arXiv subjects

Shashank Shalgar

Publications and source records attributed to Shashank Shalgar.

45 records · Page 3Linked to original sources

arXiv2015

Flavor instabilities in the multi-angle neutrino line model

Neutrino flavor oscillations in the presence of ambient neutrinos is nonlinear in nature which leads to interesting phenomenology that has not been well understood. It was recently shown that, in the two-dimensional, two-beam neutrino Line model, the inhomogeneous neutrino oscillation modes on small distance scales can become unstable at larger neutrino densities than the homogeneous mode does. We develop a numerical code to solve neutrino oscillations in the multi-angle/beam Line model with a continuous neutrino angular distribution. We show that the inhomogeneous oscillation modes can occur at even higher neutrino densities in the multi-angle model than in the two-beam model. We also find that the inhomogeneous modes on sufficiently small scales can be unstable at smaller neutrino densities with ambient matter than without, although a larger matter density does shift the instability region of the homogeneous mode to higher neutrino densities in the Line model as it does in the one-dimensional supernova Bulb model. Our results suggest that the inhomogeneous neutrino oscillation modes can be difficult to treat numerically because the problem of spurious oscillations becomes more severe for oscillations on smaller scales.

hep-ph↗

arXiv2015

Flavor instabilities in the neutrino line model

A dense neutrino medium can experience collective flavor oscillations through nonlinear neutrino-neutrino refraction. To make this multi-dimensional flavor transport problem more tractable, all existing studies have assumed certain symmetries (e.g., the spatial homogeneity and directional isotropy in the early universe) to reduce the dimensionality of the problem. In this work we show that, if both the directional and spatial symmetries are not enforced in the neutrino line model, collective oscillations can develop in the physical regimes where the symmetry-preserving oscillation modes are stable. Our results suggest that collective neutrino oscillations in real astrophysical environments (such as core-collapse supernovae and black-hole accretion discs) can be qualitatively different from the predictions based on existing models in which spatial and directional symmetries are artificially imposed.

hep-ph↗

arXiv2014

Multipole expansion method for supernova neutrino oscillations

We demonstrate a multipole expansion method to calculate collective neutrino oscillations in supernovae using the neutrino bulb model. We show that it is much more efficient to solve multi-angle neutrino oscillations in multipole basis than in angle basis. The multipole expansion method also provides interesting insights into multi-angle calculations that were accomplished previously in angle basis.

hep-ph↗

arXiv2013

Transition Magnetic Moments and Collective Neutrino Oscillations:Three-Flavor Effects and Detectability

We demonstrate the non-negligible effect of transition magnetic moments on three-flavor collective oscillations of Majorana neutrinos in the core of type-II supernovae, within the single-angle approximation. We argue that data from a galactic supernova in conjunction with terrestrial experiments can potentially give us clues about the non-zero nature of neutrino transition magnetic moments if these are Majorana fermions, even if their values are as small as those predicted by the Standard Model augmented by nonzero neutrino Majorana masses.

astro-ph.HE↗

arXiv2012

Effect of Transition Magnetic Moments on Collective Supernova Neutrino Oscillations

We study the effect of Majorana transition magnetic moments on the flavor evolution of neutrinos and antineutrinos inside the core of Type-II supernova explosions. We find non-trivial collective oscillation effects relating neutrinos and antineutrinos of different flavors, even if one restricts the discussion to Majorana transition electromagnetic moment values that are not much larger than those expected from standard model interactions and nonzero neutrino Majorana masses. This appears to be, to the best of our knowledge, the only potentially observable phenomenon sensitive to such small values of Majorana transition magnetic moments. We briefly comment on the effect of Dirac transition magnetic moments and on the consequences of our results for future observations of the flux of neutrinos of different flavors from a nearby supernova explosion.

astro-ph.HE↗

arXiv2010

Atmospheric Tau Neutrinos in a Multi-kiloton Liquid Argon Detector

An ultra-large Liquid Argon Time Projection Chamber-based neutrino detector will have the uncommon ability to detect atmospheric tau neutrino events. This paper discusses the most promising modes for identifying charged current tau neutrino interactions, and shows that, with simple kinematic cuts, ~30 tau neutrinos can be isolated in a 100 kt*yr exposure, with greater than 4 sigma significance. This sample is sufficient to perform flux-averaged total cross-section and cross-section shape parameterization measurements -- the first steps toward using tau neutrinos to search for physics beyond the Standard Model.

hep-ph↗

arXiv2010

Parameterizing Majorana Neutrino Couplings in the Higgs Sector

Nonzero masses for the active neutrinos - regardless of their nature or origin - arise only after electroweak symmetry breaking. We discuss the parameterization of neutrino couplings to a Higgs sector consisting of one SU(2)_L scalar doublet and one SU(2)_L scalar triplet, and allow for right-handed neutrinos whose Majorana mass parameters arise from the vacuum expectation value of a Standard Model scalar singlet. If the neutrinos are Majorana fermions, all Yukawa couplings can be expressed as functions of the neutrino mass eigenvalues and a subset of the elements of the neutrino mixing matrix. In the mass basis, the Yukawa couplings are, in general, not diagonal. This is to be contrasted to the case of charged-fermions or Dirac neutrinos, where couplings to the Higgs-boson are diagonal in the mass basis and proportional only to the fermion masses. Nonetheless, all physically distinguishable parameters can be reached if all neutrino masses are constrained to be positive, all mixing angles constrained to lie in the first quadrant (theta in [0,pi/2]), and all Majorana phases to lie in the first two quadrants (phi in [0,pi]), as long as all Dirac phases vary within the entire unit circle (delta in [0,2pi}). We discuss several concrete examples and comment on the Casas-Ibarra parameterization for the neutrino Yukawa couplings in the case of the type-I Seesaw Lagrangian.

hep-ph↗

arXiv2009

Implications of the Higgs Discovery in the MSSM Golden Region

If the lightest CP-even Higgs boson in the MSSM is discovered at the LHC, two measurements could be made simultaneously: the Higgs mass m_h and the event rate Bs(gg -> h -> gamma gamma). We study to what extent the combination of these two measurements would allow us to extract parameters in the stop mass matrix, including the off-diagonal mixing term, with a focus on the MSSM golden region where the stops are light and the mixing is large. Even though both the production cross-section and the decay amplitude are not sensitive to supersymmetric parameters outside of the stop sector, the branching ratio depends on the total decay width, which is dominated by the Higgs decay to b quarks and sensitive to both the pseudo-scalar mass m_A and the supersymmetric Higgs mass μ. In the end we find m_A is an important input in extracting the stop mass parameters, while a fair estimate of the off-diagonal mixing term could be obtained without prior knowledge of μ.

hep-ph↗

arXiv2007

Mass Hierarchy Determination via future Atmospheric Neutrino Detectors

We study the problem of determination of the sign of Delta m^2_{31}, or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types : (a) Megaton sized water Cerenkov detectors, which can measure the survival rates of nu_μ+ \barν_μand nu_e + \barν_e and (b) 100 kton sized magnetized iron detectors, which can measure the survival rates of ν_μand \barν_μ. For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from P_{μe}, P_{μμ} and P_{ee}, leading to an appreciable sensitivity to the hierarchy. We do a binned χ^2 analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the χ^2 over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis a vis the hierarchy determination.

hep-ph↗