arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 37 records · Page 2Linked to original sources

arXiv2025

Charmed baryon decays: $SU(3)_F$ breaking and CP violation

We present a comprehensive study of anti-triplet charmed baryon decays into octet baryons and pseudoscalar mesons using the $SU(3)_F$ flavor symmetry framework. By decomposing the flavor structure, all decay amplitudes are parametrized with a minimal set of irreducible amplitudes, and the Körner-Pati-Woo theorem further reduces the number of independent parameters from the original 35 to 19 under exact symmetry when small terms proportional to $λ_b = V_{cb}^*V_{ub}$ are neglected. The independent number becomes 27 with leading $SU(3)_F$ breaking effects. A global fit to 51 experimental measurements yields a $χ^2$/d.o.f. of 2.36 and provides precise values for the decay amplitudes. Notable discrepancies are observed in the fitted branching fractions of $Ξ_c^0 \to Ξ^- π^+$ and $Ξ_c^+ \to Ξ^- π^+ π^+$, which exceed current measurements by more than $2σ$. Incorporating final-state rescattering effects to recover the main effects of terms proportional to $λ_b$, we predict enhanced CP violation, with $A_{CP}$ reaching up to $10^{-3}$ in golden channels such as $Ξ_c^0 \to p K^-$ and $Ξ_c^0 \to Σ^+ π^-$. Our analysis also finds that the branching ratio for $Ξ_c^+ \to Λπ^+$ is enhanced to $(9.7 \pm 1.2) \times 10^{-4}$ due to significant cancellations.

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arXiv2025

$B\to K{+}$invisible, dark matter, and $CP$ violation in hyperon decays

Recently the Belle II Collaboration has reported a measurement of the $B^+\to K^+ν\barν$ rate that is higher than the standard-model expectation. Since the emitted neutrinos are unobserved, the excess could be due to the $B^+$ decaying into a $K^+$ and a dark-matter pair. We entertain this possibility in a two-Higgs-doublet model supplemented with a real singlet scalar boson acting as the dark matter. This model also accommodates strangeness-changing interactions providing new sources of $CP$ violation which can affect hyperon and kaon nonleptonic transitions. We find that the resulting $CP$ violation in the hyperon sector can be significant, reaching the current empirical bounds, after taking into account constraints from kaon mixing and decay and from dark-matter relic-density data and direct searches including the Migdal effect. We demonstrate that the hyperon and kaon processes are complementary probes of this new-physics scenario. Its prediction for sizable hyperon $CP$ violation is potentially testable in ongoing experiments, such as BESIII, Belle II, and LHCb, and in next-generation ones like PANDA and at the Super Tau Charm Facility.

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arXiv2025

Large CP violation in charmed baryon decays

This work presents the first detailed numerical predictions of CP violation in antitriplet charmed baryon decays. Adopting the flavor $SU(3)_F$ symmetry and the final state re-scattering framework, we relate the CKM-suppressed amplitudes to the leading ones, which are proportional to $λ_b$ and $λ_d$, respectively, with $λ_q = V_{uq}V^*_{cq}$. Larger-than-expected CP violations are found, reaching the order of $10^{-3}$, similar in magnitude to those in $D^0 \to π^+π^- ,K^+K^-$. This opens a promising avenue for observing CP violation in charmed baryon decays and testing the Standard Model.

