arXiv subjects

Hai-Qing Zhou

Publications and source records attributed to Hai-Qing Zhou.

At least 19 recordsLinked to original sources

arXiv2026

Strange Two-Photon Exchange in Time-Like Electromagnetic Form Factors

While two-photon exchange (TPE) effects in space-like nucleon electromagnetic form factors have been extensively studied for over two decades, their manifestations in the time-like region have yet to be definitively established. We propose the isospin-violating decay $J/ψ\to \barΣ^{0}Λ$ and its charge-conjugate process as a sensitive probe of TPE contributions to time-like hyperon electromagnetic form factors. This channel provides a clean laboratory for isolating TPE effects through sensitive and robust polarization and spin-correlation observables, which are particularly enhanced by the $Λ$-$Σ^0$ mass splitting. Combining a dispersion-relation analysis of the one-photon-exchange contribution with explicit loop calculations of the TPE amplitudes, we perform a quantitative study of TPE effects in the threshold region. Our results demonstrate that the considered observables can provide unambiguous signatures of TPE contributions and present predictions for their size together with the associated theoretical uncertainties.

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arXiv2026

Axion production in the $η\to ππa$ decay within $SU(3)$ chiral perturbation theory

We study the axion and axion-like particle production from the $η\toππa$ decay within the $SU(3)$ chiral perturbation theory up to the one-loop level. The conventional $SU(3)$ chiral low energy constants are found to be able to reabsorb all the divergences from the chiral loops in the $η\toππa$ decay amplitude, and hence render the amplitude independent of the renormalization scale. The unitarized $η\toππa$ decay amplitudes are constructed to take into account the $ππ$ final-state interactions and also properly reproduce the perturbative results from the chiral perturbation theory. Detailed analyses between the perturbative amplitudes and the unitarized ones are given in the phenomenological discussions. By taking the values of the chiral low energy constants in literature, we predict the Dalitz distributions, the spectra of the $ππ$ and $aπ$ systems, and also the branching ratios of the $η\toππa$ process by varying $m_a$ from 0 to $m_η-2m_π$.

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arXiv2026

Isospin-breaking contribution to the model-independent axion-photon-photon coupling in $U(3)$ chiral theory

We pursue the calculation of the model-independent component of the axion-photon-photon coupling in the $U(3)$ chiral perturbation theory up to next-to-leading order, with the emphasis on the isospin breaking effect. The mixing of the $π^0$-$η$-$η'$-axion system is revised as well by working out the complete linear isospin-breaking terms. Our calculation shows that the isospin-breaking correction to the axion-photon-photon coupling amounts to more than 15%, comparing with the result in the isospin limit.

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arXiv2026

Prominent enhancement of axion thermalization rate from axion-kaon interactions

The axion thermalization rate is a crucial input to determine the hot dark matter bound of axions, resulting from the scattering processes in the thermal bath of early Universe. We demonstrate that the commonly employed axion thermalization rate by including the $aπ\leftrightarrow ππ$ channel alone is significantly underestimated for the temperature $T$ above 100 MeV. This is obtained through the systematical calculation of the axion-light flavor meson scattering amplitudes within the framework of the chiral unitarization approach, paying special attention to the $a K \leftrightarrow πK$ reaction. Hadron resonances appearing in $a K \leftrightarrow πK$ amplitudes significantly enlarge the cross sections, which turn out to be much bigger than that of $aπ\leftrightarrowππ$. The axion thermalization rate is then substantially enhanced by the $a K \leftrightarrow πK$ channel for $T\gtrsim 100$ MeV. Especially at $T\simeq 130$ MeV, the contribution from the $a K \leftrightarrowπK$ reaction to the axion thermalization rate exceeds the $aπ\leftrightarrowππ$ one. Obviously more stringent constraints on the axion parameters are obtained, when confronting the number of extra relativistic degrees of freedom $ΔN_{\rm eff}$ from Planck$'$18.

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arXiv2026

$γW$-exchange correction beyond the forward-angle limit in neutron $β$ decay

In this work, we discuss the $γW$-exchange contributions in neutron $β$ decay with an elastic intermediate state, beyond the forward-angle limit (FAL). By decomposing the one-$W$-exchange and $γW$-exchange amplitudes in terms of $16$ independent Pauli-spinor structures, we calculate the $γW$-exchange corrections to the relevant coefficients. Our numerical results show that the relative corrections to the Fermi Born term $C_{\text{Born}}^{\text{F}}$ and the Gamow-Teller Born term $C_{\text{Born}}^{\text{GT}}$ are enhanced by about 8\% and 18\%, respectively. In particular, we find a non-zero contribution to $C_{\text{Born}}^{\text{GT}}$ from the axial-vector current, which is identically zero in the FAL. The corresponding effect on the extracted $V_{ud}$ from the neutron lifetime is also analyzed, and we find the correction to be at the $10^{-4}$ level.

