Subrata Samanta

Research

Perturbative Unitarity and Global Fits.— The scalar sector beyond the Standard Model (SM) contains several quartic couplings. Unlike the Yukawa and gauge couplings, these scalar quartic couplings can be large and may violate perturbativity and unitarity at high scales. For a weakly interacting extended scalar sector, perturbative unitarity delineates the region of parameter space where perturbative calculations can be trusted and therefore provides an important constraint in phenomenological studies. At the tree level, perturbative unitarity in principle allows large quartic couplings. However, such large couplings break perturbativity beyond tree level, as the higher-order corrections become comparable to the tree level contributions. We computed the one-loop corrections to all 2 → 2 bosonic scattering amplitudes in the SU(2)_L triplet scalar extensions of the SM and derived next-to-leading-order (NLO) unitarity bounds on the scalar quartic couplings. Since the scalar masses depend non-linearly on the quartic couplings, these unitarity limits cannot be translated directly into limits on the masses of the additional scalars. We therefore performed a numerical analysis with the open-source code HEPfit, in which we implemented the triplet-extended scalar sector, to determine the allowed mass ranges of the exotic scalars arising from the SU(2)_L triplets. These mass bounds have direct implications for collider studies and serve as the dominant constraints in certain regions of the parameter space. Incorporating these theoretical bounds, we performed global fits to the 125 GeV Higgs signal strengths from LHC Run 1 and Run 2, recent measurements of the 95 GeV excesses, B-physics observables, and the latest direct search limits on additional heavy and light scalars at the LHC and LEP. This combined global analysis yields the most stringent constraints on the parameter space of the triplet-extended scalar sector. In the presence of the doubly charged scalar, the di-photon rate is enhanced. As a result, the triplet-extended scalar sector can accommodate the ATLAS and CMS di-photon and LEP bb̄ excesses around 95 GeV. However, the CMS di-tau excess near 95 GeV is accommodated only at the ∼2σ level. References: arXiv:2404.18996 arXiv:2506.06427

Electroweak Precision Observables.— Electroweak quantum corrections to the relation M_W = M_Z cosθ_W (which translates into ρ = 1) are precisely calculable in the SM; hence, the precise measurement of the ρ parameter puts a constraint on the extended scalar sectors. Among the electroweak precision observables, the ρ parameter provides one of the most stringent constraints on scalar extensions containing SU(2)_L triplets. The Georgi-Machacek (GM) model, which extends the SM scalar sector by a complex and a real triplet under SU(2)_L × U(1)_Y, preserves ρ = 1 at tree level due to the global SU(2)_L × SU(2)_R symmetry in the scalar sector. However, this global symmetry is broken by the hypercharge interactions. At one loop, the symmetry-violating operators that are absent at tree level are generated radiatively. We studied the one-loop renormalization of the model and found that an additional input parameter, beyond the three SM inputs α_em(m_Z), G_μ, and m_Z, is required to completely parametrize the electroweak precision observables at one loop. We further found that the one-loop correction to the ρ parameter depends only logarithmically on the masses of the new scalars, while it is quadratically sensitive to the triplet vacuum expectation value. References: arXiv:2601.06252

Vacuum Stability.— Extended scalar sector of the SM can alleviate the instability of the electroweak vacuum up to the Planck scale. However, the question remains how can one ensure the vacuum stability of the scalar potential in the extended scalar sector? To ensure the stability of the vacuum, one of the necessary criteria is the potential to be bounded from below (BFB), where the scalar quartic couplings play the most crucial role. The necessary and sufficient BFB conditions for a generic quartic scalar potential of a large number of fields cannot always be recast into a fully analytical compact form. We compute the necessary BFB conditions in the field subspaces on the scalar quartic couplings containing triplet fields and find that numerically these BFB conditions are a very good approximation of the full set of necessary and sufficient BFB conditions while being computationally more efficient. Additionally, due to the presence of the additional scalar multiplets, the scalar potential can have several types of minima at both tree and loop level, including multiple electroweak (EW) breaking minima, as well as charge-breaking (CB) and CP-breaking minima. If our EW vacuum is not the global minimum of the potential, it is metastable and can decay via quantum tunnelling into a deeper minimum. This raises the question of the stability of our EW vacuum in extended scalar sectors. We studied this question in the Type-II two-Higgs-doublet model, one of the simplest and most popular extensions of the SM scalar sector, and analysed the stability of the EW vacuum in a gauge-independent manner at tree level and beyond. We found that there is no reason to panic, as a Bayesian global fit incorporating the NLO unitarity constraints and the latest experimental measurements excludes the regions of parameter space with a metastable EW vacuum at both tree and one-loop level. References: arXiv:2404.18996 arXiv:2607.18380

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