arXiv subjects

Shashank Shalgar

Publications and source records attributed to Shashank Shalgar.

At least 37 records · Page 2Linked to original sources

arXiv2022

Symmetry breaking induced by pairwise conversion of neutrinos in compact sources

A surprising consequence of non-linear flavor evolution is the spontaneous breaking of the initial symmetries of the neutrino gas propagating in a dense astrophysical environment. We explore the flavor conversion physics by taking into account the polar and azimuthal angular distributions of neutrinos and present the very first example of spontaneous symmetry breaking in the context of fast flavor mixing in the nonlinear regime. Intriguingly, we find that fast flavor mixing does not always develop in the proximity of the angular regions with vanishing electron lepton number, as commonly assumed, and large flavor mixing can rapidly spread through all neutrino modes. Such behavior cannot be predicted from the linear regime of the flavor evolution. These results can have major consequences on the physics of compact astrophysical objects.

hep-ph↗

arXiv2021

Fast flavor conversion of neutrinos in presence of matter bulk velocity

A dense gas of neutrinos and antineutrinos can undergo fast pairwise conversions near the decoupling regions of core-collapse supernovae and in compact binary neutron star mergers. The flavor dependent neutrino heating can play a role in sustaining convection inside the hot and dense matter. In this paper, we study the unexplored effect of the bulk velocity of matter on fast pairwise conversions and demonstrate that depending on the direction and the magnitude of the bulk velocity, neutrino flavor conversions could be significantly enhanced or suppressed. The bulk velocity of matter, which is usually neglected in the context of neutrino oscillations, can reach values of one-tenth of the speed of light in astrophysical environments. We find that bulk velocities much smaller than the maximum allowed velocities can substantially change the neutrino flavor conversion rate. The demonstration of possible enhancement of neutrino flavor conversion rate due to the bulk velocity of matter also raises several important issues that are of relevance in the supernova mechanism. Future studies with realistic velocity profiles could elucidate the possible implications on the phenomenology of core-collapse supernovae and neutron star mergers.

astro-ph.HE↗

arXiv2021

A closer look at the $pp$-chain reaction in the Sun: Constraining the coupling of light mediators to protons

The $pp$-chain of nuclear reactions is the primary route for energy production in the Sun. The first step in that reaction sequence converts two protons to a deuterium nucleus with the emission of a positron and electron neutrino. This reaction is extremely slow because it is a weak interaction, and significantly, it involves quantum tunneling through the Coulomb barrier. Though the reaction rate can be calculated with high confidence in the Standard Model, it has not been measured at solar energies. If there exist interactions that are engendered by non-standard mediators then the rate of this reaction in the Sun could be altered. We probe such non-standard interactions by comparing calculations of solar evolution to the current solar system age in the presence and absence of the non-standard mediators. These reveal ranges of non-standard mediator mass and couplings that are inconsistent with measured properties of the Sun, including solar neutrino results. Our constraints on these non-standard parameters, in many cases overlapping those derived via other considerations, could be extended further with better confidence in the value of the metalicity of the Sun and the solar neutrino CNO flux. Intriguingly, our work reveals a degeneracy between the solar metalicity and the presence of the invoked non-standard mediators.

astro-ph.HE↗

arXiv2021

The three flavor revolution in fast pairwise neutrino conversion

The modeling of fast flavor evolution of neutrinos in dense environments has been traditionally carried out by relying on a two flavor approximation for simplicity. In addition, vacuum mixing has been deemed negligible. For the first time, we highlight that the fast flavor evolution in three flavors is intrinsically different from the one obtained in the two flavor approximation. This is due to the exponential growth of flavor mixing in the $e$--$μ$ and $e$--$τ$ sectors generated by the vacuum term in the Hamiltonian. As a result, substantially larger flavor mixing is found in three flavors. Our findings highlight that the two flavor approximation is not justified for fast pairwise conversion, even if the angular distributions of non-electron type neutrinos are initially identical.

