The beauty and charm of Higgs: Search for Higgs Boson decays to charm quarks with the ATLAS experiment and search for new physics with combined measurements

Since the discovery of Higgs boson in 2012, its properties are measured with increasing details and the results are in good agreement with the prediction of Standard Model (SM) of particle physics so far. However, the SM is believed not to be the ultimate theory due to its deficiencies, such as difficulty in explaining Higgs mass hierarchy problem and missing candidate for dark matter. Numerous models have been raised to solve these issues and several of them predicts additional Higgs bosons and a different behavior of SM Higgs, including Yukawa coupling and Higgs self-coupling strength. Thus, the measurement of Higgs properties is crucial to test the SM and search for new physics. This thesis presents two works concerning the Higgs properties and new physics searches: The simultaneous measurements of Yukawa coupling of charm and bottom quarks and the searches for physics beyond Standard Model (BSM) by combining various measurements of different processes.

The Higgs couplings with third generation fermions and with vector bosons are well established. However, it’s coupling with the second generation of quarks is still not discovered yet. The search for Higgs decay to charm quarks is the next most promising decay mode to probe given its relative sizable branching ratio. It is also a good portal to search for new physics as any small deviation of Higgs-charm coupling will lead to a significant change of the Higgs phenomenology. In the first part of the thesis, the search for $H\to \overline{c}c$ decay in the vector boson associated production mode is presented. It also simultaneously measures $H\to \overline{c}c$ decay at the same time due to their similar topology and common backgrounds. The analysis use the V proton-proton collision dataset with an integrated luminosity of $140 fb^{−1}$ at 13 TeV collected by ATLAS during LHC Run-2. The searches for $H\to \overline{c}c$ yields in an observed (expected) upper limit of 11 (10) times the Standard Model prediction on the signal strength, which is the best observed results up to now. The results are also used to put constraints on a charm Yukawa coupling modifier $\left |k_{c} \right | < 4.2$ at the 95% confidence level. The simultaneous measurements provide constraint on the ratio of Higgs-charm coupling and Higgs-bottom coupling ($𝜅_{𝑐}/𝜅_{𝑏}$) to be less than 3.5 at the 95% confidence level. The analysis method is validated with the measurement of the 𝑉 𝑍 process where 𝑉 is a 𝑊 or 𝑍 boson. For the first time in ATLAS, the $VZ(\to c\overline{c})$ process is observed with a significance larger than 5 standard deviation. This study is crucial for the understanding of the Yukawa coupling structure and the hierarchy problem of fermion mass.