Sabyasachi Chakraborty

Sabyasachi Chakraborty

Assistant Professor

107-C, Old Sac Building,
Department of Physics,
Indian Institute of Technology, Kanpur.
iNSPIRE ORCID
✉: sabyac@iitk.ac.in
☎: 0512-279-2349


Physics Beyond the Standard Model

The Standard Model (SM) of particle physics, which describes the fundamental interactions among elementary particles, has been enormously successful in explaining many observed features of nature. Despite its success, the SM cannot be considered the ultimate theory due to several significant limitations. Notable issues include the observation of non-zero neutrino masses, compelling evidence for the existence of dark matter, matter-antimatter asymmetry, and fine-tuning problems. These challenges necessitate the exploration of Beyond Standard Model (BSM) physics.

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 at the Fundamendal Frontier

Axions have the potential to address several key limitations of the Standard Model (SM). They offer solutions 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:

  • Effective field theory of axions and searches at the intensity frontier : In our research, we investigated the B→Ka transition as a powerful probe of heavy QCD axions, conducting necessary two-loop calculations for the first time. We identified all relevant counterterms, which are essentially operators at the UV scale, and computed the anomalous dimension matrix to run these operators down to the electroweak scale. Additionally, we explored the phenomenological implications of this transition at flavor factories, utilizing both prompt and displaced vertex signatures. Similar studies have also been conducted for dark photons (arXiv: 2102.04474, 2108.10331, 2203.03280).

  • Wess-Zumino-Witten terms and searches at the cosmic frontier: 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. Consequently, we have studied the phenomenological implications of such WZW terms within the Standard Model and in the context of axions, particularly regarding the cooling processes of stellar objects such as neutron stars and supernovae (arXiv: 2403.12169, 2306.15872).

  • Cosmological implications with axions: In this work, 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," all without conflicting with current constraints. Additionally, this framework holds predictive power for upcoming CMB experiments (arXiv: 2108.04293).

    • 2. First Order Phase Transition

    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:

  • 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 (arXiv: 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 (arXiv:2402.02914).

    • 3. Soft Collinear Effective Field Theory for Gravity

    We have investigated two significant topics in the context of soft collinear effective theory (SCET) for gravitational interactions. First, we have extended the collinear Wilson lines, which are essential for maintaining multiple copies of diffeomorphism invariance in gravity SCET, to all orders in the SCET expansion parameter λ. Previously, these had only been understood up to O(λ) in the literature. Second, we have analyzed the implications of reparametrization invariance (RPI) for the structure of gravity SCET Lagrangians. To demonstrate the utility of RPI, we present an explicit example where the O(λ²) hard interactions of a collinear graviton are entirely predicted by RPI from its O(λ) hard interactions. Furthermore, we highlight that the combined requirements of multiple diffeomorphism invariances and RPI impose specific relations among the O(λ) terms. This reduces the number of O(λ) terms that need to be determined by matching onto the full theory initially (arXiv:1910.10738).