hep-ph↗

arXiv2021

Neutrino flavor mixing breaks isotropy in the early universe

The neutrino field is commonly assumed to be isotropic and homogeneous in the early universe. However, due to the large neutrino density, a small perturbation of the isotropy of the neutrino field could potentially be amplified by the non-linear flavor mixing caused by neutrino self-interactions. We carry out the first numerical simulations of the neutrino flavor evolution in a multi-angle anisotropic setting. Due to the computational challenges involved, we adopt a simplified framework consisting of a homogeneous universe with two angle bins -- left and right moving modes -- for neutrinos and antineutrinos, together with an approximate form for the collision term which goes beyond the commonly adopted damping approximation. By assuming a small initial left-right asymmetry of $\mathcal{O}(10^{-15})$, we convincingly demonstrate that flavor evolution can be affected in both mass orderings, with implications on the effective number of thermally excited neutrino species ($N_{\mathrm{eff}}$). Notably, the correction to $N_{\rm eff}$ is comparable to higher order corrections from finite temperature QED effects in normal ordering. In addition, by assuming an initial lepton asymmetry in the neutrino sector of the same order as the baryon one [$\mathcal{O}(10^{-9})$], we find that the neutrino-antineutrino asymmetry grows by several orders of magnitude for isotropic as well as anisotropic initial conditions. This work clearly shows that it is imperative to critically revisit standard assumptions concerning neutrino flavor mixing in the early universe, especially in the light of possible implications on the cosmological observables.

astro-ph.CO↗

arXiv2021

Core-collapse supernovae stymie secret neutrino interactions

Beyond-the-Standard-Model interactions of neutrinos among themselves -- {\it secret interactions} -- in the supernova core may prevent the shock revival, halting the supernova explosion. Besides, if supernova neutrinos en route to Earth undergo secret interactions with relic neutrinos, the neutrino burst reaching Earth may be down-scattered in energy, falling below the detection threshold. We probe secret neutrino interactions through supernova neutrinos and apply our findings to the supernova SN 1987A. We place the most stringent bounds on flavor-universal secret interactions occurring through a new mediator with mass between 10 MeV and 1 GeV.

astro-ph.HE↗

arXiv2021

A change of direction in pairwise neutrino conversion physics: The effect of collisions

Fast pairwise conversions of neutrinos may affect the flavor distribution in the core of neutrino-dense sources. We explore the interplay between collisions and fast conversions within a simplified framework that assumes angle-independent, direction-changing collisions in a neutrino gas that has no spatial inhomogeneity. Counter to expectations, we find that collisions may enhance fast flavor conversions instead of damping them. Our work highlights the need to take into account the feedback of collisions on the neutrino angular distributions self-consistently, in order to predict the flavor outcome in the context of fast pairwise conversions reliably.

astro-ph.HE↗

arXiv2021

New Developments in Flavor Evolution of a Dense Neutrino Gas

Neutrino-neutrino refraction dominates the flavor evolution in core-collapse supernovae, neutron-star mergers, and the early universe. Ordinary neutrino flavor conversion develops on timescales determined by the vacuum oscillation frequency. However, when the neutrino density is large enough, collective flavor conversion may arise because of pairwise neutrino scattering. Pairwise conversion is deemed to be fast as it is expected to occur on timescales that depend on the neutrino-neutrino interaction energy (i.e., on the neutrino number density) and is regulated by the angular distributions of electron neutrinos and antineutrinos. The enigmatic phenomenon of fast pairwise conversion has been overlooked for a long time. However, because of the fast conversion rate, pairwise conversion may possibly occur in the proximity of the neutrino decoupling region with yet to be understood implications for the hydrodynamics of astrophysical sources and the synthesis of the heavy elements. We review the physics of this fascinating phenomenon and its implications for neutrino-dense sources.

astro-ph.HE↗

arXiv2020

Multi-Dimensional Solution of Fast Neutrino Conversions in Binary Neutron Star Merger Remnants

Fast pairwise conversions of neutrinos are predicted to be ubiquitous in neutron star merger remnants with potentially major implications on the nucleosynthesis of the elements heavier than iron. We present the first sophisticated numerical solution of the neutrino flavor evolution above the remnant disk within a (2+1+1) dimensional setup: two spatial coordinates, one angular variable, and time. We look for a steady-state flavor configuration above the remnant disk. Albeit the linear stability analysis predicts flavor instabilities at any location above the remnant disk, our simulations in the non-linear regime show that fast pairwise conversions lead to minimal neutrino mixing (<1%); flavor equilibration is never achieved in our models. Importantly, fast neutrino conversions are more prominent within localized regions near the edges of the (anti)neutrino decoupling surfaces and almost negligible in the polar region of the remnant. Our findings on the role of fast pairwise conversions should be interpreted with caution because of the approximations intrinsic to our setup and advocate for further work within a more realistic framework.

