My research focuses on the field of astroparticle physics. The overarching goal of my research is to probe the physics beyond the Standard Model and gain insight into how the Universe works at the fundamental level. I work on dark matter, especially axion-like particles, as well as multi-messenger astrophysics (neutrinos, gamma-rays, cosmic rays, and gravitational waves) to investigate the properties and processes of various astrophysical sources (AGNs, GRBs, neutron stars, black holes, etc).

Dark Matter

For centuries, physicists have been on an elusive quest for that ultimate, elegant theory underlying a single equation – the “God” equation – that would unify the laws of Physics and completely describe the universe. Around the mid-1970s, the seeds of unification bore their first fruit when physicists could successfully sew three fundamental forces, except gravitational, and formulated a single theory known as the Standard Model (SM). Although the SM successfully predicted several outcomes with remarkable accuracy, it still can not be considered the fundamental theory. Some puzzles are still unsolved! One such intriguing puzzle is that SM particles make up only a tiny fraction (~ 5%) of the total mass-energy content of the universe; the rest of the universe is made up of dark matter (~ 27%) and dark energy (~ 68%), whose nature is still unknown. To address this issue, physicists proposed several “hypothetical friends,” one of which is the axion-like particles.

Axion-like particles

Axion-like particles (ALPs) are pseudoscalar bosons with a remarkable feature of converting into photons in an external magnetic field. Many laboratory and reactor experiments exploit this property to probe them; however, only upper bounds have been placed on its mass and coupling constant. In recent years, probing astrophysical sources has emerged as a promising avenue to search for these particles, mainly due to extreme astrophysical environments and energies that would be impossible to replicate by terrestrial experiments.

In astrophysical sources, the conventional mechanism to produce neutrinos and VHE gamma rays is through hadronic interactions. Neutrinos, being weakly interacting, escape the production site without any hindrance. On the other hand, gamma rays interact with ambient and low-energy background photons, leading to an attenuation in their flux. The optical depth of the attenuation depends on the photon’s energy and the source distance from the Earth. The survival probability of high-energy photons from distant (high redshift) extragalactic sources is extremely low.

However, when ALPs enter the game, the situation changes drastically. Under the photon-ALP oscillations, the photons, too, can secretly bypass the attenuation, increasing the transparency of the Universe. Even a slight decrease in optical depth significantly increases the survival probability of photons due to exponential dependence. The converted photons then reappear on Earth due to the Galactic magnetic field, thus modulating the gamma-ray spectrum. Therefore, any signature of modulation in the observed VHE gamma-ray spectrum from distant or opaque sources could be strong evidence for the existence of these particles.

Till now, no conclusive signature has been found, and several bounds have been put by studying the gamma-ray spectra of several galactic and extragalactic sources. However, some recent events hint toward some unconventional physics being involved.

If you are curious to know more about ALPs, check my papers (B. P. Pant 2024a; B. P. Pant 2024b)