arXiv subjects

Basudeb Dasgupta

Publications and source records attributed to Basudeb Dasgupta.

At least 37 records · Page 2Linked to original sources

arXiv2021

Fast Flavor Depolarization of Supernova Neutrinos

Flavor-dependent neutrino emission is critical to the evolution of a supernova and its neutrino signal. In the dense anisotropic interior of the star, neutrino-neutrino forward-scattering can lead to fast collective neutrino oscillations, which has striking consequences. We present a theory of fast flavor depolarization, explaining how neutrino flavor differences become smaller, i.e., depolarize, due to diffusion to smaller angular scales. We show that transverse relaxation determines the epoch of this irreversible depolarization. We give a method to compute the depolarized fluxes, presenting an explicit formula for simple initial conditions, which can be a crucial input for supernova theory and neutrino phenomenology.

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arXiv2021

Fast Flavor Oscillations of Astrophysical Neutrinos with $1,\,2,\,\ldots,\,\infty$ Crossings

In the early Universe, as well as in supernovae and merging neutron stars, neutrinos have such high densities that they affect each other and exhibit collective flavor oscillations. A crucial ingredient for fast collective flavor oscillations is that the electron lepton number (ELN) distribution changes its sign as a function of direction, i.e., has a zero crossing. We present a study in two dimensions and show how fast flavor oscillations depend on the ELN and its crossings. We show that a large number of crossings can inhibit flavor oscillations. This may be a natural self-limiting mechanism that stabilizes the flavor content of the dense neutrino gas in a vast majority of scenarios, especially the early Universe, where the angular distributions for all flavors are very similar and crossings occur mainly due to fluctuations.

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arXiv2020

Dark matter capture in celestial objects: light mediators, self-interactions, and complementarity with direct detection

We generalize the formalism for DM capture in celestial bodies to account for arbitrary mediator mass, and update the existing and projected astrophysical constraints on DM-nucleon scattering cross section from observations of neutron stars. We show that the astrophysical constraints on the DM-nucleon interaction strength, that were thought to be the most stringent, drastically weaken for light mediators and can be completely voided. For asymmetric DM, existing astrophysical constraints are completely washed out for mediators lighter than 5 MeV, and for annihilating DM the projected constraints are washed out for mediators lighter than 0.25 MeV. Related terrestrial direct detection bounds also weaken, but in a complementary fashion; they supersede the astrophysical capture bounds for small or large DM mass, respectively for asymmetric or annihilating DM. Repulsive self-interactions of DM have an insignificant impact on the total capture rate, but a significant impact on the black hole formation criterion. This further weakens the constraints on DM-nucleon interaction strength for asymmetric self-repelling DM, whereas constraints remain unaltered for annihilating self-repelling DM. We use the correct Hawking evaporation rate of the newly formed black hole, that was approximated as a blackbody in previous studies, and show that, despite a more extensive alleviation of collapse as a result, the observation of a neutron star collapse can probe a wide range of DM self-interaction strengths.

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arXiv2020

Neutrino and Positron Constraints on Spinning Primordial Black Hole Dark Matter

Primordial black holes can have substantial spin -- a fundamental property that has a strong effect on its evaporation rate. We conduct a comprehensive study of the detectability of primordial black holes with non-negligible spin, via the searches for the neutrinos and positrons in the MeV energy range. Diffuse supernova neutrino background searches and observation of the 511 keV gamma-ray line from positrons in the Galactic center set competitive constraints. Spinning primordial black holes are probed up to a slightly higher mass range compared to non-spinning ones. Our constraint using neutrinos is slightly weaker than that due to the diffuse gamma-ray background, but complementary and robust. Our positron constraints are typically weaker in the lower mass range and stronger in the higher mass range for the spinning primordial black holes compared to the non-spinning ones. They are generally stronger than those derived from the diffuse gamma-ray measurements for primordial black holes having masses greater than a few $\times \, 10^{16}$g.

