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Zhiguang Xiao

Publications and source records attributed to Zhiguang Xiao.

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

$3d$ flat bands and coupled $4f$ moments in the kagome-honeycomb permanent magnet Sm$_{2}$Co$_{17}$

Rare earth permanent magnets (REPMs) with both localized moments and itinerant conduction bands are not only important for fundamental research but also have significant technological applications. In particular, Sm$_{\rm 2}$Co$_{\rm 17}$ is a prototypical high-temperture REPM, where the Co atoms form a kagome-honeycomb stacked lattice. Here we report synthesis of epitaxial Sm$_{\rm 2}$Co$_{\rm 17}$ films using molecular beam epitaxy and measurements of their momentum-resolved electronic structure from \textit{in-situ} angle-resolved photoemission spectroscopy. Our results unveil two flat bands from Co $3d$ orbitals near the Fermi level ($E_F$), one at $\sim$\,--300\,meV and another right at $E_F$, which arise from orbital-selective destructive interference and strong electron correlations, respectively. In addition, our results reveal that Sm $4f$ states are far away from $E_F$ (hence mostly localized) and exhibit an anomalous temperature dependence, caused by the $3d$-$4f$ magnetic coupling. Our findings provide direct spectroscopic insights to understand the strong uniaxial ferromagnetism in Sm$_{\rm 2}$Co$_{\rm 17}$ (and REPMs alike). Our work also opens avenues to explore flat-band physics near $E_F$ and emergent phenomena in correlated kagome-honeycomb lattices.

cond-mat.str-el↗
arXiv2025

On the generalized Friedrichs-Lee model with multiple discrete and continuous states

In this study, we present several improvements of the non-relativistic Friedrichs-Lee model with multiple discrete and continuous states and still retain its solvability. Our findings establish a solid theoretical basis for the exploration of resonance phenomena in scenarios involving multiple interfering states across various channels. The scattering amplitudes associated with the continuum states naturally adhere to coupled-channel unitarity, rendering this framework particularly valuable for investigating hadronic resonant states appearing in multiple coupled channels. Moreover, this generalized framework exhibits a wide-range applicability, enabling investigations into resonance phenomena across diverse physical domains, including hadron physics, nuclear physics, optics, and cold atom physics, among others.

hep-ph↗
arXiv2025

Quantum Weak Measurement Amplifies Dispersion Signal of Rydberg Atomic System

Rydberg atoms, with their long coherence time and large electric dipole moment, are pivotal in quantum precision measurement. In the process of approaching the standard quantum limit, higher demands are placed on detection schemes. This paper presents a scheme to amplify dispersion signal of Rydberg atomic microwave detection system, using a quantum weak measurement technique together with improved dimensionless pointer. The scheme effectively mitigates the impact of technical noise and can be used to achieve a measurement precision close to the limit set by atomic shot noise in theory. Compared with the superheterodyne method based on transmission detection, our scheme has been experimentally proved to have a sensitivity increase of 5$\sim$6 dB. In this work, the Rydberg dispersion signal amplification mechanism offers a approach to enhance microwave detection sensitivity, which also facilitates deeper investigations into its dynamic processes and further applications of this mechanism in quantum communication and quantum control.

quant-ph↗
arXiv2025

Practical parametrization of two-pole structure

We suggest that the extended Lee-Friedrichs model could be directly used as a practical parametrization method for the experimental analysis of resonance structures. This parametrization incorporates the constraints of relativistic phase space and the threshold behavior, and respects both unitarity and analyticity constraints of the scattering amplitude. As such, the poles on unphysical Riemann sheets could be easily extracted. This parametrization method offers a comparable fit quality to the improved Breit-Wigner parametrization with an energy-dependent width function when the coupling strength is moderate. It is found that the parametrization could be used to correctly extract the poles near the physical region correctly. In particular, it can naturally incorporate the two-pole structure in which one pole is shifted from the discrete state and the other is dynamically generated. Moreover, the coupled-channel formulation of the extended Lee-Friedrichs parameterization is straightforward and its relationship with the Flatté parametrization form is discussed. Using $ρ(770)$, $Δ(1232)$, $K^*_0$ and $f_0$ states as illustrative examples, we demonstrate the effectiveness of this parametrization in capturing fit qualities and identifying relevant poles. It is illustrated that the $K_0^*(700)$ and $K_0^*(1430)$ could be perfectly parameterized in the Lee-Friedrichs form as a two-pole structure. A tentative investigation of $f_0$s in coupled-channel parametrization form are discussed, and a possible lineshape contributed by $χ_{c1}(3872)$ and $χ_{c1}(4012)$ is presented. The proposed parametrization scheme holds promise for future studies involving exotic hadron states near thresholds, offering a valuable tool for analyzing resonance structures in upcoming experimental investigations.

