arXiv subjects

George M. Fuller

Publications and source records attributed to George M. Fuller.

At least 19 recordsLinked to original sources

arXiv2026

Neutrino helicity oscillations in astrophysical environments: A many-body approach

Neutrino rest mass enables left-handed states to "flip" to right-handed states and vice versa. In-medium effects can enhance the probability for such spin-flip. We demonstrate that a full many-body calculation of this process in neutrino-dense environments can lead to spin-flip probabilities that exceed by orders of magnitude those calculated with mean-field treatments. We study simple configurations with a few neutrinos in well-defined momentum states, for which we show that the helicity conversion enhancement is connected to many-body momentum exchange. Such an effect would therefore be missed in a calculation that considers only forward processes. We speculate on the potential astrophysical implications of these results and the range of applicability of our calculation and its limitations.

hep-ph↗

arXiv2026

Neutrino Flavor Transformation in Collapsing Supermassive Objects

The collapse of supermassive stars (SMSs, $M\gtrsim10^4\,M_\odot$) to black holes is accompanied by a prodigious flux of neutrinos of all flavors. These are produced thermally via $e^\pm$ annihilations, mostly in the core and just before gravitational trapped surface formation. There, the ratio of fluxes for $ν_e\barν_e$-pairs to $ν_μ\barν_μ/ν_τ\barν_τ$-pairs is $\sim$\,5-to-1. This is because at SMS temperature scales, $ν_e\barν_e$ pairs have both charged and neutral current production channels, whereas $ν_μ\barν_μ/ν_τ\barν_τ$-pairs only have neutral current production channels. We point out that the typical energies of these neutrinos, and the run of density in collapsing radiation-dominated supermassive configurations, leads to Mikheyev-Smirnov-Wolfenstein (MSW) resonances inside these objects for the atmospheric neutrino mass splitting scale, $Δm^2_\mathrm{atm.}\sim2.4\times10^{-3}$ eV$^2$. In the normal neutrino mass hierarchy, adiabatic flavor transformation through the MSW resonances would then swap the fluxes $ν_e\leftrightharpoonsν_{μ,τ}$, whereas, in the inverted neutrino mass hierarchy, the anti-neutrino fluxes are swapped, $\barν_e\leftrightharpoons\barν_{μ,τ}$. We also examine the prospects for collective neutrino flavor oscillations in these environments. Implications for flavor oscillation's effects on neutrino energy deposition and neutrino-induced nucleosynthesis in the SMS's outer layers are examined, as are prospects for detections of SMS collapses through various means.

astro-ph.CO↗

arXiv2025

Early Formation of Supermassive Black Holes via Dark Star Gravitational Instability

We show that dark stars, which are dark-matter-powered stars in the early universe, can grow by accretion to masses in the range $\mathscr{O}\left ({10}^4\right )-\mathscr{O}\left ({10}^7\right)\,{M_\odot}$ before the general-relativistic Feynman-Chandrasekhar instability causes their dynamical collapse to black holes. These accreting dark star configurations avoid standard stellar nuclear- and weak-interaction evolution that would lead to their demise long before they reached this supermassive size. Remarkably, this mechanism for supermassive black hole (SMBH) genesis is relatively robust to initial dark star mass, formation epoch, accretion rate and its history. The SMBHs produced this way can serve as seeds for even larger SMBHs $({\gtrsim}10^9\,M_\odot)$ that have been discovered at high redshift.

astro-ph.CO↗

arXiv2025

Non-conservation of Lepton Numbers in the Neutrino Sector Could Change the Prospects for Core Collapse Supernova Explosions

We show that interactions violating the conservation of lepton numbers in the neutrino sector could significantly alter the standard low entropy picture for the pre-supernova collapsing core of a massive star. A rapid neutrino-antineutrino equilibration leads to entropy generation and enhanced electron capture and, hence, a lower electron fraction than in the standard model. This would affect the downstream core evolution, the prospects for a supernova explosion, and the emergent neutrino signal. If realized by lepton-number-violating neutrino self-interactions (LNV $ν$SI), the relevant mediator mass and coupling ranges can be probed by future accelerator-based experiments.

hep-ph↗

arXiv2025

Searching for MeV-mass neutrinophilic Dark Matter with Large Scale Dark Matter Detectors

