arXiv subjects

Shao-Ping Li

Publications and source records attributed to Shao-Ping Li.

35 records · Page 2Linked to original sources

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

Scattering versus Forbidden Decay in Dark Matter Freeze-in

It is generically believed that the two-body scattering is suppressed by higher-order weak couplings with respect to the two-body decay. We show that this does not always hold when a heavy particle is produced by forbidden decay in a thermal plasma, where the scattering shares the same order of couplings with the decay. We find that there is a simple and close relation between the forbidden decay and the same-order scattering. To illustrate this point, we consider freeze-in production of heavy dark matter via a light scalar mediator. We point out that, when the Boltzmann (quantum) statistics is used, the forbidden decay can contribute to the dark matter relic density at 5$\%$-24$\%$ (10$\%$-$39\%$) with a weak thermal coupling, while the contribution from the scattering channel can be several orders of magnitude larger than from the forbidden decay if the thermal coupling is much smaller. Such a relative effect between the scattering and the forbidden decay could also exist in other plasma-induced processes, such as the purely thermal generation of the right-handed neutrino dark matter, or of the lepton asymmetry in leptogenesis.

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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

Dark matter freeze-in via a light fermion mediator: forbidden decay and scattering

The connection between a hidden nonthermal sector and a thermal plasma can be established by a light thermal fermion mediator. When the fermion mediator is much lighter than the hidden species, kinematically forbidden decay of the mediator can be opened at finite temperatures to produce the hidden species. Unlike bosons having quartic couplings, renormalizable forbidden fermion decay generically shares the same order of couplings with the scattering. We present a dedicated investigation into the freeze-in dark matter production via a thermal fermion mediator. We demonstrate that the plasma-induced decay rate differs from that calculated via the tree-level amplitude, but the former can be obtained from the latter via constant rescaling. Furthermore, we find that the relative effect of the forbidden decay and the scattering on the dark matter relic density can be simply estimated via the thermal coupling between the plasma and the mediator. Applying to different thermal interactions, we show that the forbidden decay contribution can reach the level of $4\%-45\%$ for a thermal coupling at $0.1-1$.

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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

Linking the $R_{K^{(\ast)}}$ anomalies to the Hubble tension via a single right-handed neutrino

The updated measurements from the LHCb and SH0ES collaborations have respectively strengthened the deviations of the ratio $R_{K}$ in rare semi-leptonic $B$-meson decays and the present-day Hubble parameter $H_0$ in the Universe, implying tantalizing hints of new physics beyond the Standard Model. In this paper, we consider a simple flavor-specific two-Higgs-doublet model, where the long-standing $R_{K^{(*)}}$ anomalies can be addressed by a one-flavor right-handed neutrino. One of the intriguing predictions resulting from the parameter space for the $R_{K^{(*)}}$ resolution under flavor- and collider-physics constraints points toward a shift of the effective neutrino number, $ΔN_{\rm eff}=N_{\rm eff}-N_{\rm eff}^{\rm SM}$, as favored to ease the $H_0$ tension. Depending on whether the neutrino is of Dirac or of Majorana type, we show that the resulting shift is $ΔN_{\rm eff}\simeq 1.0$ for the former and $ΔN_{\rm eff}\simeq 0.5$ for the latter case, respectively. While the Dirac case is disfavored by the CMB polarization measurements, the Majorana solution is consistent with the recent studies via a combined data set from various sources. Consequently, such a simple flavor-specific two-Higgs-doublet model provides a link between the $R_{K^{(*)}}$ anomalies and the $H_0$ tension, which in turn can be readily verified or falsified by the upcoming measurements.

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arXiv2022

Scotogenic Dirac neutrino mass models embedded with leptoquarks: one pathway to address the flavor anomalies and the neutrino masses together

If the leptoquarks proposed to account for the intriguing anomalies observed in the $B$-meson decays, $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$, as well as in the anomalous magnetic moment of the muon, $(g-2)_μ$, can be embedded into the scotogenic Dirac neutrino model, all these flavor anomalies, together with the origin of neutrino masses and the nature of dark matter, would be potentially addressed in a unified picture. Among the minimal seesaw, one-loop, and two-loop realizations of the dimension-4 effective operator $\mathcal{L}_{4}$ for the Dirac neutrino masses, we show that plenty of diagrams associated with the two-loop realizations of $\mathcal{L}_{4}$ can support the coexistence of leptoquarks and dark matter candidates. After a simple match of these leptoquarks to those that can accommodate all the flavor anomalies, we establish the scotogenic Dirac neutrino models embedded with leptoquarks, which could address all the problems mentioned above.

