arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 19 recordsLinked to original sources

arXiv2026

Leptogenesis Determined By Low Energy Parameters

We study thermal leptogenesis in three predictive type-I seesaw models in which the neutrino Dirac mass matrix is equal to the mass matrix of up-type quarks, or down-type quarks, or charged leptons. In this framework, the seesaw relation permits a full reconstruction of the heavy right-handed neutrino mass matrix from low-energy neutrino parameters, which greatly reduces the parameter freedom. A systematic numerical scan based on density matrix Boltzmann equations is performed to examine whether the observed baryon asymmetry of the Universe can be obtained. Successful leptogenesis occurs for normal ordering of light neutrino masses with nonzero Majorana phases. In this case, viable solutions are found in model B, associated with down-type quarks, and model C, associated with charged leptons. Both point to a close-mass pair of heavy neutrinos satisfying $|M_i-M_j|/M_i<10^{-3}$, while remaining outside the conventional quasi-degenerate resonant regime. Four representative benchmark points are selected to show the evolution of the asymmetry and the impact of different treatments of spectator effects. Neutrinoless double beta decay is further studied for all parameter points that can generate an acceptable baryon asymmetry $η_B = (6.12 \pm 0.20)\times 10^{-10}$. The predicted effective Majorana mass for certain cases can be probed by next generation experiments with sub-10 meV sensitivity, such as LEGEND-1000, nEXO, JUNO 50 tons, and CUPID-1T. This framework therefore provides clear targets for future searches.

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arXiv2026

Transverse Tau Spin Correlations and the Weak Electric Dipole Moment at Future $Z$ Factories

Tau spin correlations at the $Z$ pole probe CP violation with a sensitivity that depends strongly on the spin projection. For a fixed $Z$ sample, transverse moments avoid the chirality suppression of longitudinal and triple-product observables, enhancing the response by a factor $m_Z/2m_τ=26$. Two transverse moments also select the axial tau charge, gaining $|a_τ/v_τ|=13.5$; the sign-weighted normal moment has a response $521$ times that of the triple product. These two transverse moments give single-parameter uncertainties no more than $14\%$ above our calculated Cramer-Rao bound for independent single-pion measurements, approaching the best precision within this analysis. A five-moment fit with $5\%$ off-peak luminosity yields statistical uncertainties of $4.5 (5.5)\times10^{-7}$ and $2.7 (3.2)\times10^{-6}$ on $\operatorname{Re}\widetilde{d}_τ^W$ and $\operatorname{Im}\widetilde{d}_τ^W$, respectively, at FCC-ee (CEPC), improving on ALEPH by three orders of magnitude. The electromagnetic-dipole uncertainty is $9.6 (12)\times10^{-6}$ on $\operatorname{Re}\widetilde{d}_τ^γ$, two orders beyond Belle. These projections assume ideal angular reconstruction and constant form factors across the scan.

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arXiv2026

Hadrophilic inelastic freeze-in dark matter in $q_1-q_2$ gauge extension and the high energy LZ event

The recent observation of a single event at nuclear recoil energy $248\pm23\pm23$ keV by the LZ collaboration has prompted dark matter (DM) model builders to explore new physics explanations. While most of the existing literature invokes an endothermic scattering by a TeV-scale DM to explain the event, the favored DM interaction strength is in tension with the indirect detection constraints for most of the models. To evade this, this work explores exothermic DM scattering in a hadrophilic $q_1-q_2$ gauge extension setup with a new $Z'$ boson as a mediator to the dark sector. This can naturally escape the indirect searches in the absence of tree-level coupling of the mediator to neutrinos, $W,τ$, and $b$ quark. We consider freeze-in production at a low reheating temperature to populate DM in the early universe, with a comparatively smaller coupling than the thermal freeze-out scenario. We identify the $Z'$ parameter space that can explain the observed relic density and LZ event simultaneously. We also discuss the relevant constraints coming from flavor mixing and FCNC.

