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S. Pakvasa

Publications and source records attributed to S. Pakvasa.

At least 19 recordsLinked to original sources
arXiv2019

The pros and cons of beyond standard model interpretations of ANITA events

The Antarctic Impulsive Transient Antenna (ANITA) experiment has observed two air shower events with energy $\sim 500~{\rm PeV}$ emerging from the Earth with exit angles $\sim 30^\circ$ above the horizon. As was immediately noted by the ANITA Collaboration, these events (in principle) could originate in the atmospheric decay of an upgoing $\tau$-lepton produced through a charged current interaction of a $\nu_\tau$ inside the Earth. However, the relatively steep arrival angles of these perplexing events create tension with the standard model (SM) neutrino-nucleon interaction cross section. Deepening the conundrum, the IceCube neutrino telescope and the Pierre Auger Observatory with substantially larger exposures to cosmic $\nu_\tau$'s in this energy range have not observed any events. This lack of observation implies that the messenger particle (MP) giving rise to ANITA events must produce an air shower event rate at least a factor of 40 larger than that produced by a flux of $\tau$-neutrinos to avoid conflicts with the upper limits reported by the IceCube and the Pierre Auger collaborations. In addition, the sensitivity of ANITA to MP-induced events must be comparable to or larger than those of IceCube and Auger to avoid conflict with the non-observation of any signal at these facilities. Beyond SM interpretations of ANITA events can be classified according to whether the MPs: (i) live inside the Earth, (ii) originate in neutrino-nucleon collisions inside the Earth, (iii) come from cosmological distances. In this communication we investigate the positive and negative facets of these three classes of models.

hep-ph↗
arXiv2014

Glashow resonance as a window into cosmic neutrino sources

The Glashow resonance at E_ν=6.3 PeV is a measure of the \barν_e content of the astrophysical neutrino flux. The fractional \barν_e content depends on the neutrino production model at the cosmic neutrino source, and the environment at the source. Thus, the strength of the Glashow resonance event rate is a potential window into astrophysical sources. We quantify the "Glashow resonometer" and comment on the significance that no Glashow events are observed in the IceCube three-year data.

astro-ph.HE↗
arXiv2014

Nonstandard Higgs decays in the E6 inspired SUSY models

We consider the exotic decays of the SM-like Higgs state within the E6 inspired supersymmetric (SUSY) models. In these models the lightest SUSY particle (LSP) can be substantially lighter than 1 eV forming hot dark matter in the Universe. The next--to--lightest SUSY particle (NLSP) also tend to be light. We argue that the NLSP with GeV scale mass may lead to the substantial branching ratio of the nonstandard decays of the SM-like Higgs boson into NLSPs.

hep-ph↗
arXiv2014

End of the cosmic neutrino energy spectrum

There may be a high-energy cutoff of neutrino events in IceCube data. In particular, IceCube does not observe either continuum events above 2 PeV, or the Standard Model Glashow-resonance events expected at 6.3 PeV. There are also no higher energy neutrino signatures in the ANITA and Auger experiments. This absence of high-energy neutrino events motivates a fundamental restriction on neutrino energies above a few PeV. We postulate a simple scenario to terminate the neutrino spectrum that is Lorentz-invariance violating, but with a limiting neutrino velocity that is always smaller than the speed of light. If the limiting velocity of the neutrino applies also to its associated charged lepton, then a significant consequence is that the two-body decay modes of the charged pion are forbidden above two times the maximum neutrino energy, while the radiative decay modes are suppressed at higher energies. Such stabilized pions may serve as cosmic ray primaries.

hep-ph↗
arXiv2013

Exotic Higgs decays in the E6 inspired SUSY models

We study the decays of the SM-like Higgs state within the E6 inspired supersymmetric (SUSY) models with exact custodial symmetry that forbids tree-level flavor-changing transitions and the most dangerous baryon and lepton number violating operators. In these models there are two states which are absolutely stable and can contribute to the dark matter density. One of them is the lightest SUSY particle (LSP) which is expected to be lighter than 1 eV forming hot dark matter in the Universe. The presence of another stable neutral state allows to account for the observed cold dark matter density. In the considered SUSY models next-to-lightest SUSY particle (NLSP) also tend to be light. We argue that the NLSP with GeV scale mass can result in the substantial branching ratio of the nonstadard decays of the lightest Higgs boson.

hep-ph↗
arXiv2013

IceCube PeV Cascade Events Initiated by Electron-Antineutrinos at Glashow Resonance

