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Xiaonu Xiong

Publications and source records attributed to Xiaonu Xiong.

At least 19 recordsLinked to original sources

Exploring the $P$-wave bottom-strange molecular pentaquarks with the complex scaling method

In this work, we perform a systematic investigation of the $P$-wave $Σ_b K/Λ_b K^{*}/Σ_b K^{*}$ and $Σ_b\bar{K}/Λ_b\bar{K}^{*}/Σ_b\bar{K}^{*}$ systems for all possible $I(J^P)$ combinations. In contrast to our earlier study of the negative-parity sector [Eur. Phys. J. C \textbf{85}, 1026 (2025)], the positive-parity sector is dominated by resonances. For the coupled-channel $I(J^P)=1/2(1/2^+)$ $Σ_b K/Λ_b K^*/Σ_b K^*$ system, we predict a narrow resonance above the $Σ_b K$ threshold. In the $I(J^P)=1/2(3/2^+)$ $Λ_b K^*/Σ_b K^*$ system, a molecular resonance candidate is found above the $Λ_b K^*$ threshold. For the higher-isospin $I=3/2$ sector, resonances are identified in the $Σ_b K^*$ subsystem for both $J^P=1/2^+$ and $3/2^+$ cases, though both states exhibit relatively compact sizes. We also extend our analysis to the $Σ_b\bar{K}/Λ_b\bar{K}^{*}/Σ_b\bar{K}^{*}$ systems, where a cutoff-sensitive resonance is found in the $I(J^P)=1/2(1/2^+)$ configuration. A broad molecular resonance candidate is found below the $Σ_b\bar{K}^*$ threshold in the $I(J^P)=1/2(3/2^+)$ system. We hope that these predictions provide useful guidance for future experimental searches for bottom-strange molecular pentaquarks by LHCb.

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Unpolarized gluon PDF of the nucleon from lattice QCD at physical point in the continuum limit

We report a state-of-the-art lattice QCD calculation of the nucleon unpolarized gluon parton distribution function employing large-momentum effective theory. The calculation is carried out on the 2+1 flavor CLQCD ensembles with five lattice spacings a={0.105,0.0897,0.0775, 0.0688, 0.0519} fm and various pion masses ranging from 136 MeV to 317 MeV, covering nulceon momenta up to 3 GeV. Distillation technique is applied to improve the signal of two-point correlators. We then apply the state-of-the-art hybrid renormalization and one-loop perturbative matching, and extrapolate the result to the continuum limit, infinite momentum limit and physical pion mass.

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Azimuthal asymmetry in $J/ψ+γ$ and $J/ψ+J/ψ$ production in ultraperipheral heavy-ion collisions at LHC

Two-photon collision in ultraperipheral heavy-ion collisions (UPCs) provides a unique and powerful platform for probing QCD with linearly polarized quasi-real photons. While photon polarization effects have been recognized in dilepton and even in light hadrons production, their consequences for heavy quarkonium production remain unexplored. In this work we investigate for the first time the $γγ\to J/ψ+γ(J/ψ)$ channels in Pb-Pb UPCs at the Large Hadron Collider (LHC), by integrating the non-relativistic QCD (NRQCD) factorization approach with the transverse-momentum-dependent (TMD) photon distributions. Based on the helicity amplitudes at lowest order in strong coupling and velocity expansion, we predict sizable $\cos(2ϕ)$ and $\cos(4ϕ)$ azimuthal asymmetries arising from the interference of linearly polarized photon states. These azimuthal-dependent observables, defined as the ratios of weighted to unweighted cross sections, are expected to be stable against including the higher-order radiative corrections and varying nonperturbative NRQCD matrix elements, thus offering a fresh test of quarkonium production mechanism and the photon TMD structure in the ultrarelativistic limit.

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A coupled-channel perspective analysis on bottom-strange molecular pentaquarks

At present work, we systematically study various bottom-strange molecular pentaquarks to search for possible bound states and resonances by adopting one-boson-exchange model within complex scaling method. According to our calculations, we predict several bound and resonant states for bottom baryon $Y_{b}(Λ_b,Σ_b) \bar K^{(*)}$ and $Y_{b} K^{(*)}$ systems. In particular, a bound state in the $I(J^P)=1/2(1/2^-)$ $Σ_{b}\bar{K}/Λ_{b}\bar{K}^*/Σ_{b}\bar{K}^*$ system may correspond to the particle $Ξ_{b}(6227)$. Meanwhile, the predicted bound state with $6303\sim6269~\rm{MeV}$ in the $I(J^P)=1/2(1/2^-)Σ_bK/Λ_bK^*/Σ_bK^*$ system is flavor exotic and does not appear in the spectroscopy of conventional baryons, which provides a practical way to clarify the nature of particle $Ξ_b(6227)$. We highly hope that our proposals can offer helpful information for the future experimental searches.