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arXiv2026

Probing the imaginary parts and their $q^2$ dependences for the tau $g-2$ and EDM

The $τ$ anomalous magnetic dipole moment (MDM) $a_τ= (g-2)_τ/2$ and electric dipole moment (EDM) $d_τ$, are precision probes of electroweak dynamics and possible new physics sources, yet both remain weakly constrained experimentally. Treated as generalized form factors, these quantities exhibit a generic $q^2$ dependence for an off-shell interacting photon. For timelike momentum transfer above the $τ^+τ^-$ threshold, $q^2 = s > 4m_τ^2$, the form factors can acquire absorptive imaginary parts. We investigate how such a $q^2$ dependence and the associated imaginary parts are generated from two complementary perspectives: the model-independent Standard Model Effective Field Theory (SMEFT) and a UV-complete Two-Higgs-Doublet Model (2HDM). The effective framework reveals the intimate correlation between $a_τ$ and $d_τ$. New CP-violating interactions which generate a non-zero $d_τ$, can also generically have non-zero contributions to $a_τ$, thereby deeply linking their phenomenological studies. Within the 2HDM, we demonstrate that sizable imaginary parts and significant $q^2$ running can be generated at levels accessible by $e^+e^-$ colliders. Motivated by these features, we propose experimental methods to extract both the real and imaginary components of the dipole form factors. Utilizing these techniques, we show that Belle II and the Super Tau-Charm Facility (STCF) can improve current bounds on $a_τ$ by more than one order of magnitude. Finally, we highlight that combining measurements across the distinct center-of-mass energies of Belle II and STCF provides a unique, previously unexplored avenue to explicitly obtain information about the $q^2$ evolution of these dipole form factors.

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arXiv2026

Nonstandard Solution for Anomaly Cancellation as Seesaw Neutrino Origin in the SM

For fixed Standard Model (SM) non-Abelian representations of 15 chiral fermions with arbitrary hypercharges, anomaly cancellation admits the usual assignment and a distinct nonstandard solution. In the latter, the colored exotic quark and exotic lepton weak doublets and exotic lepton singlet have zero hypercharge, whereas the two colored exotic quark singlets carry opposite hypercharges $-q$ and $q$. The exotic neutral lepton singlets naturally play the role of the heavy neutrinos. The minimal model with two exotic lepton copies gives a rank-two seesaw, the minimal seesaw model, with one massless active neutrino and predicts $m_{ββ} = 1.2 \text{-} 4.1 \, \mathrm{meV}$ for normal ordering or $15.9\text{-}48.9 \,\mathrm{meV}$ for inverted ordering. In a direct SM realization, generating exotic quark masses through the SM Higgs mechanism forces the exotic quarks to carry electric charges $\pm 1/2$. A separate $\mathrm{SU}(2)_{L'}$ realization of the nonstandard solution can allow exotic quarks from several TeV to $10\,\mathrm{TeV}$ with order-one Yukawa couplings. In this model, the charged exotic quarks and leptons carry electric charges $\pm q$. In both cases, the lightest exotic quark and lepton are stable, but suitable choices of their charges and masses can satisfy experimental constraints.

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arXiv2026

CP violation in $Σ^+\to p\ell^+\ell^-$ within the standard model and beyond

The LHCb collaboration has recently observed the rare hyperon decay $Σ^+\to pμ^+μ^-$. It can also measure the corresponding antihyperon channel with comparable precision and is thus in a position to extract information on $CP$ violation in this mode. Interestingly, the long-distance contributions that dominate it within the standard model provide large absorptive phases that could drive substantial $CP$ violation through interference with potential new-physics contributions. Here we explore this possibility, finding that the decay rate asymmetry is currently allowed to be as high as tens of percent, which can be probed by LHCb in the near future. We additionally consider the same with regard to the dielectron mode $Σ^+\to pe^+e^-$ as well as the related radiative one $Σ^+\to pγ$.

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arXiv2026

Decoding the Amplitude Pair with Distinct CPV Phases in Charmed Baryon Decays

In this Letter, we propose a strategy to extract information on the hierarchical amplitude pair in singly Cabibbo-suppressed (SCS) charmed baryon two-body decays, with a dominant amplitude proportional to $λ_s = V_{cs}^* V_{us}$ from tree operators and sub-leading one proportional to $λ_b = V_{cb}^* V_{ub}$ from both penguin and tree contributions. The coexistence of these two amplitudes is essential for generating nonzero CP violation (CPV) effects. Since the $λ_b$ amplitude is strongly suppressed, its experimental determination is highly challenging. However, by exploiting SU(3) flavor symmetry, which relates the well-measured Cabibbo-favored (CF) amplitudes to the SCS tree amplitudes, information on the $λ_b$ amplitude can be extracted. Using current experimental data, a conservative analysis yields $λ_b$ amplitudes can be as large as about $10\%$ of the corresponding tree amplitudes with a significance of $2.1σ$. In addition, the Lee-Yang parameters of these decays provide an independent probe of this elusive term. We further identify two golden decay channels, $Ξ_c^0 \to p K^-$ and $Ξ_c^0 \to Σ^+ π^-$, which are particularly well suited for experimental studies of CPV.

