Summaries of my recent research work are presented here. Students or postdocs who have worked on these projects are highlighted. For a complete list of my publications see my pages at Google Scholar, arXiv, and INSPIRE.

A Necessary Condition for Collective Neutrino Instability

We proved that neutrino flavor conversion can grow exponentially only if the phase space distributions of any two flavors cross each other at one (or more) locations, i.e., there is an excess of one flavor for some momenta and excess of the other at other momenta. The rich physics of collective flavor conversions, that can affect how stars explode, how elements are created, and what we will observe in neutrino telescopes looking at supernova explosions, is contingent on this simple condition.

Published in PRL (2022).

See popular coverage by Vigyan Prasar and TIFR News.

SN neutrino-induced-proton-recoil events at DLS

A Deuterated Scintillator Detector for Neutrinos

We proposed a new type of neutrino detector, based on using deuterated hydrocarbons doped with Gadolinium. It is expected to be able to detect all flavors of neutrinos from a galactic supernova. It is expected to have interesting capability for solar and atmospheric neutrino detection.

Published in JCAP (with Bhavesh Chauhan and Vivek Datar, 2021).

Extent of Fast Depolarization

Fast Depolarization of Supernova Neutrinos

We showed that neutrinos of different flavors emitted from the core of a supernova tend to take on approximately but not exactly similar spectra. This result paves the way for definitive predictions of supernova neutrino signals and inclusion of fast neutrino oscillations into supernova simulations.

Submitted (with Soumya Bhattacharyya, 2022).

Published in PRL (with Soumya Bhattacharyya, 2021).

Published in PRD (with Soumya Bhattacharyya, 2020).

Merger rate of Transmuted Black Holes

Black Holes that Break the Chandrasekhar Limit

Chandrasekhar famously showed that black holes borne of stars cannot be lighter than about 1.4 times the mass of the Sun. We show that dark matter accumulated in the core of neutron stars may undergo self-gravitating collapse and give transmuted black holes that can be lighter than the Chandrasekhar limit. We present several ways to look for these transmuted black holes, including using next-generation gravitational wave detectors.

Published in PRL (with Ranjan Laha and Anupam Ray, 2021).

This possibility became apparent to us because of a clearer understanding of dark matter capture in stars, via multiple scattering and through interactions mediated by lighter particles, as shown in these papers.

Published in JCAP (with Aritra Gupta and Anupam Ray, 2019).

Published in JCAP (with Aritra Gupta and Anupam Ray, 2020).

Constraints on Primordial Black Holes

Looking for Primordial Black Holes

Some part of dark matter could consist of small, roughly asteroid mass, black holes that formed almost immediately after the Big Bang. We have used available data from neutrino telescopes, gamma ray telescopes, as well as radio telescopes, to set strong constraints on this possibility.

Published in PRL (with Ranjan Laha and Anupam Ray, 2020).

Published in JCAP (with Shikhar Mittal, Girish Kulkarni, and Anupam Ray, 2021).

Selective Sommerfeld Enhancement

Selection Rule for Sommerfeld Enhancement of Dark Matter Annihilation

We pointed out a mechanism that selectively changes the annihilation of dark matter pairs with either odd or even angular momentum. The selection mechanism works if there are two or more dark matter species that mix with each other due to long-range interactions. As a consequence, the annihilation rate is distinctively large and strongly velocity-dependent, and can offer a unique explanation for the AMS-02 anomaly.

Published in PRL (with Anirban Das, 2017).

See Inside Science, Asian Scientist, Science Daily, and TIFR News Detail for popular coverage.

Fast Neutrino Oscilation: First Calculation

Fast Neutrino Flavor Conversion in Supernovae

Neutrino fluxes from a supernova can show substantial flavor conversions almost immediately above the core if the angular distributions are non-trivial. We showed that neutrinos traveling towards the core make fast conversions possible for a wider range of flux ratios of neutrinos and antineutrinos. Using fluxes and angular distributions predicted by supernova simulations, we found that fast conversions can occur within tens of nanoseconds. Flavor-dependent momentum-changing collisions of neutrinos play an important role in making this happen.

Published in JCAP (with Alessandro Mirizzi and Manibrata Sen, 2017)

Published in PRL (with Francesco Capozzi, Alessandro Mirizzi, Manibrata Sen, and Gunter Sigl, 2019)

See related popular coverage in The Hindu.