arXiv subjects

Shao-Ping Li

Publications and source records attributed to Shao-Ping Li.

At least 19 recordsLinked to original sources

arXiv2026

Joint probes of dark matter annihilation from neutrino detectors and CMB targets

Dark matter (DM) annihilation into neutrinos provides a promising observational channel targeted by current and forthcoming neutrino detectors. However, the detection of such neutrino fluxes alone cannot uniquely determine their astrophysical or cosmological origin, such as the recent observations from Super-Kamiokande that hint at a small excess of electron antineutrino events. We propose that the effective number of neutrino species and the spectral distortion of the cosmic microwave background (CMB) can serve as complementary observables to probe neutrino signatures from DM annihilation. Using a simple model-independent analysis, we determine the detection windows of these cosmic observables that overlap with the experimental sensitivities from the Super-Kamiokande, Jiangmen Underground Neutrino Observatory, Hyper-Kamiokande, and the Deep Underground Neutrino Experiment, showing that joint probes of large DM annihilation to neutrinos with MeV-GeV masses can be achieved by neutrino detectors and CMB experiments.

hep-ph↗

arXiv2026

Self-Interaction Bounds on Ultralight Dark Matter Couplings to Matter

Ultralight dark matter (ULDM) couplings to matter fields and ULDM self-interactions are typically treated as independent probes. However, since the ULDM-matter couplings unavoidably induce self-interactions through quantum loop corrections, bounds on self-interacting ULDM from astrophysical and cosmological observations will also limit the coupling strength to matter. Applying this argument, we find that self-interaction bounds can impose strong constraints on the linear ULDM couplings to neutrinos, excluding a large portion of parameter space that is widely considered for probing ULDM via neutrino oscillation experiments. In addition, the self-interaction bounds also limit the quadratic ULDM couplings to electrons and light quarks, which can become stronger than from the stringent test of equivalence-principle violation. Our results demonstrate that the extreme observational sensitivity of cosmic microwave background and structure formations to repulsive self-interactions can robustly translate into powerful constraints on the ULDM interactions with fundamental particles.

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arXiv2026

Low-Scale Leptogenesis from Resonant Thermal Lepton Flavour Coherences

Resonant heavy-neutrino mixing and sterile neutrino oscillations are two prominent mechanisms to realize low-scale leptogenesis, with singlet neutrino masses below TeV energies that could be probed in current and future laboratory experiments. In their minimal settings, both mechanisms require a significant degree of degeneracy in the singlet neutrino masses to compensate for the suppression that results from the small neutrino Yukawa couplings. After further developing the flavour-covariant Kadanoff-Baym formalism, we study in detail a novel dominant mechanism for low-scale leptogenesis which becomes greatly enhanced by resonant thermal lepton-flavour coherences at the two-loop level. This mechanism works successfully for both Dirac and Majorana singlet neutrinos, and it does not rely on whether these singlet neutrinos are quasi-degenerate or not. In particular, it implies that successful low-scale leptogenesis in the type-I seesaw framework can be naturally realised with heavy neutrino masses that could be as low as GeV.

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arXiv2026

Dominant Thermal Resonant Mechanism for Low-Scale Leptogenesis

We explicitly demonstrate the importance of a new thermal resonant channel in the context of low-scale leptogenesis, which goes beyond the well-known mixing and oscillation of massive singlet neutrinos. This new channel is always present when considering the thermally-induced Higgs decay to leptons and relativistic singlet neutrinos, and can become dominant thanks to thermally-generated resonant lepton-doublet flavour coherences. This mechanism, which we call Thermal Resonant Leptogenesis (TRL), can yield the observed baryon asymmetry in our universe, even if there is no resonant enhancement from quasi-degenerate sterile neutrinos. The required active-to-sterile neutrino mixing for TRL differs from other known low-scale leptogenesis scenarios and can be probed in fixed-target and long-lived particle experiments, and by displaced vertex searches at high-energy colliders.

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arXiv2025

Neutrinogenic CMB spectral distortions

Extra radiation injection after neutrino decoupling in the early Universe contributes to the effective number of neutrino species that can be constrained by the cosmic microwave background (CMB). However, any effective neutrino number itself cannot uniquely determine the underlying source. We argue that the degeneracy can be relaxed by CMB spectral distortions, which are caused by energy exchange between the extra radiation and photons. We consider neutrinogenic CMB spectral distortions, where extra energy is released in the form of neutrinos but still creates the CMB spectral distortions via electroweak interactions. The synergy between the effective neutrino number and CMB spectral distortions provides a complementary probe of hidden sectors that dominantly couple to neutrinos, opening up parameter space that can be targeted by joint CMB anisotropy and spectral distortion experiments.

