Sabyasachi Chakraborty
Assistant Professor
107-C, Old Sac Building,
Department of Physics,
Indian Institute of Technology, Kanpur.
Physics Beyond the Standard Model
Assuming the scale of new physics (NP) to be within the experimentally accessible range, our objective is to address these issues and leverage all available resources to detect any signatures of BSM physics across fundamental frontiers, namely the intensity, cosmic, energy, and theory frontiers. Below, I outline some of my recent work in this area.
Recent Works
1. Axions
Axions have the potential to address several key limitations of the Standard Model (SM). They offer solution to the strong-CP problem, serve as an excellent candidate for cold dark matter, and help explain the matter-antimatter asymmetry of the universe. In certain specific models, axions also address the hierarchy problem within the SM. Motivated by these possibilities, extensive searches for axions are being conducted across all fundamental frontiers. My current research focuses on the following aspects related to axions:
- Renormalization schemes in axion EFT: We formulated axion effective field theory within the BMHV scheme, analyzing the renormalization of fermionic dimension-five operators. Employing Ward identities for chiral currents, we determined the required finite renormalization factors and clarified the role of evanescent operators in maintaining consistency of the theory (arXiv: 2603.26880).
Effective field theory of axions and searches at the intensity frontier :
We investigated the B→Ka transition as a powerful probe of heavy QCD axions, conducting necessary two-loop calculations (2102.04474), identifying all relevant counterterms, anomalous dimension matrix and the cancellation of IR divergences is explicitly demonstrated (2412.09678). We also explored the phenomenological implications of this transition at flavor factories (2108.10331).Astrophysical implications of axions:
We highlighted that, in addition to prototypical interactions, axions can also possess Wess-Zumino-Witten (WZW) terms, which introduce a coupling between axions, photons, and vector mesons. These interactions can lead to the photo-production of axions in nucleon-rich stellar objects as well as in terrestrial experiments (2403.12169).Cosmological implications of axions:
We explored a chiral gauge theory in which light composite Dirac neutrinos dynamically emerge, complemented by a QCD axion to address the strong CP problem. We demonstrated that the interplay between the composite sector and the axion can simultaneously achieve successful baryogenesis and generate the correct dark matter abundance through a mechanism, known as "co-genesis" (2108.04293).
2. SCET
Soft-Collinear Effective Theory (SCET) is a powerful framework for analyzing high-energy processes with soft and collinear degrees of freedom. It systematically organizes perturbative and non-perturbative effects through effective operators and factorization theorems. SCET plays a central role in the theoretical description of exclusive and inclusive processes in particle physics and gravity:
- Axion SCET: We develop a soft-collinear effective theory (SCET) framework for heavy QCD axions, identifying the leading-power soft-overlap and spectator-scattering contributions to rare B-meson decays. We show that a derivative-gluon operator generated from aG̃G gives rise to a leading spectator contribution amounting to about 20-30% of the soft term. We further demonstrate how the low-energy realization of the axion interactions determines whether spectator scattering or the soft form-factor contribution dominates (2608.12467).
- Gravity SCET: We investigated two aspects of soft-collinear effective theory (SCET) for gravitational interactions. We extended the collinear Wilson lines required by the multiple diffeomorphism symmetries of gravity SCET to all orders in the expansion parameter λ. We also studied reparametrization invariance and showed that it relates higher-order hard interactions to lower-order terms, thereby reducing the number of structures that must be determined by matching (1910.10738).
- More to follow:
3. Model of Mesons
Mesons provide an important laboratory for probing strong interactions and searching for physics beyond the Standard Model. Owing to the interplay of perturbative and non-perturbative QCD effects, heavy–light meson systems such as B and D mesons are particularly well suited for effective field theory techniques. My research focuses on the dynamics of mesons within frameworks such as HQET, SCET, and chiral perturbation theory, with applications to flavor physics and precision studies at the intensity frontier.
- Framework for Heavy Light Pseudoscalar Mesons: We develop a symmetry-guided model of pseudoscalar mesons to study charged-current weak decays of heavy-light mesons. The framework combines chiral symmetry in the light sector with heavy-quark flavor symmetry, while CKM elements are incorporated as spurions. It organizes the leading dimension-six current-current operators relevant for leptonic, semileptonic, and hadronic decays, reproduces known heavy-quark scaling relations, and provides a hadron-level description of non-factorizable effects. (2605.26880).
Hilbert Series for Mesons:
We construct a complete non-redundant operator basis for heavy-meson weak decays using the Hilbert series method, with the CKM matrix elements treated as spurion fields consistent with the underlying flavor symmetry. The framework provides a model-independent description of two- and three-body decays, from which amplitude sum rules follow directly. It reproduces known flavor relations, predicts new symmetry-driven identities, and can be readily extended to axions, vector mesons, etc. (2608.11118).More to follow:
4. FOPT
Generic gauge theories utilize phase transitions to induce masses for particles through spontaneous symmetry breaking. First-order phase transitions (FOPT) have garnered significant attention recently for two principal reasons. Firstly, FOPT signify a departure from thermal equilibrium, which is crucial for elucidating baryogenesis. Secondly, the physics associated with FOPT can be probed across various frontiers, including within the gravitational wave spectrum. Our recent contributions have encompassed the following areas:
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Ultra-relativistic bubbles in SM with a real singlet scalar field:
We conducted an analysis of phase transitions within the minimal extension of the Standard Model incorporating a real singlet scalar field. Our study innovatively focused on identifying and comprehensively examining the parameter space region conducive to first-order phase transitions (FOPT). Specifically, we investigated scenarios where bubbles containing the true vacuum can attain relativistic velocities. This particular parameter regime holds significant interest due to its potential implications for novel baryogenesis and dark matter production mechanisms recently under discussion (2207.02230). Ultra-relativistic bubbles in SMEFT:
In this study, we extend our analysis using a model-independent approach within the framework of the Standard Model Effective Field Theory (SMEFT) incorporating dimension-6 and dimension-8 operators. We utilize a thermally corrected and renormalization group improved potential to examine its impact on the nucleation temperature. Subsequently, we outline the dynamics of bubble formation leading to ultra-relativistic bubble wall velocities, primarily motivated by the implications for gravitational wave detection. We emphasize the ranges of the Wilson coefficients that result in such bubble wall velocities and predict the gravitational wave spectra generated by these phase transitions, which can be tested in future experimental observations (2402.02914).