Search for charged Higgs Bosons decaying via H+→τlep.+ν{H^+ \to \tau_{lep.} + \nu} in ttˉ{t\bar{t}} events at s=7 TeV{\sqrt{s}=7\textrm{ TeV}} in ATLAS

This thesis for the degree of Master of Science by Research presents the theory, methodology and results of a search for charged Higgs bosons decaying via ${H^+ \to \tau_{lep.} + \nu}$ using single lepton and two lepton channels in ${t}$ quark pair (${t\bar{t}}$) events with a leptonically decaying ${\tau}$ in the final state based on ${1.03\textrm{ fb}^{-1}}$ of proton-proton collision data at centre of mass energy ${\sqrt{s}=7\textrm{ TeV}}$ from ATLAS, an experiment of the LHC, with special attention given to the statistical analysis and calculation involved in the limit setting process. For the single lepton channel, the expected number of Standard Model-like\linebreak${t\bar{t} \to b\bar{b}W^{+}W^{-}}$ background events lies between $0.99$ and $1.03$ times the Standard Model prediction, with uncertainties in the range ${2\%}$ -- ${3\%}$. For the two lepton channel, the expected number of Standard Model-like background events lies between $0.99$ and $1.03$ times the Standard Model prediction, with uncertainties in the range ${5\%}$ -- ${25\%}$. Assuming the branching fraction ${\mathcal{B}\left(H^{+} \to \tau\mu\right)=1}$, the upper limits on the branching fraction ${\mathcal{B}\left(t \to bH^{+}\right)}$ at the $95\%$ confidence level are between ${5.2\%}$ and ${14.1\%}$ for charged Higgs boson masses in the range ${90\textrm{ GeV}\leq m_{H^{+}}\leq 160\textrm{ GeV}}$. In the context of the ${m^{\textrm{max.}}_{h}}$ scenario of the MSSM, values of ${\tan\beta}$ greater than ${30}$--${56}$ are excluded in the mass range ${90\textrm{ GeV}\leq m_{H^{+}}\leq 140\textrm{ GeV}}$. The compatibility with background of the combination of the single lepton and two lepton channels ranges between ${26\%}$ and ${50\%}$, as measured by ${p_{0}}$ values. Hence, no indication of a ${H^{+}}$-like excess is found

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

Measurement of the inclusive isolated-photon cross section in pp collisions at √s = 13 TeV using 36 fb−1 of ATLAS data

The differential cross section for isolated-photon production in pp collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 36.1 fb. The differential cross section is presented as a function of the photon transverse energy in different regions of photon pseudorapidity. The differential cross section as a function of the absolute value of the photon pseudorapidity is also presented in different regions of photon transverse energy. Next-to-leading-order QCD calculations from Jetphox and Sherpa as well as next-to-next-to-leading-order QCD calculations from Nnlojet are compared with the measurement, using several parameterisations of the proton parton distribution functions. The predictions provide a good description of the data within the experimental and theoretical uncertainties. [Figure not available: see fulltext.