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Jun-Xu Lu

Publications and source records attributed to Jun-Xu Lu.

At least 37 records · Page 2Linked to original sources
arXiv2025

From bare two-nucleon interaction to nuclear matter and finite nuclei in a relativistic framework

Understanding nuclear forces, infinite nuclear matter, and finite nuclei within a unified framework has remained a central challenge in nuclear physics for decades. While most \textit{ab initio} studies employ nonrelativistic Schrödinger-equation frameworks, this work offers a relativistic perspective. Using a leading-order (LO) relativistic chiral interaction, we describe two-nucleon scattering via the Thompson equation, symmetric nuclear matter, and medium-mass nuclei (Ca, Ni, Zr, Sn) via the relativistic Brueckner-Hartree-Fock theory. Systematic uncertainties from regulator cutoffs and interaction parameters are analyzed. The empirical saturation region of nuclear matter is reproduced, and the binding energies and charge radii of medium-mass nuclei agree reasonably well with experimental data, significantly improving the ``Coester line". These results highlight that the relativistic approach, employing a leading-order chiral force with only four low-energy constants and no three-nucleon forces, can capture the most important dynamics and offer a complementary pathway to address longstanding challenges in nuclear \textit{ab initio} studies.

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arXiv2025

Nuclear and neutron matter in the relativistic Brueckner-Hartree-Fock theory with next-to-leading order covariant chiral nuclear force

The symmetric nuclear matter and pure neutron matter are investigated by the relativistic Brueckner-Hartree-Fock (RBHF) theory with the covariant chiral nuclear forces up to the next-to-leading order~(NLO). A fitting scheme to ensure the naturalness of the low-energy constants is proposed, which plays a crucial role in the proper description of nuclear matter. With a momentum cutoff $Λ=590$ MeV, the empirical saturation energy and density, as well as the incompressibility coefficient at the saturation density are reproduced well. The EoSs show less dependence on the momentum cutoff and become softer at densities above saturation density, in comparison with the previous leading order results. Given the good description for the saturation properties of nuclear matter, the present work encourages future studies of the finite nuclei in the framework of the RBHF theory with the NLO covariant chiral nuclear forces.

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arXiv2025

Pole trajectories of the $Λ(1405)$ helps establish its dynamical nature

The $Λ(1405)$ has been one of the most controversial exotic baryons. If the $Λ(1405)$ possesses a two-pole molecular structure, these poles are expected to evolve differently towards the SU(3) limit. From an analysis of a recent LQCD simulation on the $πΣ-\bar{K}N$ scattering for $I=0$ and the study of the quark mass dependence of the octet baryon masses, we determine for the first time the trajectories of these poles towards the symmetric point over the $\mathrm{Tr}[M]=C$ trajectory accurately. At $m_π\simeq 200$ MeV, our results are consistent with the lattice simulations, and the extrapolations to the physical point, based on the NLO chiral Lagrangians, agree well with existing experimental analyses. We predict qualitatively similar trajectories at LO and up to NLO, consistent with the LO interaction's dominance. At the SU(3) symmetric point of this trajectory, both poles are on the physical sheet, and the lower pole is located at $E^{(1)}=1573(6)(6)$ MeV, becoming a SU(3) singlet, while the higher pole at $E^{(8a)}=1589(7)(5)$ MeV couples to the octet representation. Moreover, we make predictions in $I=1$ for the $Σ^*$ resonance. We find a resonance pole that evolves into a bound state around $m_π=415$ MeV in this sector. The results presented here are crucial to shed light on the molecular nature of exotic strange baryon resonances and can be tested in future LQCD simulations.

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arXiv2025

Two-pole structures in QCD -- a universal phenomenon governed by chiral dynamics

We illustrate how the two-pole structures of the $Λ(1405)$ emerge from the underlying universal chiral dynamics that describe the coupled-channel interactions between octet baryons and pseudo-Nambu-Goldstone bosons. Specifically, we attribute this phenomenon to the form of the leading-order chiral potential, which is of the Weinberg-Tomozawa type. We reveal how the underlying chiral dynamics can be exposed by examining the light-quark mass evolution of the two poles. The latest lattice QCD simulations have indeed found evidence for the existence of the two poles of $Λ(1405)$, in qualitative agreement with our predictions. We briefly mention a recent work in which lattice QCD simulations are studied more quantitatively, along with a proposal for how the SU(3) flavor content of the two poles of $Λ(1405)$ can be experimentally verified.

