Forays Beyond the Standard Model: From Particle Astrophysics to Cosmology

The Standard Model of particle physics, established over several decades, successfully describes the interactions of all known particles at energies accessible to today's experiments. However, there is definitive observational evidence that most of the matter content of the Universe is composed of an entity (given the name "dark matter") that cannot fit into the framework of the Standard Model. Understanding the identity and cosmological history of dark matter constitutes one of the most important problems in theoretical physics. Moreover, this quest is likely to unveil new physical laws or selection principles, shedding light on a host of associated fundamental questions in particle physics. As such, it is a matter of great national interest. In his research, Professor Sinha will explore the potential of future gravitational wave detectors and X-ray telescopes to probe dark matter physics, studying two leading candidates: the axion and primordial black holes. Through his work, Professor Sinha will thus be investigating ways to discover the most abundant particle in the Universe. This project will involve postdoctoral scholars as well as graduate and undergraduate students, providing critical training for the next generation of junior physicists. He will disseminate his work through public lectures and develop new courses based on his research findings, further enhancing the project's broader impacts. More technically, Professor Sinha will study axions from neutron stars by leveraging data from X-ray polarization missions on the one hand, as well as astrophysical modelling of the hard X-ray tails of magnetar emission on the other. The work relies on the theoretical formalism for axion-induced polarization in the presence of dipolar magnetic fields that has been recently developed by his group. Professor Sinha will also study axions produced in neutron star mergers, performing detailed studies of the temporal information of the photons coming from decays of axions produced from mergers using a combination of temporal-spectral data to probe hitherto unconstrained parts of axion parameter space. In his research on primordial black holes, Professor Sinha will be studying the effect of asteroid-mass primordial black holes on stellar and pulsar binaries, probing a crucial gap in the dark matter parameter space. In his research on gravitational waves, Professor Sinha will perform precision calculations of the gravitational wave spectrum arising from phase transitions in the early Universe, and the possible connection of such phase transitions with the generation of dark matter. He will also investigate the possibility of constraining dark sector particles and mediators with gravitational wave signals from black hole inspirals and other sources. In his work on dark matter and stellar evolution, Professor Sinha will investigate the effect of axions and millicharged particles on the evolution of Cepheids, specifically the effects on the blue loop stage and pulsation period. Professor Sinha will also study cosmological moduli and early matter-dominated eras, especially in the context of axions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.