Some phenomenological studies of the Higgs boson(s) in the Standard Model and beyond

Abstract

The fundamental particles and their interactions are described by the Standard Model (SM) of particle physics. Various experiments has been performed over the years and they indicate towards the consistency of the SM with the observed particles. Recently, the Higgs boson has been observed at the Large Hadron Collider (LHC). Its couplings to other SM particles are being measured with great precision. To understand the electroweak symmetry breaking mechanism, one needs to measure the Higgs boson self-coupling directly. A measurement of this coupling is beyond the reach of the current runs of LHC mainly because of the very small production rate. Several experimental observations and theoretical issues compel us to explore beyond the SM (BSM) signatures. This thesis investigates the Higgs boson self-coupling in the context of future runs of the LHC in chapters 2 and 3. We then examine the Higgs sector's various properties in well-motivated BSM models, namely, the minimal supersymmetric extension of the Standard Model (MSSM) and the Georgi-Machacek model, in chapter 4, 5 and 6.

First, we analyse the Higgs pair production at two different LHC centre of mass energies, the high luminosity run of the 14 TeV LHC, HL-LHC in chapter 2, and the 27 TeV high energy LHC (HE-LHC) in chapter 3. The Higgs pair production is a direct probe to measure Higgs boson self-coupling. We select various di-Higgs final states depending on their cleanliness and production rates, perform standard cut-based analysis and more sophisticated multivariate analysis. Our study suggests that the prospect of observing Higgs pair production at the HL-LHC is bleak, while the HE-LHC would be sensitive enough to observe the Higgs pair production. We also investigate the ramifications upon any modification in the Higgs self-coupling, potentially changing the double Higgs final states' kinematics.

We explore the MSSM Higgs sector in chapter 4 and 5. The Higgs sector of MSSM contains five physical states, namely, two CP even Higgs bosons (h, H), one CP odd Higgs boson (A) and two singly charged Higgs bosons. The lightest CP even Higgs, h, behaves as a SM-like Higgs boson in the alignment limit. The gauginos and higgsinos are the superpartners of weak gauge bosons and Higgs bosons, respectively. The gauginos and higgsinos can mix among themselves and give rise to the physical mass eigenstates, four neutralinos and two charginos. They are collectively called electroweakinos. We first study the SM decay modes of heavy CP even and CP odd Higgs bosons in chapter 4 and constrain the MSSM parameter space at the HL-LHC. We also examine the final states coming from MSSM Higgs decaying via electroweakinos in chapter 5. For this purpose, we consider backgrounds coming from SM processes and direct electroweakino production via SM mediators. The case of wino-like long-lived chargino decaying from MSSM Higgs is also discussed, improving the sensitivity in disappearing track searches at the LHC.

Till now, we have only discussed about the neutral Higgs bosons. We study the charged Higgs bosons in chapter 6. The coupling of charged Higgs to the gauge bosons appears at the one-loop level except for models extended by a real and complex scalar triplet. One such example is the Georgi-Machacek model. We study the gauge interaction of charged Higgs with W and Z boson. We choose two charged Higgs production modes where the interaction is present: vector boson fusion and associated production with gauge bosons. We explore the prospect of this interaction at the HE-LHC in the WZ and tb final states. Finally, we recast our results in the context of the Georgi-Machacek model.