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

ATLAS Searches for VH/HH Resonances

The discovery of a Higgs boson at the Large Hadron Collider (LHC) motivates searches for physics beyond the Standard Model (SM) in channels involving coupling to the Higgs boson. A search for a massive resonance decaying into a standard model Higgs boson (h) and a W or Z boson or two a standard model Higgs bosons is performed. The results of a search for non-resonant Higgs boson pair production in a combination of six decay modes with 36fb-1 is given, along with two new and ground-breaking results with 140fb-1

A Search for Resonant and Non-Resonant di-Higgs Production in the γγbbˉ\gamma \gamma b \bar{b} Channel Using the ATLAS Detector

This dissertation presents a search for resonant and non-resonant di-Higgs production in the $\gamma \gamma b \bar{b}$ final state using data from the ATLAS detector at the Large Hadron Collider (LHC). The search is performed on 36.1 fb$^{-1}$ of data from proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV collected in 2015 and 2016. No significant excesses are observed in this search. The non-resonant analysis sets limits on the $HH\rightarrow\gamma\gamma b\bar{b}$ cross-section times branching ratio, with an upper observed (expected) limit of 0.73 (0.93) pb. The observed (expected) limits on the Higgs boson trilinear coupling at 95% CL are set at $-8.2 < \kappa_{\lambda} < 13.3$ ($-8.5 < \kappa_{\lambda} < 13.7$). A model-independent resonant search is also presented, setting limits on a generic scalar resonance under the narrow-width approximation. These limits cover mass hypotheses ranging from 260 GeV to 1000 GeV, and the observed (expected) limits set are 0.85 (0.92) pb at the lowest mass hypothesis to 0.13 (0.15) pb at the highest mass hypothesis. Work toward the future of this analysis is presented. Improvements in photon identification are studied, investigating optimization through two approaches. First, through adding the moments of topological clusters as inputs, which show additional discriminating power. Second, through using a multivariate approach to define photon identification, studying a Boosted Decision Tree (BDT) and a Neural Network (NN). Through these additional inputs and the employment of a BDT, an improvement of as much as 27% background rejection for the same signal efficiency as the current tight working point is shown. Additionally, improvements to the analysis through studying the Vector Boson Fusion (VBF) production mode are shown. This provides handles on new couplings, and a dedicated signal region targeting this mode can improve overall Asimov significance. To define this signal region, a multiclass BDT is used, with classes to model the VBF HH production mode, along with gluon-gluon fusion HH production, as well as the dominant $\gamma\gamma$-continuum background, and $ttH$ mono-Higgs background. By adding this signal region, a 9.7% improvement in Asimov significance is achieved using the full 140 fb$^{-1}$ of Run 2 data

Higgs boson potential at colliders: status and perspectives

This document summarises the current theoretical and experimental status of the di-Higgs boson production searches, and of the direct and indirect constraints on the Higgs boson self-coupling, with the wish to serve as a useful guide for the next years. The document discusses the theoretical status, including state-of-the-art predictions for di-Higgs cross sections, developments on the effective field theory approach, and studies on specific new physics scenarios that can show up in the di-Higgs final state. The status of di-Higgs searches and the direct and indirect constraints on the Higgs self-coupling at the LHC are presented, with an overview of the relevant experimental techniques, and covering all the variety of relevant signatures. Finally, the capabilities of future colliders in determining the Higgs self-coupling are addressed, comparing the projected precision that can be obtained in such facilities. The work has started as the proceedings of the Di-Higgs workshop at Colliders, held at Fermilab from the 4th to the 9th of September 2018, but it went beyond the topics discussed at that workshop and included further developments. Part III of the document reviews the capabilities of future colliders to establish the the size of Higgs self-coupling both qualitatively and quantitatively