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arXiv2020

Galactic Positron Excess from Selectively Enhanced Dark Matter Annihilation

Precision measurements of the positron flux in cosmic ray have revealed an unexplained bump in the spectrum around $E\simeq 300\,\mathrm{GeV}$, not clearly attributable to known astrophysical processes. We propose annihilation of dark matter of mass $m_χ= 780\,\mathrm{GeV}$ with a late-time cross section $σv = 4.63\times 10^{-24}\,\mathrm{cm^3\,s^{-1}}$ as a possible source. The nonmonotonic dependence of the annihilation rate on dark matter velocity, owing to a selective $p$-wave Sommerfeld enhancement, allows such a large signal from the Milky Way without violating corresponding constraints from CMB and dwarf galaxy observations. We briefly explore other signatures of this scenario, and outline avenues to test it in future experiments.

hep-ph↗

arXiv2020

Fast Neutrino Flavor Conversion at Late Time

We study the fully nonlinear fast flavor evolution of neutrinos in 1+1 dimensions. Our numerical analysis shows that at late time the system reaches an approximately steady state. Using the steady state approximation we analytically show that the spatial variation of the polarization vectors is given by their precession around a common axis, which itself has a motion reminiscent of a gyroscopic pendulum. We then show that the steady state solution to the equations of motion cannot be separated in position and velocity, that is the motion is not collective in the usual sense. However, the fast evolution allows spectral-swap-like dynamics leading to partial decoherence over a range of velocities, constrained by conservation of lepton number(s). Finally, we numerically show that at late time the transverse components of the polarization vectors become randomly oriented at different spatial locations for any velocity mode and lepton asymmetry.

hep-ph↗

arXiv2020

Fast Neutrino Flavor Instability in the Neutron-star Convection Layer of Three-dimensional Supernova Models

Neutrinos from a supernova (SN) might undergo fast flavor conversions near the collapsed stellar core. We perform a detailed study of this intriguing possibility, analyzing time-dependent state-of-the-art 3D SN models of 9 and 20 Msun. Both models were computed with multi-D three-flavor neutrino transport based on a two-moment solver, and both exhibit the presence of the lepton-number emission self-sustained asymmetry (LESA). The transport solution does not provide the angular distributions of the neutrino fluxes, which are crucial to track the fast flavor instability. To overcome this limitation, we use a recently proposed approach based on the angular moments of the energy-integrated electron lepton-number distribution. With this method we find the possibility of fast neutrino flavor instability at radii <~20 km, which is well interior to the neutrinosphere. Our results confirm recent observations in a 2D SN model and in 2D/3D models with fixed matter background, which were computed with Boltzmann neutrino transport. However, the flavor unstable locations are not isolated points as discussed previously, but thin skins surrounding volumes where electron antineutrinos are more abundant than electron neutrinos. These volumes grow with time and appear first in the convective layer of the proto-neutron star (PNS), where a decreasing electron fraction (Ye) and high temperatures favor the occurrence of regions with negative neutrino chemical potential. Since Ye remains higher in the LESA dipole direction, where convective lepton-number transport out from the nonconvective PNS core slows down the deleptonization, flavor unstable conditions become more widespread in the opposite hemisphere. This interesting phenomenon deserves further investigation, since its impact on SN modeling and possible consequences for SN dynamics and neutrino observations are presently unclear. (abridged)

astro-ph.HE↗

arXiv2019

Ballistic Dark Matter oscillates above $Λ$CDM

Dark matter may have been relativistic and collisional until relatively late times and become cold and collisionless after a phase transition before the matter-radiation equality of the standard $Λ$CDM cosmology. We show that such a dark matter has large peculiar velocities due to acoustic oscillations before the phase transition, and evolves ballistically after the phase transition in the collisionless phase until the initial acoustic velocities are redshifted away. We show that this Ballistic Dark Matter (BDM) results in new non-trivial interesting features in the cosmological observables. In particular, the linear matter power spectrum exhibits acoustic oscillations on scales smaller than the Hubble scale at the time of phase transition, and for fast transitions the power at the acoustic peaks in the matter power spectrum exceeds that in a $Λ$CDM cosmology. If BDM only forms a part of the total dark matter, an odd vs. even acoustic peak asymmetry becomes prominent. We give an approximate analytical treatment of the linear perturbations in BDM, explaining these features. We also discuss the possibility to constrain BDM using cosmic microwave background and large scale structure data.

astro-ph.CO↗

arXiv2019

Collisional triggering of fast flavor conversions of supernova neutrinos

Fast flavor conversions of supernova neutrinos, possible near the neutrinosphere, depends on an interesting interplay of collisions and neutrino oscillations. Contrary to naive expectations, the rate of self-induced neutrino oscillations, due to neutrino-neutrino forward scattering, comfortably exceeds the rate of collisions even deep inside the supernova core. Consistently accounting for collisions and oscillations, we present the first calculations to show that collisions can create the conditions for fast flavor conversions of neutrinos, but oscillations can continue without significant damping thereafter. This may have interesting consequences for supernova explosions and the nature of its associated neutrino emission.