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arXiv2022

Next-to-tribimaximal mixing against CP violation and baryon asymmetry signs

Four next-to-tribimaximal (NTBM) mixing patterns are widely considered as the feasible candidates for the observed leptonic mixing structure. With the recent measurements from T2K, as well as intensive global fits, the interval of $\sinδ_{ CP} > 0$ for the Dirac CP-violating phase is persistently small in the normal ordering while $\sinδ_{ CP} < 0$ is successively obtained up to $3σ$ level in the inverted ordering. In this paper, we advocate the fitting results of Dirac CP-violating phase as a constraint, and show that, it can basically rule out half of the regions allowed by the constraint from three mixing angles only. In addition, given the small $\sinδ_{ CP} > 0$ interval, we find that a unique NTBM pattern can be selected out of the four candidates within $3σ$ uncertainty of the latest NuFIT 5.1, when the patterns are exposed to a Yukawa texture-independent leptogenesis for the baryon asymmetry of the Universe. For the surviving pattern, we find an interesting correlation between the Dirac CP-violating phase and the octant of atmospheric angle $θ_{23}$ , where $θ_{23}>45^\circ$ can be predicted once $5π/6 < δ_{CP} <π$ in the normal ordering can be confirmed in the future measurements.

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arXiv2022

Effective neutrino number shift from keV-vacuum neutrinophilic 2HDM

If the Dirac neutrino masses are generated by a new scalar doublet with a vacuum at keV order, there would be rare hope to probe the framework in low-energy flavor physics, such as the lepton-flavor violating processes. However, the predicted neutrino Yukawa couplings being around the order of electronic Yukawa can realize a purely thermal Dirac leptogenesis, and thus render a more direct link between the high- and the low-energy CP violation. Despite its inert property in the low-energy flavor processes, the keV-vacuum induced Dirac neutrino model can generate a significant shift of the effective neutrino number, which can in turn be probed by the big-bang nucleosynthesis and cosmic microwave background epochs. We show that such a keV-vacuum induced Dirac neutrino model is already constrained by the current observations and will be probed with forecast sensitivity, serving therefore as a complementary avenue to test the Dirac leptogenesis.

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arXiv2021

Baryogenesis from Hierarchical Dirac Neutrinos

We present here a novel and testable mechanism for leptogenesis, which is characterized by a purely thermal generation of lepton asymmetry via scalar decay. Guided by the thermal mass matrix diagonalization in the finite-temperature regime, we propose a scenario in which the baryon asymmetry is formulated in terms of the masses and mixing from hierarchical Dirac neutrinos, as well as a vacuum scale accountable for the sub-eV neutrino masses. This allows a natural and direct link between the high-scale \textit{CP} asymmetry and the low-energy Dirac \textit{CP}-violating phase without involved model structures, and thus circumvents the haunting problem encountered in general leptogenesis scenarios. The mechanism can also be applied as a prototype for solving the baryon asymmetry problem to a broad class of well-motivated model buildings.

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arXiv2021

Simple estimate of BBN sensitivity to light freeze-in dark matter

We provide a simple analysis of the big-bang nucleosynthesis (BBN) sensitivity to the light dark matter (DM) generated by the thermal freeze-in mechanism. It is shown that the ratio of the effective neutrino number shift $ΔN_ν$ over the DM relic density $ω\equiv Ωh^2$, denoted by $R_χ\equivΔN_ν/ω$, cancels the decaying particle mass and the feeble coupling, rendering therefore a simple visualization of $ΔN_ν$ at the BBN epoch in terms of the DM mass. This property drives one to conclude that the shift with a sensitivity of $ΔN_ν\simeq \mathcal{O}(0.1)$ cannot originate from a single warm DM under the Lyman-$α$ forest constraints. For the cold-plus-warm DM scenarios where the Lyman-$α$ constraints are diluted, the ratio $R_χ$ can be potentially used to test the thermal freeze-in mechanism in generating a small warm component of DM and a possible excess at the level of $ΔN_ν\simeq \mathcal{O}(0.01)$.

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arXiv2021

Freeze-in self-interacting dark matter in warped extra dimension

A classically scale-invariant scalar singlet can be a MeV-scale dark matter, with a feeble Higgs portal coupling at $\mathcal{O}(10^{-10})$. Besides, an $\mathcal{O}(0.1)$ self-interaction coupling could further serve to alleviate the small-scale problems in the Universe. We show that, such a dark matter candidate can naturally arise in the warped extra dimension, with the huge span of parameter space predicted well within $\mathcal{O}(1)$ fundamental parameters.

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arXiv2021

Power-aligned 2HDM: a correlative perspective on $(g-2)_{e,μ}$

With the hypothesis of minimal flavor violation, we find that there exists a power-aligned relation between the Yukawa couplings of the two scalar doublets in the two-Higgs-doublet model with Hermitian Yukawa matrices. Within such a power-aligned framework, it is found that a simultaneous explanation of the anomalies observed in the electron and muon anomalous magnetic moments can be reached with TeV-scale quasi-degenerate Higgs masses, and the resulting parameter space is also phenomenologically safer under the B-physics, $Z$ and $τ$ decay data, as well as the current LHC bounds. Furthermore, the flavor-universal power that enhances the charged-lepton Yukawa couplings prompts an interesting correlation between the two anomalies, which makes the model distinguishable from the (generalized) linearly aligned and the lepton-specific two-Higgs-doublet models that address the same anomalies but in a non-correlative manner, and hence testable by future precise measurements.

