arXiv subjects

Xun-Jie Xu

Publications and source records attributed to Xun-Jie Xu.

At least 37 records · Page 2Linked to original sources

arXiv2024

Looping Around Neutrino Charge Radius at Ultra-Near Reactor Experiments

We scrutinize the potential of upcoming ultra-near reactor neutrino experiments to detect radiative corrections in the elastic neutrino-electron scattering channel, focusing on the JUNO-TAO and CLOUD detectors, which employ advanced scintillator detection technologies. Previous reactor experiments have already constrained the electron neutrino charge radius, which is a neutrino property associated with a certain subset of the total radiative corrections, and have achieved limits that are only about an order of magnitude away from the Standard Model prediction. Our study demonstrates that JUNO-TAO and CLOUD could discover the neutrino charge radius in the near future, considering the established treatment of the charge radius. However, we show that it is necessary to go beyond this standard treatment. By including the complete set of one-loop level radiative corrections, we find a partial cancellation with the charge radius effect, reducing the experimental sensitivity to this quantity. Nevertheless, JUNO-TAO and CLOUD still have the potential to achieve a $5σ$ discovery but over longer timescales within a reasonable operational timeframe.

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arXiv2024

Impact of the cosmic neutrino background on long-range force searches

Light bosons can mediate long-range forces. We show that light bosonic mediators interacting with a background medium, in particular, with the cosmic neutrino background (C$ν$B), may induce medium-dependent masses which could effectively screen long-range forces from detection. This leads to profound implications for long-range force searches in e.g. the Eöt-Wash, MICROSCOPE, and lunar laser-ranging (LLR) experiments. For instance, we find that when the coupling of the mediator to neutrinos is above $3\times10^{-10}$ or $5\times10^{-13}$, bounds from LLR and experiments employing the Sun as an attractor, respectively, would be entirely eliminated. Larger values of the coupling can also substantially alleviate bounds from searches conducted at shorter distances.

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arXiv2024

A comprehensive calculation of the Primakoff process and the solar axion flux

The Primakoff process plays a crucial role in axion production in astrophysical environments and laboratories. Given the rising interest in axion physics and many on-going experimental activities, we conduct a comprehensive calculation of this process and carefully examine several aspects that have been neglected in the literature. In particular, our calculation is valid for axions with significantly large masses, which would be of importance to axion searches utilizing crystal and liquid xenon detectors. We present the most updated calculation of the Primakoff solar axion flux, with a simple parametrization that is applicable to a broad range of axion masses up to a few tens of keV. Our code is publicly available at GitHub.

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arXiv2024

The neutrino force in neutrino backgrounds: Spin dependence and parity-violating effects

The neutrino force results from the exchange of a pair of neutrinos. A neutrino background can significantly influence this force. In this work, we present a comprehensive calculation of the neutrino force in various neutrino backgrounds with spin dependence taken into account. In particular, we calculate the spin-independent and spin-dependent parity-conserving neutrino forces, in addition to the spin-dependent parity-violating neutrino forces with and without the presence of a neutrino background for both isotropic and anisotropic backgrounds. Compared with the vacuum case, the neutrino background can effectively violate Lorentz invariance and lead to additional parity-violating terms that are not suppressed by the velocity of external particles. We estimate the magnitude of the effect of atomic parity-violation experiments, and it turns out to be well below the current experimental sensitivity.

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arXiv2024

Imprints of light dark matter on the evolution of cosmic neutrinos

Neutrinos are often considered as a portal to new physics beyond the Standard Model (SM) and might possess phenomenologically interesting interactions with dark matter (DM). This paper examines the cosmological imprints of DM that interacts with and is produced from SM neutrinos at temperatures below the MeV scale. We take a model-independent approach to compute the evolution of DM in this framework and present analytic results which agree well with numerical ones. Both freeze-in and freeze-out regimes are included in our analysis. Furthermore, we demonstrate that the thermal evolution of neutrinos might be substantially affected by their interaction with DM. We highlight two distinctive imprints of such DM on neutrinos: (i) a large, negative contribution to $N_{\rm eff}$, which is close to the current experimental limits and will readily be probed by future experiments; (ii) spectral distortion of the cosmic neutrino background (C$ν$B) due to DM annihilating into neutrinos, a potentially important effect for the ongoing experimental efforts to detect C$ν$B.