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arXiv2018

Normal-mode Analysis for Collective Neutrino Oscillations

In an interacting neutrino gas, collective modes of flavor coherence emerge that can be propagating or unstable. We derive the general dispersion relation in the linear regime that depends on the neutrino energy and angle distribution. The essential scales are the vacuum oscillation frequency $ω=Δm^2/(2E)$, the neutrino-neutrino interaction energy $μ=\sqrt{2}G_{\rm F} n_ν$, and the matter potential $λ=\sqrt{2}G_{\rm F} n_e$. Collective modes require non-vanishing $μ$ and may be dynamical even for $ω=0$ ('fast modes'), or they may require $ω\not=0$ ('slow modes'). The growth rate of unstable fast modes can be fast itself (independent of $ω$) or can be slow (suppressed by $\sqrt{|ω/μ|}$). We clarify the role of flavor mixing, which is ignored for the identification of collective modes, but necessary to trigger collective flavor motion. A large matter effect is needed to provide an approximate fixed point of flavor evolution, while spatial or temporal variations of matter and/or neutrinos are required as a trigger, i.e., to translate the disturbance provided by the mass term to seed stable or unstable flavor waves. We work out explicit examples to illustrate these points.

hep-ph↗

arXiv2018

A few moments to diagnose fast flavor conversions of supernova neutrinos

Neutrinos emitted from a supernova may undergo flavor conversions almost immediately above the core, with possible consequences for supernova dynamics and nucleosynthesis. However, the precise conditions for such fast conversions can be difficult to compute and require knowledge of the full angular distribution of the flavor-dependent neutrino fluxes, that is not available in typical supernova simulations. In this paper, we show that the overall flavor evolution is qualitatively similar to the growth of a so-called `zero mode', determined by the background matter and neutrino densities, which can be reliably predicted using only the second angular moments of the electron lepton number distribution, i.e., the difference in the angular distributions of $ν_e$ and $\barν_e$ fluxes. We propose that this zero mode, which neither requires computing the full Green's function nor a detailed knowledge of the angular distributions, may be useful for a preliminary diagnosis of possible fast flavor conversions in supernova simulations with modestly resolved angular distributions

hep-ph↗

arXiv2018

Model-independent diagnostic of self-induced spectral equalization versus ordinary matter effects in supernova neutrinos

Self-induced flavor conversions near the supernova (SN) core can make the fluxes for different neutrino species become almost equal, potentially altering the dynamics of the SN explosion and washing out all further neutrino oscillation effects. We present a new model-independent analysis strategy for the next galactic SN signal that will distinguish this flavor equalization scenario from a matter effects only scenario during the SN accretion phase. Our method does not rely on fitting or modelling the energy-dependent fluences of the different species to a known function, but rather uses a model-independent comparison of charged-current and neutral-current events at large next-generation underground detectors. Specifically, we advocate that the events due to elastic scattering on protons in a scintillator detector, which is insensitive to oscillation effects and can be used as a model-independent normalization, should be compared with the events due to inverse beta decay of $\barν_e$ in a water Cherenkov detector and/or the events due to charged-current interactions of $ν_e$ in an Argon detector. The ratio of events in these different detection channels allow one to distinguish a complete flavor equalization from a pure matter effect, for either of the neutrino mass orderings, as long as the spectral differences among the different species are not too small.

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arXiv2018

Sterile Neutrinos with Secret Interactions -- Cosmological Discord?

Several long-standing anomalies from short-baseline neutrino oscillation experiments -- most recently corroborated by new data from MiniBooNE -- have led to the hypothesis that extra, 'sterile', neutrino species might exist. Models of this type face severe cosmological constraints, and several ideas have been proposed to avoid these constraints. Among the most widely discussed ones are models with so-called 'secret interactions' in the neutrino sector. In these models, sterile neutrinos are hypothesized to couple to a new interaction, which dynamically suppresses their production in the early Universe through finite-temperature effects. Recently, it has been argued that the original calculations demonstrating the viability of this scenario need to be refined. Here, we update our earlier results from arXiv:1310.6337 [JCAP 1510 (2015) no.10, 011] accordingly. We confirm that much of the previously open parameter space for secret interactions is in fact ruled out by cosmological constraints on the sum of neutrino masses and on free-streaming of active neutrinos. We then discuss possible modifications of the vanilla scenario that would reconcile sterile neutrinos with cosmology.