The indirect detection of dark matter (DM) through its annihilation products is one of the primary strategies for DM detection. One of the least constrained classes of models is neutrinophilic DM, because the annihilation products, weakly interacting neutrinos, are challenging to observe. Here, we consider a scenario where MeV-mass DM exclusively annihilates to the third neutrino mass eigenstate, which is predominantly of tau and muon flavor. In such a scenario, the potential detection rate of the neutrinos originating from the DM annihilation in our Galaxy in the conventional detectors would be suppressed by up to approximately two orders of magnitude. This is because the best sensitivity of such detectors for neutrinos with energies below approximately 100 MeV is for electron neutrino flavor. In this work, we highlight the potential of large-scale DM detectors in uncovering such signals in the tens of MeV range of DM masses. In addition, we discuss how coincident signals in direct detection DM experiments and upcoming neutrino detectors such as DUNE, Hyper-Kamiokande, and JUNO could provide new perspectives on the DM problem.

hep-ph↗

arXiv2024

Dark Matter-Powered Stars and the High-Redshift Tidal Disruption Event Rate

Tidal disruption events (TDEs) result from stars being gravitationally-scattered into low angular momentum orbits around massive black holes. We show that the short lifetimes of massive Population III stars at high redshifts could significantly suppress the volumetric TDE rate because they are too short-lived to reach disruption-fated orbits. However, this suppression can be alleviated if captured dark matter (DM) within stellar interiors provides an additional energy source, thereby extending stellar lifetimes. We find that this TDE rate revival is most pronounced for DM particles with mass $\mathcal{O}({\rm MeV})$, as this particle mass scale is optimal in the competing processes of DM accretion and evaporation in stars.

astro-ph.HE↗

arXiv2024

Dark Matter and General Relativistic Instability in Supermassive Stars

We calculate the extent to which collisionless dark matter impacts the stability of supermassive stars $(M\gtrsim10^4\,M_\odot)$. We find that, depending on the star's mass, a dark matter content in excess of ${\sim}1\%$ by mass throughout the entire star can raise the critical central density for the onset general relativistic instability, in some cases by orders of magnitude. We consider implications of this effect for the onset of nuclear burning and significant neutrino energy losses.

astro-ph.CO↗

arXiv2024

Cosmic Neutrino Decoupling and its Observable Imprints: Insights from Entropic-Dual Transport

Very different processes characterize the decoupling of neutrinos to form the cosmic neutrino background (C$ν$B) and the much later decoupling of photons from thermal equilibrium to form the cosmic microwave background (CMB). The C$ν$B emerges from the fuzzy, energy-dependent neutrinosphere and encodes the physics operating in the early universe in the temperature range $T\sim 10\,{\rm MeV}$ to $T\sim10\,{\rm keV}$. This is the epoch where beyond Standard Model (BSM) physics may be influential in setting the light element abundances and the necessarily distorted fossil neutrino energy spectra. Here we use techniques honed in extensive CMB studies to analyze the C$ν$B as calculated in detailed neutrino energy transport and nuclear reaction simulations. Our moment method, relative entropy, and differential visibility approach can leverage future high precision CMB and primordial abundance measurements to provide new insights into the C$ν$B and any BSM physics it encodes. We demonstrate that the evolution of the energy spectrum of the C$ν$B throughout the weak decoupling epoch is accurately captured in the Standard Model by only three parameters per species, a non-trivial conclusion given the deviation from thermal equilibrium. Furthermore, we can interpret each of the three parameters as physical characteristics of a non-equilibrium system. The success of our compact description within the Standard Model motivates its use also in BSM scenarios. We demonstrate how observations of primordial light element abundances can be used to place constraints on the C$ν$B energy spectrum, deriving response functions that can be applied for general C$ν$B spectral distortions. Combined with the description of those deviations that we develop here, our methods provide a convenient and powerful framework to constrain the impact of BSM physics on the C$ν$B.

astro-ph.CO↗

arXiv2024

Dark population transfer mechanism for sterile neutrino dark matter

We present a mechanism for producing a cosmologically-significant relic density of one or more sterile neutrinos. This scheme invokes two steps: First, a population of "heavy" sterile neutrinos is created by scattering-induced decoherence of active neutrinos; Second, this population is transferred, via sterile neutrino self-interaction-mediated scatterings and decays, to one or more lighter mass ($\sim 10\,{\rm keV}$ to $\sim 1\,{\rm GeV}$) sterile neutrinos that are far more weakly (or not at all) mixed with active species and could constitute dark matter. Dark matter produced this way can evade current electromagnetic and structure-based bounds, but may nevertheless be probed by future observations.

hep-ph↗

arXiv2023

Probing self-interacting sterile neutrino dark matter with the diffuse supernova neutrino background

