Publications
13) Photons from neutrinos: the gamma ray echo of a supernova neutrino burst.
When a star undergoes core collapse, a vast amount of energy is released in a ~10 s long burst of neutrinos of all species. Inverse beta decay in the star's hydrogen envelope causes an electromagnetic cascade which ultimately results in a flare of gamma rays - an "echo" of the neutrino burst - at the characteristic energy of 0.511 MeV. We study the phenomenology and detectability of this flare. Its luminosity curve is characterized by a fast, seconds-long, rise and an equally fast decline, with a minute- or hour-long plateau in between. For a near-Earth star (distance D<1 kpc) the echo will be observable at near future gamma ray telescopes with an effective area of 10^3 cm^2 or larger. Its observation will inform us on the envelope size and composition. In conjunction with the direct detection of the neutrino burst, it will also give information on the neutrino emission away from the line of sight and will enable tests of neutrino propagation effects between the stellar surface and Earth.
The next generation gravitational wave (GW) detectors - Einstein Telescope (ET) and Cosmic Explorer (CE), will have distance horizons up to order 10 Gpc for detecting binary neutron star (BNS) mergers. This will make them ideal for triggering high-energy neutrino searches from BNS mergers at the next generation neutrino detectors, such as IceCube-Gen2. We calculate the distance limits as a function of the time window of neutrino analysis, up to which meaningful triggers from the GW detectors can be used to minimize backgrounds and collect a good sample of high-energy neutrino events at the neutrino detectors, using the sky localization capabilities of the GW detectors. We then discuss the prospects of the next generation detectors to work in synergy to facilitate coincident neutrino detections or to constrain the parameter space in the case of non-detection of neutrinos. We show that good localization of GW events, which can be achieved by multiple third generation GW detectors, is necessary to detect a GW-associated neutrino event or put a meaningful constraint (∼ 3 σ confidence level) on neutrino emission models. Such a model independent analysis can also help constrain physical models and hence provide insights into neutrino production mechanisms in binary neutron star mergers.
11) On probing turbulence in core-collapse supernovae in upcoming neutrino detectors
Neutrino propagation through a turbulent medium can be highly non-adiabatic leading to distinct signatures in the survival probabilities. A core-collapse supernova can be host to a number of hydrodynamic instabilities which occur behind the shockfront. Such instabilities between the forward shock and a possible reverse shock can lead to cascades introducing turbulence in the associated matter profile, which can imprint itself in the neutrino signal. In this work, we consider realistic matter profiles and seed in the turbulence using a randomization scheme to study its effects on neutrino propagation in an effective two-flavor framework. In particular, we find that the double-dip feature, originally predicted in the neutrino spectra associated with forward and reverse shocks, can be completely washed away in the presence of turbulence, leading to total flavor depolarization. We also study the sensitivity of upcoming neutrino detectors - DUNE and Hyper-Kamiokande- to the power spectrum of turbulence to check for deviations from the usual Kolmogorov (
5/3) inverse power law. We find that while these experiments can effectively constrain the parameter space for the amplitude of the turbulence power spectra, they will only be mildly sensitive to the spectral index.
10) Multi-messenger signatures of delayed choked jets in tidal disruption events.
Recent radio observations and coincident neutrino detections suggest that some tidal disruption events (TDEs) exhibit late-time activities, relative to the optical emission peak, and these may be due to delayed outflows launched from the central supermassive black hole. We investigate the possibility that jets launched with a time delay of days to months, interact with a debris that may expand outwards. We discuss the effects of the time delay and expansion velocity on the outcomes of jet breakout and collimation. We find that a jet with an isotropic-equivalent luminosity of $\leq 5 x 10^{45}$ erg/s is likely to be choked for a delay time of ~ 3 months. We also study the observational signatures of such delayed choked jets. The jet-debris interaction preceding the breakout would lead to particle acceleration and the resulting synchrotron emission can be detected by current and near-future radio, optical and X-ray telescopes, and the expanding jet-driven debris could explain late-time radio emission. We discuss high-energy neutrino production in delayed choked jets, and the time delay can significantly alleviate the difficulty of the hidden jet scenario in explaining neutrino coincidences.
9) Neutrinos from the brightest gamma-ray burst?
We discuss implications that can be obtained by searches for neutrinos from the brightest gamma- ray burst, GRB 221009A. We derive constraints on GRB model parameters such as the cosmic-ray loading factor and dissipation radius, taking into account both neutrino spectra and effective areas. The results are strong enough to constrain proton acceleration near the photosphere, and we find that the single burst limits are comparable to those from stacking analysis. Quasithermal neutrinos from subphotospheres and ultrahigh-energy neutrinos from external shocks are not yet constrained. We show that GeV-TeV neutrinos originating from analysis on these neutrinos with DeepCore and neutron collisions are detectable, and urge dedicated IceCube as well as ORCA and KM3NeT.
8) Memory-triggered supernova neutrino detection
In this work, we demonstrated that observations of the gravitational memory from core collapse supernovae at future Deci-Hz interferometers enables time-triggered searches of supernova neutrinos at Mt-scale detectors. Achieving a sensitivity to characteristic strains of at least ~ 10^{-25} at $f ~ 0.3 Hz -- e.g., by improving the noise of DECIGO by one order of magnitude -- will allow robust time triggers for supernovae at distances up to 10 - 100 Mpc, resulting in a nearly background-free sample of ~ 3 - 30 neutrino events per Mt per decade of operation. This sample would bridge the sensitivity gap between rare galactic supernova bursts and the cosmological diffuse supernova flux, allowing detailed studies of the neutrino emission of supernovae in the local Universe.