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arXiv2026

Corrections to the Forward Limit Dispersion Relations for $γZ$-Exchange Contributions

The weak charge of the proton $Q_{\textrm{w}}$ is one of the most fundamental quantities in physics. It can be determined by measuring the parity asymmetry $A_{\textrm{PV}}$ in elastic $ep$ scattering, where the $γZ$-exchange contributions are crucial. For the past fifteen years, dispersion relations (DRs) in the forward limit have been widely used as a model-independent method to estimate these contributions. In this work, we study corrections to these forward-limit DRs. We first estimate these corrections using pointlike interactions as an illustrative example. We then estimate the $γZ$-exchange contributions for the upcoming P2 experiment through both direct calculation and the forward-limit DRs, within the framework of low-energy effective interactions. The results indicate that the correction to the forward-limit DR for $\Box_{γZ}^{V}$ is around 47\% for the upcoming P2 experiment, which will significantly modify the extracted value of $Q_{\textrm{w}}$.

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arXiv2026

$γZ$-exchange contribution in elastic $ep$ scattering by perturbative QCD

In this study, we calculate the $γZ$-exchange contribution to elastic $ep$ scattering at large momentum transfer within perturbative QCD. We present analytical expressions for the $γZ$-exchange contributions to the amplitudes. We also estimate the asymptotic behaviors of the amplitude contributions and of the physical quantity $A_{\text{PV}}$ at high momentum transfer. These asymptotic behaviors determine the subtraction order in the dispersion relations (DRs) satisfied by the amplitudes. We find that the DR usually used in the literature for the axial-vector part of the amplitude is not valid at high $Q^2$ and should be modified to a once-subtracted form. Within the present pQCD framework and the adopted proton distribution amplitudes, these high-energy properties also provide nontrivial constraints on low-energy DR assumptions.

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arXiv2026

Axion-like particle production from lepton-nucleon scattering in chiral effective theory

In this work we study the axion/axion-like particle production from the lepton-nucleon scattering in the low-energy region, i.e., the $\ell N\to \ell N a$ processes, $\ell$ being the electron or muon and $N$ the proton or neutron. We simultaneously include three different types of axion interaction couplings within the chiral effective field theory, namely the axion-nucleon-nucleon couplings $g_{aNN}$, axion-photon-photon coupling $g_{aγγ}$ and axion-photon-vector meson resonances couplings $g_{ρaγ}$ and $g_{ωaγ}$. Vast inputs from the lattice QCD and hadron phenomenological studies are used to fix the unknown couplings. The relative strengths of different axion interactions in the $\ell N\to \ell N a$ processes are then revealed. We provide detailed predictions for the differential cross sections with respect to various angles and axion energy, as well as the total cross sections in the low-energy region around production thresholds, both for the Kim-Shifman-Vainstein-Zakharov (KSVZ) and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) axion models.

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arXiv2025

Relativistic correction to the binding energies of two-body hadronic molecular states

This study presents a systematic estimation of the relativistic correction to the binding energies of two-body hadronic molecular states by comparing the numerical solutions of the three-dimensional (3D) Schr{ö}dinger, 3D Salpeter, and fully relativistic four-dimensional (4D) Bethe-Salpeter (BS) equations derived from the same underlying interaction. The numerical results reveal a counter-intuitive property: for hadronic molecular states whose binding energies are in the MeV range, the relativistic correction is unexpectedly large. This finding contradicts the conventional expectation that a heavier exchanged mass in the interaction implies suppressed relativistic effects. Specifically, we first benchmark the results using the Wick-Cutkosky model with a one-boson-exchange (OBE) interaction of mass $m_{ex}$, and then extend the analysis to the physical $D\bar{D}$ system. We find within the $1\sim 50$ MeV binding energy region, the relativistic correction is substantial, amounting to $-90\% \sim -70\%$ of the non-relativistic result. Such a significant correction strongly suggests that analyses based solely on the 3D Schr{ö}dinger or 3D Salpeter equations for hadronic molecular states should be treated with caution.