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arXiv2025

Effect of a repulsive three-body interaction on the $DD^{(*)}K$ molecule

The hadronic molecular picture of the observed exotic states has inspired numerous investigations into few-body systems. Recently, the lattice effective field theory studied the effect of a three-body interaction on the binding energy of the $DD^{*}K$ system, revealing an intriguing phenomenon in the binding energy. This work uses the Gaussian expansion method to explore the underlying physics. Our results show that as the repulsive three-body interaction strengthens, the spatial size of the $DD^{(*)}K$ bound state gradually increases. Further enhancement of the three-body interaction causes the $DD^{(*)}K$ three-body bound state to break into a $D^{(*)}K$ two-body bound state, accompanied by a distant $D$ meson. The identical nature of the two $D$ mesons leads to the fact that the $DDK$ system consistently resembles an isosceles triangle-shaped spatial configuration.

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arXiv2025

Relativistic chiral nuclear forces: status and prospects

Understanding nuclear structure, reactions, and the properties of neutron stars from \textit{ab initio} calculations from the nucleon degrees of freedom has always been a primary goal of nuclear physics, in which the microscopic nuclear force serves as the fundamental input. So far, the Weinberg chiral nuclear force, first proposed by the Nobel laureate Weinberg, has become the \textit{de facto} standard input for nuclear \textit{ab initio} studies. However, compared to their non-relativistic counterparts, relativistic \textit{ab initio} calculations, which describe better nuclear observables, have only begun. The lack of modern relativistic nucleon-nucleon interactions is an important issue restricting their development. In this work, we briefly review the development and status of the Weinberg chiral nuclear force, as well as its limitations. We further present a concise introduction to the relativistic chiral nuclear force, show its description of the scattering phase shifts and observables such as differential cross sections, and demonstrate its unique features. Additionally, we show that the relativistic framework could be naturally extended to the antinucleon-nucleon interaction.

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arXiv2024

Three ways to decipher the nature of exotic hadrons: multiplets, three-body hadronic molecules, and correlation functions

In the past two decades, a plethora of hadronic states beyond the conventional quark model of $q\bar{q}$ mesons and $qqq$ baryons have been observed experimentally, which motivated extensive studies to understand their nature and the non-perturbative strong interaction. Since most of these exotic states are near the mass thresholds of a pair of conventional hadrons, the prevailing picture is that they are primarily hadronic molecules. In principle, one can verify the molecular nature of these states by thoroughly comparing their masses, decay widths, and production rates in a particular picture with experimental data. However, this is difficult or impossible. First, quantum mechanics allows for the mixing of configurations allowed by symmetries and quantum numbers. Second, data are relatively scarce because of their small production rates and the many difficulties in the experimental measurements. As a result, other alternatives need to be explored. This review summarizes three such approaches that can help disentangle the nature of the many exotic hadrons discovered. In the first approach, based on the molecular interpretations for some exotic states, we study the likely existence of multiplets of hadronic molecules related by various symmetries, such as isospin symmetry, SU(3)-flavor symmetry, heavy quark spin/flavor symmetry, and heavy antiquark diquark symmetry. In the second approach, starting from some hadronic molecular candidates, one can derive the underlying hadron-hadron interactions. With these interactions, one can study related three-body systems and check whether three-body bound states/resonances exist. In the third approach, one can turn to the femtoscopy technique to derive the hadron-hadron interactions, hence inaccessible. This technique provided an unprecedented opportunity to understand the interactions between unstable hadrons.

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arXiv2024

Reexamination of antinucleon-nucleon interactions in covariant chiral effective field theory

Motivated by the recent progress in developing high-precision relativistic chiral nucleon-nucleon interactions, we study the antinucleon-nucleon interaction in a hybrid approach where the real part of the potential is constructed in the leading-order covariant chiral effective field theory, and the imaginary part is described following the procedure adopted in the heavy baryon chiral effective field theory. The phase shifts and inelasticities with $J\leq 1$ are obtained and compared to those calculated in the next-to-leading order heavy baryon chiral effective field theory. For most partial waves, the descriptions of phase shifts and inelasticities in the hybrid approach are comparable to those in the next-to-leading order heavy baryon chiral effective field theory, confirming the relatively faster convergence of the relativistic approach observed in the nucleon-nucleon sector. In addition, we search for bound states/resonances near the $\bar{N}N$ threshold and find several structures that can be associated with those states recently observed by the BESIII Collaboration.