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arXiv2025

Constraints on $ΔL=2$ Vector Bosons with Tree Couplings to SM Particles

We investigate phenomenological implications of vector bosons $V$ transforming as (1, 2, -3/2) under the standard model (SM) product gauge group $SU(3)_C$, $SU(2)_L$ and $U(1)_Y$. These vector bosons can couple to two SM leptons at tree-level forming dimension-4 operators. These operators dictate $V$ to have two units of global lepton number, $ΔL = 2$. The operators generated conserve the global lepton number but can violate generational lepton numbers. We study constraints on the couplings $Y$ of $V$ to SM particles using tree-level processes such as $l_α^{-} \to l_β^{+} l_ρ^{-} l_σ^{-}$, muonium and antimuonium oscillation, neutrino trident scattering, inverse muon decay, $e^- e^+ \to l^- l^+$, and also one-loop level processes such as the magnetic dipole moment of a charged lepton and $l_i \to l_j γ$. Strong constraints are obtained from $l_α^{-} \to l_β^{+} l_ρ^{-} l_σ^{-}$ with $\left|Y_{e e} Y^{*}_{μe}\right| < 3.29 \times 10^{-11}\left(m_{V}/ \mathrm{GeV}\right)^{2},\left|Y_{e e} Y^{*}_{e μ}\right| < 3.29 \times 10^{-11}\left(m_{V}/ \mathrm{GeV}\right)^{2}$ and from $l_i \to l_j γ$ with $ \left|Y_{τe}Y_{μτ}^{*}\right|<3.46\times10^{-12}(m_{V}/ \mathrm{GeV})^2, \left|Y_{e τ}Y_{τμ}^{*}\right|<3.46\times10^{-12}\left(m_{V}/ \mathrm{GeV}\right)^2$, respectively. Interestingly, the imaginary part of the coupling constant in our model induces CP violation, which is constrained by experimental limits on the electric dipole moment.

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arXiv2025

Large CP violation in $Λ^0_b \to pK^-π^+π^-$ and its U-spin partner decays

The LHCb Collaboration has recently found a large CP-violating rate asymmetry in the $b$-baryon decay $Λ^0_b \to pK^-π^+π^-$. This is the first observation of CP violation in baryon processes, opening a new window to test its standard model origin. Many more baryon decays are expected to exhibit observable signals of CP violation. We show that there also exists large CP violation in the U-spin partner decay mode, $Ξ^0_b \to Σ^+π^- K^+K^-$, with rate asymmetry $$A_{CP}(Ξ^0_b \to Σ^+π^- K^+ K^-) = - A_{CP}(Λ^0_b \to p K^- π^+ π^-)~ \frac{Br(Λ^0_b \to p K^- π^+ π^-)}{Br(Ξ^0_b \to Σ^+ π^- K^+ K^-)}~ \frac{τ^{Ξ_b}}{τ^{Λ_b}} $$ in the U-spin symmetry limit. By neglecting a subleading contribution in the amplitudes, we obtain $$A_{CP}(Λ^0_b \to p π^+ π^- π^-) = A_{CP}( Ξ^0_b \to Σ^+π^- K^+ K^-) = -(12 \pm 3 ) \%. $$ These predictions provide crucial tests for the standard model.

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arXiv2025

Precise measurement of CP violating $τ$ EDM through $e^+ e^- \to γ^*, ψ(2s) \to τ^+ τ^-$

A nonzero electric dipole moment of a tauon, $d_τ$, signals CP violation and provides an important probe for new physics. We study methods to measure $d_τ$ at low energy $e^+ e^-$ colliders through the processes $e^+e^- \to γ^*, ψ(2S) \to τ^+τ^-$ with $τ^\pm$ decays into a charged hadron and a tau neutrino. We point out that, with measuring energies of the charged hadron, Im$(d_τ)$ can be measured. On the other hand, selecting events of $τ$ decays after traveling more than the detector resolution distance, Re$(d_τ)$ can also be determined. We find that the precision at Super Tau-Charm Facility (STCF) running at the center energy of $m_{ψ(2S)}$ for 10 year data accumulation, the precision of Im$(d_τ)$ and Re$(d_τ)$ are found to be 1.8 and 11 in unit of $ 10^{-18}~e\,\text{cm}$, respectively. The sensitivity for $d_τ$ measurement precision at the STCF can be reached its optimum at a central energy of $6.3~\text{GeV}$, achieving a precision of $0.7$ for Im$(d_τ)$ and $2.8$ for Re$(d_τ)$ in unit of $ 10^{-18}~e\,\text{cm}$.

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arXiv2025

CP violation studies at Super Tau-Charm Facility

Charge-parity ($C\!P$) violation in the tau-charm energy region is a promising area for sensitive tests of Standard Model (SM) predictions and searches for new, beyond the SM physics. A future Tau-Charm Facility that operates at center-of-mass energies between 2.0 and 7.0 GeV, with a peak luminosity of $0.5\times10^{35}$~cm$^{-2}$s$^{-1}$, would provide huge numbers of hadrons and tau ($τ$) leptons that are produced in low-background environments and with well understood kinematic properties. In this report, prospects for unique studies of $C\!P$ violation in the decay of charmed hadrons, and in the production and decay of hyperons and $τ$ leptons at a next-generation tau-charm facility are discussed. In addition, opportunities for improved tests of $CPT$ invariance test in $K^{0}-\bar{K}^{0}$ mixing are presented.