astro-ph.CO↗

arXiv2025

The mass-coupling effect in leptogenesis

Particle decay in leptogenesis provides a simple avenue to explain the baryon asymmetric universe, where the decaying particle can provide the out-of-equilibrium condition to create a net lepton asymmetry. It is widely anticipated that the lepton asymmetry would be changed significantly by varying couplings and the decaying particle mass, especially in the weak washout regime. Contrary to this naive expectation, we demonstrate a general phenomenon in a class of leptogenesis scenarios from heavy particle decay, where varying the mass and couplings would not modify the lepton asymmetry in a noticeable way, as these mass and coupling effects are largely canceled out from the evolution of the decaying particle. It points out that a much broader parameter space in the mass and couplings will open automatically once leptogenesis is realized in a benchmark point; however, tuning the mass and couplings to boost leptogenesis will be challenging.

hep-ph↗

arXiv2025

Bounds and detection of MeV-scale dark matter annihilation to neutrinos

Current and most upcoming neutrino detectors can only reach a dark matter annihilation cross section to neutrinos larger than the standard freeze-out value, but they open intriguing detection avenues for non-standard dark matter paradigms. An important corollary of these non-standard scenarios is relic dark matter annihilation after neutrino decoupling, which was previously overlooked in constraining MeV-scale dark matter. However, by combining the contributions from entropy injection during neutrino decoupling and from nonthermal neutrino energy release after decoupling, we derive significant constraints on the annihilation cross section to neutrinos, which in some mass regimes become stronger than the current bounds. Furthermore, we find that the lower bounds on dark matter masses become inconclusive under the recent data releases from the DESI, SPT-3G, and ACT collaborations. These bounds determine the extent to which upcoming neutrino detectors will probe dark matter annihilation into neutrinos.

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arXiv2025

Resonant Forbidden CP Asymmetry from Soft Leptons

To explain the baryon asymmetry in the early universe via leptogenesis, quantum corrections to new particles are commonly invoked to generate the necessary CP asymmetry. We demonstrate, however, that a large CP asymmetry can already arise from Standard Model leptons. The mechanism relies on resummation of soft leptons at finite temperatures. The CP asymmetry, which is kinematically forbidden in vacuum, can be resonantly enhanced from thermally resummed leptons by seven orders of magnitude. Contrary to the resonance from exotic particles, we show that the resonant enhancement from soft leptons is protected by controlled widths under finite-temperature perturbation theory. We quantify such CP asymmetries in leptogenesis with secluded flavor effects and comment on the significance and application. The mechanism exploits the maximal role of leptons themselves, featuring low-scale leptogenesis, minimal model buildings and dark matter cogenesis.

hep-ph↗

arXiv2025

Particle decay as asymptotic narrow parametric resonance

Parametric resonance can produce particles from oscillating scalar field, where an exponential growth of the particle number density can be developed. While it has been noticed that stimulated decay in Boltzmann equations exhibits similar parametric dependence of the exponential growth, it is not quantitatively clear yet under what circumstance can the two phenomena reconcile. We demonstrate that trilinear particle interaction in the Boltzmann equation can provide a good approximation to describe the distribution function and the number density in the asymptotic regime of narrow parametric resonance. We find that the crucial treatment leading to the quantitative agreement is a proper Gaussian simulation of the momentum spread inherited from nonrelativistic particle decay. With the simple particle picture, the analytic Boltzmann approximation can be applied to explosive photon production from axions/axion-like particles and to dark matter production from oscillating fields.

hep-ph↗

arXiv2025

Asteroid-mass soliton as the dark matter-baryon coincidence solution

Nontopological solitons formed during first-order phase transitions can serve as macroscopic dark matter candidates, with their stability ensured by a charge asymmetry traditionally assumed to originate from baryogenesis. Following this generic pattern, we demonstrate that solitogenesis after baryogenesis makes the solitons a coincident dark matter candidate, providing new explanations for the coincidence problem between baryon and dark matter energy densities. We derive a novel and robust conclusion: asteroid-mass coincident soliton dark matter is always accompanied by detectable gravitational waves observable by LISA, $\mu$Ares, and Theia, providing a new candidate beyond primordial black holes in this mass window. Additionally, we propose a simple neutrino-ball scenario that addresses baryon asymmetry, dark matter, and neutrino masses, featuring new particles below the electroweak scale and correlated observable signals, including lensing, gravitational waves, and soliton evaporation or collisions.

hep-ph↗

arXiv2024

Forbidden neutrinogenesis

The origin of neutrino masses can be simply attributed to a new scalar beyond the Standard Model. We demonstrate that leptogenesis can explain the baryon asymmetry of the universe already in such a minimal framework, where the electroweak scalar is favored to enhance the baryon asymmetry. Different from traditional leptogenesis, the realization here exploits the thermal behavior of leptons at finite temperatures, which is otherwise kinetically forbidden in vacuum. We present detailed calculations of the CP asymmetry in the Schwinger-Keldysh Closed-Time-Path formalism, and compute the asymmetry evolution via the Kadanoff-Baym equation. Such minimal forbidden neutrinogenesis establishes a direct link between the baryon asymmetry and the CP-violating phase from neutrino mixing, making the scenario a compelling target in neutrino oscillation experiments. Complementary probes from cosmology, flavor physics and colliders are also briefly discussed.