astro-ph.HE↗

arXiv2020

Dispelling a myth on dense neutrino media: fast pairwise conversions depend on energy

Pairwise conversions of neutrinos have complicated the challenging goal of quantifying the relevance of neutrino microphysics in compact astrophysical objects, limiting the ability to perform numerical simulations and encouraging the employment of semi-analytical tools, such as the linear stability analysis. Given the high neutrino density, the dependence of pairwise conversions on the neutrino energy has been deemed to play a negligible role. We show that fast pairwise conversions are affected by the neutrino energy, especially in the non-linear regime. An earlier onset of flavor conversions and a higher oscillation frequency are found as the vacuum frequency increases (i.e., the neutrino energy decreases); the oscillation periodicity, otherwise present, is gradually destroyed as the vacuum frequency increases. Such effects are, however, not further exacerbated by the inclusion of spectral energy distributions for neutrinos, contrary to what was found for "slow" neutrino-neutrino conversions. In addition, our findings highlight the dependence of fast pairwise conversions on the neutrino mass ordering for realistic values of the neutrino vacuum frequency. Our findings highlight the limitations intrinsic to widely adopted approximations and extrapolations based on the linear regime of fast conversions. In order to gauge their possible impact on the source physics, a more sophisticated modeling of pairwise conversions is necessary.

astro-ph.HE↗

arXiv2020

Neutrino propagation hinders fast pairwise flavor conversions

Neutrino flavor conversions may dramatically affect the inner working of compact astrophysical objects as well as the synthesis of the heavier elements. We present the first sophisticated numerical solution of the neutrino flavor conversion within a (2+1+1) dimensional setup: we include the advective term in the neutrino equations of motion and track the flavor evolution in two spatial dimensions, one angular variable, and time. Notably, the advective term hinders the development of neutrino pairwise conversions, if the conditions favoring such conversions (i.e., crossings between the angular distributions of $ν_e$ and $\barν_e$ or a non-negligible flux of neutrinos traveling backward with respect to the main propagation direction) exist for time scales shorter than the typical time scale of the advective term. As a consequence, fast pairwise conversions can only occur when the conditions favoring flavor conversions are self-sustained and global, such as the ones induced by the lepton emission self-sustained asymmetry (LESA) in core-collapse supernovae. Our work highlights the major impact of the dynamical evolution of the neutrino field on the growth of flavor instabilities and the strong interplay between classical and quantum effects. Critical limitations of the linear stability analysis, used to predict neutrino flavor instabilities, are also pointed out.

astro-ph.HE↗

arXiv2020

Bounds on secret neutrino interactions from high-energy astrophysical neutrinos

Neutrinos offer a window to physics beyond the Standard Model. In particular, high-energy astrophysical neutrinos, with TeV-PeV energies, may provide evidence of new, "secret" neutrino-neutrino interactions that are stronger than ordinary weak interactions. During their propagation over cosmological distances, high-energy neutrinos could interact with the cosmic neutrino background via secret interactions, developing characteristic energy-dependent features in their observed energy distribution. For the first time, we look for signatures of secret neutrino interactions in the diffuse flux of high-energy astrophysical neutrinos, using 6 years of publicly available IceCube High Energy Starting Events (HESE). We find no significant evidence for secret neutrino interactions, but place competitive upper limits on the coupling strength of the new mediator through which they occur, in the mediator mass range of 1-100 MeV.

astro-ph.HE↗

arXiv2019

On the Occurrence of Crossings Between the Angular Distributions of Electron Neutrinos and Antineutrinos in the Supernova Core