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arXiv2024

Fundamental Tests of P and CP Symmetries Using Octet Baryons at the $J/ψ$ Threshold

We systematically investigate tests of the parity P and the combined parity and charge-conjugate CP symmetries from differential angular distributions of $J/ψ$ decaying into the lowest-lying baryon pairs at BESIII and the next-generation super tau-charm facilities (STCFs). Large corrections from $Z$ and $W$ exchange induced parity violating effects are found for $J/ψ$ decays with large logarithms resummed up to $\mathcal{O}(α_s)$. The parity-violating asymmetries on the production and the decay sides of $J/ψ$ are both estimated to be of $\mathcal{O}(10^{-4})$, thus barely observable with the 10 billion $J/ψ$ events currently collected at BESIII. Nevertheless, these asymmetries utilizing the current BESIII data already permit a measurement of the weak mixing angle with an absolute uncertainty $δs_w^2\approx{0.09}$, corresponding to the first determination of $s_w^2$ at the $J/ψ$ threshold. In the future, STCFs are estimated to improve this bound by a factor of $\sim\,20$ to $δs_w^2\approx{0.005}$ within one year based on luminosity rescaling. We also obtain the 95% confidence level upper bounds on the electric dipole moments of the octet baryons, which are of $\mathcal{O}(10^{-18})\,e{\rm\,cm}$ for BESIII and $\mathcal{O}(10^{-19})\,e{\rm\,cm}$ for STCFs. These bounds are improved by two to three orders of magnitude in comparison with the only existing one on $Λ$ from Fermilab. The method discussed in this work also paves a way for a first and direct measurement of the $Ξ$ and $Σ$ electric dipole moments.

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arXiv2024

Impact of parity violation on quantum entanglement and Bell nonlocality

Quantum entanglement (QE), evidenced by Bell inequality (BI) violations, reveals the nonlocality of nature. Fundamental interactions manifest in various forms, each with distinct effects on QE and BI, but have not yet been studied in depth. We investigate in detail the relationship between QE, Bell nonlocality, and parity-violating interactions in spin-1/2 bipartite systems arising from the decays of spin-0 and spin-1 particles within the quantum field theory (QFT). Our findings reveal that parity (P) violation can completely disentangle particle pairs, rendering Bell tests ineffective in distinguishing between classical and quantum theories. In the spin-0 case, complete disentanglement occurs at maximal P violation, which is similarly true for spin-1 decays. Without restrictions from the QFT, the predicted relation between entanglement and the Bell nonlocality may no longer be valid and we propose promising methods for testing it. Additionally, we emphasize the previously overlooked influence of magnetic fields within detectors, which alters predictions for QE and Bell nonlocality. This environmental effect induces spurious P and charge-parity (CP) violations and thus has to be subtracted for genuine P, CP, and Bell tests.

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arXiv2024

The SM expected branching ratio for $h \to γγ$ and an excess for $h \to Z γ$

The recent measurements of $h \to Z γ$ from ATLAS and CMS show an excess of the signal strength $μ_Z = (σ\cdot{\cal B})_{\mathrm{obs}}/(σ\cdot{\cal B})_{\mathrm{SM}}=2.2\pm 0.7$, normalized as 1 in the standard model~(SM). If confirmed, it would be a signal of new physics (NP) beyond the SM. We study NP explanation for this excess. In general, for a given model, it also affects the process $h \to γγ$. Since the measured branching ratio for this process agrees well with the SM prediction, the model is severely constrained. We find that a minimally fermion singlets and doublet extended NP model can explain simultaneously the current data for $h \to Z γ$ and $h\to γγ$. There are two solutions. Although both solutions enhance the amplitude of $h \to Z γ$ to the observed one, in one of the solutions the amplitude of $h \to γγ$ flips sign to give the observ ed branching ratio. This seems to be a contrived solution although cannot be ruled out simply using branching ratio measurements alone. However, we find another solution that naturally enhances $h \to Z γ$ to the measured value, but keeps the amplitude of $h \to γγ$ close to its SM prediction. We also comment on the phenomenology associated with these new fermions.