In this work, we develop a phenomenological (analytical) toy model for the supernova neutrino memory effect, which is overall consistent with the results of numerical simulations. General Relativity predicts that the passage of matter or radiation from an asymmetrically-emitting source should cause a permanent change in the local space-time metric. This phenomenon, called the gravitational memory effect, has never been observed, however supernova neutrinos have long been considered a promising avenue for its detection in the future. With the advent of deci-Hertz gravitational wave interferometers, observing the supernova neutrino memory will be possible, with important implications for multimessenger astronomy and for tests of gravity. Our description is generalized to several case studies of interest. We find that, for a galactic supernova, the dimensionless strain, h(t), is of order ~ 10^{-22} - 10^{-21} , and develops over a typical time scale that varies between ~ 0.1 - 10 s, depending on the time-evolution of the anisotropy of the neutrino emission. The characteristic strain, has a maximum at a frequency f_{max} ~ O(0.1) - O(1) Hz. The detailed features of the time- and frequency-structure of the memory strain will inform us of the matter dynamics near the collapsed core, and allow to distinguish between different stellar collapse scenarios. Next generation gravitational wave detectors like DECIGO and BBO will be sensitive to the neutrino memory effect for supernovae at typical galactic distances and beyond; with Ultimate DECIGO exceeding a detectability distance of 10 Mpc.
6) Presupernova neutrinos: directional sensitivity and prospects for progenitor identification
For this work, we explored the potential of current and future liquid scintillator neutrino detectors of kt mass to localize a presupernova neutrino signal in the sky.
In the hours preceding the core collapse of a nearby star (at distance kpc), tens to hundreds of inverse beta decay events will be recorded, and their reconstructed topology in the detector can be used to estimate the direction to the star. Although the directionality of inverse beta decay is weak (∼8% forward−backward asymmetry for currently available liquid scintillators), we find that for a fiducial signal of 200 events (which is realistic for Betelgeuse), a positional error of ∼60° can be achieved, resulting in the possibility to narrow the list of potential stellar candidates to less than 10, typically. For a configuration with improved forward−backward asymmetry (∼40%, as expected for a lithium-loaded liquid scintillator), the angular sensitivity improves to ∼15°, and—when a distance upper limit is obtained from the overall event rate—it is in principle possible to uniquely identify the progenitor star. Any localization information accompanying an early supernova alert will be useful to multimessenger observations and to particle physics tests using collapsing stars.
5) Resonance structures in kink-antikink scattering in a quantum vacuum
We investigate kink-antikink scattering in the λφ4 model in the presence of an additional scalar field, ψ, that is in its quantum vacuum and interacts with φ via a ξ φ^2ψ^2 term where ξ is the coupling. The final state of such a scattering is either a bound state with eventual annihilation or a reflection of the kink-antikink pair. Without the ψ field, the outcome is known to depend fractally on the initial velocity of the kink-antikink pair. In the quantum vacuum of the ψ field, the fractal dependence gets modified and disappears above a critical interaction strength, ξ ≈ 0.1.
4) Kink-antikink scattering in a quantum vacuum
We study kink-antikink scattering in the sine-Gordon model in the presence of interactions with an additional scalar field, $\psi$, that is in its quantum vacuum. In contrast to the classical scattering, now there is quantum radiation of $\psi$ quanta and the kink-antikink may form bound states that resemble breathers of the sine-Gordon model. We quantify the rate of radiation and map the parameters for which bound states are formed. Even these bound states radiate and decay, and eventually there is a transition into long-lived oscillons.
3) Quantum Formation of Topological Defects
In this work, we consider quantum phase transitions with global symmetry breakings that result in the formation of topological defects. We evaluate the number densities of kinks, vortices, and monopoles that are produced in d=1, 2, 3 spatial dimensions, respectively, and find that they scale as t^{-d/2} and evolve toward attractor solutions that are independent of the quench timescale. For d=1 our results apply in the region of parameters λτ/m≪1 where λ is the quartic self-interaction of the order parameter, τ is the quench timescale, and m is the mass parameter.
2) Emergence of classical structures from the quantum vacuum
After a quantum phase transition, the quantum vacuum can break up to form classical topological defects. For this work, we examine this process for scalar field models with Z_2 symmetry for different quench rates for the phase transition. We find that the number density of kinks at late times universally scales as Cm^{1/2}t^{-1/2}, where m is a mass scale in the model and C ≈ 0.22; it does not depend on the quench timescale in contrast to the Kibble-Zurek scaling for thermal phase transitions. A subleading correction ∝t^{-3/2} to the kink density depends on the details of the phase transition.
1) Rolling classical scalar field in a linear potential coupled to a quantum field
In this work, we studied the dynamics of a classical scalar field that rolls down a linear potential as it interacts bi-quadratically with a quantum field. We explicitly solve the dynamical problem by using the classical-quantum correspondence (CQC). Rolling solutions on the effective potential are shown to compare very poorly with the full solution. Spatially homogeneous initial conditions maintain their homogeneity and small inhomogeneities in the initial conditions do not grow.