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arXiv2025

Critical properties of bound states with one-boson-exchange potential

In this study, we discuss some general critical properties of bound states with one-boson-exchange potential. For simplicity, we first take a system with two identical scalar particles as an example. The interaction between these two scalar particles is described by the exchange of another massive scalar meson under the instantaneous approximation, which results in the Yukawa potential. A highly accurate numerical method is used to determine the critical screening mass value of the system. The resulting critical screening mass for the ground state is consistent with those reported in the literature, agreeing to about 30 significant figures. The highly accurate results for the $l=1$ case are also presented, which are significantly more precise than those previously reported in the literature. Furthermore, we extend the discussion to physical hadronic molecule states, where form factors are introduced in the interaction to describe the structure of hadrons. Our numerical results show that although the binding energies of the hadronic molecule states depend on the cutoff in the form factors, the number of hadronic molecule states is almost independent of the cutoffs across a very wide physical region. This indicates a strong and important property: the number of hadronic molecule states is almost solely determined by the coupling constants and the masses of the exchange particles. This highly accurate numerical method can also be straightforwardly applied to higher $l$ cases or other systems.

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arXiv2025

Relativistic corrections to energy spectrum of hydrogen due to the full one-photon-exchange interaction

In this work, we present expressions for the full effective potential corresponding to the one-photon exchange interaction within the framework of an effective Schrödinger-like equation, which is derived exactly from the Bethe-Salpeter equation in quantum electrodynamics. The final effective potential is expressed in terms of eight scalar functions. When these eight scalar functions are expanded in terms of velocities order by order, we can return to the non-relativistic effective potential systematically organized by velocities. By retaining the exact momentum dependence in the effective potential, we estimate its corrections to the energy spectrum of hydrogen using a highly precise numerical method. The comparison of the numerical results with those obtained using conventional bound-state perturbative theory is discussed. Our calculations suggest that it is possible to accurately account for all relativistic contributions using this method. It would be interesting to extend these calculations to positronium, muonic hydrogen, and scenarios involving nuclear structure and radiative corrections.

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arXiv2025

$γW$-exchange contributions in neutron $β$ decay in the forward-angle limit

In this work, the contributions from $γW$-exchange in neutron $β$ decay are estimated at the amplitude level. Using a general form for the electromagnetic (EM) form factors (FFs) of the proton, the EM FFs of the neutron, and the weak FFs of the $Wnp$ interaction as inputs, we present analytical expressions for the inner part of the $γW$-exchange amplitude under the forward angle limit. The differences and relations between our method and those used in the references are discussed. To compare our numerical results with those provided in the references, we consider three types of FFs as examples. The numerical results show that when the favored EM FFs are used, our result for the contribution from the Fermi part is consistent with those reported in the references, while our results for the Gamow-Teller parts are about 7\% and 13\% larger than those reported in Ref.~\cite{Hayen-2021}.

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arXiv2024

In-depth Study of Spin- and Momentum-Dependent Interaction Potentials Between Two Spin-1/2 Fermions Mediated by Light Spin-0 Particles

We present a calculation by including the relativistic and off-shell contributions to the interaction potentials between two spin-1/2 fermions mediated by the exchange of light spin-0 particles, in both momentum and coordinate spaces. Our calculation is based on the four-point Green function rather than the scattering amplitude. Among the sixteen potential components, eight that vanish in the non-relativistic limit are shown to acquire nonzero relativistic and off-shell corrections. In addition to providing relativistic and off-shell corrections to the operator basis commonly used in the literature, we introduce an alternative operator basis that facilitates the derivation of interaction potentials in coordinate space. Furthermore, we calculate both the long-range and short-range components of the potentials, which can be useful for future experimental analyses at both macroscopic and atomic scales.

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arXiv2024

$γZ$-Exchange Contributions in Low-Energy Parity-Violating $ep$ Scattering

In this work, the $γZ$-exchange contributions in the low-energy elastic parity-violating $ep$ scattering are discussed with the approximation $m_e=0$. By expanding the $γpp$ and $Zpp$ interactions on the momentum of photon and considering both the leading-order and the next-to-leading order interactions, we calculate the amplitudes of the $γZ$-exchange diagrams. After performing the loop integral, we expand the results in the low energy limit, and obtain the analytic expressions for the amplitudes. Numerical comparisons show that the analytic expressions are very close to the full results over a large region. We investigate the power behaviors of these contributions and find that some are enhanced by a kinematic factor in the low energy limit. Additionally, in some cases, the imaginary parts of the contributions from the next-to-leading-order interactions are at the same order as those from the leading-order interactions. Furthermore, the corresponding contributions to the physical observable quantity $A_{\textrm{PV}}$ are also discussed. Combining all the properties together, we conclude that these analytic expressions describe the leading-order contributions of all $γZ$-exchange helicity amplitudes in the region with $α_e\ll Q/M_N\sim δ/M_N^2\ll 1$, where $α_{e}$ is the fine structure constant, $M_N$ is the mass of proton, $Q$ and $δ$ are the small quantities related to the momentum transfer and the center-of-mass energy.

