Department of Physics
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Item Models of decaying FIMP Dark Matter: potential links with the Neutrino Sector(2020-08) Mondal, TanmoyThe absolute stability of a dark matter (DM) particle is not a binding requirement. Here we suggest a few scenarios where the DM particle is liable to decay via extremely feeble interactions. This can happen via inexplicably small Yukawa couplings in the simplest conjectures. After setting down such a model, we go beyond it, thus treading onto scenarios where the spontaneous breakdown of some gauged U(1) symmetry may lead to intermediate scales, and suitably suppressed effective operators which allow the DM particle to decay slowly. The constraints from particle physics as well as cosmology are taken into account in each case. The last and more involved scenario, studied in detail, suggest a link between the model parameters that govern neutrino physics on one side, and the dynamics of a quasi-stable DM particle on the other.Item Using supernova neutrinos to probe strange spin of proton with JUNO and THEIA(2022-11) Chauhan, BhaveshThe strange quark contribution to proton's spin (Δs) is a fundamental quantity that is poorly determined from current experiments. Neutrino-proton elastic scattering (pES) is a promising channel to measure this quantity, and requires an intense source of low-energy neutrinos and a low-threshold detector with excellent resolution. In this paper, we propose that neutrinos from a galactic supernova and their interactions with protons in large-volume scintillation detectors can be utilized to determine Δs. The spectra of all flavors of supernova neutrinos can be independently determined using a combination of DUNE and Super-(Hyper-)Kamiokande. This allows us to predict pES event rates in JUNO and THEIA, and estimate Δs by comparing with detected events. We find that the projected sensitivity for a supernova at 1 kpc (10 kpc), is approximately ±0.01 (±0.15). Interestingly, the limits from a nearby supernova would be comparable to the results from lattice QCD, and better than polarized deep-inelastic scattering experiments. Using supernova neutrinos provides a true Q2→0 measurement, and thus an axial-mass independent determination of Δs.Item Large Energy Singles at JUNO from Atmospheric Neutrinos and Dark Matter(American Physical Society, 2021-11) Chauhan, BhaveshLarge liquid scintillator detectors, such as JUNO, present a new opportunity to study neutral current events from the low-energy end of the atmospheric neutrinos, and possible new physics signals due to light dark matter. We carefully study the possibility of detecting ``Large Energy Singles'' (LES), i.e., events with visible scintillation energy >15\,MeV, but no other associated tags. For an effective exposure of 20 kton-yr and considering only Standard Model physics, we expect the LES sample to contain ∼40 events from scattering on free protons and ∼108 events from interaction with carbon, from neutral-current interactions of atmospheric neutrinos. Backgrounds, largely due to β-decays of cosmogenic isotopes, are shown to be significant only below 15 MeV visible energy. The LES sample at JUNO can competitively probe a variety of new physics scenarios, such as boosted dark matter and annihilation of galactic dark matter to sterile neutrinos.Item Constraining R Violation from Anomalous Abelian Family Symmetry(ARXIV, 2002-03) Vaidya, RishikeshThe patterns of R violation resulting from imposition of a gauged U(1) horizontal symmetry, on the minimal supersymmetric standard model are systematically analyzed. We concentrate on a class of models with integer U(1) charges chosen to reproduce the quark masses and mixings as well as charged lepton masses exactly or approximately. The U(1) charges are further restricted from the requirement that very large bilinear lepton number violating terms should not be allowed in the super-potential. It is shown that all the trilinear λ′ijk and all but at most two trilinear λijk couplings vanish or are enormously suppressed.Item Radiative B decays in Supersymmetry without R-parity(ARXIV, 2004-11) Vaidya, RishikeshWe present a systematic analysis of all the contributions at the leading log order to the branching ratio of the inclusive radiative B decay, B --> X_s + gamma in the framework of supersymmetry without R-parity. The relevant set of four-quark operators involved in QCD running are extended from 6 (within SM and MSSM) to 24, with also many new contributions to the Wilson coefficients of (chromo)magnetic penguins for either chiral structure. We present complete analytical results here without any a priori assumptions on the form of R-parity violation. Mass eigenstate expressions are given, hence the results are free from the commonly adopted mass-insertion approximation. In the numerical analysis, we focus here only on the influence of the trilinear lambda-prime couplings and report on the possibility of a few orders of magnitude improvement for the bounds on a few combinations of the lambda-prime couplings. Our study shows that the Wilson coefficients of the current-current operators due to R-parity violation dominate over the direct contributions to the penguins. However, the inter-play of various contributions is complicated due to the QCD corrections which we elaborate here.Item Quark Loop Contribution to Neutron EDM from R-parity Violation(ARXIV, 2005-05) Vaidya, RishikeshWe present a detailed analysis together with numerical calculations on one-loop contributions to the neutron electric dipole moment from supersymmetry without R parity, focusing on the quark-scalar loop contributions. Complete formulas are given for the various contributions through the quark dipole operators. Analytical expressions illustrating the explicit role of the R-parity violating parameters are given following perturbative diagonalization of mass-squared matrices for the scalars. Dominant contributions come from the combinations B∗iλ′ij1 for which we obtain robust bounds. Even if the R-parity violating couplings are real, CKM phase does induce RPV contribution and for some cases such a contribution is as strong as contribution from phases in the R-parity violating couplings. Hence, we have bounds directly on |B∗ił′ij1| even if the parameters are all real.Item Tanbeta enhanced contributions to b--> s gamma in SUSY without R-parity(ARXIV, 2004-11) Vaidya, RishikeshWe present a systematic analysis of the decay b --> s gamma at the leading log within the framework of Supersymmetry without R-parity. We point out some new contributions in the form of bilinear-trilinear combination of R-parity violating (RPV) couplings that are enhanced by large tanbeta. We also improve by a few orders of magnitude, bounds on several combinations of RPV parameters.Item B--> X_s + gamma in Supersymmetry without R-parity(ARXIV, 2004-11) Vaidya, RishikeshWe present a systematic analysis of the decay B--> X_s + gamma at the leading log within the framework of Supersymmetry without R-parity. We point out some new contributions in the form of bilinear-trilinear combination of R-parity violating (RPV) couplings that are enhanced by large tanbeta. We also improve by a few orders of magnitude, bounds on several combinations of R-parity violating parameters