two hardware based hit collection methods targeting long-lived particles are studied for a regional track trigger in anticipation of the HL-LHC upgrade. The efficiency is estimated to be ~20% for a wide range of track parameters for tracks originating from a displaced vertex. The prospect of using GPU-computing on the LHC GRID is considered. Extensions of the ARC middleware Information Providers integrating local GPU information are demonstrated

Comparison of dilepton events in simulation and pppp collision data at \sqrt s = 8\tev gathered by the ATLAS detector at the LHC

This thesis presents the results of a comparison between collision data and simulations based on Monte Carlo methods. The experimental dataset consists of $20.3\,\mathrm{fb}^{-1}$ of proton-proton collision data at \sqrt s = 8\tev collected during 2012 by the ATLAS experiment located at the Large Hadron Collider. The final state used is $e\mu + \mathrm{jets}$. Four regions are defined, pretag ($\geq0$ jets, $\geq0$ $b$-jets), $\geq1$-tag ($\geq1$ jets, $\geq1$ $b$-jets), $\geq2$-jet ($\geq2$ jets, $\geq0$ $b$-jets), and 2-tag ($\geq2$ jets, 2 $b$-jets). Data and simulations are consistent in all regions considered

Studies of a neutral Higgs boson produced in gluon-gluon fusion and vector boson fusion

This paper presents an outline of the generation of mass for the massive Standard Model particles (fermions, $W^\pm$, $Z^0$) through electroweak symmetry breaking via the Higgs mechanism, and how the Higgs boson emerges from this framework. A Monte Carlo study was done on the decay $H\rightarrow\tau\tau$, with one leptonically and one hadronically decaying tau, with two different production channels for the $H$, gluon-gluon fusion (gg) and vector boson fusion (VBF), at \sqrt s = 7\tev with a Higgs mass m_H = 120\gev. The kinematics of these two production channels were compared and it was found that the transverse momentum of muons produced in VBF were higher on average than those produced in gg. This differance was greater in muons originating from the leptonically decaying tau in the Higgs decay, than those produced by other processes in the underlying event. In the latter, however, the difference was still noticable. Jets were slightly more abundant in VBF than in gg, and were in VBF more distributed along the beam axis. The separation in pseudorapidity between the two jets with highest transverse momentum was found to be greater in VBF than in gg. An attempt to reconstruct the Higgs mass using Monte Carlo data run through a simulation of the ATLAS detector was done. The estimator used was the transverse mass of the system consisting of the visible part of the hadronically decaying tau, the lepton from the leptonically decaying tau and the total missing transvese energy. In gg the mean of the transvese mass distribution was found to be 89.26\gev with a root mean square uncertainty (RMS) of 23.86\gev. In VBF the mean was found to be 85.57\gev with RMS 27.08\gev