arXiv subjects

Feng-Kun Guo

Publications and source records attributed to Feng-Kun Guo.

At least 55 records · Page 3Linked to original sources

arXiv2025

Two-Loop Extraction of the Pion-Nucleon Sigma Term

The pion-nucleon sigma term, characterizing the mass component of Higgs origin related to $u$ and $d$ quarks inside the nucleon, is investigated within relativistic baryon chiral perturbation theory at leading two-loop order using the extended-on-mass-shell renormalization scheme. The two-loop representation of the sigma term is derived from the nucleon mass via the Feynman-Hellmann theorem and verified through a direct calculation of the forward isoscalar-scalar nucleon matrix element. We apply the derived chiral expression to extract the physical pion-nucleon sigma term by extrapolating $N_f=2+1$ lattice quantum chromodynamics (QCD) data at unphysical quark masses. We find that, at the two-loop level, the long-standing tension between lattice QCD and dispersive determinations can be naturally resolved, owing to the incorporation of intermediate $ππ$ rescattering effects that begin to contribute at two-loop order. Our final result for the nucleon sigma term based on recent lattice QCD calculations is $σ_{πN}=55.9(2.5)$ MeV. It is compatible with the result of the Roy-Steiner equation analysis and thus provides a satisfactory resolution to the previous debate between lattice QCD and phenomenological determinations.

hep-ph↗

arXiv2025

Nucleon mass in covariant baryon chiral perturbation theory at leading two-loop order

We calculate the nucleon mass within a manifestly relativistic formulation of baryon chiral perturbation theory (BChPT), extending the framework to leading two-loop order ($\mathcal{O}(p^5)$). By employing dimensional counting analysis and rigorously verifying the extended on-mass-shell scheme at this order, we obtain a complete chiral representation of the nucleon mass that preserves analyticity, respects proper power counting, and maintains renormalization-scale independence. The resulting expression exhibits excellent convergence, with $\mathcal{O}(p^5)$ contributions remaining small ($\sim 8~\rm{MeV}$). This formulation provides a robust foundation for chiral extrapolation, demonstrating remarkable agreement with lattice QCD data across a wide range of pion masses ($M_π\lesssim 300~\rm{MeV}$). The success of this calculation establishes two-loop relativistic BChPT as a precision tool for studying nucleon structure and related properties.

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arXiv2025

Rigorous Roy-Steiner equation analysis of $πK$ scattering at unphysical quark masses

We perform a rigorous analysis of the $πK$ scattering at an unphysical pion mass 391 MeV using the Roy-Steiner equations, which satisfy unitarity, analyticity and crossing symmetry, for the first time. Stable solutions of the Roy-Steiner equations with different quantum numbers of isospin and angular momentum are obtained in the elastic energy region, by taking inputs from the $πK$ lattice data in the inelastic region, the lattice data from the crossed $ππ\to K\bar{K}$ channels, the masses of $f_0(500)$ and $K^*$ at the same pion mass from previous study, and the Regge model. Predictions on the elastic $πK$ scattering phase shifts and the $K_0^*(700)$ pole content are made. Contrary to the virtual pole scenario obtained using the $K$-matrix method in the literature, we find that lightest strange scalar meson $K_0^*(700)$ remains a broad resonance at $m_π=391$ MeV. The cross-channel dynamics is found to play a crucial role in deriving the proper pole position.

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arXiv2025

Revisiting Roy-Steiner-equation analysis of pion-kaon scattering from lattice QCD data

A comprehensive analysis of $πK\rightarrow πK$ and $ππ\rightarrow K\bar K$ amplitudes at large unphysical pion mass for all important partial waves is presented. A set of crossing-symmetric partial-wave hyperbolic dispersion relations is used to describe lattice QCD data at $m_π=391$ MeV. In the present analysis, the amplitudes for the $S$- and $P$-waves are formulated by combining the constraints of analyticity, unitarity, and crossing symmetry, fulfilling Roy-Steiner-type equations. We use these results to investigate the low-lying strange-meson resonances and resolve the instability problem tied to analytic continuation in prior lattice QCD studies based on the $K$-matrix formalism. At $m_π=391$ MeV, the rigorous Roy-Steiner-type equation approach allows us to determine the $S$-wave scattering lengths, $m_πa_0^{1/2}=\left(0.92_{-0.28}^{+0.06}\right)$, $m_πa_0^{3/2}=-\left(0.32_{-0.02}^{+0.05}\right)$, and the $κ$ (also known as $K_0^*(700)$) pole position, $\sqrt{s_κ}=\left(966_{-24}^{+41}-i 198_{-17}^{+38}\right)$ MeV. We also provide a detailed analysis of the complex validity domain of the Roy-Steiner-type equations.