We propose an interpretation of the two neutrino initiated cascade events with PeV energies observed by IceCube: Ultra-high energy cosmic ray protons (or Fe nuclei) scatter on CMB photons through the Delta-resonance (the Berezinsky-Zatsepin process) yielding charged pions and neutrons. The neutron decays give electron-antineutrinos which undergo neutrino oscillations to populate all antineutrino flavors, but the electron-antineutrino flux remains dominant. At 6.3 PeV electron-antineutrino energy their annihilation on electrons in the IceCube detector is enhanced by the Glashow resonance (the W-boson) whose decays can give the PeV showers observed in the IceCube detector. The two observed showers with ~1 PeV energies would need to be from W leptonic decays to electrons and taus. An order of magnitude higher event rate of showers at 6.3 PeV is predicted from W to hadron decays. This interpretation can be tested in the near term. It has significant physics implications on the origin of the highest energy cosmic rays, since neutrino events and cosmic ray events likely share a common origin.

astro-ph.HE↗
arXiv2011

Nonstandard Higgs Decays and Dark Matter in the E6SSM

We study the decays of the lightest Higgs boson within the exceptional supersymmetric (SUSY) standard model (E6SSM). The E6SSM predicts three families of Higgs-like doublets plus three SM singlets that carry U(1)_{N} charges. One family of Higgs-like doublets and one SM singlet develop vacuum expectation values. The fermionic partners of other Higgs-like fields and SM singlets form inert neutralino and chargino states. Two lightest inert neutralinos tend to be the lightest and next-to-lightest SUSY particles (LSP and NLSP). The considered model can account for the dark matter relic abundance if the lightest inert neutralino has mass close to half the Z mass. In this case the usual SM-like Higgs boson decays more than 95% of the time into either LSPs or NLSPs. As a result the decays of the lightest Higgs boson into l^{+} l^{-} + X might play an essential role in the Higgs searches. This scenario also predicts other light inert chargino and neutralino states below 200 GeV and large LSP direct detection cross-sections which is on the edge of observability of XENON100.

hep-ph↗
arXiv2011

A model of superluminal neutrinos

Motivated by the tentative observation of superluminal neutrinos by the OPERA experiment, we present a model of active-sterile neutrino oscillations in which sterile neutrinos are superluminal and active neutrinos appear superluminal by virtue of neutrino mixing. The model demonstrates some interesting possibilities and challenges that apply to a large class of models aiming to explain the OPERA result.

hep-ph↗
arXiv2011

Collider phenomenology of the E6SSM

We consider collider signatures of the exceptional supersymmetric (SUSY) standard model (E6SSM). This E6 inspired SUSY model is based on the SM gauge group together with an extra U(1) gauge symmetry under which right--handed neutrinos have zero charge. To ensure anomaly cancellation and gauge coupling unification the low energy matter content of the E6SSM involve extra exotic matter beyond the MSSM. We discuss the collider signatures associated with the production of new particles predicted by the E6SSM and consider the implications of this model for dark matter and Higgs phenomenology. Since exotic quarks in the E6SSM can be either diquarks or leptoquarks they may provide spectacular new physics signals at the LHC.

hep-ph↗
arXiv2011

Measurement of the 8B Solar Neutrino Flux with the KamLAND Liquid Scintillator Detector

We report a measurement of the neutrino-electron elastic scattering rate from 8B solar neutrinos based on a 123 kton-day exposure of KamLAND. The background-subtracted electron recoil rate, above a 5.5 MeV analysis threshold is 1.49+/-0.14(stat)+/-0.17(syst) events per kton-day. Interpreted as due to a pure electron flavor flux with a 8B neutrino spectrum, this corresponds to a spectrum integrated flux of 2.77+/-0.26(stat)+/-0.32(syst) x 10^6 cm^-2s^-1. The analysis threshold is driven by 208Tl present in the liquid scintillator, and the main source of systematic uncertainty is due to background from cosmogenic 11Be. The measured rate is consistent with existing measurements and with Standard Solar Model predictions which include matter enhanced neutrino oscillation.

hep-ex↗
arXiv2011

Novel Higgs Decays and Dark Matter in the E(6)SSM

The Exceptional Supersymmetric (SUSY) Standard Model predicts three families of Higgs doublets plus three Higgs singlets, where one family develops vacuum expectation values (VEVs), while the remaining two which do not are called Inert. The model can account for the dark matter relic abundance if the two lightest Inert neutralinos, identified as the (next-to) lightest SUSY particles ((N)LSPs), have masses close to half the Z mass. In this case we find that the usual SM-like Higgs boson decays more than 95% of the time into either LSPs or NLSPs. The latter case produces a final state containing two leptons l^{+}l^{-} with an invariant mass less than or about 10 GeV. We illustrate this scenario with a set of benchmark points satisfying phenomenological constraints and the WMAP dark matter relic abundance. This scenario also predicts other light Inert chargino and neutralino states below 200 GeV, and large LSP direct detection cross-sections close to current limits and observable soon at XENON100.

