arXiv subjects

Sovan Chakraborty

Publications and source records attributed to Sovan Chakraborty.

45 records · Page 3Linked to original sources

arXiv2012

Probing the neutrino mass hierarchy with the rise time of a supernova burst

The rise time of a Galactic supernova (SN) bar-nue lightcurve, observable at a high-statistics experiment such as the IceCube Cherenkov detector, can provide a diagnostic tool for the neutrino mass hierarchy at "large" 1-3 leptonic mixing angle theta_13. Thanks to the combination of matter suppression of collective effects at early postbounce times on one hand and the presence of the ordinary Mikheyev-Smirnov-Wolfenstein effect in the outer layers of the SN on the other hand, a sufficiently fast rise time on O(100) ms scale is indicative of an inverted mass hierarchy. We investigate results from an extensive set of stellar core-collapse simulations, providing a first exploration of the astrophysical robustness of these features. We find that for all the models analyzed (sharing the same weak interaction microphysics) the rise times for the same hierarchy are similar not only qualitatively, but also quantitatively, with the signals for the two classes of hierarchies significantly separated. We show via Monte Carlo simulations that the two cases should be distinguishable at IceCube for SNe at a typical Galactic distance 99% of the times. Finally, a preliminary survey seems to show that the faster rise time for inverted hierarchy as compared to normal hierarchy is a qualitatively robust feature predicted by several simulation groups. Since the viability of this signature ultimately depends on the quantitative assessment of theoretical/numerical uncertainties, our results motivate an extensive campaign of comparison of different code predictions at early accretion times with implementation of microphysics of comparable sophistication, including effects such like nucleon recoils in weak interactions.

astro-ph.SR↗

arXiv2011

No collective neutrino flavor conversions during the supernova accretion phase

The large neutrino fluxes emitted with a distinct flavor hierarchy from core-collapse supernovae (SNe) during the post-bounce accretion phase, offer the best opportunity to detect effects from neutrino flavor oscillations. We perform a dedicated study of the SN neutrino flavor evolution during the accretion phase, using results from recent neutrino radiation hydrodynamics simulations. In contrast to what expected in the presence of only neutrino-neutrino interactions, we find that the multi-angle effects associated with the dense ordinary matter suppress collective oscillations. This is related to the high matter densities during the accretion phase in core-collapse SNe of massive iron-core progenitors. The matter suppression implies that neutrino oscillations will start outside the neutrino transport region and therefore will have a negligible impact on the neutrino heating and the explosion dynamics. Furthermore, the possible detection of the next galactic SN neutrino signal from the accretion phase, based on the usual Mikheyev- Smirnov-Wolfenstein effect in the SN mantle and Earth matter effects, can reveal the neutrino mass hierarchy in the case that the mixing angle $θ_{13}$ is not very small.

hep-ph↗

arXiv2011

On the Observability of Collective Flavor Oscillations in Diffuse Supernova Neutrino Background

Collective flavor oscillations are known to bring multiple splits in the supernova (SN) neutrino and antineutrino spectra. These spectral splits depend not only on the mass hierarchy of the neutrinos but also on the initial relative flux composition. Observation of spectral splits in a future galactic supernova signal is expected to throw light on the mass hierarchy pattern of the neutrinos. However, since the Diffuse Supernova Neutrino Background (DSNB) comprises of a superposition of neutrino fluxes from all past supernovae, and since different supernovae are expected to have slightly different initial fluxes, it is pertinent to check if the hierarchy dependent signature of collective oscillations can survive this averaging of the flux spectra. Since the actual distribution of SN with initial relative flux spectra of the neutrinos and antineutrinos is unknown, we assume a log-normal distribution for them. We study the dependence of the hierarchy sensitivity to the mean and variance of the log-normal distribution function. We find that the hierarchy sensitivity depends crucially on the mean value of the relative initial luminosity. The effect of the width is to reduce the hierarchy sensitivity for all values of the mean initial relative luminosity. We find that in the very small mixing angle ($θ_{13}$) limit considering only statistical errors even for very moderate values of variance, there is almost no detectable hierarchy sensitivity if the mean relative luminosities of $ν_e$ and $\barν_e$ are greater than 1.

hep-ph↗

arXiv2011

Analysis of matter suppression in collective neutrino oscillations during the supernova accretion phase

The usual description of self-induced neutrino flavor conversions in core collapse supernovae (SNe) is based on the dominance of the neutrino density n_nu over the net electron density n_e. However, this condition is not met during the post-bounce accretion phase, when the dense matter in a SN is piled up above the neutrinosphere. As recently pointed-out, a dominant matter term in the anisotropic SN environment would dephase the flavor evolution for neutrinos traveling on different trajectories, challenging the occurrence of the collective behavior in the dense neutrino gas. Using the results from recent long term simulations of core-collapse SN explosions, based on three flavor Boltzmann neutrino transport in spherical symmetry, we find that both the situations of complete matter suppression (when n_e >> n_nu) and matter-induced decoherence (when n_e \gtrsim n_nu) of flavor conversions are realized during the accretion phase. The matter suppression at high densities prevents any possible impact of the neutrino oscillations on the neutrino heating and hence on the dynamics of the explosion. Furthermore, it changes the interpretation of the Earth matter effect on the SN neutrino signal during the accretion phase, allowing the possibility of the neutrino mass hierarchy discrimination at not too small values of the leptonic mixing angle θ_{13} (i.e. \sin^2θ_{13} \gtrsim 10^{-3}).

hep-ph↗

arXiv2010

Constraining Scalar Singlet Dark Matter with CDMS, XENON and DAMA and Prediction for Direct Detection Rates