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arXiv2024

Saturation of nuclear matter in the relativistic Brueckner Hatree-Fock approach with a leading order covariant chiral nuclear force

Nuclear saturation is a crucial feature in nuclear physics that plays a fundamental role in understanding various nuclear phenomena, ranging from properties of finite nuclei to those of neutron stars. However, a proper description of nuclear saturation is highly nontrivial in modern nonrelativistic~\textit{ab initio}~studies because of the elusive three-body forces. In this letter, we calculate the equation of state for nuclear matter in the relativistic Brueckner-Hartree-Fock (RBHF) framework with the leading order covariant chiral nuclear force. We show that a simultaneous description of the nucleon-nucleon scattering data and the saturation of the symmetric nuclear matter can be achieved. In this regard, the relativistic effects nicely explain the saturation of nuclear matter. As a result, the present study based on the covariant chiral nuclear force shows that in the RBHF framework, one can achieve saturation with a leading order covariant chiral nuclear force with only two-body forces, in contrast to the vast majorities of studies in the non-relativistic framework, where the next-to-next-to-leading order two-body and three-body chiral forces are needed. This study sets the foundation for studying nuclear saturation with the covariant chiral force in the RBHF framework, which allows for a systematic understanding of one of the key features of nuclear physics more microscopically.

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arXiv2023

Dynamical origin of universal two-pole structures and their light quark mass evolution

Two-pole structures refer to the fact that two dynamically generated states are located close to each other between two coupled channels and have a mass difference smaller than the sum of their widths. Thus, the two poles overlap in the invariant mass distribution of their decay products, creating the impression that only one state exists. This phenomenon was first noticed for the $Λ(1405)$ and the $K_1(1270)$, and then for several other states. This report explicitly shows how the two-pole structures emerge from the underlying universal chiral dynamics describing the coupled-channel interactions between a heavy matter particle and a pseudo-Nambu-Goldstone boson. Furthermore, we predict similar two-pole structures in other systems dictated by chiral symmetry, such as the isospin $1/2$ $\bar{K}Σ_c-πΞ'_c$ coupled channel, awaiting experimental discoveries.

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arXiv2023

Systematic studies of $DDKK$ and $D\bar{D}K\bar{K}$ four-hadron molecules

Assuming that $D_{s0}^{*}(2317)$ is a $DK$ molecular state with a binding energy of 45 MeV, we investigate the existence of four-hadron molecules, $DDKK$ and $D\Bar{D}K\Bar{K}$, with the Gaussian expansion method. Their binding energies are $138\sim155$ MeV and $123\sim163$ MeV below the mass thresholds of $DDKK$ and $D\Bar{D}K\Bar{K}$. The $D\Bar{D}K\Bar{K}$ state has a decay width of $36\sim54$ MeV due to the complex $K\Bar{K}$ interaction. Further theoretical studies of and experimental searches for such four-hadron molecules can help deepen the understanding of the nonperturbative strong interaction in a nontrivial way.

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arXiv2023

Two-pole structures as a universal phenomenon dictated by coupled-channel chiral dynamics

In the past two decades, one of the most puzzling phenomena discovered in hadron physics is that a nominal hadronic state can actually correspond to two poles on the complex energy plane. This phenomenon was first noticed for the $Λ(1405)$, then for $K_1(1270)$, and to a lesser extent for $D_0^*(2300)$. In this Letter, we show explicitly how the two-pole structures emerge from the underlying universal chiral dynamics describing the coupled-channel interactions between heavy matter particles and pseudo Nambu-Goldstone bosons. In particular, the fact that two poles appear in between the two dominant coupled channels can be attributed to the particular form of the leading order chiral potentials of the Weinberg-Tomozawa form. Their lineshapes overlap with each other because the degeneracy of the two coupled channels is only broken by explicit chiral symmetry breaking of higher order. We predict that for light-quark~(pion) masses heavier than their physical values (e.g., about 200 MeV in the $Λ(1405)$ case studied), the lower pole becomes a virtual state, which can be easily verified by future lattice QCD simulations. Furthermore, we anticipate similar two-pole structures in other systems, such as the isopin $1/2$ $\bar{K}Σ_c-πΞ'_c$ coupled channel, which await for experimental discoveries.

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arXiv2023

Long-range $S$-wave $DD^*$ interaction in covariant chiral effective field theory

Motivated by the recent lattice QCD study of the $DD^*$ interaction at unphysical quark masses, we perform a theoretical study of the $DD^*$ interaction in covariant chiral effective field theory (ChEFT). In particular, we calculate the relevant leading-order two-pion exchange contributions. The results compare favorably with the lattice QCD results, supporting the conclusion that the intermediate-range $DD^*$ interaction is dominated by two-pion exchanges and the one-pion exchange contribution is absent. At a quantitative level, the covariant ChPT results agree better with the lattice QCD results than their non-relativistic counterparts, showing the relevance of relativistic corrections in the charm sector.