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arXiv2024

Wash-in leptogenesis after the evaporation of primordial black holes

Wash-in leptogenesis is a powerful mechanism to generate the baryon asymmetry of the Universe that treats right-handed-neutrino interactions on the same footing as electroweak sphaleron processes: as mere spectator processes acting on the background of chemical potentials in the Standard Model plasma. Successful wash-in leptogenesis requires this chemical background to be CP-violating, which can be achieved by violating any of the more than ten global charges that are conserved in the Standard Model at very high temperatures. In this paper, we demonstrate that the primordial charge asymmetries required for wash-in leptogenesis can be readily produced by evaporating primordial black holes (PBHs). Our argument is based on the fact that the Hawking radiation emitted by PBHs contains more or less any state in the particle spectrum. Therefore, if heavy states with CP-violating decays are present in the ultraviolet, PBH evaporation will unavoidably lead to the production of these states. We illustrate this scenario by means of a simple toy model where PBH evaporation leads to the production of heavy particles that we call asymmetrons and whose decay results in a primordial charge asymmetry for right-handed electrons, which in turn sets the initial conditions for wash-in leptogenesis. We focus on the parameter region where the decay of the initial thermal asymmetron abundance occurs long before PBH evaporation and only results in a negligible primordial charge asymmetry. PBH evaporation at later times then serves as a mechanism to resurrect the asymmetron abundance and ensure the successful generation of the baryon asymmetry after all. We conclude that PBHs can act as asymmetry-producing machines that grant access to whatever CP-violating physics may be present in the ultraviolet, rekindling it at lower energies where it can be reprocessed into a baryon asymmetry by right-handed neutrinos.

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arXiv2024

The $ν_{R}$-philic scalar dark matter

Right-handed neutrinos ($ν_{R}$) offer an intriguing portal to new physics in hidden sectors where dark matter (DM) may reside. In this work, we delve into the simplest hidden sector involving only a real scalar exclusively coupled to $ν_{R}$, referred to as the $ν_{R}$-philic scalar. We investigate the viability of the $ν_{R}$-philic scalar to serve as a DM candidate, under the constraint that the coupling of $ν_{R}$ to the standard model is determined by the seesaw relation and is responsible for the observed DM abundance. By analyzing the DM decay channels and solving Boltzmann equations, we identify the viable parameter space. In particular, our study reveals a lower bound ($2.6\times10^{5}$ GeV) on the mass of $ν_{R}$ for the $ν_{R}$-philic scalar to be DM. The DM mass may vary from sub-MeV to sub-GeV. Within the viable parameter space, monochromatic neutrino lines from DM decay can be an important signal for DM indirect detection.

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arXiv2024

Shedding light on neutrino self-interactions with solar antineutrino searches

Solar antineutrinos are absent in the standard solar model prediction. Consequently, solar antineutrino searches emerge as a powerful tool to probe new physics capable of converting neutrinos into antineutrinos. In this study, we highlight that neutrino self-interactions, recently gaining considerable attention due to their cosmological and astrophysical implications, can lead to significant solar antineutrino production. We systematically explore various types of four-fermion effective operators and light scalar mediators for neutrino self-interactions. By estimating the energy spectra and event rates of solar antineutrinos at prospective neutrino detectors such as JUNO, Hyper-Kamiokande, and THEIA, we reveal that solar antineutrino searches can impose stringent constraints on neutrino self-interactions and probe the parameter space favored by the Hubble tension.

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arXiv2024

Neutrino forces in neutrino backgrounds

The Standard Model predicts a long-range force, proportional to $G_F^2/r^5$, between fermions due to the exchange of a pair of neutrinos. This quantum force is feeble and has not been observed yet. In this paper, we compute this force in the presence of neutrino backgrounds, both for isotropic and directional background neutrinos. We find that for the case of directional background the force can have a $1/r$ dependence and it can be significantly enhanced compared to the vacuum case. In particular, background effects caused by reactor, solar, and supernova neutrinos enhance the force by many orders of magnitude. The enhancement, however, occurs only in the direction parallel to the direction of the background neutrinos. We discuss the experimental prospects of detecting the neutrino force in neutrino backgrounds and find that the effect is close to the available sensitivity of the current fifth force experiments. Yet, the angular spread of the neutrino flux and that of the test masses reduce the strength of this force. The results are encouraging and a detailed experimental study is called for to check if the effect can be probed.

