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

Search for the Higgs boson in the WH production mode with H→WW* decay using the ATLAS detector

© 2018 Dr. Petar RadosIn order for fundamental particles to attain mass, the electroweak symmetry of the Standard Model (SM) of particle physics must be broken. The simplest way of breaking this symmetry is via the SM Higgs Mechanism, and it predicts the existence of a new particle called the SM Higgs boson. This particle should be experimentally accessible at the high-energy frontier, and so its discovery is considered as one of the most important goals in modern particle physics. This goal was partially achieved in July 2012, when the ATLAS and CMS experiments at the Large Hadron Collider (LHC) announced the discovery of a Higgs-like particle with a mass of around 125 GeV. The next step for both experiments is to scrutinize the properties of this new particle in order to determine whether it is, in fact, the SM Higgs boson. One high-priority objective of the ATLAS experiment is to confirm the SM predictions for Higgs boson production at the LHC. The main result presented in this thesis contributes to this objective by searching for the WH production mode with subsequent H→WW* decay. The H→WW* decay channel provides a sensitive probe of WH production due to its large branching ratio and clean detector signature. Moreover, this signal process provides important information on the Higgs boson couplings, since it only involves couplings to W bosons at both the production and decay vertices at tree-level. The search for this signal process was conducted using LHC proton-proton collision data collected by the ATLAS detector. This data was recorded at centre-of-mass energies of 7, 8 and 13 TeV corresponding to integrated luminosities of 4.5, 20.3 and 5.8 fb−1, respectively. The analysis strategy was first developed using the 7 and 8 TeV data samples and it was used to measure the relative signal strength with respect to the SM expectation. For a Higgs boson of mass 125 GeV, the observed value of the signal strength was determined to be 0.72 +1.2−1.1 (stat.) +0.4−0.3 (sys.). The analysis method was then extended to perform the first measurement of the signal strength at 13 TeV, with this quantity measured to be 3.2 +3.7−3.2 (stat) +2.3−2.7 (sys). An overall excess was observed at 0.66σ (0.77σ) significance with respect to the background-only hypothesis in the 7+8 TeV (13 TeV) data. All measurements are in agreement with the SM expectations

Trigger efficiency measurement using e^+e^-→^+^- events at Belle II

The Belle II detector was completed with the installation of a silicon vertex detector that covers most of the solid angle around the interaction region. In 2019 Physics Run before summer shutdown, 5.15 /fb of data were collected at a center of mass energy corresponding to the mass of the Y(4S). We present the measurement of the L1 and high-level trigger efficiencies at Belle IIusing the τ-pair events. The 1x3, 3x3, 1x5 and 1x1 τ-pair decay modes are considered. These measurements were performed using the e+e collision data recorded by Belle II during the early Phase III of data taking