The neutrinos in the diffuse supernova neutrino background (DSNB) travel over cosmological distances and this provides them with an excellent opportunity to interact with dark relics. We show that a cosmologically-significant relic population of keV-mass sterile neutrinos with strong self-interactions could imprint their presence in the DSNB. The signatures of the self-interactions would be ``dips" in the otherwise smooth DSNB spectrum. Upcoming large-scale neutrino detectors, for example Hyper-Kamiokande, have a good chance of detecting the DSNB and these dips. If no dips are detected, this method serves as an independent constraint on the sterile neutrino self-interaction strength and mixing with active neutrinos. We show that relic sterile neutrino parameters that evade X-ray and structure bounds may nevertheless be testable by future detectors like TRISTAN, but may also produce dips in the DSNB which could be detectable. Such a detection would suggest the existence of a cosmologically-significant, strongly self-interacting sterile neutrino background, likely embedded in a richer dark sector.

hep-ph↗

arXiv2023

Signatures of a High Temperature QCD Transition in the Early Universe

Beyond Standard Model extensions of QCD could result in quark and gluon confinement occurring well above a temperature of $\sim$GeV. These models can also alter the order of the QCD phase transition. The enhanced production of primordial black holes (PBHs) that can accompany the change in relativistic degrees of freedom at the QCD transition therefore could favor the production of PBHs with mass scales smaller than the Standard Model QCD horizon scale. Consequently, and unlike PBHs associated with a standard GeV-scale QCD transition, such PBHs can account for all the dark matter abundance in the unconstrained asteroid-mass window. This links beyond Standard Model modifications of QCD physics over a broad range of unexplored temperature regimes ($\sim 10-10^3$ TeV) with microlensing surveys searching for PBHs. Additionally, we discuss implications of these models for gravitational wave experiments. We show that a first order QCD phase transition at $\sim7$ TeV is consistent with the Subaru Hyper-Suprime Cam candidate event, while a $\sim 70$ GeV transition is consistent with OGLE candidate events, and also could account for the claimed NANOGrav gravitational wave signal.

astro-ph.CO↗

arXiv2023

Big Bang Nucleosynthesis

As the early universe expands and cools the rates of the weak interactions that keep neutrinos in thermal equilibrium with the matter and the related rates of the reactions that inter-convert neutrons and protons decrease. Eventually, these rates fall below the expansion rate -- they freeze out. Likewise, the rates of the strong and electromagnetic nuclear reactions that build up and tear down nuclei, though fast enough to maintain equilibrium early on, slow down and ultimately lead to freeze out. Together these freeze out processes comprise the epoch of Big Bang Nucleosynthesis (BBN). The relics emerging from this early time include the light element abundances, for example of helium and deuterium, and a background of decoupled neutrinos, a "C$ν$B" , roughly analogous to the Cosmic Microwave Background, the CMB. These fossil relics encode the history of the physics operating in the early universe. Consequently, BBN has emerged as a key tool for constraining new, beyond-standard-model (BSM) physics. BBN may become an even finer probe of BSM physics, given the anticipated higher precision in measurements of the primordial abundances of deuterium and helium afforded by the advent of large optical telescopes and Stage-4 CMB experiments. The latter experiments will also provide higher precision determinations of $N_{\rm eff}$, a measure of the relativistic energy density at the photon decoupling epoch and, hence, an important probe of the C$ν$B.

astro-ph.CO↗

arXiv2022

Snowmass2021 CMB-HD White Paper

CMB-HD is a proposed millimeter-wave survey over half the sky that would be ultra-deep (0.5 uK-arcmin) and have unprecedented resolution (15 arcseconds at 150 GHz). Such a survey would answer many outstanding questions about the fundamental physics of the Universe. Major advances would be 1.) the use of gravitational lensing of the primordial microwave background to map the distribution of matter on small scales (k~10 h Mpc^(-1)), which probes dark matter particle properties. It will also allow 2.) measurements of the thermal and kinetic Sunyaev-Zel'dovich effects on small scales to map the gas density and velocity, another probe of cosmic structure. In addition, CMB-HD would allow us to cross critical thresholds: 3.) ruling out or detecting any new, light (< 0.1 eV) particles that were in thermal equilibrium with known particles in the early Universe, 4.) testing a wide class of multi-field models that could explain an epoch of inflation in the early Universe, and 5.) ruling out or detecting inflationary magnetic fields. CMB-HD would also provide world-leading constraints on 6.) axion-like particles, 7.) cosmic birefringence, 8.) the sum of the neutrino masses, and 9.) the dark energy equation of state. The CMB-HD survey would be delivered in 7.5 years of observing 20,000 square degrees of sky, using two new 30-meter-class off-axis crossed Dragone telescopes to be located at Cerro Toco in the Atacama Desert. Each telescope would field 800,000 detectors (200,000 pixels), for a total of 1.6 million detectors.