Neutrino fast pairwise conversions have been postulated to occur in the dense core of a core-collapse supernova (SN), possibly having dramatic consequences on the SN mechanism and the observable neutrino signal. One crucial condition favoring pairwise conversions is the presence of crossings between the electron neutrino and antineutrino angular distributions (i.e., electron neutrino lepton number crossings, ELN crossings). A stationary and spherically symmetric SN toy-model is constructed to reproduce the development of the neutrino angular distributions in the dense SN core in the absence of perturbations induced by hydrodynamical instabilities. By iteratively solving the neutrino Boltzmann equations including the collisional term, our model predicts that ELN crossings can develop only in the proximity of the decoupling region and for a sharp radial evolution of the baryon density, when the electron neutrino and antineutrino number densities are comparable. Such conditions are likely to occur only in the late SN stages. Interestingly, flavor instabilities induced by spatial or temporal perturbations are unlikely to generate ELN crossings dynamically within our simplified setup.

astro-ph.HE↗

arXiv2018

Matter parametric neutrino flavor transformation through Rabi resonances

We consider the flavor transformation of neutrinos through oscillatory matter profiles. We show that the neutrino oscillation Hamiltonian in this case describes a Rabi system with an infinite number of Rabi modes. We further show that, in a given physics problem, the majority of the Rabi modes have too small amplitudes to be relevant. We also go beyond the rotating wave approximation and derive the relative detuning of the Rabi resonance when multiple Rabi modes with small amplitudes are present. We provide an explicit criterion of whether an off-resonance Rabi mode can affect the parametric flavor transformation of the neutrino.

hep-ph↗

arXiv2018

Collective neutrino oscillations with the halo effect in single-angle approximation

We perform a self-consistent calculation of collective neutrino oscillations including the effect of back scattered neutrinos (Halo effect) in the `single-angle' approximation, within a spherically symmetric supernova model. We find that due to the Halo effect the onset of flavor transformations is pushed to smaller radii, by a few kilometers. The celebrated phenomenon of the spectral split is found to be robust under the present inclusion of the Halo effect.

hep-ph↗

arXiv2018

Multi-angle calculation of the matter-neutrino resonance near an accretion disk

We perform a numerical study of the matter-neutrino resonance in a multi-angle calculation in the vicinity of an accretion disk. We assume thermally distributed neutrino and anti-neutrino fields emitted by two-dimensional disk that is homogeneous and isotropic; the electrons are assumed to be at constant density. We compare the the result of this computation to that obtained using single-angle approximation. We investigate the robustness of matter-neutrino resonance in environment surrounding accretion disks by progressively relaxing the single angle approximation. We find that the multi-angle results in the present simplified model do not support a robust resonance mechanism as suggested by the single angle treatment. We also discuss the context under which matter-neutrino resonance may be important in future studies.

hep-ph↗

arXiv2017

Effect of collisions on neutrino flavor inhomogeneity in a dense neutrino gas

We investigate the stability, with respect to spatial inhomogeneity, of a two-dimensional dense neutrino gas. The system exhibits growth of seed inhomogeneity due to nonlinear coherent neutrino self-interactions. In the absence of incoherent collisional effects, we observe a dependence of this instability growth rate on the neutrino mass spectrum: the normal neutrino mass hierarchy exhibits spatial instability over a larger range of neutrino number density compared to that of the inverted case. We further consider the effect of elastic incoherent collisions of the neutrinos with a static background of heavy, nucleon-like scatterers. At small scales, the growth of flavor instability can be suppressed by collisions. At large length scales we find, perhaps surprisingly, that for inverted neutrino mass hierarchy incoherent collisions fail to suppress flavor instabilities, independent of the coupling strength.

hep-ph↗

arXiv2016

Indirect Signals from Solar Dark Matter Annihilation to Long-lived Right-handed Neutrinos

We study indirect detection signals from solar annihilation of dark matter (DM) particles into light right-handed (RH) neutrinos with a mass in a $1-5$ GeV range. These RH neutrinos can have a sufficiently long lifetime to allow them to decay outside the Sun and their delayed decays can result in a signal in gamma rays from the otherwise `dark' solar direction, and also a neutrino signal that is not suppressed by the interactions with solar medium. We find that the latest Fermi-LAT and IceCube results place limits on the gamma ray and neutrino signals, respectively. Combined photon and neutrino bounds can constrain the spin-independent DM-nucleon elastic scattering cross section better than direct detection experiments for DM masses from 200 GeV up to several TeV. The bounds on spin-dependent scattering are also much tighter than the strongest limits from direct detection experiments.

hep-ph↗