arXiv subjects

Xun-Jie Xu

Publications and source records attributed to Xun-Jie Xu.

At least 19 recordsLinked to original sources

arXiv2026

Solar Constraints on Heavy Neutral Leptons with $ν_τ$ Mixing

The mixing of heavy neutral leptons (HNLs) with tau neutrinos remains largely unconstrained compared to their mixing with electron and muon neutrinos. In this work, we investigate the potential of solar neutrinos to improve constraints on HNL-$ν_τ$ mixing in the MeV mass range. Due to neutrino oscillations, the Sun is a copious source of neutrinos of all flavors, and the partial conversion of electron neutrinos into muon and tau neutrinos occurs already during their propagation through the solar interior. Tau neutrinos can produce HNLs through scattering with protons in the Sun, provided nonzero HNL-$ν_τ$ mixing is present. The HNLs escape the Sun and subsequently decay through the same interaction. For $\mathcal{O}(10)$ MeV HNL masses, the dominant visible decay yields electrons and positrons that can be detected by space-based solar observatories. Using data from the Solar and Heliospheric Observatory (SOHO), we search for such signals and derive constraints on the squared HNL-$ν_τ$ mixing matrix element, $|U_{τN}|^2$, reaching below the $10^{-2}$ level for HNL masses of $\sim 5$ MeV. These constraints improve upon existing terrestrial limits in this mass range by more than an order of magnitude.

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arXiv2026

A Unified Bogoliubov Approach to Primordial Gravitational Waves: From Inflation to Reheating

We present an effective numerical method that can be used to straightforwardly calculate the full spectrum of primordial gravitational waves produced during inflation and reheating. Our method is based on the Bogoliubov approach with several key improvements to overcome its shortcomings such as numerical errors at high frequencies and issues with tachyonic modes. We also present a few useful analytical examples from which one can gain crucial insights into the numerical errors. The improved method allows us to demonstrate that anharmonicity of inflaton oscillations can leave interesting fingerprints on the high-frequency part of the GW spectrum. Our numerical code is publicly available on GitHub https://github.com/xunjiexu/Unified-Bogoliubov.git.

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arXiv2026

High-frequency gravitational waves from axion inflation in the weak-backreaction regime

Axion inflation, characterized by a Chern-Simons interaction between the inflaton and a gauge field, provides a powerful mechanism for generating primordial gravitational waves (GWs) through tachyonic enhancement of the gauge field. While recent literature has predominantly focused on the Strong Backreaction (SB) regime to maximize GW signals for future interferometers, this regime suffers from computational complexities as well as the risk of overproducing scalar perturbations. In this work, we investigate gauge field amplification and GW production strictly within the theoretically safer Weak Backreaction (WB) regime, with a particular focus on the largely unexplored non-instantaneous reheating phase. Because the tachyonic enhancement during slow-roll typically increases as inflation approaches its end, it is crucial to investigate how the production of GWs behaves at the very end of inflation and thereafter. By continuously tracking the evolution from slow-roll through reheating to radiation domination, we present a complete picture of inflationary and post-inflationary GW production in this framework. A particularly interesting feature of the post-inflationary phase is that the oscillatory behavior of the inflaton during reheating leads to frequent sign-flips of the instability parameter $ξ$, exciting both helical modes of the gauge field. Our analysis reveals that axion inflation can naturally generate one of the strongest known primordial GW signals at high-frequency bands. The yield is relevant for future precision measurements of the effective number of neutrino species, $N_{\rm eff}$, and also strongly motivates the development of novel high-frequency GW detectors.

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arXiv2026

Widen the Resonance at Ultra-High Energies: Novel Probes of Neutrino Self-interactions in the High-Mass Regime

Neutrino self-interaction beyond the Standard Model is well motivated by the nonzero masses of neutrinos, which are the only known particles guaranteed to have new physics. Cosmic messengers, especially neutrinos, play a central role in probing new physics, as they provide experimental conditions far beyond the reach of laboratories and serve as the link between laboratory fundamental-physics discoveries and their roles in the Universe, where many new physics motivations originate. In this work, we propose a novel probe of neutrino self-interactions through ultra-high-energy neutrinos scattering off the cosmic neutrino background when the lightest neutrino species remains relativistic today. This allows us to ``Widen the Resonance'' of such scattering. Meanwhile, we also provide a semi-analytic framework for cosmogenic UHE neutrino production, avoiding computationally intensive simulations and yielding results precise enough for BSM studies. The widened resonance enables future ultrahigh-energy neutrino telescopes, in particular GRAND, to probe mediator masses from MeV to GeV, reaching couplings down to $g \sim 10^{-3}$ -- up to two orders of magnitude beyond current bounds. Our results enhance the discovery potential of $ν$SI in the high-mass regime, potentially offering crucial insights into the connections between the neutrino sector and dark sector.