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Quark Transverse Spin-Momentum Correlation of the Nucleon from Lattice QCD: The Boer-Mulders Function

We present the first lattice QCD calculation of the quark transverse spin-momentum correlation, i.e., the naive time-reversal-odd Boer-Mulders function, of the nucleon, using large-momentum effective theory (LaMET). The calculation is carried out on an ensemble with lattice spacing $a=0.098$ fm and pion mass $338$ MeV, at various proton momenta up to $2.11$ GeV. We have implemented perturbative matching up to the next-to-next-to-leading order together with a renormalization-group resummation improvement. The result exhibits a decay behavior with increasing transverse separation $b_\perp$. We also compare the results in the nucleon and pion.

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Vector meson production associated with a lepton pair in $e^+$ $e^-$ annihilation

In this work, we investigate a novel production mechanism of vector mesons, exemplified by the production of a neutral vector meson associated with a lepton pair in $e^+e^-$ annihilation, i.e., $e^+e^-\to V l^+l^-$ ($V=J/ψ, ρ^0, ω, ϕ$, and $l=μ, τ$). These vector meson production channels can be precisely accounted within QED. The production rates of these processes are dominated by those diagrams where the vector meson is emitted from either the incident electron or positron, which exhibit a $\ln^2 m_l^2$ enhancement stemming from the triple collinear limit of leptons. Our numerical analysis indicates that the corresponding production rates are substantial enough to warrant the observation of these novel vector meson production channels at BESIII and Belle II experiments in near future.

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Strange-antistrange and charm-anticharm asymmetries of pion in 't Hooft model

As a sequel of our preceding work [S. Hu et al., Phys. Rev. D 108 (2023) 9, 094040], we investigate the strange-antistrange and charm-anticharm asymmetries in the parton distribution functions (PDFs) of a light flavored meson, exemplified by the first excited pion in the 't Hooft model, {\it viz.}, QCD in two spacetime dimensions with infinite number of colors. Counted as an ${\cal O}(1/N_c)$ effect, the intrinsic strange content necessarily originates from the higher Fock component of the light flavored meson, which entails infinite towers of $K$ and $\overline{K}$ mesons. Numerical studies reveal that, with $m_u/m_d=1/2$, the $s$-$\bar{s}$ and $c$-$\bar{c}$ asymmetries of the first excited $π^-$ can reach per cents level. While the $s$-$\bar{s}$ asymmetry predicted from the meson cloud model (MCM) grossly align with the rigorous approach, there exists severe discrepancy between two approaches on the $c$-$\bar{c}$ asymmetry.

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Quark Transverse Spin-Momentum Correlation of the Pion from Lattice QCD: The Boer-Mulders Function

We present the first lattice QCD calculation of the quark transverse spin-momentum correlation, i.e., the T-odd Boer-Mulders function, of the pion, using large-momentum effective theory (LaMET). The calculation is done at three lattice spacings $a=(0.098, 0.085, 0.064)$ fm and pion masses $\sim350$ MeV, with pion momenta up to $1.8$ GeV. The matrix elements are renormalized in a state-of-the-art scheme and extrapolated to the continuum and infinite momentum limit. We have implemented the perturbative matching up to the next-to-next-to-leading order and carried out a renormalization-group resummation. Our results provide valuable input for phenomenological analyses of the Boer-Mulders single-spin asymmetry.

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Exclusive production of double light neutral mesons at the $e^+e^-$ colliders

In this work we investigate the exclusive production of a pair of light neutral mesons in $e^+e^-$ annihilation, where the final state bears an even $C$-parity. The production processes can be initiated via the photon fragmentation or the non-fragmentation mechanism. While the fragmentation contribution can be rigorously accounted, the non-fragmentation contributions are calculated within the framework of collinear factorization, where only the leading-twist light-cone distribution amplitudes (LCDAs) of mesons are considered. Mediately solely by the non-fragmentation mechanism, the production rates of double light neutral pseudoscalar mesons are too small to be observed at the commissioning $e^+e^-$ facilities. In contrast, the production rates of a pair of light neutral vector mesons are greatly amplified owing to the significant kinematic enhancement brought by the fragmentation mechanism. It is found that, at $\sqrt{s}=3.77$ GeV, after including the destructive interference between the non-fragmentation and fragmentation contributions, the production rates for $e^+e^-\to ρ^{0}ρ^{0}$ and $ρ^0ω$ can be lowered by about 10\% and 30\% relative to the fragmentation predictions. Future precise measurement of these exclusive double neutral vector meson production channels at {\tt BESIII} experiment may provide useful constraints on the LCDAs of light vector mesons.