hep-ph↗

arXiv2024

Photon proliferation from multi-body dark matter annihilation

Multi-body dark matter annihilation is commonly expected to be suppressed by higher-order couplings and phase-space factors, therefore being ignored thus far. We show that, however, this does not hold for a class of nonthermal dark matter scenarios, where the dark matter particle becomes nonrelativistic at temperatures much higher than its mass. We exemplify such a multi-body process via ultralight pseudoscalar dark matter annihilation to diphotons, which leads to a novel photon proliferation effect in the early Universe. As a phenomenological application, we consider the photon temperature shift after neutrino decoupling, showing that the photon proliferation effect can render bounds on the ultralight dark matter couplings stronger than the existing constraints by several orders of magnitude. Our research can be extended to other interactions and dark matter candidates, highlighting the importance of multi-body processes in the early Universe.

hep-ph↗

arXiv2024

Observability of CMB spectrum distortions from dark matter annihilation

Even after dark matter chemically freezes out in the early universe, electromagnetic cascades from dark matter annihilation can still perturb the background photon spectrum when the universe temperature cools down to 0.5 keV. We revisit the CMB spectrum distortions caused by $s$-wave dark matter annihilation under the updated Planck data and the future CMB sensitivity, concluding that $s$-wave annihilation cannot create observable distortions under forecast sensitivities of the (Super-)PIXIE missions. We further detail the case of $p$-wave dark matter annihilation, demonstrating the observability of the primordial $μ$-distortion. Taking current constraints from primordial light elements, structure formations, cosmic electron-positron rays, and gamma rays, we find that the $μ$-distortion reaching the observational limit as large as $μ\simeq 3\times 10^{-8}$ can only be realized with a dark matter mass at 10--50 MeV and a kinetic decoupling temperature around 1 keV. The upper bound of the $p$-wave annihilation cross section can be strengthened by an order of magnitude if the $μ$-distortion is not detected.

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arXiv2024

A cosmological sandwiched window for lepton-number breaking scale

A singlet majoron can arise from the seesaw framework as a pseudo-Goldstone boson when the heavy Majorana neutrinos acquire masses via the spontaneous breaking of global ${\rm U}(1)_L$ symmetry. The resulting cosmological impacts are usually derived from the effective majoron-neutrino interaction, and the majoron abundance is accumulated through the freeze-in neutrino coalescence. However, a primordial majoron abundance can be predicted in a minimal setup and lead to distinctive cosmological effects. In this work, we consider such a primordial majoron abundance from relativistic freeze-out and calculate the modification to the effective neutrino number $N_{\rm eff}$. We demonstrate that the measurements of $N_{\rm eff}$ will constrain the parameter space from a primordial majoron abundance in an opposite direction to that from neutrino coalescence. When the contributions from both the primordial abundance and the freeze-in production coexist, the ${\rm U}(1)_L$-breaking scale (seesaw scale) $f$ will be pushed into a ''sandwiched window''. Remarkably, for majoron masses below 1 MeV and above the eV scale, the future CMB-S4 experiment will completely close such a low-scale seesaw window for $f\in [1,10^5]~{\rm GeV}$. We highlight that any new light particle with a primordial abundance that couples to SM particles may lead to a similar sandwiched window, and such a general phenomenon deserves careful investigation.

hep-ph↗

arXiv2024

Dark phase transition from WIMP: complementary tests from gravitational waves and colliders

A dark sector is an interesting place where a strong first-order phase transition, observable gravitational waves and/or a dark matter candidate could arise. However, the experimental tests for such a dark sector could be ambiguous due to the dark content, largely unconstrained parameter space and the connection to the visible world. We consider a minimal dark scalar-vector boson plasma to realize the three mentioned phenomena, with a unique connection to the Standard Model via the Higgs portal coupling. We discuss the important features of the Higgs portal in such a minimal dark sector, namely the dark thermalization, collider tests, and direct detection of dark matter. We perform numerical analyses of the dark phase transition associated with stochastic gravitational waves and dark matter, discussing the complementarity of collider detection, dark matter direct detection and space-based/terrestrial interferometers as a promising avenue to hear and see the minimal dark sector.

hep-ph↗

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.

hep-ph↗

arXiv2023

A collider test of nano-Hertz gravitational waves from pulsar timing arrays

A cosmic first-order phase transition (FOPT) occurring at MeV-scale provides an attractive explanation for the nano-Hertz gravitational wave (GW) background indicated by the recent pulsar timing array data from the NANOGrav, CPTA, EPTA and PPTA collaborations. We propose this explanation can be further tested at the colliders if the hidden sector couples to the Standard Model sector via Higgs portal. Through a careful analysis of the thermal history of the hidden sector, we demonstrate that in order to successfully explain the observed GW signal, the portal coupling must be sizable that it can be probed through Higgs invisible decay at the LHC or future lepton colliders such as CEPC, ILC, and FCC-ee. Our research offers a promising avenue to uncover the physical origin of the nano-Hertz GWs through particle physics experiments.

hep-ph↗

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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