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arXiv2025

Entanglement suppression and emergent symmetries in hadron scatterings

Recently entanglement suppression was proposed to be one potential origin of emergent symmetries. In this work, we extend this theoretical framework to accommodate particles with arbitrary spins and/or arbitrary group representations. As case studies, we discuss recent efforts to test the entanglement-suppression conjecture in two hadron systems that exhibit possible emergent symmetries. The first concerns interactions involving spin-3/2 baryons, where entanglement suppression gives rise to symmetries such as $\text{SU}(40)$ spin-flavor symmetry. The second system involves low-energy scattering of heavy mesons, where entanglement suppression leads to an enhancement of the inherent heavy-quark spin symmetry to a light-quark spin symmetry, predicting additional siblings for the prominent exotic double-charm meson $T_{cc}(3875)^+$. These predictions should be confronted against experimental data and lattice results to further test the minimal-entanglement conjecture.

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arXiv2025

Flavor Physics at the CEPC: a General Perspective

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $τ$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

hep-ex↗

arXiv2025

Discontinuity calculus and applications to two-body coupled-channel scattering

We present a novel method, termed discontinuity calculus, for computing discontinuities of complex functions. This framework enables a systematic investigation of both analytic continuation and the topological structure of Riemann surfaces. We apply this calculus to analyze the analytic continuation of partial-wave amplitudes in two-body coupled-channel scattering problems and discuss their uniformization of the corresponding Riemann surfaces. This methodology offers new perspectives and tools for analyzing coupled-channel scattering problems in quantum scattering theory.

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arXiv2025

Dispersive Determination of Nucleon Gravitational Form Factors

Being closely connected to the origin of the nucleon mass, the gravitational form factors of the nucleon have attracted significant attention in recent years. We present the first model-independent determinations of the gravitational form factors of the pion and nucleon at the physical pion mass, using a data-driven dispersive approach. The so-called "last global unknown property" of the nucleon, the $D$-term, is determined to be $-3.38^{+0.34}_{-0.35}$. The root mean square radius of the scalar trace density inside the nucleon is determined to be $(0.97 \pm0.03)~\text{fm}$. Notably, this value is larger than the proton charge radius, suggesting a modern structural view of the nucleon where gluons, responsible for most of the nucleon mass, are distributed over a larger spatial region than quarks, which dominate the charge distribution, indicating that the radius of the trace density may be regarded as a confinement radius. We also predict the nucleon angular momentum and mechanical radii, providing further insights into the intricate internal structure of the nucleon.

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arXiv2025

Effective range expansion with a long-range force

The validity range of the time-honored effective range expansion can be very limited due to the presence of a left-hand cut close to the two-particle threshold. Such a left-hand cut arises in the two-particle interaction involving a light particle exchange with a mass small or slightly heavier than the mass difference of the two particles, identified as a long-range force, a scenario encountered in a broad range of systems. This can hinder a precise extraction of low-energy scattering observables and resonance poles. To address this issue, we propose a new parameterization for the low-energy scattering amplitude that accounts for the left-hand cut. The parameterization is like a Padé approximation but with nonanalytic terms from the left-hand cut and can be regarded as an extension of the effective range expansion. This parameterization is versatile and applicable to a broad range of systems with Yukawa-type interactions -- including particle, hadronic, nuclear, cold atom and quantum gas systems. In particular, it should be invaluable in understanding various near-threshold hadron resonances. As byproducts, we also show that the parameterization can be used to extract the couplings of the exchanged particle to the scattering particles, and derive expressions for amplitude zeros caused by the interplay between the short- and long-range interactions.

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arXiv2025

Exploring Efimov states in $D^D^D^$ and $DD^D^*$ three-body systems

The Efimov effect is an intriguing three-body quantum phenomenon. Searching for Efimov states within the realms of nuclear and hadronic physics presents a challenge due to the inherent inability of natural physical systems to exhibit adjustable two-body scattering lengths. In this study, we examine the potential existence of Efimov states in the $D^*D^*D^*$ and $DD^*D^*$ three-hadron systems. Utilizing a pionless effective field theory framework, we determine that the presence of Efimov states in the spectrum of the $D^*D^*D^*$ system is contingent upon the existence of an $(I,J)=(1,2)$ $D^*D^*$ two-body bound system. If only the $T_{cc}^+$ and its heavy quark spin partner $T_{cc}^{*+}$ exist while there is no near-threshold pole in all other $S$-wave $D^{(*)}D^*$ scattering amplitudes, no Efimov effect is expected in the $D^{(*)}D^*D^*$ systems.

