Research

The large quantum corrections to the mass of the Higgs boson within the standard model, leading to a hierarchy of two mass scales, find a natural remedy if there exists new physics around a TeV. This attractive proposal has driven the search for new physics beyond SM in the last few decades. As no new elementary particle has been discovered in the LHC, this idea does not seem favourable anymore. However, the existing problems demand new physics at energy scales not reachable with the present generation of accelerators. This requirement, combined with the fact that at energies 100 GeV or so the SM describes the Nature quite well, establishes the SM as a successful low energy effective field theory (EFT). Construction of various EFTs, testing their theoretical consistencies, confronting them with the various experiments help explore new physics beyond SM in a model-independent manner.

The LHC has so far unravelled the existence of only one Higgs boson, that looks like the one in the standard model. The Higgs, being the only known fundamental scalar particle, needs further attention. Its interactions need to be precisely measured in the forthcoming experiments to reveal hints of new physics. It can act as a portal to the sector that holds the key to the unanswered questions of particle physics like the flavour puzzle, existence of dark matter, non-zero neutrino mass, etc. The scalar sector can be more elaborate as the inclusion of a single scalar doublet in the SM is a choice made only to keep the sector as minimal as possible. Finding the existence of additional scalars is an important venture physicists have undertaken, which might be possible with the commissioning of a 100 TeV collider at CERN.

The traditional astronomy with the photons corresponding to the radio waves to gamma rays cease to operate at energies more than tens of TeVs as they interact with the diffused photon background and get absorbed. Neutrinos, being weakly interacting, can bring in information from the distant corners of the Universe unhindered and hence, acts as a reliable messenger for doing astronomy. We already have achieved this feat by observing the neutrinos from the Sun, which helped establish the standard solar model. With the observation of extra-terrestrial neutrinos at the IceCube experiment stationed at Antarctica, a new era has started in the neutrino astronomy, which may eventually enable us to understand the inner workings of the monstrous astrophysical objects like Active Galactic Nuclei, Gamma Ray Bursts etc. A neutrino telescope like the IceCube can compete with the collider experiments in probing new physics models.

Today, the existence of dark matter is quite convincing from various experiments. It leads to anomalous galactic rotation curves. Its existence is important for structure formation during the earlier ages of this Universe. The bullet cluster observations have helped us to accept the existence of dark matter conclusively. The cosmological observations even suggest that the contributions to the energy density from dark matter is roughly five to six times more than the ordinary matter. As the standard model of particle physic does not offer a suitable candidate for a dark matter particle, it is important to build models beyond SM. Several experiments attempting a direct detection of these particles are currently in operation throughout the world that can reveal the identity of these particles. The dark sector could be the new physics sector that can offer solutions to various problems in particle physics that cannot be solved only within the context of the visible sector.

Within the purview of the SM one cannot explain why muon is heavier than the electron. More interestingly, although we know that it is the Higgs boson that is responsible for the masses of the fermions, the magnitude of them is dictated by the Yukawa couplings, which is fed into the SM as inputs only. The physics behind the observed pattern of masses and mixing of the fermions is likely to lie in a sector not reachable to us with the current experimental facilities. The theory behind it is also quite unknown. Unlike the gauge interactions, the flavour dynamics must distinguish between different generations of fermions. On top of that, neutrinos, being neutral particles and having tiny masses, are believed to get masses from a different mechanism as the mixing pattern of neutrinos is drastically different from that of the quarks. The CP-violating parameter embedded in the quark sector is quite small. But the one in the lepton sector can be large. Thus it is rather intriguing to understand the origin of flavour and its underlying dynamics.

The Universe we observe today is made up of matter only. Whereas in particle physics interactions, the same amount of matter and antimatter are produced. One needs a sizable amount of CP violation and lepton number violation for a successful leptogenesis. The lepton asymmetry thus generated can then be converted to a baryon asymmetry through sphaleron transitions. Hence, this too requires consideration of physics beyond SM. It may be related to an asymmetry in the dark sector as well. This might be a likely possibility as the amount of matter and dark matter are of the same order. This, in turn, indicates that asymmetry in both of them could have been originated from the same dynamics.

Of late, gravitational wave detectors have achieved unprecedented success in tracing back events like mergers of black holes and neutron stars at the early Universe. Like neutrinos, gravity waves has now become another important mediator in the program of multimessenger astronomy. Phase transitions at the early Universe can lead to gravitational waves that can be observed in the upcoming gravitational wave detectors. With the initial success of gravitational waves, this avenue to explore new physics beyond the standard model seems quite promising. It has been proposed that even models for grand unified theories can be probed using these waves. This will enable us to understand the pattern of the electroweak phase transition even better and shed light on the extended scalar sectors. At present, the possibilities look quite rich. The future prospect of this will depend on the sensitivity of the upcoming satellite-based detectors.