Role of SU(2) Triplet Scalars in Electroweak Phenomenology

Ghosh, Rituparna (2025) Role of SU(2) Triplet Scalars in Electroweak Phenomenology. PhD thesis, Indian Institute of Science Education & Research Kolkata.

Text (PhD thesis of Rituparna Ghosh (20RS072)) 20RS072.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

The Standard Model (SM) of particle physics has proven to be the most successful theory in describing elementary particles and their interactions. However, it still faces significant challenges, from issues like the fine-tuning problem, the baryon asymmetry of the universe (BAU), the existence of neutrino mass, and dark matter, etc. Numerous theoretical models, such as supersymmetry and composite Higgs, have been proposed to address these issues. In this thesis, we adopt a bottom-up approach to explore beyond Standard Model (BSM) physics, with electroweak precision measurements serving as one of our key guiding principles. The ρ− parameter imposes a strong constraint on the non-doublet vacuum expectation values (VEV), suggesting that models with doublets are strongly favored by nature. However, are the scenarios with SU(2) doublet scalars, the only viable options? In this work, we focus on a triplet scenario, specifically the Georgi-Machacek(GM) model, which allows for a large triplet scalar VEV while still respecting the ρ-parameter. We will first derive the constraints on the parameter space of this scenario and then investigate its phenomenological implications. As the Large Hadron Collider (LHC) continues its search for BSM physics, the parameter spaces of proposed BSM theories are being increasingly constrained. In this thesis, we examine the constraints on the GM scenario using available data, which includes measurements of the 125-GeV scalar, indirect limits from rare heavy flavor decays, and theoretical considerations such as vacuum stability and unitarity. We analyze three distinct mass hierarchies in the scalar sector of the model, allowing the decay of H±± into W±H± and H±H± and thus suppresses the branching ratio of H±± into W±W±, thereby relaxing the constraints from searches for the doubly charged scalar decaying into like-sign W-boson pairs. Our findings show that the upper limit for the triplet contribution to the W and Z masses (denoted sH) is approximately 0.4 for a mass range of 200 GeV to 500 GeV, which is higher than the value indicated in previous studies for the similar mass range. Unitarity constraints further suppress sH for large masses of the doubly charged scalar (H₅±± ). Additionally, the H±W∓Z couplings can be enhanced when the doubly charged scalar has multiple decay channels. When the custodial SU(2) symmetry is broken, allowing unequal VEV for the real and complex triplets, a wider range of parameter points becomes accessible, and the observed features are more broadly visible. The scalar H₃± also couples to W∓Z bosons. Overall, despite the various constraints, the GM scenario still presents intriguing possibilities for phenomenological exploration. The diphoton decay channel of the custodial SU(2) singlet scalar H has been found to produce promising signals at the LHC. Key factors enhancing this signal are contributions from doubly and singly-charged scalar loops, suppression of destructive fermion loops, enhanced trilinear scalar couplings in certain regions, and suppression of fermion and gauge boson pair decays due to the dominant triplet composition of H. While the W±W± decay channel of H±± is often considered the best probe of the GM scenario, the diphoton final state becomes more useful when sH is small or when H±± has notable branching ratios into other channels. The diphoton signal, with its invariant mass peaking at mH, offers an independent search channel for the GM model, crucial for multichannel analyses. Major backgrounds include SM contributions to γγ, jγ, and jj production, which were considered in the simulation. The parameter space was scanned to ensure consistency with data from the 125 GeV scalar, extended scalar sector searches, and various theoretical and indirect constraints. Signal significance calculations show that the best regions for detection at the LHC correspond to mH in the ranges 160-180 GeV and 220-240 GeV. Regions with at least 3σ significance were identified for integrated luminosities of 3000 fb⁻¹ and 300 fb⁻¹. The analysis using a neural network expands the explorable region significantly. The results demonstrate that the diphoton final state is a valuable part of the GM scenario search and could potentially show a 5σ excess before the high luminosity LHC (HL-LHC) becomes operational. We further investigated the viability of this scenario in generating the observed BAU. With resonant leptogenesis being the mechanism to generate the lepton asymmetry, the leptogenesis scale could be lowered sufficiently so that the corresponding region of parameter space can be explored at LHC itself. Specifically we have studied the leptogenesis potential of a 500 GeV scalar. The strength of the trilinear coupling responsible for leptogenesis, along with the corresponding permissible values of the triplet VEV, has been determined through numerical analysis.

Actions (login required)

View Item