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arXiv2023

Beyond Tree Level with Solar Neutrinos: Towards Measuring the Flavor Composition and CP Violation

After being produced as electron neutrinos ($ν_e$), solar neutrinos partially change their flavor to $ν_μ$ and $ν_τ$ en route to Earth. Although the flavor ratio of the $ν_e$ flux to the total flux has been well measured, the $ν_μ:ν_τ$ composition has not yet been experimentally probed. In this work we investigate the potential of the next-generation experiments for measuring the $ν_μ:ν_τ$ flavor ratio by utilizing flavor-dependent radiative corrections in the cross sections for $ν_μ$ and $ν_τ$ scattering. Since the transition probabilities of $ν_e$ to $ν_μ$ and $ν_τ$ depend on the leptonic CP phase, we also investigate the sensitivity to the CP phase and show that a statistical significance of $\sim1 σ$ could be reached through precision measurements of solar neutrino spectra.

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arXiv2023

$N_{\rm eff}$ constraints on light mediators coupled to neutrinos: the dilution-resistant effect

We investigate the impact of new light particles, carrying significant energy in the early universe after neutrino decoupling, on the cosmological effective relativistic neutrino species, $N_{\rm eff}$. If the light particles are produced from decoupled neutrinos, $N_{\rm eff}$ is predominantly modified through the dilution-resistant effect. This effect arises because the energy stored in the mass of new particles is less diluted than the photon and neutrino energy as the universe expands. Our study comprehensively explores this effect, deriving $N_{\rm eff}$ constraints on the couplings of light mediators with neutrinos, encompassing both scalar and vector mediators. We find that the dilution-resistant effect can increase $N_{\rm eff}$ by 0.118 and 0.242 for scalar and vector mediators, respectively. These values can be readily reached by forthcoming CMB experiments. Upon reaching these levels, future $N_{\rm eff}$ constraints on the couplings will be improved by many orders of magnitude.

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arXiv2023

Production rates of dark photons and $Z'$ in the Sun and stellar cooling bounds

Light weakly interacting particles could be copiously produced in the Sun which, as a well-understood star, could provide severe constraints on such new physics. In this work, we calculate the solar production rates of light gauge bosons (e.g. dark photon) arising from various $U(1)$ extensions of the standard model. It is known that the dark photon production rate is suppressed by the dark photon mass if it is well below the plasmon mass of the medium. We show that for more general $U(1)$ gauge bosons, this suppression is absent if the couplings are not in alignment with those of the photon. We investigate a few frequently discussed $U(1)$ models including $B-L$, $L_μ-L_τ$, and $L_{e}-L_{μ(τ)}$, and derive the stellar cooling bounds for these models.

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arXiv2023

Dark matter produced from right-handed neutrinos

Right-handed neutrinos (RHNs) provide a natural portal to a dark sector accommodating dark matter (DM). In this work, we consider that the dark sector is connected to the standard model only via RHNs and ask how DM can be produced from RHNs. Our framework concentrates on a rather simple and generic interaction that couples RHNs to a pair of dark particles. Depending on whether RHNs are light or heavy in comparison to the dark sector and also on whether one or both of them are in the freeze-in/out regime, there are many distinct scenarios resulting in rather different results. We conduct a comprehensive and systematic study of all possible scenarios in this paper. For illustration, we apply our generic results to the type-I seesaw model with the dark sector extension, addressing whether and when DM in this model can be in the freeze-in or freeze-out regime. Some observational consequences in this framework are also discussed.

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arXiv2023

Neutrino forces and experimental probes

Neutrinos as almost massless particles could mediate long-range forces, known as neutrino forces. In this talk, I will introduce some theoretical aspects of neutrino forces, including why the potential of a neutrino force has the $1/r^{5}$ form and how it may vary under different circumstances. Experimental probes and possible implications for cosmology are also briefly discussed.