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$a_0(1710)$-$f_0(1710)$ mixing effect in the $D_{s}^{+} \rightarrow K_S^{0} K_S^{0} π^{+}$ decay

With the measurements of the decay $D^+_s \rightarrow K^0_S K^0_S π^+$ by the BESIII Collaboration, we investigate this three-body weak decay via the chiral unitary approach for the final state interaction, where the resonances $S(980)$ and $S(1710)$ are dynamically reproduced with the interaction of eleven coupled channels, and the $W$-external and -internal emission mechanisms are considered at the quark level. Besides, we also take into account the contribution from the $P$-wave resonance $K^*(892)^+$ and make a combined fit of the $K^0_S K^0_S$ and $K^0_S π^+$ invariant mass spectra measured by the BESIII Collaboration. The fitted results show that the enhancement around 1.7 GeV in $K^0_S K^0_S$ mass spectrum is overlapped with two visible peaks, indicating the mixing signal originated from the resonances $a_0(1710)$ and $f_0(1710)$ due to their different poles (masses). Thus, the decay $D^+_s \rightarrow K^0_S K^0_S π^+$ is helpful to reveal their molecular nature with the mixing signal, which can be more precisely measured in the future.

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Light-cone and quasi generalized parton distributions in the 't Hooft model

We present a comprehensive study of the light-cone generalized parton distribution (GPD) and quasi-GPD of a flavor-neutral meson in the 't Hooft model, {\it i.e.}, two-dimensional QCD (\QCDtw) in the $N_c\to\infty$ limit. With the aid of the Hamiltonian approach, we construct the light-cone GPD in terms of the meson's light-cone wave function in the framework of light-front quantization, and express the quasi-GPD in terms of the meson's Bars-Green wave functions and the chiral angle in the framework of equal-time quantization. We show that, both analytically and numerically, the quasi-GPD does approach the light-cone GPD when the meson is boosted to the infinite momentum frame, which justifies the tenet underlying the large momentum effective theory for the off-forward parton distribution. Upon taking the forward limit, the light-cone and quasi-GPDs reduce to the light-cone and quasi-PDFs. As a bonus, we take this chance to correct the incomplete expression of the quasi-PDFs in the 't Hooft model reported in our preceding work [Y. Jia et al. Phys. Rev. D 98, 054011 (2018)].

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Heavy quark fragmentation function in 't Hooft Model

We carry out a comprehensive study of the quark-to-meson fragmentation function in the 't Hooft model, i.e., the two-dimensional Quantum Chromodynamics (QCD) in $N_c\to \infty$ limit, following the operator definition pioneered by Collins and Soper. We apply the Hamiltonian approach as well as the diagrammatic approach to construct the functional form of the quark-to-meson fragmentation function in terms of the meson's light-cone wave function. For the sake of comparison, we also investigate the heavy quark fragmentation into quarkonium in two-dimensional QCD within the framework of the nonrelativistic QCD (NRQCD) factorization, at the lowest order in quark velocity. In the heavy quark limit, the quark fragmentation function obtained from the {\it ab initio} method agrees well, both analytically and numerically, with that obtained from the NRQCD approach. This agreement might be regarded as a nontrivial justification for the validity of both field-theoretical approaches to compute the heavy quark fragmentation function.

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Nucleon Transversity Distribution in the Continuum and Physical Mass Limit from Lattice QCD

We report a state-of-the-art lattice QCD calculation of the isovector quark transversity distribution of the proton in the continuum and physical mass limit using large-momentum effective theory. The calculation is done at four lattice spacings $a=\{0.098,0.085,0.064,0.049\}$~fm and various pion masses ranging between $220$ and $350$ MeV, with proton momenta up to $2.8$ GeV. The result is non-perturbatively renormalized in the hybrid scheme with self renormalization which treats the infrared physics at large correlation distance properly, and extrapolated to the continuum, physical mass and infinite momentum limit. We also compare with recent global analyses for the nucleon isovector quark transversity distribution.

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Parton distribution of intrinsic charm in two dimensional QCD

We present a detailed investigation on the intrinsic charm content in a light meson within the 't Hooft model, namely, the two-dimensional QCD in large $N_c$ limit. The intrinsic charm parton distribution function (PDF) of a light meson, which first arises at order $N_c^{-1}$, is explicitly expressed in terms of the 't Hooft wave functions of the light meson and an infinite towers of excited charmed mesons. We also derive the functional forms from the two-dimensional counterparts of the meson cloud model (MCM) and Brodsky-Hoyer-Peterson-Sakai (BHPS) model. We then make a quantitative comparison between our rigorous results and model predictions. We also study how the profile of the intrinsic charm PDF varies with charm quark mass. The average momentum fraction carried by the charm quark inside a light meson is found to decrease faster than $m_c^{-4}$ with increasing charm quark mass.

