On aspects of weakly interacting physics : neutrino oscillation and dark sector

Abstract

The Standard Model (SM) of particle physics describes the electromagentic, weak and strong interactions nicely in terms of particles: bosons, which are mediators of these interactions and fermions, which are the matter content. With no concrete hints for new physics beyond the SM from the experimental side, weakly interacting physics are a subject of focus in the recent times since weakly coupled sector particles may have eluded our detectors hitherto. Among the fermions in the SM, neutrinos are massless and electrically neutral particles which participate only in weak inter actions. The experimentally observed phenomenon of neutrino oscillation establishes neutrinos having mass and there is a mixing between the three neutrino flavours. The point to note is the neutrino mass terms in the action introduces off-diagonal elements in the neutrino mass matrix in the flavour basis which depends on neutrino mass and mixing parameters. Measurements of the neutrino oscillation parameters may lead to discovering the underlying symmetries of the neutrino mass matrix thus providing a lead in the ultra-violet (UV) physics beyond the Standard Model. In current and fu ture neutrino oscillation experiments, the oscillation parameters, namely the neutrino mass-squared differences and the mixing angles are being measured with precision. The main three unknowns in the neutrino oscillation in the standard three generation framework are: the neutrino mass hierarchy, octant of θ23 and the leptonic phase δCP . Among several ongoing & future experiments where these unknown oscillation parameters can be probed, Deep Underground Neutrino Experiment (DUNE) is an ambitiously planned experiment and consists of the largest Liquid Argon-TPC neu trino detector. It will have high intensity νµ−beam from Fermilab directed towards Sanford Underground Research Facility (SURF) laboratory. Using νµ-disappearance and νe appearance channels, it will be able to measure the three unknowns mentioned earlier with high statistical significance. Beside this, DUNE detector is capable to detecting tau-flavored neutrinos, the species of neutrinos which have been identified very less number of times in any experiment til now. DUNE is going to surpass this with huge tau statistics. In the first half of the thesis we considered the physics po tential using this νµ → ντ appearance channel. We studied the physics capabilities in the standard oscillation framework and the sensitivities of the measurement using the ντ -appearance channel. Alongside we investigated the impact of systematics, tau detection efficiency and various experimental reaches on the ντ -appearance channel. DUNE collaboration also has plans for a tau-optimized flux which will be able to provide almost 4-times larger τ -events. We also performed studies based on this flux. Apart from the standard physics scenarios, new physics especially such coupling to the third generation leptons are weakly constrained and can be concretely probed using the ντ -appearance channel. We investigated the measurement sensitivities of new physics in terms the non-standard interaction (NSI) parameters and predicted the bounds DUNE will be able to pose on these. In the second half of the thesis we focused on the hidden weakly coupled sec tor physics. Several theoretical models envisage various types of dark matter (DM), among which of particular interest is the possibility that there is a specific type of particle that may serve as a portal between the dark particles of the new type of matter and the ordinary matter of the SM through a new very weak interaction. In a theoretical scenario of DM particles, this new sector can be constituted of possible DM candidates that can be classified based on the masses of the mediators being in the TeV, sub-GeV, eV or lighter. Due to the unsuccessful results from the traditional searches for WIMP particles, there are at least two possible reasons why a DM par ticle of a new sector has not been discovered yet: first, the mass scale of the new particles, including the mediators of the new forces, is well above the energy scale reached so far in laboratory experiments, mainly investigated in collider experiments. Second, the mass scale is within experimental reaches, but the couplings between the new particles and the SM ones are so feeble that the whole new sector has so far remained hidden. We reviewed the search for dark photon (mediator between the SM and the hidden sector) in various experiments and observations and studied the current bounds. Dark photon (DP) is particular of interested in terms of anomalies of the discrepancy between the theoretical and experiment value (g − 2)µ and the electron-positron observed in decays of excited 88 Be nuclei In particular, the last anomaly can be resolved by hypothesizing the emission of a 17 MeV dark photon (DP) A0 , in the decay 8 Be ∗ →8 Be + A0 followed by A0 → e + + e−. We developed a novel technique to search for this DP in the PADME experiment in the Frascati National laboratory. This method involves the DP being produced by resonance in positron beam-dump experiment. We investigated the sensitivity of PADME using this process and compared it with the bremsstrahlung method of production. Both of them are complementary in the sense that these strategies of DP search will be able to probe different regions of the parameter space. Keywords: Weakly interacting, Neutrino Physics, Neutrino Oscillation, Long Baseline Neutrino Experiments (LBL), Deep Underground Neutrino Experiment (DUNE), Leptonic CP Phase, CP Violation, Non-Standard Interaction (NSI), tau, ντ appear ance, Liquid Argon detector, Fermilab. BSM U(1), dark sector, dark photon, dark matter, PADME, resonance, bremsstrahlung, ATOMKI, Be-anomaly.