hep-ph↗

arXiv2025

Production of $1^{-+}$ exotic charmonium-like states in electron-positron collisions

The absence of observed charmonium-like states with the exotic quantum numbers $J^{PC}=1^{-+}$ has prompted us to investigate the production rates of the $1^{-+}$ $D\bar D_1(2420)$ and $D^*\bar D_1(2420)$ hadronic molecules, which we refer to as $η_{c1}$ and $η_{c1}^{\prime}$, respectively, in electron-positron collisions. Assuming a hadronic molecular nature for the vector charmonium-like states $ψ(4360)$ and $ψ(4415)$, we evaluate the radiative decay widths of $ψ(4360)\toγη_{c1}$ and $ψ(4415)\toγη_{c1}^{\prime}$. Using these decay widths, we estimate the cross sections for producing $η_{c1}$ and $η_{c1}^{\prime}$ in electron-positron annihilations, as well as the event numbers at the planned Super $τ$-Charm Facility. Our results suggest that the ideal energy region for observing these states is around $4.44$ and $4.50$ GeV, just above the $D^* \bar D_1(2420)$ and $D^*\bar D_2^*(2460)$ thresholds, respectively.

hep-ph↗

arXiv2025

Confirmation of the existence of an exotic state in the $πD$ system

In recent years many candidates for states beyond the most simple realization of the quark model were found in various experiments around the world. However, so far no consensus exists on their structure, although there is strong evidence that at least some of those are dynamically generated from meson-meson interactions. In this Letter we provide an important missing piece from the theoretical side to prove that the lightest open charm strange and non-strange scalars $D_{s0}^*(2317)$ and $D_0^*$ as well as their axial-vector partner states can all be understood as emerging from the interactions between Goldstone bosons stemming from the spontaneous breaking of chiral symmetry and the ground state charmed mesons. For that purpose we exploit the flavor multiplet structure of the lightest open-charm positive-parity scalar states in an SU(3) symmetric lattice QCD simulation at large pion masses to establish that there exists a bound state in the flavor-sextet representation, which cannot emerge for quark-antiquark states but appears naturally for four-quark configurations. Moreover, we find repulsion in the $[15]$ representation and thus no single-particle state in this representation exists, falsifying the expectation for tetraquark models. The findings establish the pattern predicted for the interactions of Goldstone bosons with $D$ mesons from chiral symmetry and the paradigm of the lowest-lying positive-parity charmed mesons as dynamically generated states.

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arXiv2025

Exotics in the $πD$ system

In this proceedings we consider several states, namely the $D^*_{s0}(2317)$, $D_{s1}(2460)$, $D^*_{0}(2300)$ and $D_{1}(2430)$, which appear to defy description as simple quark-antiquark pairs. Theoretical input from unitarized chiral perturbation theory suggests they can be understood as emerging from Goldstone-Boson--$D$-meson scattering. We present results from an $SU(3)$ flavor-symmetric lattice QCD simulation at large pion masses suggesting that there exists a $πD$ bound state in the flavor-sextet representation that cannot emerge for quark-antiquark states, but that appears naturally from the multiquark states. Moreover, we find repulsion in the [15] representation, which establishes the pattern predicted for the interactions of Goldstone bosons with $D$ mesons. This suggests these states may have the structure of hadronic molecules.

hep-lat↗

arXiv2025

Evidence for the existence of a flavor-sextet charmed meson?

Recently, the LHCb Collaboration reported a signal for a new resonance in the $D_s^+ π^\pm$ invariant mass distribution in the decays $B\to \bar{D}^{(*)} D_s^+ π^+ π^-$ (arXiv:2411.03399). This could be a direct observation of an SU(3) flavor-sextet charmed meson with $J^P=0^+$. Such an SU(3) multiplet, is beyond the conventional quark-antiquark picture, and thus a verification of this observation is important.

hep-ph↗

arXiv2025

$P$-wave charmonium contribution to hidden-charm states from reanalysis of lattice QCD data

