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Highlights


Selected highlights of my research are presented here. See my publications and talks for a complete overview.

Selection Rule for Sommerfeld Enhancement of Dark Matter Annihilation

We pointed out a selection rule for enhancement/suppression of odd/even partial waves of dark matter co-annihilation or annihilation using Sommerfeld effect. Using this, the usually velocity-suppressed p-wave annihilation can dominate the annihilation signals in the present Universe. The selection mechanism is a manifestation of the exchange symmetry of identical incoming particles, and generic for multi-state DM with off-diagonal long-range interactions. As a consequence, the relic and late-time annihilation rates are parametrically different and a distinctive phenomenology, with large but strongly velocity-dependent annihilation rates, is predicted.

Published in Physical Review Letters (with Anirban Das, 2017).

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




Fast Neutrino Flavor Conversion in Supernovae

Neutrino fluxes from a supernova can show substantial flavor conversions almost immediately above the core when the angular distributions are non-trivial. We performed a detailed study of these fast conversions, focussing on the region just above the supernova core, and showed that neutrinos travelling 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.

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

Using the newly advocated language of dispersion relations, we showed that complex wave-numbers or frequencies of neutrino flavor conversions in space and time do not necessarily imply an instability. Using the properties of the Green's function, as originally done in plasma physics, we gave more refined and quantitative criteria for identifying instabilities and showed how to classify them as being absolute or convective. We performed analytical and numerical studies, with neutrinos plane waves as well as wavepackets, and showed how the above criteria can be used fruitfully.

Published in Physical Review D (with Francesco Capozzi, Eligio Lisi, Antonio Marrone, and Alessandro Mirizzi, 2017)
  
Fluctuations



Impact of Noise on Neutrino Flavor Conversion in Supernovae

We have recently started studying what happens when the supernova medium is not static and smooth, but instead fluctuates with time and has inhomogeneities. The remarkable new effect we find is that the temporal fluctuations can make the flavor evolution unstable at much higher densities, where it was thought to be stable. This can have important consequences on supernova explosion dynamics as well as synthesis of heavy elements in these stars.

Published in Physical Review D (with Alessandro Mirizzi, 2015)


We followed up this up with a more detailed study showing that the flavor instability is dragged down to lower Fourier modes as the density of surrounding matter decreases slowly. If the background matter and neutrino density decrease much faster than the instability spreads in Fourier space, the instability may not be able to grow. We also performed detailed comparisons of the fully nonlinear and linear solutions, and showed that they agree where they ought to.

Published in JCAP (with Francesco Capozzi and Alessandro Mirizzi, 2016)
 
DMandDR
Improved Dark Matter Halos from Dark Radiation

We showed that if dark matter interacts with some form of dark radiation, which can be a Goldstone boson of a broken symmetry under which the dark matter particle was charged, it naturally avoids the problems of overly cuspy and overly numerous small scale halos associated with cold dark matter. We provided the simplest model that solves these small-scale problems and explains dark radiation.

Published in Physical Review Letters (with Xiaoyong Chu, 2014).
   
NuSterile


Hidden Sterile Neutrinos and Dark Matter

We showed that sterile neutrinos of 1eV remain cosmologically viable if coupled to a new MeV-mass gauge boson. It gets a thermal mass larger than the neutrino oscillation frequency and is not produced by oscillations. This not only evades cosmological constraints on number of radiation-like degrees of freedom at BBN and CMB epochs, but can in fact help cure some of the problems of a Lambda-Cold-Dark-Matter cosmology at the smallest scales, i.e., the problems of overly cuspy and numerous dark matter halos.

Published in Physical Review Letters as Editors' Suggestion (with Joachim Kopp, 2014)

Following the considerable interest in our above proposal and based on improved treatment of collisional processes by other groups, we revisited our study and made more precise estimates for the number of radiation like degrees of freedom and on the sum of neutrino masses in our model. We showed that the model is in tension with cosmological data, except perhaps in two parameter ranges where the tensions could be lower.

Published in JCAP (with Xiaoyong Chu and Joachim Kopp, 2015).