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arXiv2023

Probing the $ν_{R}$-philic $Z'$ at DUNE near detectors

We consider a hidden $U(1)$ gauge symmetry under which only the right-handed neutrinos ($ν_{R}$) are charged. The corresponding gauge boson is referred to as the $ν_{R}$-philic $Z'$. Despite the absence of direct gauge couplings to ordinary matter at tree level, loop-induced couplings of the $ν_{R}$-philic $Z'$ via left-right neutrino mixing can be responsible for its experimental accessibility. An important feature of the $ν_{R}$-philic $Z'$ is that its couplings to neutrinos are generally much larger than its couplings to charged leptons and quarks, thus providing a particularly interesting scenario for future neutrino experiments such as DUNE to probe. We consider two approaches to probe the $ν_{R}$-philic $Z'$ at DUNE near detectors via (i) searching for $Z'$ decay signals, and (ii) precision measurement of elastic neutrino-electron scattering mediated by the $Z'$ boson. We show that the former will have sensitivity comparable to or better than previous beam dump experiments, while the latter will improve current limits substantially for large neutrino couplings.

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arXiv2023

Neutrino Magnetic Moments Meet Precision $N_{\rm eff}$ Measurements

In the early universe, Dirac neutrino magnetic moments due to their chirality-flipping nature could lead to thermal production of right-handed neutrinos, which would make a significant contribution to the effective neutrino number, $N_{\rm eff}$. We present in this paper a dedicated computation of the neutrino chirality-flipping rate in the thermal plasma. With a careful and consistent treatment of soft scattering and the plasmon effect in finite temperature field theories, we find that neutrino magnetic moments above $2.7\times 10^{-12}μ_B$ have been excluded by current CMB and BBN measurements of $N_{\rm eff}$, assuming flavor-universal and diagonal magnetic moments for all three generation of neutrinos. This limit is stronger than the latest bounds from XENONnT and LUX-ZEPLIN experiments, and comparable with those from stellar cooling considerations.

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arXiv2022

Timing and Multi-Channel: Novel Method for Determining the Neutrino Mass Ordering from Supernovae

One of the few remaining unknowns in the standard three-flavor neutrino oscillation paradigm is the ordering of neutrino masses. In this work we propose a novel method for determining neutrino mass ordering using the time information on early supernova neutrino events. In a core-collapse supernova, neutrinos are produced earlier than antineutrinos and, depending on the mass ordering which affects the adiabatic flavor evolution, may cause earlier observable signals in $ν_e$ detection channels than in others. Hence, the time differences are sensitive to the mass ordering. We find that using the time information on the detection of the first galactic supernova events at future detectors like DUNE, JUNO and Hyper-Kamiokande, the mass ordering can already be determined at $\sim 2 σ$ CL, while $\mathcal{O}(10)$ events suffice for the discovery. Our method does not require high statistics and could be used within the supernova early warning system (SNEWS) which will have access to the time information on early supernova neutrino events recorded in a number of detectors. The method proposed in this paper also implies a crucial interplay between the mass ordering and the triangulation method for locating supernovae.

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arXiv2022

Neutrino bound states and bound systems

Yukawa interactions of neutrinos with a new light scalar boson $ϕ$ can lead to formation of stable bound states and bound systems of many neutrinos ($ν$-clusters). For allowed values of the coupling $y$ and the scalar mass $m_ϕ$, the bound state of two neutrinos would have the size larger than $10^{12}$ cm. Bound states with sub-cm sizes are possible for keV scale sterile neutrinos with coupling $y > 10^{-4}$. For $ν$-clusters we study in detail the properties of final stable configurations. If there is an efficient cooling mechanism, these configurations are in the state of degenerate Fermi gas. We formulate and solve equations of the density distributions in $ν$-clusters. In the non-relativistic case, they are reduced to the Lane-Emden equation. We find that (i) stable configurations exist for any number of neutrinos, $N$; (ii) there is a maximal central density $\sim 10^9$ cm$^{-3}$ determined by the neutrino mass; (iii) for a given $m_ϕ$ there is a minimal value of $Ny^3$ for which stable configurations can be formed; (iv) for a given strength of interaction, $S_ϕ= (ym_ν/m_ϕ)^2$, the minimal radius of $ν$-clusters exists. We discuss the formation of $ν$-clusters from relic neutrino background in the process of expansion and cooling of the Universe. One possibility realized for $S_ϕ> 700$ is the development of instabilities in the $ν$-background at $T < m_ν$ which leads to its fragmentation. For allowed values of $y$, cooling of $ν$-clusters due to $ϕ$-bremsstrahlung and neutrino annihilation is negligible. The sizes of $ν$-clusters may range from $\sim$ km to $\sim 5$ Mpc. Formation of clusters affects perspectives of detection of relic neutrinos.

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