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arXiv2026

Dark Photons in the Early Universe: From Thermal Production to Cosmological Constraints

Dark photons, a generic class of light gauge bosons that interact with the Standard Model (SM) exclusively through kinetic mixing, arise naturally in many gauge extensions of the SM. Motivated by these theoretical considerations, we present a comprehensive analysis of their thermal production in the early universe. Our calculation covers a broad range of dark photon masses from 0.1 keV to 100 MeV and include inverse decay, annihilation, and semi-Compton processes. Wherever possible, we present analytical estimates of the production rates and yields, and verify their accuracy numerically. For dark photons lighter than twice the electron masses (around 1 MeV), we find that our analytical estimate of the freeze-in yield based on resonant production is very accurate, implying that off-resonance contributions can be neglected in practice. For heavy dark photons, although this conclusion no longer holds, we derive an interesting ratio, $4πe/27\approx0.14$, with $e$ the coupling constant of QED, that can be used to estimate the relative importance of on- and off-resonance contributions. Finally, using the calculated abundance of dark photons in the early universe, we derive cosmological constraints on the dark photon mass and kinetic mixing. Compared with bounds from stellar cooling and supernovae, the cosmological constraints are most stringent in the mass range from 0.1 MeV to 6 MeV, within which kinetic mixing at the level of $10^{-12}\sim10^{-10}$ can be probed.

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arXiv2026

Dark Photons from Red Dwarfs

Light dark photons can be produced in stellar systems and thus contribute to the stellar cooling rate. The additional cooling changes the evolution of the star and has an impact on various observable properties such as radius, photon luminosity or the emitted neutrino fluxes. This has been exploited before to derive limits based on observations of the Sun, horizontal branch stars and red giants. Given the wealth of astrophysical data collected in the last decade and the improvements in modeling stellar evolution it is interesting to investigate whether other stellar systems offer a complementary avenue towards testing dark photons. In this work, we study the effect on an alternative class of stars. We focus on the impact of dark photon induced cooling on red dwarfs, i.e. the lowest mass stars on the Hydrogen main sequence. Running simulations of the evolution of red dwarfs with dark photon cooling we determine the impact on the mass-radius relation. Combining our simulations with precise determination of mass and radius derived from observations of eclipsing binaries that have recently become available, we derive competitive limits which outperform the solar ones in a significant part of the parameter space.

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arXiv2026

Probing Bose-enhanced Inflaton Decay with Gravitational Waves

We investigate cosmic reheating dynamics in the presence of a transient condensate formed by bosonic decay products of the inflaton. We show that the emergence of such a condensate and the corresponding Bose enhancement can dramatically increase the efficiency of inflaton decay, giving rise to qualitatively new reheating dynamics beyond the standard perturbative picture. As a consequence, graviton production from inflaton decay processes is significantly amplified by Bose enhancement effects, leading to a stochastic gravitational-wave background with a potentially observable amplitude, even in the low-frequency regime.

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arXiv2026

Primordial Black Holes Evaporating before Big Bang Nucleosynthesis

Primordial black holes (PBHs) formed from the collapse of density fluctuations provide a unique window into the physics of the early Universe. Their evaporation through Hawking radiation around the epoch of Big Bang nucleosynthesis (BBN) can leave measurable imprints on the primordial light-element abundances. In this work, we analyze in detail the effects of PBHs evaporating before BBN, with various intermediate steps understood analytically, and obtain the BBN constraint on PBHs within a transparent and reproducible framework. We find that, to produce observable effects on BBN, the PBH mass must exceed $10^{9}$ g, a threshold higher than that reported in some earlier studies. Slightly above $10^{9}$ g, the BBN sensitivity rapidly increases with the mass and then decreases, with the turning point occurring at $2\times10^{9}$ g. For PBHs in the mass range $[10^{9},\ 10^{10}]$ g, current measurements of BBN observables set an upper bound on the initial mass fraction parameter $β$ ranging from $10^{-17}$ to $10^{-19}$. To facilitate future improvements, we make our code publicly available, enabling straightforward incorporation of updated nuclear reaction rates, particle-physics inputs, and cosmological data.

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arXiv2026

Solar neutrino physics

As a free, intensive, weakly interacting, and well directional messenger, solar neutrinos have been driving both solar physics and neutrino physics developments for more than half a century. Since more extensive and advanced neutrino experiments are under construction, being planned or proposed, we are striving toward an era of precise and comprehensive measurement of solar neutrinos in the next decades. In this article, we review recent theoretical and experimental progress achieved in solar neutrino physics. We present not only an introduction to neutrinos from the standard solar model and the standard flavor evolution, but also a compilation of a variety of new physics that could affect and hence be probed by solar neutrinos. After reviewing the latest techniques and issues involved in the measurement of solar neutrino spectra and background reduction, we provide our anticipation on the physics gains from the new generation of neutrino experiments.