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Four-lepton decays of neutral vector mesons

We investigate the rare electromagnetic decays of neutral vector mesons (exemplified by $J/ψ$, $Υ$, $ρ^0$, $ω,ϕ$, \ldots) into four charged leptons ($2(e^+e^-)$, $2(μ^+μ^-)$, $2(τ^+τ^-)$, $e^+e^-μ^+μ^-$, $e^+e^-τ^+τ^-$, \ldots) at lowest order in QED. In contrast to the case of vector meson decay into a single pair of leptons, the lepton mass must be retained in these four-lepton decay channels to cutoff the mass singularity. Owing to more pronounced collinear enhancement, the branching fractions of vector mesons decays into $ 2(e^+e^-)$ are considerably greater than those for decays into $2(μ^+μ^-)$. The decay channels $J/ψ(Υ)\to 2(e^+e^-), e^+e^-μ^+μ^-$ are predicted to have branching fractions of order $10^{-5}$, which appear to have bright observation prospect at {\tt BESIII}, {\tt Belle 2} and {\tt LHC} experiments.

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Fragmentation production of fully-charmed tetraquarks at LHC

The $X(6900)$ resonance, very recently discovered in the double-$J/ψ$ channel at LHCb experiment, has spurred intensive interest in unravelling the nature of the fully charmed tetraquark state. The aim of this paper is to present a model-independent theoretical framework to study the inclusive production of this novel species of exotic hadrons, the resonances composed of four heavy quark (commonly referred to as $T_{4c}$), at large $p_T$ in hadron collision experiments. Appealing to asymptotic freedom and the fact $m_c\gg Λ_{\rm QCD}$, we propose that the nonpertubative yet universal gluon-to-$T_{4c}$ fragmentation function, can be decomposed into the product of the perturbatively calculable short-distance coefficient and the long-distance NRQCD matrix elements. We compute the short-distance coefficient at lowest-order in $α_s$ and velocity expansion. Adopting the diquark ansatz to roughly estimate those not-yet-known NRQCD matrix elements, together with the standard QCD factorization theorem, we predict the differential production rates for the $T_{4c/4b}(0^{++})$ and $T_{4c/4b}(2^{++})$ at large $p_T$ in $pp$ collision, which eagerly awaits the confrontation with the future LHC experiments.

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$W$ radiative decay to heavy-light mesons in HQET factorization through ${\cal O}(α_s)$

Analogous to NRQCD factorization for heavy quarkonium exclusive production, in this work we propose to employ the heavy-quark-effective-theory (HQET) factorization, which has been predominantly applied to account for exclusive $B$ decays, to study the exclusive production of the heavy-flavored mesons. We take $W\to B(D_s)+γ$ as a prototype process. The validity of the HQET factorization rests upon the presumed scale hierarchy: $m_W\sim m_b\gg Λ_{\rm QCD}$. Through an explicit analysis at next-to-leading order in $α_s$ yet at leading order in $1/m_b$, we verify that the decay form factors can indeed be expressed as the convolution between perturbatively calculable hard-scattering kernel and the $B$ meson light-cone distribution amplitude (LCDA) defined in HQET. It is observed that the factorization scale dependence becomes reduced after incorporating the NLO perturbative correction. An interesting future investigation is to identify and resum large collinear logarithms of $m_W/m_b$ that arise ubiquitously in the fixed-order expressions of the hard-scattering kernel in HQET factorization.

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A factorization theorem connecting the light-cone distribution amplitudes of heavy-flavor mesons in QCD and HQET

The light-cone distribution amplitude (LCDA) of a heavy-light meson defined in heavy quark effective theory (HQET), is a fundamental nonperturbative input to account for innumerable $B$ meson exclusive decay and production processes. On the other hand, the conventional heavy-flavored meson LCDA defined in QCD, also ubiquitously enters the factorization formula for hard exclusive $B$ production processes. Inspired by the observation that these two LCDAs exhibit the identical infrared behaviors, yet differ in the ultraviolet scale of order $m_b$ or greater, we propose a novel factorization theorem for the heavy-light mesons, that the LCDA defined in QCD can be further expressed as a convolution between the LCDA in HQET and a perturbatively calculable coefficient function thanks to asymptotic freedom. This refactorization program can be invoked to fully disentangle the effects from three disparate scales $Q$, $m_b$ and $Λ_{\rm QCD}$ for a hard exclusive $B$ production process, particularly to facilitate the resummation of logarithms of type $\ln Q/m_b$ and $\ln m_b/Λ_{\rm QCD}$ in a systematic fashion.

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