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arXiv2023

Distinguishing the spins of $P_c(4440)$ and $P_c(4457)$ with femtoscopic correlation functions

The spins of the pentaquark states $P_c(4440)$ and $P_c(4457)$ play a decisive role in unraveling their nature, but remain undetermined experimentally. Assuming that they are $Σ_c\bar{D}^{*}$ bound states, we demonstrate how one can determine their spins by measuring the $Σ_c^+\bar{D}^{(*)0}$ correlation functions. We show that one can use the $Σ_c^+\bar{D}^0$ correlation function to fix the size of the Gaussian source and then determine the strength of the $Σ_c^+\bar{D}^{*0}$ interaction of spin $1/2$ and $3/2$ and therefore the spins of the $P_c(4440)$ and $P_c(4457)$ states. The method proposed can be applied to decipher the nature of other hadronic molecules and thus deepen our understanding of the nonperturbative strong interaction.

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arXiv2023

New insights into the pole parameters of the $Λ(1380)$, the $Λ(1405)$ and the $Σ(1385)$

A coupled-channel S- and P-wave next-to-leading order chiral-unitary approach for strangeness $S=-1$ meson-baryon scattering is extended to include the new data from the KLOE and AMADEUS experiments as well as the $Λπ$ mass distribution of the $Σ(1385)$. The positions of the poles on the second Riemann sheet corresponding to the $Σ(1385)$ pole and the $Λ(1380)$ and $Λ(1405)$ poles as well as the couplings of these states to various channels are calculated. We find that the resonance positions and branching ratios are on average determined with about 20\% higher precision when including the KLOE and AMADEUS data. Additionally, for the first time, the correlations between the parameters of the poles are investigated and shown to be relevant. We also find that the $Σ(1385)$ has negligible influence on the properties of the $Λ$ states given the available data. Still, we identify isospin-1 cusp structures in the present solution in light of new measurements of $π^\pmΛ$ line-shapes by the Belle collaboration.

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arXiv2023

Study of the $DK$ interaction with femtoscopic correlation functions

The $DK$ interaction in isospin zero is known to be attractive to such an extent that a bound state can be generated, which can be associated with the mysterious $D_{s0}^*(2317)$. In this work, we calculate the $DK$ femtoscopic correlation function in the coupled-channel framework for different source sizes that can directly probe the strongly attractive $DK$ interaction, which is otherwise inaccessible due to the unstable nature of $D$ and $K$ mesons, and therefore can help elucidate the nature of $D_{s0}^*(2317)$. We further generalize the study of source size dependence to various interactions, ranging from repulsive, weakly attractive, moderately attractive, and strongly attractive, in a square-well model. We hope that our study can motivate future experimental measurements of the $DK$ correlation function and other interactions relevant to the understanding of the nature of the many exotic hadrons discovered so far.

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arXiv2023

Cross-channel constraints on resonant antikaon-nucleon scattering

Chiral perturbation theory and its unitarized versions have played an important role in our understanding of the low-energy strong interaction. Yet, so far, such studies typically deal exclusively with perturbative or nonperturbative channels. In this letter, we report on the first global study of meson-baryon scattering up to one-loop order. It is shown that covariant baryon chiral perturbation theory, including its unitarization for the negative strangeness sector, can describe meson-baryon scattering data remarkably well. This provides a highly non-trivial check on the validity of this important low-energy effective field theory of QCD. We show that the $\bar{K}N$ related quantities can be better described in comparison with those of lower-order studies, and with reduced uncertainties due to the stringent constraints from the $πN$ and $K N$ phase shifts. In particular, we find that the two-pole structure of $Λ(1405)$ persists up to one-loop order reinforcing the existence of two-pole structures in dynamically generated states.

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arXiv2022

$Z_{cs}(3985)$ in next-to-leading-order chiral effective field theory -- the first truncation uncertainty analysis

We revisit the $D_s^-D^{*0}$/$D_s^{*-}D^0$ interaction and the $Z_{cs}(3985)$ state in chiral effective field theory(EFT) up to the next-to-leading order. We examine the relative importance of the leading-order contact, one-eta exchange, next-to-leading-order contact, and two-kaon-exchange contributions. We show that the leading-order and next-to-leading-order contact contributions play the most important role such that the $Z_{cs}(3985)$ state qualifies as a $D_s^-D^{*0}$/$D_s^{*-}D^0$ resonance. On the other hand, the weak one-eta-exchange and weakly energy-dependent two-kaon-exchange contributions are less important in dynamically generating the $Z_{cs}(3985)$ state, indicating that chiral EFT is less predictive in the present situation. Furthermore, we apply the Bayesian method to estimate chiral truncation uncertainties and find that they are of similar magnitude as their statistical counterparts. Our study shows that if $Z_c(3900)$ exists, then SU(3)-flavor symmetry also predicts $Z_{cs}(3985)$ with a certain robustness, i.e., both can be accommodated in the chiral EFT with dominant contact contributions.

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