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arXiv2026

Freeze-in $SU(2)$ vector dark matter at low reheating temperature

The freeze-in mechanism for dark matter (DM) requires extremely feeble interactions with the Standard Model (SM), preventing thermal equilibrium in the early Universe and typically evading experimental detection. However, for sufficiently low reheating temperatures ($T_{\rm RH}$), the observed relic abundance can be realized with larger couplings, opening prospects for experimental searches. In this work, we investigate freeze-in production of $SU(2)_{\rm HS}$ vector dark matter (VDM) in a low-$T_{\rm RH}$ cosmology. The framework naturally contains three mass-degenerate stable VDM candidates without the need for any additional discrete symmetry. We perform a systematic study of the dark matter phenomenology and identify the parameter space consistent with the observed relic abundance. In contrast to conventional freeze-in scenarios, the required DM couplings can be sizable, rendering part of the parameter space already constrained by existing direct searches like PandaX-4T and LZ, while a significant region remains within the reach of future experiments such as DARWIN. Though one can realize the freeze-in mechanism for an abelian $U(1)_X$ vector DM models as well, we find that the non-abelian structure of the $SU(2)_{\rm HS}$ scenario leads to a distinct feature due to a larger number of dark matter particles, resulting in an enlarged viable parameter space due to the multiplicity of dark matter states.

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arXiv2026

Complex $τ$ Electric Dipole Moment from GeV-Scale New Physics

Among the charged leptons, the $τ$ electric dipole moment~($d_τ$) is the least constrained. We show that the Im[$d_τ$] imposes strong constraints on new physics that have yet to be discussed. Motivated in particular by the Super Tau-Charm Facility (STCF), which will provide a uniquely clean environment for precision $τ$-physics, we study the momentum-transfer dependence of $d_τ(q^2)$ and compare the projected sensitivities of STCF and Belle II. Our analysis shows that an axion-like coupling of the $τ$ lepton can induce sizable real and imaginary components of the EDM. The predicted EDM values may approach the present experimental sensitivities, making them accessible to future measurements at Belle II and the STCF.

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arXiv2026

Spin-flavor entanglement in $Λ_b \to ΛD$ and weak phase extraction

We identify a new spin-flavor entanglement structure in $Λ_b\toΛD$ decays, formed by the correlation between the $Λ$ spin and the $D$ flavor states ($D=D^0,\overline D^0,D_1,D_2$). The entanglement information is encoded in the decay rates and Lee-Yang parameters of the four neutral-$D$ modes. We then show that the same spin-flavor structure provides a new method to determine the weak phase $γ$, a key angle of the Cabibbo-Kobayashi-Maskawa unitarity triangle. We find that the experimental uncertainty scales as $σ_γ\propto 1/{\cal C}$, where ${\cal C}$ is the Wootters concurrence, thereby quantitatively relating the precision of the weak-phase extraction to the amount of spin-flavor entanglement.

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arXiv2026

Revisiting $μ$-$e$ conversion in $R$-parity violating SUSY

The $μ$-$e$ conversion process is one of the most powerful ways to test lepton-flavor-violating (LFV) interactions involving charged leptons. The standard model with massive neutrinos predicts an extremely low rate for $μ$-$e$ conversion, making this process an excellent probe for testing LFV arising from new physics. Among many theoretical models that can induce LFV, the Supersymmetric model with R-parity violating interactions is one of the most studied for $μ$-$e$ conversion. In this work, we revisit trilinear R-parity violating interactions for $μ$-$e$ conversion, considering renormalization group (RG) running effects from high to low energy scales. The $μ$-$e$ conversion, $μ\to e γ$, and $μ\to eee$ experimental data are compared to give upper limits on the relevant 15 combinations of the trilinear $λ^{\prime}$ couplings and 6 combinations of the $λ$ couplings, certain of which are underexplored in previous studies. We find that RG running effects influence the limits by no more than 30\% in most cases, but can improve constraints by $\sim$80\% in certain combinations, which cannot be neglected. In the near future, COMET and Mu2e are expected to begin data-taking and aim to provide the most stringent constraints on $μ$-$e$ conversion. These next-generation $μ$-$e$ experiments have the ability to give much more comprehensive examinations on most trilinear coupling combinations than the $μ\to eγ$ and $μ\to 3e$ decay experiments. The $μ$-$e$ experiments will not only deepen our understanding of LFV but also provide a crucial way to examine the underlying new physics contributions.

