arXiv subjects

Han-Qing Zheng

Publications and source records attributed to Han-Qing Zheng.

At least 19 recordsLinked to original sources

arXiv2026

Lattice QCD study of the $K^*(892)$ resonance at the physical point

We present a lattice QCD study of the $K^*(892)$ resonance using eight $N_f=2+1$ Wilson-Clover ensembles with three lattice spacings and six pion masses ranging from 135 to 320 MeV. For each ensemble, a large number of finite volume energy levels in the $P$-wave $Kπ$ channel are determined. The energy dependence of the scattering phase shift is then obtained from Lüscher's finite-volume method. To systematically assess parametrization dependence, the amplitude is described using three different models, which yield consistent results. The resulting phase shifts show a clear resonant behavior for all ensembles, and the corresponding $K^*(892)$ resonance pole is identified on the second Riemann sheet in the complex energy plane. The pole positions are extrapolated to the physical pion mass and the continuum limit, yielding a $K^*(892)$ resonance located at $\sqrt{s_0} = [883(22)-i20(13)]\mathrm{MeV}$, which is in excellent agreement with the experimental value. This study provides a first-principles QCD determination of the $K^*(892)$ mass and width with controlled systematic uncertainties.

hep-lat↗

arXiv2026

Studies of low energy $l+p\to l+p+γ$ process in covariant chiral perturbation theory

This study presents a tree-level calculation of the scattering amplitude for the $lp\to lpγ$ (with a hard photon) process within the framework of Chiral Perturbation Theory. Our calculations, based on the $O(p^2)$ and $O(p^3)$ nucleon-pion Lagrangians, aim to provide a theoretical prediction for the differential cross-section. The result shows that explicit inclusion of the nonzero lepton mass significantly influences the low energy differential cross section for $μp\to μp γ$ process. The kinematic region of the present experimental data is beyond the validity domain of the $χ$PT and is therefore not suitable for determining the low-energy constants (LECs). By comparing our results with future experimental data, we expect to determine the values of the LECs as a further test of $χ$PT as an effective low-energy theory of QCD. The process is of significant interest as it can help to determine the generalized polarizabilities of the nucleon.

hep-ph↗

arXiv2025

Studies on quark-mass dependence of the $N^*(920)$ pole from $πN$ $χ$PT amplitudes

The quark-mass dependence of the $N^*(920)$ pole is analyzed using $K$-matrix method, with the $πN$ scattering amplitude calculated up to $O(p^3)$ order in chiral perturbation theory. As the quark mass increases, the $N^*(920)$ pole gradually approaches the real axis in the complex $w$-plane (where $w=\sqrt{s}$). Eventually, in the $O(p^2)$ case, it crosses the $u$-cut on the real axis and enters the adjacent Riemann sheet when the pion mass reaches $526~{\rm MeV}$. At order $O(p^3)$, the rate at which it approaches the real axis slows down; however, we argue that it will ultimately cross the $u$-cut and enter the adjacent Riemann sheet as well. Additionally, the trajectory of the $N^*(920)$ pole is in qualitative agreement with the results from the linear $σ$ model calculation.

hep-ph↗

arXiv2025

Review on recent progress in the study of the $N^*(920)$ subthreshold singularity and the $σ/f_0(500)$ meson

We summarize recent results on studies of $ππ$ and $πN$ scatterings. They include the finding of a negative-parity nucleon pole with a mass lower than the nucleon mass, and the pole trajectory of $f_0(500)$ as the pion mass varies. The results are obtained from model-independent dispersion analyses. We also study the thermal properties of $f_0(500)$ based on the $O(N)$ $σ$ model and $N/D$ method.

hep-ph↗

arXiv2025

On the pole trajectory of the subthreshold negative parity nucleon with varying pion masses

We study the pole trajectory of the recently established subthreshold negative parity nucleon pole, namely the $N^*(920)$, with varying pion masses, in the scheme of linear $σ$ model with nucleons using the $N/D$ unitarization method. We find that as the pion mass increases, the pole moves toward the real axis. For larger pion masses, at tree level, the pole falls to a specific point on $u$-channel cut and crosses to the adjacent Riemann sheet defined by the logarithmic $u$ channel cut. At one-loop level, the pole does not meet the $u$-cut up to $m_π=0.36$GeV. We also re-examined the $σ$ pole trajectory and find it in good agreement with Roy equation analysis result.

hep-ph↗

arXiv2025

Double pion photoproduction off nucleons in covariant chiral perturbation theory

The double pion photoproduction off nucleons near threshold is analyzed in a covariant baryon chiral perturbation theory up to next to leading order, where the $Δ(1232)$, $N^*(1400)$ and $ρ(770)$ resonances are included as explicit degrees of freedom. For the process $γp \to π^+ π^0 n$, the chiral results of total cross sections, invariant-mass distributions and beam-helicity asymmetry are in good agreement with the experimental data within uncertainties. For the process $γp \to π^0 π^0 p$, the prediction of total cross section deviates from the existing experimental data. Once the final-state interaction of $ππ$ in the isoscalar S-wave channel is taken into account, a good description of the cross section is achieved. The effect of the Roper resonance always turns out be negligible, and hence can be thrown away in future study of this process.

