Large Neutrino Secret Interactions Have a Small Impact on Supernovae.

When hypothetical neutrino secret interactions (νSI) are large, they form a fluid in a supernova (SN) core, flow out with sonic speed, and stream away as a fireball. For the first time, we tackle the complete dynamical problem and solve all steps, systematically using relativistic hydrodynamics. The impact on SN physics and the neutrino signal is remarkably small. For complete thermalization within the fireball, the observable spectrum changes in a way that is independent of the coupling strength. One potentially large effect beyond our study is quick deleptonization if νSI violate lepton number. By present evidence, however, SN physics leaves open a large region in parameter space, where laboratory searches and future high-energy neutrino telescopes will probe νSI.

Strong Supernova 1987A Constraints on Bosons Decaying to Neutrinos.

Majoron-like bosons would emerge from a supernova (SN) core by neutrino coalescence of the form νν→ϕ and ν[over ¯]ν[over ¯]→ϕ with 100-MeV-range energies. Subsequent decays to (anti)neutrinos of all flavors provide a flux component with energies much larger than the usual flux from the "neutrino sphere." The absence of 100-MeV-range events in the Kamiokande-II and Irvine-Michigan-Brookhaven signal of SN 1987A implies that less than 1% of the total energy was thus emitted and provides the strongest constraint on the Majoron-neutrino coupling of g≲10^{-9} MeV/m_{ϕ} for 100 eV≲m_{ϕ}≲100 MeV. It is straightforward to extend our new argument to other hypothetical feebly interacting particles.

Low-Energy Supernovae Severely Constrain Radiative Particle Decays.

The hot and dense core formed in the collapse of a massive star is a powerful source of hypothetical feebly interacting particles such as sterile neutrinos, dark photons, axionlike particles (ALPs), and others. Radiative decays such as a→2γ deposit this energy in the surrounding material if the mean free path is less than the radius of the progenitor star. For the first time, we use a supernova (SN) population with particularly low explosion energies as the most sensitive calorimeters to constrain this possibility. These SNe are observationally identified as low-luminosity events with low ejecta velocities and low masses of ejected ^{56}Ni. Their low energies limit the energy deposition from particle decays to less than about 0.1 B, where 1 B(bethe)=10^{51} erg. For 1-500 MeV-mass ALPs, this generic argument excludes ALP-photon couplings G_{aγγ} in the 10^{-10}-10^{-8} GeV^{-1} range.

Axionlike Particles from Hypernovae.

It was recently pointed out that very energetic subclasses of supernovae (SNe), like hypernovae and superluminous SNe, might host ultrastrong magnetic fields in their core. Such fields may catalyze the production of feebly interacting particles, changing the predicted emission rates. Here we consider the case of axionlike particles (ALPs) and show that the predicted large scale magnetic fields in the core contribute significantly to the ALP production, via a coherent conversion of thermal photons. Using recent state-of-the-art supernova (SN) simulations, including magnetohydrodynamics, we find that, if ALPs have masses m_{a}∼O(10) MeV, their emissivity in such rare but exciting conditions via magnetic conversions would be over 2 orders of magnitude larger than previously estimated. Moreover, the radiative decay of these massive ALPs would lead to a peculiar delay in the arrival times of the daughter photons. Therefore, high-statistics gamma-ray satellites can potentially discover MeV ALPs in an unprobed region of the parameter space and shed light on the magnetohydrodynamical nature of the SN explosion.

Exploring Primordial Black Holes from the Multiverse with Optical Telescopes.

Primordial black holes (PBHs) are a viable candidate for dark matter if the PBH masses are in the currently unconstrained "sublunar" mass range. We revisit the possibility that PBHs were produced by nucleation of false vacuum bubbles during inflation. We show that this scenario can produce a population of PBHs that simultaneously accounts for all dark matter, explains the candidate event in the Subaru Hyper Suprime-Cam (HSC) data, and contains both heavy black holes as observed by LIGO and very heavy seeds of supermassive black holes. We demonstrate with numerical studies that future observations of HSC, as well as other optical surveys, such as LSST, will be able to provide a definitive test for this generic PBH formation mechanism if it is the dominant source of dark matter.

Tunable Axion Plasma Haloscopes.

We propose a new strategy for searching for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes, we estimate competitive sensitivity for axion masses of 35-400 µeV, at least.