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arXiv2024

Scalar dark matter explanation of the excess in the Belle II $B^+\to K^+ +$ invisible measurement

Recently Belle II reported the first measurement of $B^+\to K^++{\rm invisible (inv)}$, which is $2.7σ$ above the standard model (SM) prediction. If confirmed, this calls for new physics beyond SM. In the SM, the invisible particles are neutrino-anti-neutrino pairs. There are more possibilities when going beyond the SM. In this work, we focus on decays to dark matter (DM) and show that the $B\to K +\mathrm{inv}$ excess from Belle II and DM relic density can be simultaneously explained in a simple extension of the SM. The model introduces a real scalar singlet $ϕ$ acting as a DM candidate, and two heavy vector-like quarks $Q,D$ with the same quantum numbers as the SM left-handed quark doublet and right-handed down-type quark singlet, respectively. All these new particles are odd under a $\mathbb{Z}_2$ symmetry while the SM particles are even. The model can successfully explain the Belle II anomaly and DM relic density for TeV-scale heavy quarks with hierarchical Yukawa couplings involving $b$ and $s$ quarks. At the same time, it can easily satisfy other flavour physics constraints. Direct detection searches utilizing the Migdal effect constrain some of the parameter space.

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arXiv2024

Majorana Phase And Matter Effects In Neutrino Chiral Oscillation

Due to finite masses and mixing, for neutrinos propagation in space-time, there is a chiral oscillation between left- and right- chiral neutrinos, besides the usual oscillation between different generations. The probability of chiral oscillation is suppressed by a factor of $m^2/E^2$ making the effect small for relativistic neutrinos. However, for non-relativistic neutrinos, this effect can be significant. In matter, the equation of motion is modified. When neutrinos produced in weak interaction pass through the matter, the eigen-energies are split into two different ones depending on the helicity of the neutrino. This results in different oscillation behavior for neutrinos with different helicity, in particular there is a new resonant effect related to the helicity state of neutrino different than the usual MSW effect. For Majorana neutrinos, chiral oscillation also depends on Majorana phases.

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arXiv2024

Revisiting models that enhance $B^+\to K^+ ν\barν$ in light of the new Belle II measurement

Belle II has recently reported the new measurement ${\cal B}(B^+\to K^+ν\barν)=(2.3\pm0.7)\times 10^{-5}$ \cite{Belle-II:2023esi} which is two times larger than their previous result (although consistent within errors) and about $2.7\,σ$ higher than the SM prediction. We re-examine new physics scenarios we have discussed previously which can enhance this rate to determine if they can accommodate the higher value reported in the new measurement. We use consistency with existing bounds on $B\to K^*ν\barν$, $b\to s \ell^+\ell^-$, $B\to D^{(*)}\ell\barν$ and $B_s$ mixing to limit possible explanations for the excess. For the case of LFV neutrino couplings, we find that only two leptoquarks remain viable requiring a large $C_{9^\prime}^{ττ}=-C_{10^\prime}^{ττ}$. For models with different types of light dark matter particle pairs (scalar, fermion, or vector), the preliminary $q^2$ distribution from Belle II, which shows that the excess appears mostly for bins with $3\leq q^2\leq7$ GeV$^2$ \cite{Belle-II:2023esi}, implies only the vector current operators with scalar or vector dark matter particles with masses in the hundreds of MeV can match the anomaly.

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arXiv2024

Complete determination of $SU(3)_F$ amplitudes and strong phase in $Λ_c^+ \to Ξ^0 K^+$

The BESIII collaboration has recently reported the first time measurement of the decay asymmetry $α(Λ_c^+ \to Ξ^0 K^+) = 0.01 \pm 0.16(stat.) \pm 0.03(syst.)$ and also a sizable phase shift of $δ_P-δ_S = -1.55 \pm 0.25$ or $1.59\pm 0.25$ between S- and P-wave amplitudes. This implies significant strong phase shifts in the decay amplitudes. The strong phases indicate the existence of rescattering or loop effects, which are challenging to calculate due to non-perturbative effects. By employing the flavor $SU(3)_F$ symmetry and applying the Körner-Pati-Woo theorem to reduce the number of parameters, we find that the current data already allow us to obtain, for the first time, model-independent decay amplitudes and their strong phases. The establishment of the existence of sizable strong phases opens a window for future investigations into CP violation. In our fit, a notable discrepancy emerges in the branching ratio of $Ξ_c^0 \to Ξ^- π^+$. The direct relationship between $Γ(Λ_c^+ \to Λe^+ν_e)$ and $Γ(Ξ_c^0 \to Ξ^- e^+ν_e)$, along with newly discovered $SU(3)_F$ relations, collectively suggests an underestimation of $\mathcal{B}(Ξ_c^0 \to Ξ^- π^+)$ in experimental findings.