We reanalyze, considering the contribution of $P$-wave charmonia, lattice data for the $D \bar{D}$-$D_s\bar{D}_s$ coupled-channel of S. Prelovsek et al. [JHEP 06, 035 (2021)] and $D\bar{D}^*$ systems of S. Prelovsek et al. [Phys. Rev. Lett. 111, 192001 (2013)] with $m_π\simeq 280$ and $266$ MeV, and $L=24a/32a$ ($a\simeq 0.09$ fm) and $L=16a$ ($a\simeq0.1239(13)$ fm), respectively. The hidden-charm states with $J^{PC}=0^{++}$, $1^{++}$, and $2^{++}$ quantum numbers are then searched for. For $0^{++}$, the analysis reveals three poles in the $D\bar{D}$-$D_s\bar{D}_s$ coupled-channel amplitude, corresponding to three states. Two of these poles, located near the $D\bar{D}$ and $D_s\bar{D}_s$ thresholds, can be interpreted as mostly molecular states. A third pole above the $D_s\bar{D}_s$ threshold is originated from the $P$-wave $χ_{c0}(2P)$ charmonium state. The number of poles found in the $D\bar D$-$D_s \bar D_s$ system is the same as that found in the original lattice analysis though the position of the third pole changes sizeably. In the $1^{++}$ sector, we find two poles in the complex energy plane. The first one is related to the molecular $X(3872)$ state, with a compositeness exceeding $90\%$, while the second one, stemming from the $χ_{c1}(2P)$ charmonium, appears above the $D\bar{D}^*$ threshold and it likely corresponds to the recently discovered $χ_{c1}(4010)$ state. In the $2^{++}$ sector, we also report two poles and find that the dressed $χ_{c2}(2P)$ is lighter than the $D^*\bar{D}^*$ molecular state, with the dynamics of the latter closely related to that of the heavy-quark spin-symmetry partner of the $X(3872)$. Our exploratory study of the $1^{++}$ and $2^{++}$ sectors offers valuable insights into their dynamics, but given that the fits that we carry out are underconstrained, more lattice data are required to draw robust conclusions.

hep-ph↗

arXiv2025

Exclusion of a diquark-antidiquark structure for the lightest positive-parity charmed mesons

The nature of low-lying scalar and axial-vector charmed mesons has been debated for decades, with hadronic molecular and compact tetraquark models being prominent candidates. These two models predict quite different features for the accessible SU(3) multiplets in the scalar and axial-vector sectors, which can be tested through lattice calculations at SU(3) symmetric points. In this work, we perform lattice calculations for both scalar and axial-vector charmed mesons with an SU(3) symmetric pion mass about 613 MeV for the SU(3) $[6]$ and $[\overline{15}]$ multiplets. We find that the $[6]$ multiplet exhibits attractive interactions in both scalar and axial-vector sectors, while the $[\overline{15}]$ multiplet shows repulsive interactions in both sectors. The energy shifts in the scalar and axial-vector sectors are compatible with each other within uncertainties. These results are fully consistent with the hadronic molecular picture, while challenging the compact tetraquark model, which predicts the existence of low-lying $[\overline{15}]$ states in the axial-vector sector but not in the scalar sector.

hep-lat↗

arXiv2025

Chiral Representation of the Nucleon Mass at Leading Two-loop Order

We calculate the nucleon mass in a manifestly relativistic baryon chiral perturbation theory up to the leading two-loop order. Through dimensional counting analysis, we perform the chiral expansion and verify the validity of the extended-on-mass-shell scheme at the two-loop level. As a result, we obtain the complete chiral representation of the nucleon mass up to $\mathcal{O}(p^5)$, which preserves the original analytic properties and satisfies the correct power counting. The obtained chiral result is well-suited for chiral extrapolation and provides an excellent description of lattice QCD data across a broad range of pion masses. We find that the $\mathcal{O}(p^5)$ contribution is small, approximately $10$ MeV, and varies only mildly with increasing pion mass, demonstrating good convergence of the nucleon mass up to pion masses of about 350 MeV at two-loop order.

hep-ph↗

arXiv2025

Classification of Coupled-Channel Near-Threshold Structures

Since 2003, plenty of resonant structures have been observed in the heavy quarkonium regime. Many of them are close to the thresholds of a few pairs of heavy hadrons. They are candidates of exotic hadrons and have attracted immense attentions. Based on a coupled-channel nonrelativistic effective field theory, we classify the near-threshold structures of a symmetry-related two-channel system by studying the evolution of the scattering amplitude line shapes and pole positions with the variation of the single-channel scattering length and channel coupling strength. We show that the evolution of the scattering amplitude line shapes can be understood from the pole trajectories in the complex energy plane, and the pole evolution can be traced back to the renormalization group fixed points. We provide a dictionary of correspondence between the evolution of line shapes and pole trajectories along with varying interaction and channel coupling strengths, which can be used to understand the experimental observations of the near-threshold structures.

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