hep-ph↗

arXiv2018

New Dissipation Mechanisms from Multi-level Dark Matter Scattering

Multi-level dark matter with diagonal and off-diagonal interactions shows a rich phenomenology in its self-scattering. If the interactions are mediated by a particle that is less massive than the dark matter, Sommerfeld effect can lead to resonant enhancement of the scattering. For mediators lighter than the level separation, dark matter particles can upscatter to excited states and de-excite by emitting these mediators. We compute these cross-sections, both above and below the kinematic threshold, in a generic two-component dark matter model and identify the large inelastic cross-section as a result of maximal mixing between the two states. A new route for cooling of large dark matter halos and a new drag force between two colliding halos are identified and shown to arise purely from the inelastic scattering.

hep-ph↗

arXiv2018

Fast Neutrino Flavor Conversion as Oscillations in a Quartic Potential

Neutrinos in dense environments undergo collective pair conversions $ν_e\barν_e \leftrightarrow ν_x\barν_x$, where $x$ is a non-electron flavor, due to forward scattering off each other that may be a crucial ingredient for supernova explosions. Depending on the flavor-dependent local angular distributions of the neutrino fluxes, the conversion rate can be "fast", i.e., of the order $μ=\sqrt{2}G_F n_ν$, which can far exceed the usual neutrino oscillation frequency $ω=Δm^2/(2E)$. Until now, this surprising nonlinear phenomenon has only been understood in the linear regime and explored further using numerical experiments. We present an analytical treatment of the simplest system that exhibits fast conversions, and show that the conversion can be understood as the dynamics of a particle rolling down in a quartic potential governed dominantly by $μ$, but seeded by slower oscillations.

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arXiv2017

Fast flavor conversions of supernova neutrinos: Classifying instabilities via dispersion relations

Supernova neutrinos can exhibit a rich variety of flavor conversion mechanisms. In particular, they can experience "fast" self-induced flavor conversions almost immediately above the core. Very recently, a novel method has been proposed to investigate these phenomena, in terms of the dispersion relation for the complex frequency and wave number ($ω$,$k$) of disturbances in the mean field of the $ν_eν_x$ flavor coherence. We discuss a systematic approach to such instabilities, originally developed in the context of plasma physics, and based of the time-asymptotic behavior of the Green's function of the system. Instabilities are typically seen to emerge for complex $ω$, and can be further characterized as convective (moving away faster than they spread) and absolute (growing locally), depending on $k$-dependent features. Stable cases emerge when $k$ (but not $ω$) is complex, leading to disturbances damped in space, or when both $ω$ and $k$ are real, corresponding to complete stability. The analytical classification of both unstable and stable modes leads not only to qualitative insights about their features but also to quantitative predictions about the growth rates of instabilities. Representative numerical solutions are discussed in a simple two-beam model of interacting neutrinos. As an application, we argue that supernova and binary neutron star mergers exhibiting a "crossing" in the electron lepton number would lead to an absolute instability in the flavor content of the neutrino gas.

hep-ph↗

arXiv2017

Selection Rule for Enhanced Dark Matter Annihilation

We point out a selection rule for enhancement (suppression) of odd (even) partial waves of dark matter coannihilation or annihilation using Sommerfeld effect. Using this, the usually velocity-suppressed p-wave annihilation can dominate the annihilation signals in the present Universe. The selection mechanism is a manifestation of the exchange symmetry of identical incoming particles, and generic for multi-state DM with off-diagonal long-range interactions. As a consequence, the relic and late-time annihilation rates are parametrically different and a distinctive phenomenology, with large but strongly velocity-dependent annihilation rates, is predicted.

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arXiv2016

Fast neutrino flavor conversions near the supernova core with realistic flavor-dependent angular distributions

It has been recently pointed out that neutrino fluxes from a supernova can show substantial flavor conversions almost immediately above the core. Using linear stability analyses and numerical solutions of the fully nonlinear equations of motion, we perform a detailed study of these fast conversions, focussing on the region just above the supernova core. We carefully specify the instabilities for evolution in space or time, andfind that neutrinos travelling towards the core make fast conversions more generic, i.e., possible for a wider range of flux ratios and angular asymmetries that produce a crossing between the zenith-angle spectra of $ν_e$ and ${\barν_e}$. Using fluxes and angular distributions predicted by supernova simulations, we find that fast conversions can occur within tens of nanoseconds, only a few meters away from the putative neutrinospheres. If these fast flavor conversions indeed take place, they would have important implications for the supernova explosion mechanism and nucleosynthesis.

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