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arXiv2026

A light DM model for large $B \to K + \mbox{invisible}$ and $K \to π+ \mbox{invisible}$ decays and its implications for $B_s-\bar B_s$ mixing and neutron EDM

We study the implications for $B_s - \bar B_s$ mixing and the neutron electric dipole moment (EDM) in a light dark matter model with sizable invisible rare meson decays to accommodate the recent possible deviations from Standard Model (SM) predictions observed in $B^+\to K^+ν\barν$ by Belle II and $K^+\toπ^+ν\barν$ by NA62. Given that the neutrinos in these decays escape detection, they can be replaced by other invisible final states. Based on effective operator analysis, it has been proposed that branching ratios for $B^+\to K^+ +\mbox{invisible}$ and $K^+\toπ^+ +\mbox{invisible}$ can naturally be larger than the SM predictions due to the emission of light dark matter pairs. We demonstrate that this scenario can be realized within a UV-complete two-Higgs-doublet model (2HDM) where neutral Higgs bosons mediating dark matter interactions induce significant low-energy effects especially for $B_s-\bar B_s$ mixing and neutron EDM. Within the allowed parameter space, we find non-negligible contributions to $B_s - \bar B_s$ mixing. For neutron EDM, there is a cancellation due to the exchange of neutral spin-zero particle, but QCD renormalization group evolution will lift this cancellation which in fact is generally true for any neutral Higgs contribution. However, we demonstrate that such a cancellation does not occur for charged scalar contributions. Ultimately, the allowed CP-violating phases in the Yukawa sector can generate a neutron EDM at a level consistent with current bound.

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arXiv2026

Perturbative QCD Prediction of the Hyperon EDM from CP-violating Dipole Interactions

Electric dipole moment (EDM) of baryons provides a sensitive probe of CP-violating interactions beyond the Standard Model. Motivated by the recent BESIII measurement on the $Λ$-hyperon EDM [1], we present the first perturbative QCD analysis of the $Λ$ EDM form factor to elucidate its origin in CP-violating quark dipole interactions. In particular, we derive a QCD factorization formula that relates the $Λ$ EDM form factor to quark EDMs and chromo-electric dipole moments (CEDMs) through convolutions with the light-cone distribution amplitudes of $Λ$. These connections allow us to extract constraints on CP-violating dipole couplings from current and future hyperon EDM measurements. Our numerical analysis demonstrates that the $Λ$ EDM exhibits unique sensitivity to the strange-quark CEDM, providing complementary information to that obtained from the neutron EDM.

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arXiv2026

CP violation in the hyperon decays $Σ\to Nπ$

The study of $CP$ violation in hyperon transitions has a long history. In the early 2000s the HyperCP experiment made a major effort to seek $CP$-odd signals in the decay sequence $Ξ^-\toΛπ^-$ and $Λ\to pπ^-$, which motivated more searches. Most recently the BESIII and LHCb Collaborations have acquired or improved the upper bounds on $CP$ violation in a variety of hyperon nonleptonic processes, including $Σ^+\to nπ^+$ and $Σ^+\to pπ^0$. These measurements have not reached the standard-model level yet, but have stimulated a renewed interest in $CP$-violating new physics in strange-quark decay beyond what is constrained by the parameters $\varepsilon$ and $\varepsilon^\prime$ from the kaon sector. In this paper, after updating the standard-model expectations for $CP$-odd observables in the modes $Σ^\pm\to Nπ$, we revisit new-physics scenarios that could enhance the corresponding quantities in $Λ\to Nπ$ and $Ξ\toΛπ$ and apply them to the $Σ^\pm$ modes. We find that the $CP$ asymmetries in the latter can be significantly increased over the standard-model expectations, at levels which may be tested in the ongoing BESIII experiment and in future endeavors such as PANDA and the Super Tau Charm Facility.