hep-ph↗
arXiv2010

Nonstandard Higgs decays in the E(6)SSM

We study the nonstandard decays of the lightest Higgs state within the Exceptional Supersymmetric Standard Model (E6SSM). We argued that the SM--like Higgs boson can decay predominantly into dark matter particles while its branching ratios into SM particles varies from 2% to 4%. This scenario also implies the presence of other relatively light Inert chargino and neutralino states in the particle spectrum with masses below 200 GeV. We argue that in this case the decays of the lightest Higgs boson into l^{+} l^{-} + X may play an essential role in the Higgs searches.

hep-ph↗
arXiv2008

Hanohano:A Deep Ocean Antineutrino Observatory

This paper presents the science potential of a deep ocean antineutrino observatory being developed at Hawaii and elsewhere. The observatory design allows for relocation from one site to another. Positioning the observaory some 60 km distant from a nuclear reactor complex enables preecision measurement of neutrino mixing parameters, leading to a determination of neutrino mass hierarchy and theta_13. At a mid-Pacific location, the observatory measures the flux of uranium and thorium decay series antineutrinos from earth's mantle and performs a sensitive search for a hypothetical natural fission reactor in earth's core. A subequent deployment at another mid-ocean location would test lateral homogeneity of uranium and thorium in earth's mantle. These measurements have significance for earth energy studies.

hep-ex↗
arXiv2006

Earth Radioactivity Measurements with a Deep Ocean Anti-neutrino Observatory

We consider the detector size, location, depth, background, and radio-purity required of a mid-Pacific deep-ocean instrument to accomplish the twin goals of making a definitive measurement of the electron anti-neutrino flux due to uranium and thorium decays from Earth's mantle and core, and of testing the hypothesis for a natural nuclear reactor at the core of Earth. We take the experience with the KamLAND detector in Japan as our baseline for sensitivity and background estimates. We conclude that an instrument adequate to accomplish these tasks should have an exposure of at least 10 kilotonne-years (kT-y), should be placed at least at 4 km depth, may be located close to the Hawaiian Islands (no significant background from them), and should aim for KamLAND radio-purity levels, except for radon where it should be improved by a factor of at least 100. With an exposure of 10 kT-y we should achieve a 24% measurement of the U/Th content of the mantle plus core. Exposure at multiple ocean locations for testing lateral heterogeneity is possible.

hep-ex↗
arXiv2005

Neutrino physics from new SNO and KamLAND data and future prospects

We analyze the cumulative data from the latest SNO, KamLAND and other solar neutrino experiments in the standard scenario of three oscillating active neutrinos. We determine the solar neutrino oscillation parameters and obtain new bounds on $θ_x$. We also place constraints on the fraction of oscillating solar neutrinos that transform to sterile neutrinos with the $^8$B flux normalization left free. Concomitantly, we assess the sensitivity of future data from the SNO and KamLAND experiments to $θ_x$ and to the sterile neutrino content of the solar flux.

hep-ph↗
arXiv2004

Working Group Report: Neutrino and Astroparticle Physics

This is the report of neutrino and astroparticle physics working group at WHEPP-8. We present the discussions carried out during the workshop on selected topics in the above fields and also indicate progress made subsequently. The neutrino physics subgroup studied the possibilites of constraining neutrino masses, mixing and CPT violation in lepton sector from future experiments. Neutrino mass models in the context of abelian horizontal symmetries, warped extra dimensions and in presence of triplet Higgs were studied. Effect of threshold corrections on radiative magnification of mixing angles was investigated. The astroparticle physics subgroup focused on how various particle physics inputs affect the CMBR fluctuation spectrum, and on brane cosmology. This report also contains an introduction on how to use the publicly available code CMBFAST to calculate the CMBR fluctuations.

hep-ph↗
arXiv2003

Neutrino Decays and Neutrino Telescopes

It is shown how deviations of neutrino flavor mix from the canonical 1 : 1 : 1 in high energy astrophysical neutrinos can provide evidence for neutrino decays with lifetimes as long as $10^4 s/eV$.

hep-ph↗