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arXiv2021

Muon $g-2$ in a $U(1)$-symmetric Two-Higgs-Doublet Model

We show in this paper that, in a $U(1)$-symmetric two-Higgs-doublet model (2HDM), the two additional neutral Higgs bosons would become nearly degenerate in the large $\tanβ$ regime, under the combined constraints from both theoretical arguments and experimental measurements. As a consequence, the excess observed in the anomalous magnetic moment of the muon could not be addressed in the considered framework, following the usual argument where these two neutral scalars are required to manifest a large mass hierarchy. On the other hand, we find that, with an $\mathcal{O}(1)$ top-Yukawa coupling and a relatively light charged Higgs boson, large contributions from the two-loop Barr-Zee type diagrams can account for the muon $g-2$ anomaly at the 1$σ$ level, in spite of a large cancellation between the scalar and pseudoscalar contributions. Furthermore, the same scenario can survive the tight constraints from the $B$-physics observables.

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arXiv2020

Freeze-in Dirac neutrinogenesis: thermal leptonic CP asymmetry

We present a freeze-in realization of the Dirac neutrinogenesis in which the decaying particle that generates the lepton-number asymmetry is in thermal equilibrium. As the right-handed Dirac neutrinos are produced non-thermally, the lepton-number asymmetry is accumulated and partially converted to the baryon-number asymmetry via the rapid sphaleron transitions. The necessary CP-violating condition can be fulfilled by a purely thermal kinetic phase from the wavefunction correction in the lepton-doublet sector, which has been neglected in most leptogenesis-based setup. Furthermore, this condition necessitates a preferred flavor basis in which both the charged-lepton and neutrino Yukawa matrices are non-diagonal. To protect such a proper Yukawa structure from the basis transformations in flavor space prior to the electroweak gauge symmetry breaking, we can resort to a plethora of model buildings aimed at deciphering the non-trivial Yukawa structures. Interestingly, based on the well-known tri-bimaximal mixing with a minimal correction from the charged-lepton or neutrino sector, we find that a simultaneous explanation of the baryon-number asymmetry in the Universe and the low-energy neutrino oscillation observables can be attributed to the mixing angle and the CP-violating phase introduced in the minimal correction.

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arXiv2020

Probing New Physics Signals with Symmetry-Restored Yukawa Textures

We implement a symmetry violation guideline into a two-Higgs-doublet model embedded with three right-handed neutrinos, and exploit the generic Yukawa structures of the model via a hypothetical symmetry restoration of a global $U_Q(1)^3\times U_L(1)^3$ symmetry. We then apply a mass-powered parametrization to construct the phenomenologically motivated Yukawa interactions, which enables us to incorporate correlatively the neutrino mass, dark matter, as well as the lepton-flavor universality violations in $R_{K^{(*)}}$ and $R_{D^{(*)}}$. Specifically, two atmospheric-scale neutrino masses are generated by a low-scale seesaw mechanism, while the much lighter one, being of $\mathcal{O}(10^{-6})~{\rm eV}$, is fixed by a $7.1~{\rm keV}$ sterile neutrino dark matter produced primordially by the freeze-in mechanism. On the other hand, the neutrino and the charged-lepton mass hierarchies encoded in the mass-powered textures can naturally account for the $R_{K^{(*)}}$ and the $R_{D^{(*)}}$ anomalies, respectively. As a further application, a milder discrepancy of the muon $g-2$ is observed, which has also been implied by the recently refined lattice results.

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arXiv2018

$R_{D^{(\ast)}}$, $R_{K^{(\ast)}}$ and neutrino mass in the 2HDM-III with right-handed neutrinos

Given that the two-Higgs-doublet model of type III (2HDM-III) has the potential to address the $R_{D^{(\ast)}}$ anomalies while the resolution to the $R_{K^{(\ast)}}$ deficits requires new degrees of freedom within this framework, we consider in this paper a unified scenario where the low-scale type-I seesaw mechanism is embedded into the 2HDM-III, so as to accommodate the $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$ anomalies as well as the neutrino mass. We first revisit the $R_{D^{(\ast)}}$ anomalies and find that the current world-averaged results can be addressed at $2σ$ level without violating the bound from the branching ratio $\mathcal{B}(B_c^-\rightarrow τ^- \barν)\leqslant 30\%$. The scenario predicts two sub-eV neutrino masses based on a decoupled heavy Majorana neutrino and two nearly degenerate Majorana neutrinos with mass around the electroweak scale. For the $R_{K^{(\ast)}}$ anomalies, the same scenario can generate the required Wilson coefficients in the direction $ C_{9 μ}^{\rm NP}=-C_{10 μ}^{\rm NP}<0$, with $\mathcal{O}(1)$ Yukawa couplings for the muon and the top quark.

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