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arXiv2026

No Hiding in the Dark: Cosmological Bounds on Heavy Neutral Leptons with Dark Decay Channels

Heavy neutral leptons (HNLs) are well-motivated new physics candidates. The mixing of sub-GeV HNLs with active neutrinos is severely constrained by cosmology. In particular, the success of Big Bang Nucleosynthesis (BBN) requires the HNL lifetime to be shorter than about 0.02 sec if they were in thermal equilibrium, thus excluding a wide range of mixing angles accessible to terrestrial experiments. In order to justify the laboratory searches in this cosmologically-forbidden region, it is often argued that adding new dark sector decay modes of HNLs can evade the stringent BBN constraint. Here we rule out this possibility and show that, contrary to the naive expectation, HNLs with significant dark decay modes actually lead to stronger cosmological bounds. This is mainly because of the increase in the extra radiation energy density in the Universe around the BBN epoch, which causes observable effects in the primordial helium fraction and $ΔN_{\rm eff}$. Our result has major implications for laboratory searches of HNLs.

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arXiv2025

Strongly Coupled Quantum Forces

Quantum forces are long-range interactions originating from vacuum fluctuations of mediator fields. Such forces inevitably arise between ordinary matter particles whenever they couple to light mediator species. Conventional computations of quantum forces rely on evaluating one-loop Feynman diagrams of the relevant scattering processes. In this work, we introduce a novel framework to compute quantum forces. Instead of relying on perturbative scattering amplitudes, we directly evaluate the quantum fluctuations of the mediator field by solving its quantized equation of motion with appropriate boundary conditions. This approach remains valid beyond the Born approximation and thus applies to regimes of strong coupling between the mediator and matter fields. In the weak-coupling limit, our results reproduce the known expressions from the Feynman diagram approach. In the strong-coupling regime, the result is modified by a factor that can suppress or enhance the effect. In contrast to classical forces, quantum forces intrinsically violate the superposition principle. Our approach may therefore offer a useful tool for probing non-perturbative effects in the infrared regime.

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arXiv2025

Widen the Resonance: Probing a New Regime of Neutrino Self-Interactions with Astrophysical Neutrinos

Neutrino self-interactions beyond the standard model have profound implications in astrophysics and cosmology. In this Letter, we study an uncharted scenario in which one of the three neutrino species has a mass smaller than the temperature of the cosmic neutrino background. This results in a relativistic component that significantly broadens the absorption feature on the astrophysical neutrino spectra, in contrast to the sharply peaked absorption expected in the extensively studied scenarios assuming a fully nonrelativistic cosmic neutrino background. By solving the Boltzmann equations for neutrino absorption and regeneration, we demonstrate that this mechanism provides novel sensitivity to sub-keV mediator masses, well below the traditional $\sim 1$--100 MeV range. Future observations of the diffuse supernova neutrino background with Hyper-Kamiokande could probe coupling strengths down to $g \sim 10^{-8}$, surpassing existing constraints by orders of magnitude. These findings open new directions for discoveries and offer crucial insights into the interplay between neutrinos and the dark sector.

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arXiv2025

Full-Spectrum Analysis of Gravitational Wave Production from Inflation to Reheating

In this work, we systematically study gravitational wave (GW) production during both the inflationary and post-inflationary epochs. While inflationary GWs can be readily derived from tensor perturbations during inflation, post-inflationary GWs arise from a variety of processes during reheating and require detailed treatment for quantitative analysis. We consider four distinct production channels: $(i)$ pure inflaton annihilation, $(ii)$ graviton bremsstrahlung from inflaton decay, $(iii)$ radiation-catalyzed inflaton-graviton conversion, and $(iv)$ scattering among fully thermalized radiation particles. For each channel, we solve the corresponding Boltzmann equation to obtain the GW spectrum and derive a simple yet accurate analytical expression for it. By employing a consistent treatment of all production channels, our analysis yields for the first time the full spectrum of GWs produced during the inflationary and post-inflationary epochs. We find that, while inflationary GWs dominate at low frequencies, post-inflationary processes generally produce high-frequency GWs with considerably high energy densities that may significantly exceed that of inflationary GWs.

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arXiv2025

Pre-thermalized Gravitational Waves

We investigate a novel gravitational wave (GW) production mechanism from gravitons generated during the pre-thermal phase of cosmic reheating, where the energy density is dominated by non-thermalized inflaton decay products, dubbed reheatons. We consider multiple production channels, including: $i)$ pure inflaton-inflaton annihilation, $ii)$ graviton Bremsstrahlung from inflaton decay, $iii)$ scatterings between an inflaton and a reheaton, and $iv)$ scatterings among reheatons. To determine the resulting GW spectrum, we solve the Boltzmann equation to obtain the graviton phase-space distribution for each channel. We find that the third channel, $iii)$, dominates due to the large occupation number of reheatons at highly-energetic states during the pre-thermalization phase. Notably, in scenarios with a low inflaton mass, the GW spectrum could fall within the sensitivity range of future experiments such as the Einstein Telescope, the Cosmic Explorer, the Big Bang Observer, and ultimate DECIGO.

