Search for nonresonant Higgs boson pair production in final states with two bottom quarks and two photons with CMS at √s = 13 TeV

Abstract

This thesis describes a search for nonresonant production of Higgs boson pairs (HH) using proton-proton collision data collected at a center-of-mass energy of $\sqrt{s} = 13$ TeV and recorded by the CMS detector at the LHC. Events where one Higgs boson decays to a bottom quark-antiquark pair and the other decays to a pair of photons are explored to investigate the main production mechanisms, gluon-gluon fusion (ggF) and vector boson fusion (VBF). HH production gives a unique direct access to the Higgs boson trilinear self-coupling, and, in addition, is especially sensitive to the presence of physics beyond the standard model. A considerable effort has been devoted to the development of an algorithm for an accurate prediction of the b jet energy. A regression algorithm based on a deep neural network was designed to simultaneously provide b jet energy correction and per-jet resolution estimator. The algorithm is trained on a large sample of simulated b jets, and employs jet composition and shape information. It was validated on data recorded by the CMS detector, and shown to bring a significant improvement in b jet energy resolution. The algorithm is being used by the CMS Collaboration in several physics analyses targeting final states containing b jets, including the observation of the Higgs boson decay to bottom quarks, and searches for HH production. The search for HH production is performed using data corresponding to an integrated luminosity of 137 fb$^{-1}$. The analysis targets the main HH production modes: ggF and VBF. Both modes are analyzed following similar strategies. The nonresonant and single Higgs boson production backgrounds are reduced using different multivariate techniques. A dedicated categorization is developed and optimised to enhance the sensitivity of the search. The described analysis advances the previous search by a factor of four, benefiting equally from the larger collected data sets, and the innovative analysis techniques. No significant deviation from the background-only hypothesis is observed. An upper limit at 95% confidence level is set on the product of the Higgs boson pair production cross section and branching fraction. The observed (expected) upper limit is determined to be 7.7 (5.2) times the standard model prediction. This search has the highest sensitivity to Higgs boson pair production to date. Most stringent constraints to date are set on the Higgs boson self-coupling. The coupling between a pair of Higgs bosons and a pair of vector bosons is also constrained. Numerous hypotheses on coupling modifiers beyond the standard model were explored. The production of Higgs boson pairs was also combined with the top quark-antiquark pair associated production of a single Higgs boson.