hep-ph↗

arXiv2024

Revisiting $O(N)$ $σ$ model at unphysical pion masses and high temperatures. II. The vacuum structure and thermal $σ$ pole trajectory with cross-channel improvements

The effective potential of the $O(N)$ model at large $N$ limit is reinvestigated with varying pion mass and temperature. For large pion masses and high temperatures, we find the phenomenologically favored vacuum, located on the upper branch of the double-branched effective potential for physical $m_π$, moves to the lower branch and becomes no longer a local minimum but a saddle point. The existence and running of the tachyon pole are also discussed. These phenomena indicate that the applicable energy range of $O(N)$ model is more and more limited as $m_π$ becoming larger and temperature going higher. With the effective coupling constant defined from the effective potential, the possible correspondence between the two branches of the effective potential and the two phases of the theory (distinguished by positive or negative coupling) is verified even with nonzero explicit symmetry breaking and at finite temperature. Also, we generalize the $N/D$ modified $O(N)$ model to study the thermal trajectory of the $σ$ pole with the cross-channel contributions considered and find the thermal $σ$ pole trajectory resembles its counterpart with varying pion mass at zero temperature.

hep-ph↗

arXiv2024

Revisiting $O(N)$ $σ$ model at unphysical pion masses and high temperatures

Roy-equation analyses on lattice data of $ππ$ scattering phase shifts at $m_π=391$MeV reveals that the lowest $f_0$ meson becomes a bound state under this condition. In addition, there is a pair of complex poles below threshold generated by crossing symmetry [X.-H. Cao et al., Phys. Rev. D 108, 034009 (2023)]. We use the $N/D$ method to partially recover crossing symmetry of the $O(N)$ $σ$ model amplitude at leading order of $1/N$ expansion, and qualitatively reproduce the pole structure and pole trajectories with varying pion masses as revealed by Roy-equation analyses. The $σ$ pole trajectory with varying temperature is also discussed and found to be similar to its properties when varying $m_π$. As the temperature increases, the complex $σ$ poles firstly move from the second Riemann sheet to the real axis becoming two virtual state poles, and then one virtual state pole moves to the first sheet turning into a bound state pole and finally tends to the pion pole position at high temperature which is as expected from the chiral symmetry restoration. Our results provide further evidences that the lowest $f_0$ state extracted from experiments and lattice data plays the role of $σ$ meson in the spontaneous breaking of chiral symmetry. Finally, we also briefly discuss the problems of the effective potential in the situation when $m_π$ and temperature get large.

hep-ph↗

arXiv2024

Some remarks on compositeness of $T^+_{cc}$

Recently LHCb experimental group find an exotic state $T^+_{cc}$ from the process $p\bar{p} \to D^0D^0π^+ + X$. A key question is if it is just a molecule or may have confined tetraquark ingredient. To investigate this, different methods are taken, including two channel ($D^{*+}D^0$ and $D^{*0}D^+$) K-matrix unitarization and single channel Flatté-like parametrization method analysed by pole counting rule and spectral density function sum rule. It demonstrates that $T^+_{cc}$ is a molecular state, though the possibility that there may exist elementary ingredient can not be excluded, by rough analysis on its production rate.

hep-ph↗

arXiv2023

A discussion on the anomalous threshold enhancement of $J/ψ$ -- $ψ(3770)$ couplings and $X(6900)$ peak

The attractive interaction between $J/ψ$ and $ψ(3770)$ has to be strong enough to form the molecular resonant structure, i.e., $X(6900)$. We argue that since $ψ(3770)$ decays predominantly into a $D\bar D$ pair, the interactions between $J/ψ$ and $ψ(3770)$ may be significantly enhanced due to the three point $D\bar D$ loop diagram. The enhancement comes from the anomalous threshold located at $t=-1.288$GeV$^2$, which effect propagates into the $s$-channel partial wave amplitude in the vicinity of $\sqrt{s}\simeq 6.9$GeV. This effect may be helpful in forming the $X(6900)$ peak.

hep-ph↗

arXiv2023

Roy equation analyses of $ππ$ scatterings at unphysical pion masses

An extended Roy equation including a bound state pole is used to study $ππ$ scatterings at unphysical large pion masses when $σ$ becomes a bound state in one situation and stays as a broad resonance in the other case. The coupled integral equations at large pion masses are solved by taking the lattice driving terms and the Regge amplitudes as inputs. Relying on the solutions of Roy equations that respect unitarity, analyticity and crossing symmetry, we give predictions to the phase shifts with $IJ=00,11,20$ in the elastic energy region. We then perform analytic continuation into the complex $s$ plane to search for various poles, all of which are inside the validity domain of the Roy equation. This is the first time that lattice data at unphysical large pion masses are analyzed within the rigorous Roy equation method.