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 new look at $ψ(4160)$ and $ψ(4230)$

By simultaneously analyzing the cross section data of $e^+e^-\rightarrow D\bar D, D\bar D^*, D^*\bar D^*, D\bar Dπ$ in a coupled-channel scheme with unitarity, we found that, in contrast to the conventional wisdom, the pole of $ψ(2^3D_1)$ might be located at about $\sqrt{s}=4222-32i\mathrm{MeV}$. This observation implies a possibility that the two resonances, dubbed the $ψ(4160)$ and $ψ(4230)$ in the PDG table now, might be the same $ψ(2^3D_1)$ state. Such a suggestion could provide more insight to our understanding the enigmatic decay properties of $ψ(4160)$ and $ψ(4230)$. Furthermore, this coupled-channel scheme could be applied to study other phenomena with several interfering resonances.

hep-ph↗
arXiv2023

ARPES signature of the competition between magnetic order and Kondo effect in CeCoGe3

The competition between magnetic order and Kondo effect is essential for the rich physics of heavy fermion systems. Nevertheless, how such competition is manifested in the quasiparticle bands in a real periodic lattice remains elusive in spectroscopic experiments. Here we report a high-resolution photoemission study of the antiferromagnetic Kondo lattice system CeCoGe3 with a high TN1 of 21K. Our measurements reveal a weakly dispersive 4f band at the Fermi level near the Z point, arisingfrom moderate Kondo effect. The intensity of this heavy 4f band exhibits a logarithmic increase with lowering temperature and begins to deviate from this Kondo-like behavior below 25 K, just above TN1, and eventually ceases to grow below 12 K. Our work provides direct spectroscopic evidence for the competition between magnetic order and the Kondo effect in a Kondo lattice system with local-moment antiferromagnetism, indicating a distinct scenario for the microscopic coexistence and competition of these phenomena, which might be related to the real-space modulation.

cond-mat.str-el↗
arXiv2022

Quasiparticle characteristics of the weakly ferromagnetic Hund's metal MnSi

Hund's metals are multi-orbital systems with $3d$ or $4d$ electrons exhibiting both itinerant character and local moments, and they feature Kondo-like screenings of local orbital and spin moments, with suppressed coherence temperature driven by Hund's coupling $J_H$. They often exhibit magnetic order at low temperature, but how the interaction between the Kondo-like screening and long-range magnetic order is manifested in the quasiparticle spectrum remains an open question. Here we present spectroscopic signature of such interaction in a Hund's metal candidate MnSi exhibiting weak ferromagnetism. Our photoemission measurements reveal renormalized quasiparticle bands near the Fermi level with strong momentum dependence: the ferromagnetism manifests through possibly exchange-split bands (Q1) below $T_C$ , while the spin/orbital screenings lead to gradual development of quasiparticles (Q2) upon cooling. Our results demonstrate how the characteristic spin/orbital coherence in a Hund's metal could coexist and compete with the magnetic order to form a weak itinerant ferromagnet, via quasiparticle bands that are well separated in momentum space and exhibit distinct temperature dependence. Our results imply that the competition between the spin/orbital screening and the magnetic order in a Hund's metal bears intriguing similarity to the Kondo lattice systems.

cond-mat.str-el↗
arXiv2022

Scrutinizing $ππ$ scattering in light of recent lattice phase shifts

In this paper, the $IJ=00, 11, 20$ partial wave $ππ$ scattering phase shifts determined by the lattice QCD approach are analyzed by using a novel dispersive solution of the S-matrix, i.e. the PKU representation, in which the unitarity and analyticity of scattering amplitudes are automatically satisfied and the phase shifts are conveniently decomposed into the contributions of the cuts and various poles, including bound states, virtual states and resonances. The contribution of the left-hand cut is estimated by the $SU(2)$ chiral perturbation theory to $\mathcal{O}(p^4)$. The Balanchandran-Nuyts-Roskies relations are considered as constraints to meet the requirements of the crossing symmetry. It is found that the $IJ=00$ $ππ$ scattering phase shifts obtained at $m_π=391$ MeV by Hadron Spectrum Collaboration (HSC) reveal the presence of both a bound state pole and a virtual state pole below the $ππ$ threshold rather than only one bound state pole for the $σ$. To reproduce the lattice phase shifts at $m_π=391$ MeV, a virtual-state pole in the $IJ=20$ channel is found to be necessary in order to balance the left-hand cut effects from the chiral amplitudes. Similar discussions are also carried out for the lattice results with $m_π=236$ MeV from HSC. The observed behaviors of the pole positions with respect to the variation of the pion masses can provide deep insights into our understanding of the dynamical origin of $σ$ resonance.

hep-ph↗
arXiv2021

Two-pole structures in a relativistic Friedrichs-Lee-QPC scheme

A general appearance of two-pole structures is exhibited in a relativistic Friedrichs-Lee model combined with a relativistic quark pair creation model in a consistent manner. This kind of two-pole structure could be found when a $q\bar q$ state couples to the open-flavor continuum state in the $S$ partial wave. We found that many enigmatic states, such as $f_0(500)/σ$, $K_0^*(700)/κ$, $a_0(980)$, $f_0(980)$, $D_0^*(2300)$, $D_{s0}^*(2317)$, and $X(3872)$, together with another higher state for each, all result from this kind of two-pole structures. Furthermore, an interesting observation is that this kind of two-pole structure will contribute roughly a total of 180$^\circ$ phase shift for the scattering process in a single channel approximation. This relativistic scheme may provide more insights into the understanding of the properties of non-$q\bar q$ state. It is also suggested that such two-pole structure could be a common phenomenon which deserves studying both from theoretical and experimental perspectives.

hep-ph↗