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arXiv2025

Heating the dark matter halo with dark radiation from supernovae

Supernova explosions are among the most extreme events in the Universe, making them a promising environment in which to search for the effects of light, weakly coupled new particles. As significant sources of energy, they are known to have an important effect on the dynamics of ordinary matter in their host galaxies but their potential impact on the dark matter (DM) halo remains less explored. In this work, we investigate the possibility that some fraction of the supernova energy is released via the form of dark radiation into the DM halo. Based on evaluation of energetics, we find that even a small fraction of the total SN energy is sufficient to change the overall shape of the DM halo and transform a cuspy halo into a cored one. This may help to explain the cores that are observed in some dwarf galaxies. Alternatively, one can interpret the upper limit on the size of a possible DM core as an upper limit on the energy that can go into light particles beyond the SM. These arguments are largely independent of a concrete model for the new physics. Nevertheless, it is important to ensure that the conditions we need, i.e.~significant supernova emissivity of dark radiation and the opacity of DM halo to the dark radiation, can be met in actual models. To demonstrate this, we study four simple benchmark models: the dark photon, dark Higgs, and gauged $B-L$ and $L_μ- L_τ$ models -- all provide light weakly coupled particles serving as the dark radiation. Assuming a sizable coupling of the dark radiation to DM, we find that all of the benchmark models have a significant part of the parameter space that meets the conditions. Interestingly, the couplings allowed by observations of SN1987A can have a significant effect on the halo of dwarf spheroidal galaxies.

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arXiv2025

Gravitational charge production

Wash-in leptogenesis is an attractive mechanism to produce the baryon asymmetry of the Universe. It treats right-handed-neutrino interactions as spectator processes, on the same footing as electroweak sphalerons, that reprocess primordial charge asymmetries in the thermal plasma into a baryon-minus-lepton asymmetry. The origin of these primordial charges must be accounted for by new $CP$-violating dynamics at very high energies. In this paper, we propose such a scenario of chargegenesis that, unlike earlier proposals, primarily relies on new interactions in the gravitational sector. We point out that a coupling of a conserved current to the divergence of the Ricci scalar during reheating can lead to nonzero effective chemical potentials in the plasma that, together with a suitable charge-violating interaction, can result in the production of a primordial charge asymmetry. Gravitational chargegenesis represents a substantial generalization of the idea of gravitational baryogenesis. We provide a detailed analysis of a generic and minimal realization that is consistent with inflation and show that it can successfully explain the baryon asymmetry of the Universe.

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arXiv2025

High-Energy and Ultra-High-Energy Neutrinos from Primordial Black Holes

Primordial Black Holes (PBHs) are capable of emitting extremely energetic particles independent of their interactions with the Standard Model. In this work, we investigate whether PBHs evaporating in the early universe could be responsible for some of the observed high-energy neutrinos above the TeV or PeV scale in the present universe. We compute the energy spectrum of neutrinos directly emitted by PBHs with a monochromatic mass function and estimate the wash-out point, which determines the maximum energy of the spectrum. We find that the spectrum generally extends to high energies following a power law of $E_ν^{-3}$ until it reaches the wash-out point, which crucially depends on the PBH mass. For PBHs of $10^{13}$ grams, the spectrum can extend up to the PeV scale, though the flux is too low for detection. We also consider an indirect production mechanism involving dark particles that are emitted by PBHs and decay into neutrinos at a much later epoch. This mechanism allows lighter (such as those in the gram to kilogram range) PBHs to produce more energetic neutrino fluxes without being washed out by the thermal plasma in the early universe. In this scenario, we find that ultra-high-energy neutrinos around or above the EeV scale can be generated, with sufficiently high fluxes detectable by current and future high-energy neutrino observatories such as IceCube and GRAND.

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arXiv2025

Solar neutrinos

Solar neutrinos, generated abundantly by thermonuclear reactions in the solar interior, offer a unique tool for studying astrophysics and particle physics. The observation of solar neutrinos has led to the discovery of neutrino oscillation, a topic currently under active research, and it has been recognized by two Nobel Prizes. In this pedagogical introduction to solar neutrino physics, we will guide readers through several key questions: How are solar neutrinos produced? How are they detected? What is the solar neutrino problem, and how is it resolved by neutrino oscillation? This article also presents a brief overview of the theory of solar neutrino oscillation, the experimental achievements, new physics relevant to solar neutrinos, and the prospects in this field.

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