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arXiv2023

Forbidden Dark Matter Combusted Around Supermassive Black Hole

The forbidden dark matter cannot annihilate into a pair of heavier partners, either SM particles or its partners in the dark sector, at the late stage of cosmological evolution by definition. We point out the possibility of reactivating the forbidden annihilation channel around supermassive black holes. Being attracted towards a black hole, the forbidden dark matter is significantly accelerated to overcome the annihilation threshold. The subsequent decay of the annihilation products to photon leaves a unique signal around the black hole, which can serve as a smoking gun for the forbidden dark matter. For illustration, the Fermi-LAT data around Sgr $A^*$ provides a preliminary constraint on the thermally averaged cross section of the reactivated forbidden annihilation that is consistent with the DM relic density requirement.

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arXiv2023

Global analysis of measured and unmeasured hadronic two-body weak decays of antitriplet charmed baryons

A large amount of data on hadronic two body weak decays of anti-triplet charmed baryons $T_{c\bar 3}$ to an octet baryon $T_8$ and an octet or singlet pseudoscalar meson $P$, $T_{c \bar 3} \to T_8 P$, have been measured. The SU(3) flavor symmetry has been applied to study these decays to obtain insights about weak interactions for charm physics. However not all such decays needed to determine the SU(3) irreducible amplitudes have been measured forbidding a complete global analysis. Previously, it has been shown that data from measured decays can be used to do a global fit to determine all except one parity violating and one parity conserving amplitudes of the relevant SU(3) irreducible amplitudes causing 8 hadronic two body weak decay channels involving $Ξ^0_c$ to $η$ or $η'$ transitions undetermined. It is important to obtain information about these decays in order to guide experimental searches. In this work using newly measured decay modes by BESIII and Belle in 2022, we carry out a global analysis and parameterize the unknown amplitudes to provide the ranges for the branching ratios of the 8 undetermined decays. Our results indicate that the SU(3) flavor symmetry can explain the measured data exceptionally well, with a remarkable minimal $χ^2/d.o.f.$ of 1.21 and predict 80 observables in 45 decays for future experimental data to test. We then vary the unknown SU(3) amplitudes to obtain the allowed range of branching ratios for the 8 undetermined decays. We find that some of them are within reach of near future experimental capabilities. We urge our experimental colleagues to carry out related searches.

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arXiv2023

$ΔS=2$ nonleptonic hyperon decays as probes of new physics

Hyperon nonleptonic decays that change strangeness by two units, such as $Ξ\to Nπ$ and $Ω^-\to nK^-,Λπ^-,Σ^{(*)}π$, are highly suppressed in the standard model. Only a few of them have been searched for to date, leading to experimental upper bounds which are many orders of magnitude above the expectations of the standard model. This leaves ample opportunity to look for indications of new physics in these processes. At the same time, most, but not all, $ΔS=2$ interactions beyond the standard model are severely constrained by kaon-mixing data. We present two scenarios where new physics satisfying the kaon-mixing constraints can enhance the hyperon decay rates to levels that can be probed in future quests by BESIII and LHCb and at the proposed Super Tau-Charm Factory. Both scenarios require significant fine-tuning.

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arXiv2023

A Search for Light Fermionic Dark Matter Absorption on Electrons in PandaX-4T

We report a search on a sub-MeV fermionic dark matter absorbed by electrons with an outgoing active neutrino using the 0.63 tonne-year exposure collected by PandaX-4T liquid xenon experiment. No significant signals are observed over the expected background. The data are interpreted into limits to the effective couplings between such dark matter and electrons. For axial-vector or vector interactions, our sensitivity is competitive in comparison to existing astrophysical bounds on the decay of such dark matter into photon final states. In particular, we present the first direct detection limits for an axial-vector (vector) interaction which are the strongest in the mass range from 25 to 45 (35 to 50) keV/c$^2$.

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