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arXiv2025

Modified Tri-bimaximal neutrino mixing confronted by JUNO $θ_{12}$ measurement

The JUNO collaboration has released its first measurement of reactor neutrino oscillations results, obtaining $\sin^2θ_{12} = 0.3092\pm 0.0087$, an improvement in precision by a factor of 1.6 over previous combined results. We confront the minimally modified tri-bimaximal mixing pattern with the new data. Before the measurement of a non-zero $θ_{13}$ mixing angle, the tri-bimximal mixing pattern is one of the most popular simple mixing scheme for neutrinos. Modifications have been proposed to keep some features of tri-bimaximal mixing and make it to be consistent with data. Minimal modifications preserving one column of the tri-bimaximal mixing matrix, yielding three patterns: a) unchanged third, b) unchanged second and c) unchanged first column. Pattern a) is excluded since it keeps $θ_{13}=0$. Pattern b) predicts $V_{e2} = 1/\sqrt{3}$ and specific CP phase correlations, but JUNO's smaller $|V_{e2}|$ disfavors it at more than 3.5$σ$. Pattern c), predicting $V_{e2} = \cosτ/\sqrt{3}$, can be in agreement with current data within 1$σ$, and implies $\sin^2θ_{12} = (1-3\sin^2θ_{13})/3(1-\sin^2θ_{13})$ and CP violating quantity $\sin θ= \pm 0.998$. The negative sign favors the inverted neutrino mass hierarchy. Upcoming experiments can further test this scenario.

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arXiv2025

Predictions for CP violation in anti-triplet beauty baryon to charmonium decays

Motivated by the recent 3.9$σ$ evidence for CP violation from the LHCb collaboration in decays of an anti-triplet beauty baryon to a charmonium, an octet baryon, and a pseudoscalar meson, we perform, for the first time, a systematic analysis of this class of decays within the framework of flavor $SU(3)$ symmetry. Several predictions for branching ratios and CP violating relations which can be tested in future experiments are found, in particular $ΔA_{CP} = A_{CP}(Λ_b^0 \to p π^- J/ψ) - A_{CP}(Λ_b^0 \to p K^- J/ψ) = A_{CP}(Λ_b^0 \to n π^0 J/ψ) - A_{CP}(Λ_b^0 \to (n K_S, n K_L) J/ψ)$. Our results provide guidance to test the availability of $SU(3)$ symmetry and search for possible CP violation effects in this class of decays.

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arXiv2025

The $U(1)_{L_μ-L_τ}$ model meets the new $(g-2)_μ$ data and muon neutrino trident scattering

The Muon $g-2$ collaboration at Fermilab has announced their final result of the anomalous magnetic moment of the muon. By adopting the lattice-QCD evaluation of the leading-order hadronic-vacuum-polarization, this result is now in agreement with the latest theoretical prediction to the $1σ$ level. This new result further constrains the allowed parameter space, but does not rule out all possible new physics contributions the muon $g-2$. We study the implications for one of the relevant models, the gauged $U(1)_{L_μ- L_τ}$. When using this model to resolve the previous $4σ$ tension, results from muon neutrino trident (MNT) scattering experiments would restrict the mass of the new gauge boson ($Z'$) to be less than $300$ MeV. Since the theory and experimental data difference for muon $g-2$ is lowered down to $1σ$, the requirement for $m_{Z'}\lesssim 300\,{\rm MeV}$ is much relaxed. Within the updated allowed range of $Z'$ boson mass, we study the models implications for electron and tauon $g-2$ as well as future muon colliders. We find that muon collider can effectively probe the $U(1)_{L_μ- L_τ}$.

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arXiv2025

Light dark-matter window constrained by \boldmath$K^+\toπ^+$$+$$\not{\!\!E}$

We explore the constraints on new physics from the recent NA62 observation of the kaon decay $K^+\toπ^+$+$\not{\!\!E}$ with missing energy $\not{\!\!E}$ in the context of a dark-matter (DM) scenario recently used to accommodate the Belle II finding of an enhanced rate of the $b$-meson decay $B^+\to K^+$+$\not{\!\!E}$ compared to the standard-model expectation. Specifically, assuming that a light real scalar boson $ϕ$ plays the role of DM and working in an effective field-theory framework, we study model independently the impact of operators involving $ϕ$ and ordinary quarks on the aforementioned transitions over the kaon mode's kinematical mass region of $m_ϕ< (m_K - m_π)/2 = 177$ MeV. Such a DM particle is subject to significant restrictions from the observed relic abundance and from DM direct-detection experiments incorporating the Migdal effect, as well as from indirect searches in cosmic microwave background data and collider experiments, except when its mass is between 110 and 146 MeV. We demonstrate that $K^+\toπ^+ϕϕ$ can saturate the new-physics window in the NA62 result if $m_ϕ$ lies in the 110-130 MeV portion of the range left by the DM constraints, thus providing a complementary constraint on this scenario. Improved data from future Belle II and NA62 measurements and DM quests can test it more stringently. In particular, expanding the NA62 signal window into the region that is now removed due to three-body decay background modes could further explore the remaining mass window for this type of invisible particle, $130 < m_ϕ< 177$ MeV.

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