astro-ph.CO↗

arXiv2022

A modified Brink-Axel hypothesis for astrophysical Gamow-Teller transitions

Weak interaction charged current transition strengths from highly excited nuclear states are fundamental ingredients for accurate modeling of compact object composition and dynamics, but are difficult to obtain either from experiment or theory. For lack of alternatives, calculations have often fallen back upon a generalized Brink-Axel hypothesis, that is, assuming the strength function (transition probability) is independent of the initial nuclear state but depends only upon the transition energy and the weak interaction properties of the parent nucleus ground state. Here we present numerical evidence for a modified `local' Brink-Axel hypothesis for Gamow-Teller transitions for $pf$-shell nuclei relevant to astrophysical applications. Specifically, while the original Brink-Axel hypothesis does not hold globally, strength functions from initial states nearby in energy are similar within statistical fluctuations. This agrees with previous work on strength function moments. Using this modified hypothesis, we can tackle strength functions at previously intractable initial energies, using semi-converged initial states at arbitrary excitation energy. Our work provides a well-founded method for computing accurate thermal weak transition rates for medium-mass nuclei at temperatures occurring in stellar cores near collapse. We finish by comparing to previous calculations of astrophysical rates.

nucl-th↗

arXiv2021

Exploring the Origin of Supermassive Black Holes with Coherent Neutrino Scattering

Collapsing supermassive stars ($M \gtrsim 3 \times 10^4 M_{\odot}$) at high redshifts can naturally provide seeds and explain the origin of the supermassive black holes observed in the centers of nearly all galaxies. During the collapse of supermassive stars, a burst of non-thermal neutrinos is generated with a luminosity that could greatly exceed that of a conventional core collapse supernova explosion. In this work, we investigate the extent to which the neutrinos produced in these explosions can be observed via coherent elastic neutrino-nucleus scattering (CE$ν$NS). Large scale direct dark matter detection experiments provide particularly favorable targets. We find that upcoming $\mathcal{O}(100)$ tonne-scale experiments will be sensitive to the collapse of individual supermassive stars at distances as large as $\mathcal{O}(10)$ Mpc.

astro-ph.HE↗

arXiv2021

Effects of an Intermediate Mass Sterile Neutrino Population on the Early Universe

The hot and dense early Universe combined with the promise of high-precision cosmological observations provide an intriguing laboratory for Beyond Standard Model (BSM) physics. We simulate the early Universe to examine the effects of the decay of thermally populated sterile neutrino states into Standard Model products around the time of weak decoupling. These decays deposit a significant amount of entropy into the plasma as well as produce a population of high-energy out-of-equilibrium active neutrinos. As a result, we can constrain these models by their inferred value of $N_{\rm eff}$, the effective number of relativistic degrees of freedom. In this work, we explore a variety of models with $N_{\rm eff}{}$ values consistent with CMB observations, but with vastly different active neutrino spectra which will challenge the standard cosmological model, affect lepton capture rates on free nucleons, and may significantly affect Big Bang Nucleosynthesis (BBN).

hep-ph↗

arXiv2021

Where, when and why: occurrence of fast-pairwise collective neutrino oscillation in three-dimensional core-collapse supernova models

Fast-pairwise collective neutrino oscillation represents a key uncertainty in the theory of core-collapse supernova (CCSN). Despite the potentially significant impact on CCSN dynamics, it is usually neglected in numerical models of CCSN because of the formidable technical difficulties of self-consistently incorporating this physics. In this paper, we investigate the prospects for the occurrence of fast flavor conversion by diagnosing electron neutrino lepton number (ELN) crossing in more than a dozen state-of-the-art three-dimensional CCSN models. ELN crossings is a necessary condition for triggering flavor conversion. Although only zeroth and first angular moments are available from the simulations, our new method enables us to look into the angular distributions of neutrinos in momentum space and provide accurate insight into ELN crossings. Our analysis suggests that fast flavor conversion generally occurs in the post-shock region of CCSNe, and that explosive models provide more favorable conditions for the flavor conversion than failed CCSNe. We also find that there are both common and progenitor-dependent characteristics. Classifying ELN crossings into two types, we analyze the generation mechanism of each case by scrutinizing the neutrino radiation field and matter interactions. We find key ingredients of CCSN dynamics driving the ELN crossings: proto-neutron star (PNS) convection, asymmetric neutrino emission, neutrino absorptions and scatterings. This study suggests that we need to accommodate fast flavor conversions in realistic CCSN models.

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↗