We consider a simplest extension of the Standard Model (SM) through the incorporation of a real scalar singlet and an additional discrete $Z_2$ symmetry. The model admits the neutral scalar singlet to be stable and thus, a viable component of dark matter. We explore the parameter space of the model keeping in view the constraints arise from different dark matter direct detection experiments through WIMP-nucleon scattering. First of all, we have utilised the data obtained from CDMS, XENON-10 and XENON-100 collaborations. We further constraint the parameter space from the DAMA collaboration results (both with and without channelling) and CoGeNT collaboration results. Throughout our analysis, the constraint arises due to the observed relic density of dark matter reported by WMAP experiment, is also incorporated. Utilising all those constraints, on the model parameter space, we calculate the event rates and the annual variation of event rates in the context of a Liquid Argon Detector experiment.

hep-ph↗

arXiv2010

Collective Flavor Oscillations Of Supernova Neutrinos and r-Process Nucleosynthesis

Neutrino-neutrino interactions inside core-collapse supernovae may give rise to collective flavor oscillations resulting in swap between flavors. These oscillations depend on the initial energy spectra, and relative fluxes or relative luminosities of the neutrinos. It has been observed that departure from energy equipartition among different flavors can give rise to one or more sharp spectral swap over energy, termed as splits. We study the occurrence of splits in the neutrino and antineutrino spectra, varying the initial relative fluxes for different models of initial energy spectrum, in both normal and inverted hierarchy. These initial relative flux variations give rise to several possible split patterns whereas variation over different models of energy spectra give similar results. We explore the effect of these spectral splits on the electron fraction, $Y_e$, that governs r-process nucleosynthesis inside supernovae. Since spectral splits modify the electron neutrino and antineutrino spectra in the region where r-process is postulated to happen, and since the pattern of spectral splits depends on the initial conditions of the spectra and the neutrino mass hierarchy, we show that the condition $Y_e < 0.5$ required for successful r-process nucleosynthesis will lead to constraints on the initial spectral conditions, for a given neutrino mass hierarchy.

hep-ph↗

arXiv2010

Interpreting the bounds on Solar Dark Matter induced muons at Super-Kamiokande in the light of CDMS results

We consider the recent limits on dark matter - nucleon elastic scattering cross section from the analysis of CDMS II collaboration using the two signal events observed in CDMS experiment. With these limits we try to interpret the Super-Kamiokande (SK) bounds on the detection rates of up-going muons induced by the neutrinos that are produced in the sun from the decay of annihilation products of dark matter (WIMPs) captured in the solar core. Calculated rates of up-going muons for different annihilation channels at SK using CDMS bounds are found to be orders below the predicted upper limits of such up-going muon rates at SK. Thus there exists room for enhancement (boost) of the calculated rates using CDMS limits for interpreting SK bounds. Such a feature is expected to represent the PAMELA data with the current CDMS limits. We also show the dependence of such a possible enhancement factor (boost) on WIMP mass for different WIMP annihilation channels.

hep-ph↗

arXiv2009

Effect of Collective Flavor Oscillations on the Diffuse Supernova Neutrino Background

Collective flavor oscillations driven by neutrino-neutrino self interaction inside core-collapse supernovae have now been shown to bring drastic changes in the resultant neutrino fluxes. This would in turn significantly affect the diffuse supernova neutrino background (DSNB), created by all core-collapse supernovae that have exploded in the past. In view of these collective effects, we re-analyze the potential of detecting the DSNB in currently running and planned large-scale detectors meant for detecting both electron neutrinos and antineutrinos. The next generation detectors should be able to observe DSNB fluxes. Under certain conducive conditions, one could learn about neutrino parameters. For instance, it might be possible to determine the neutrino mass hierarchy, even if theta_{13} is almost zero.

hep-ph↗

arXiv2008

Upper Limit on the Cosmic Gamma-Ray Burst Rate from High Energy Diffuse Neutrino Background

We derive upper limits on the ratio $f_{GRB/CCSN}(z) \equiv R_{GRB}(z)/R_{CCSN}(z) \equiv f_{GRB/CCSN}(0)(1+z)^α$, the ratio of the rate, $R_{GRB}$, of long-duration Gamma Ray Bursts (GRBs) to the rate, $R_{CCSN}$, of core-collapse supernovae (CCSNe) in the Universe ($z$ being the cosmological redshift and $α\geq 0$), by using the upper limit on the diffuse TeV--PeV neutrino background given by the AMANDA-II experiment in the South Pole, under the assumption that GRBs are sources of TeV--PeV neutrinos produced from decay of charged pions produced in $pγ$ interaction of protons accelerated to ultrahigh energies at internal shocks within GRB jets. For the assumed ``concordance model'' of cosmic star formation rate, $R_{SF}$, with $R_{CCSN}(z) \propto R_{SF}(z)$, our conservative upper limits are $f_{GRB/CCSN}(0)\leq 5.0\times10^{-3}$ for $α=0$, and $f_{GRB/CCSN}(0)\leq 1.1\times10^{-3}$ for $α=2$, for example. These limits are already comparable to (and, for $α\geq 1$ already more restrictive than) the current upper limit on this ratio inferred from other astronomical considerations, thus providing a useful independent probe of and constraint on the CCSN-GRB connection. Non-detection of a diffuse TeV--PeV neutrino background by the up-coming IceCube detector in the South pole after three years of operation, for example, will bring down the upper limit on $f_{GRB/CCSN}(0)$ to below few $\times10^{-5}$ level, while a detection will confirm the hypothesis of proton acceleration to ultrahigh energies in GRBs and will potentially also yield the true rate of occurrence of these events in the Universe.

astro-ph↗