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arXiv2023

Axion-pion scattering at finite temperature in chiral perturbation theory and its influence in axion thermalization

Axion-pion scattering amplitudes at finite temperatures are calculated within chiral perturbation theory up to the one loop level. Unitarization procedure is implemented to these amplitudes in order to extend the applicable range of energy and temperature. The influence of the thermal axion-pion scattering amplitudes on the $aπ\toππ$ cross sections and the axion thermalization rate is investigated, with the emphasis on the comparison with the zero-temperature-amplitude case. A brief discussion on the cosmological implication of the axion thermalization rate, that is calculated by using the $aπ\toππ$ amplitudes at finite temperatures, is also given. The thermal corrections to the axion-pion scattering amplitudes can cause around a $10\%$ shift of the determination of the axion decay constant $f_a$ and its mass $m_a$, comparing with the results by using the $aπ\toππ$ amplitudes at zero temperature.

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arXiv2023

Axion-meson mixing in light of recent lattice $η$-$η'$ simulations and their two-photon couplings within $U(3)$ chiral theory

We study the mixing of the QCD/QCD-like axion and light-flavor mesons $π^0, η, η'$ within the framework of $U(3)$ chiral perturbation theory up to next-to-leading order in this work. The axion-meson mixing formulas are calculated order by order in the $U(3)$ $δ$-expansion scheme, namely the joint expansions of the momentum, light-quark masses and $1/N_C$. We provide axion-meson mixing relations in terms of the $π^0$-$η$-$η'$ mixing parameters and their masses. The recent lattice simulations on the $η$-$η'$ systems turn out to be able to offer valuable inputs to constrain the unknown low-energy constants. The relation of the mass and decay constant of the axion is then further explored based on our updated calculations. The two-photon couplings of the light-flavor mesons, together with the axion, are also investigated in the $U(3)$ chiral theory up to next-to-leading order in the $δ$-counting scheme.

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arXiv2023

Recoil contributions in neutron beta decay and their corrections to the correlation coefficients

In this work, the recoil corrections to the correlation coefficients in the neutron beta decay with polarized neutron and electron are evaluated to order O(m_n^-2) in the Born approximation. Different from the usual calculations applied in the literatures where the phase space factor and the amplitude are expanded independently, we directly expand the full differential scattering cross section and then integrate the solid angles of the neutrino. Furthermore, the most general coupling forms in Wpn are kept in the calculation and the analytic expressions for the angular correlation coefficients are given. The numerical comparisons between the results by the different treatments of the phase space factor and the different choices of the coupling parameters are also presented. The results show that the careful treatment of the expansion on m_n^-1 and the inclusion of the general coupling forms are necessary when aiming to reach the precision 10^-5

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arXiv2022

Two-Photon-Exchange effect in $ep\rightarrow enπ^+$ at small $-t$ with the hadronic model and dispersion relation approach

In this work, the two-photon-exchange (TPE) effect in $ep\rightarrow enπ^+$ at small $-t$ is discussed. In the previous work, the TPE contribution with one $π$ intermediate state is estimated numerically within a hadronic model under the pion-dominance approximation. Here we extend the discussion to include one $ρ$ intermediate state. The TPE contribution can be described by one scalar function in the limit $m_e\rightarrow 0$, the dispersion relation (DR) satisfied by this scalar function is analysed. The analytic expressions for the imaginary parts of the TPE contributions from one $π$ or one $ρ$ intermediate state are given within the hadronic model. Combining these analytic expressions and the DR, the corresponding real parts of the TPE contributions can be estimated easily at any available region. This can help the further experimental analysis to include the TPE contributions in a convenient way. The numeric results show that the TPE correction with one $ρ$ intermediate state is much smaller than that with one $π$ intermediate state in the current energy region. These results suggest that the TPE contribution with an elastic state is the main TPE contribution in $ep\rightarrow enπ^+$ at small $-t$.

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