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 energy resolution and determination of the constant term of the ATLAS ECAL using electrons from \$Z \rightarrow ee\$ events

In this project, the energy resolution and the constant term of the electromagnetic calorimeter of the ATLAS experiment have been determined. In order to do this, the peak if the Z boson in the dielectron invariant mass spectrum has been analysed for both data and Monte-Carlo samples. It has been examined how different fitting methods affect the energy resolution and constant term. By this, information about the systematic uncertainty on these quantities can be obtained. Also the constant terms for 2011 samples and for 2012 samples have been compared. Moreover, it has been analyzed how a cut on the variable fbrem, which quantifies energy losses by bremsstrahlung, affects the energy resolution and the constant term

Selection of Vector Boson Fusion Events in the H→γγH→γγ Decay Channel Using an Inclusive Event Shape

Present-day measurements of the signal strength of the Higgs-boson production modevector boson fusion tend to suffer from large theoretical uncertainties on the gluon fusioncontamination in the vector boson fusion selection. The measurements are still limited by thestatistical uncertainties, but as more data will be recorded, the statistical uncertainties will bereduced. Therefore it is of importance to think of alternative vector boson fusion selectionswith a better control over the theoretical uncertainty. Vector boson fusion selections use acentral jet veto. Such a veto can be implemented via the inclusive event shape 2-jettiness asan alternative to the presently used central-jet veto.In this thesis, the properties of this event shape inH!ggevents and its use in a vectorboson fusion selection are examined. The theoretical uncertainties accompanying thisevent-shape based selection are studied. Finally, the vector boson fusion signal strength isdetermined using8 TeVATLAS data with a vector boson fusion selection which relies oncuts on the 2-jettiness

Measurement and Interpretation of Higgs Boson Differential Cross Sections in the Diphoton Decay Channel and Measurement of the Photon Identification Efficiency in the ATLAS Experiment

he properties of the scalar boson that was first found in 2012 by the ATLAS and CMS experiments at the LHC are so far consistent with those of the Higgs boson, predicted by the Standard Model of particle physics. Further investigations of the boson's properties with increased accuracy are of immediate interest, as any deviation from Standard Model predictions could lead the way to a more fundamental theory of nature. The comparison of predicted and measured cross sections, both inclusive and differential, allows important tests of the Standard Model. One of the decay channels of the Higgs boson that is particularly well suited for measurements of Higgs boson production cross sections is the decay into a pair of photons, $H\rightarrow\gamma\gamma$.Based on proton-proton collision data collected with the ATLAS experiment in the data-taking periods in 2015 -- 2017 at a center-of-mass energy of $\sqrt{s}=13$\,TeV, corresponding to an integrated luminosity of 79.8/fb, inclusive and differential Higgs boson production cross sections are measured in the $H\rightarrow\gamma\gamma$ decay channel in a fiducial phase-space volume. The observed fiducial inclusive cross section of Higgs production with the Higgs decaying to a pair of photons corresponds to $(60.4 \pm 8.6)$\,fb, which is in good agreement with the Standard Model prediction of $(63.5 \pm 3.3)$\,fb.The differential cross section in the Higgs boson transverse momentum, $p_\mathrm{T}^H$, is sensitive to the Yukawa couplings between the Higgs boson and quarks; therefore, limits for these couplings can be set by performing a fit on the observed $p_\mathrm{T}^H$ distribution. In this work, limits are set on the Yukawa couplings between both bottom and charm quarks and the Higgs boson, using ATLAS $pp$ collision data collected in the years 2015 -- 2018, corresponding to an integrated luminosity of 139.0/fb. The charm quark Yukawa coupling modifier, $\kappa_c = y_c/y_c^{SM}$, has been constrained to $\kappa_c \in [-19,\,25]$ at $95\,$~CL, while for the bottom quark Yukawa coupling modifier, $\kappa_b = y_b/y_b^{SM}$, the corresponding confidence interval was determined to be $\kappa_b \in [-6,\,16]$.The measurement of photon identification efficiencies and a comparison with efficiencies in simulation are an important input for photon-based analyses of ATLAS data such as the above mentioned measurement of $H\rightarrow\gamma\gamma$ cross sections. Using the data recorded in the years 2015 -- 2017, corresponding to an integrated luminosity of 79.8/fb, photon identification efficiencies are measured using a method that relies on applying transformations to variables parametrizing the shape of the electromagnetic showers of electrons and photons. A pure and unbiased sample of electrons to which these transformations are applied is selected with a tag-and-probe method. The transformed electrons are photon-like objects and as such can be used to measure photon identification efficiencies. Depending on the considered region of pseudorapidity and photon transverse momentum, the uncertainties on these efficiencies range from $0.5\,\%$ to $3\,\%$

Measurement of H→γγH\rightarrow \gamma\gamma Differential Cross Sections and Charm Yukawa Coupling Interpretation with ATLAS

The measurement of differential cross sections of the Higgs boson that has been found in 2012 is an important way of testing the hypothesis of it being the Standard Model Higgs boson. In this presentation, results for such a measurement are shown based on the diphoton decay channel, using data collected at a center-of-mass energy of $\sqrt{s}\,=\,13\,\mathrm{TeV}$. The measured differential cross sections are model independent and can be interpreted in various frameworks. Examples for such interpretations will also be presented

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

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

Operation and performance of the ATLAS semiconductor tracker in LHC Run 2

Abstract The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015–2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb-1 to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achieved a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules.</jats:p