hep-ph↗

arXiv2023

On the nature of X(3960)

A near-threshold enhancement in the $D_s^+ D_s^-$ system, dubbed as $X(3960)$, is observed by the LHCb collaboration recently. A combined analysis on $χ_{c0}(3930)~(\to D^+ D^-)$, $X(3960)~(\to D_s^+ D_s^-)$, and $X(3915)~(\to J/ψω)$ is performed using both a $K$-matrix approach of $D_{(s)}\bar{D}_{(s)}$ four-point contact interactions and a model of Flatté-like parameterizations. The use of the pole counting rule and spectral density function sum rule indicate, under current statistics, that this $D_s^+ D_s^-$ near-threshold state has probably the mixed nature of a $c\bar{c}$ confining state and $D_s^+ D_s^-$ continuum.

hep-ph↗

arXiv2023

The $X(6900)$ peak could be a molecular state

The analyses of the LHCb data on $X(6900)$ found in di-$J/ψ$ and $J/ψψ(3686)$ systems are performed using a momentum-dependent {Flatté}-like parameterization. The use of the pole counting rule and spectral density function sum rule give consistent conclusions that $X(6900)$ may not be a molecule of $J/ψψ(3686)$. Nevertheless it is still possible that $X(6900)$ be a molecule of higher states, such as $J/ψψ(3770)$, $χ_{c0}χ_{c2}$, etc.

hep-ph↗

arXiv2023

Identifying Hadronic Molecular States with a Neural Network

Neural networks are trained to judge whether or not an exotic state is a hadronic molecule of a given channel according its line-shapes. This method performs well in both trainings and validation tests. As applications, it is applied to study $X(3872)$, $X(4260)$ and $Z_c(3900)$. The results show that $Z_c(3900)$ should be regarded as a $\bar{D}^* D$ molecular state but $X(3872)$ not. As for $X(4260)$, it can not be a molecular state of $χ_{c0}ω$. Some discussions on $X_1(2900)$ are also provided.

hep-ph↗

arXiv2022

A possible subthreshold pole in $S_{11}$ channel from $πN$ Roy-Steiner equation analyses

The hyperbolic version of Roy-Steiner equation describing low energy $πN$ scatterings, with larger analyticity domain in the complex $s$ plane is solved. The numerical results on phase shifts of low partial waves are in agreement with that of Hoferichter et al. [Phys. Rept. 625 (2016) 1]. A subthreshold pole in $S_{11}$ channel is found located at $\sqrt{s}=(918 \pm 3)-i (163 \pm 9)$~MeV.

hep-ph↗

arXiv2022

Singularities and Accumulation of Singularities of $π$N Scattering amplitudes

It is demonstrated that for the isospin $I=1/2$ $π$N scattering amplitude, $T^{I=1/2}(s,t)$, $s={(m_N^2-m_π^2)^2}/{m_N^2}$ and $s=m_N^2+2m_π^2$ are two accumulation points of poles on the second sheet of complex $s$ plane, and are hence accumulation of singularities of $T^{I=1/2}(s,t)$. For $T^{I=3/2}(s,t)$, $s={(m_N^2-m_π^2)^2}/{m_N^2}$ is the accumulation point of poles on the second sheet of complex $s$ plane. The proof is valid up to all orders of chiral expansions.

nucl-th↗

arXiv2022

Radiative Correction to Lepton Proton Scatterings in Manifestly Lorentz-Invariant Chiral Perturbation Theory

Manifestly Lorentz-invariant baryon chiral perturbation theory is used to calculate the radiative correction of low energy elastic lepton proton scatterings. Corrections of differential cross section and charge asymmetry are given at chiral next-to-leading order $(\mathcal{O}(p^2))$ with a nonzero lepton mass, which are infrared and ultraviolet finite. The results are basically consistent with previous predictions based on hadron model calculation, but they are somewhat different from calculations based on heavy baryon chiral perturbation theory, especially in charge asymmetry.

hep-ph↗

arXiv2022

On Lowest Lying $\frac{1}{2}^-$ Octet Baryons

The recently proposed $N^*(890)$ $1/2^-$ baryon is studied in a flavor $SU(3)$ scheme with $K$ matrix unitarization, by fitting to low energy cross section and phase shift data. It is found that $N^*(890)$ co-exists with low lying poles in other channels, which have been extensively discussed in the literature, though they belong to different octets, in the $SU(3)$ limit. Hence the existence of $N^*(890)$ is further verified.

hep-ph↗