arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 19 recordsLinked to original sources

arXiv2024

Molecular components in the $J/ψ$ and the $ρ$-$π$ puzzle

Motivated by the large branching fractions of $J/ψ\to f_0 (1710) ω/f_0(1710) ϕ$ and the light exotic candidates, we find that there may exist molecular states composed of $f_0(1710) ω$ and $f_0 (1710) ϕ$, which correspond to $X(2440)$ and $X(2680)$ observed in a few decades before. The branching fraction of $X(2440)$ and $X(2680)$ to various $PV$ channels and $KKω(ϕ)$ channels are estimated in the molecular scenario. In addition, the large branching fractions of $J/ψ\to f_0 (1710) ω/f_0(1710) ϕ$ indicate the sizable molecular components in the $J/ψ$ state. Thus, we consider the $J/ψ$ as the supperposition of $c\bar{c}(1S)$, $f_0(1710) ω$ and $f_0 (1710) ϕ$ molecular states, and these molecular components have significant impact on the light hadron decays of $J/ψ$, which may shield light on the long standing $ρ-π$ puzzle.

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arXiv2023

Study on the possible molecular states composed of $Λ_c\bar D^$, $Σ_c\bar D^$, $Ξ_c\bar D^$ and $Ξ_c'\bar D^$ in the Bethe-Salpeter frame based on the pentaquark states $P_c(4440)$, $P_c(4457)$ and $P_{cs}(4459)$

The measurements on a few pentaquarks states $P_c(4440)$, $P_c(4457)$ and $P_{cs}(4459)$ excite our new interests about their structures. Since the masses of $P_c(4440)$ and $P_c(4457)$ are close to the threshold of $Σ_c\bar D^*$, in the earlier works, they were regarded as molecular states of $Σ_c\bar D^*$ with quantum numbers $I(J^P)=\frac{1}{2}(\frac{1}{2}^-)$ and $\frac{1}{2}(\frac{3}{2}^-)$, respectively. In a similar way $P_{cs}(4459)$ is naturally considered as a $Ξ_c\bar D^*$ bound state with $I=0$. Within the Bethe-Salpeter (B-S) framework we systematically study the possible bound states of $Λ_c\bar D^*$, $Σ_c\bar D^*$, $Ξ_c\bar D^*$ and $Ξ_c'\bar D^*$. Our results indicate that $Σ_c\bar D^*$ can form a bound state with $I(J^P)=\frac{1}{2}(\frac{1}{2}^-)$, which corresponds to $P_c(4440)$. However for the $I(J^P)=\frac{1}{2}(\frac{3}{2}^-)$ system the attraction between $Σ_c$ and $\bar D^*$ is too weak to constitute a molecule, so $P_{c}(4457)$ may not be a bound state of $Σ_c\bar D^*$ with $I(J^P)=\frac{1}{2}(\frac{3}{2}^-)$. As $Ξ_c\bar D^*$ and $Ξ_c'\bar D^*$ systems we take into account of the mixing between $Ξ_c$ and $Ξ'_c$ and the eigenstets should include two normal bound states $Ξ_c\bar D^*$ and $Ξ_c'\bar D^*$ with $I(J^P)=\frac{1}{2}(\frac{1}{2}^-)$ and a loosely bound state $Ξ_c\bar D^*$ with $I(J^P)=\frac{1}{2}(\frac{3}{2}^-)$. The conclusion that two $Ξ_c\bar D^*$ bound states exist, supports the suggestion that the observed peak of $P_{cs}(4459)$ may hide two states $P_{cs}(4455)$ and $P_{cs}(4468)$. Based on the computations we predict a bound state $Ξ_c'\bar D^*$ with $I(J^P)=\frac{1}{2}(\frac{1}{2}^-)$ but not that with $I(J^P)=\frac{1}{2}(\frac{3}{2}^-)$. Further more accurate experiments will test our approach and results.

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arXiv2022

Possible molecular states of $\bar D^{}K^{}$ ($ D^{}K^{}$) and the new exotic states $X_0(2900)$ and $X_1(2900)$ ($T^a_{cs0}(2900)^0$ and $T^a_{cs0}(2900)^{++}$)

Two iso-singlet hadron states $X_0(2900)$ and $X_1(2900)$ with $J=0$ and 1 respectively, discovered by the LHCb collaboration in 2020, were identified as molecular bound states of $\bar D^*K^*$. Recently two structures $T^a_{cs0}(2900)^0$ and $T^a_{cs0}(2900)^{++}$ have been observed at the hadron spectra, one would suspect if they also are molecular states of $D^*$ and $K^*$. As long as they were of the molecular structures of $D^*K^*$, the hadron states must be in an iso-vector, namely $T^a_{cs0}(2900)^0$ and $T^a_{cs0}(2900)^{++}$ were $I_3=-1, 1$ components of the iso-vector. If it is the case, the corresponding $T^a_{cs0}(2900)^+$ of ($I=1,I_3=0$) and $T^{'a}_{cs0}(2900)^{+}$ of $I=0,I_3=0$ so far evade experimental observation, but should be found by the future experiments. To testify this ansatz, in this paper we study the possible molecular structures of $\bar D^{*}K^{*}$ and $D^{*}K^{*}$ within the Bethe-Salpeter (B-S) framework. With reasonable input parameters it is found that $\bar D^{*}K^{*}$ iso-scalar systems with $J^P=0^+$ and $1^+$ are solutions. The result supports the ansatz of $X_0(2900)$ ($X_1(2900)$) being molecular states of $\bar D^*K^{*}$. Whereas for the system of $ D^{*}K^{*}$ with $I=1$ the corresponding B-S equation has no solution. Thus we can draw a clear conclusion that $T^a_{cs0}(2900)^0$ and $T^a_{cs0}(2900)^{++}$ should not be bound states of $ D^{*}$ and $K^{*}$. The two structures observed by the LHCb collaboration may be caused by dynamics, such as the well-recognized triangle anomalies or other mechanisms.

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arXiv2022

Revisiting the transition $Ξ^{+}_{cc}\toΞ^{(')+}_c$ to understand the data from LHCb

The LHCb collaboration newly measured the decay rate of doubly charmed baryon $Ξ^{++}_{cc}\toΞ^{'+}π^+$ and a ratio of its branching fraction with respect to that of the decay $Ξ^{++}_{cc}\toΞ^{+}π^+$ is reported as $1.41\pm 0.17\pm 0.10$. This result conflicts with the theoretical predictions made by several groups. In our previous work, following the prescription given in early literature where the $us$ diquark in $Ξ^{+}_{c}$ is assumed to be a scalar whereas in $Ξ^{'+}_{c}$ is a vector i.e. the spin-flavor structure of $Ξ^{+}_{c}$ is $[us]_0 c$ and that of $Ξ^{'+}_{c}$ is $[us]_1 c$, we studied the case of $Ξ^{++}_{cc}\toΞ^{(')+}$ with the light front quark model. Numerically we obtained $Γ(Ξ^{++}_{cc}\toΞ^{'+}π^+)/Γ(Ξ^{++}_{cc}\toΞ^{+}π^+)=0.56\pm0.18$ which is about half of the data. While abandoning the presupposition, we suppose the spin-flavor structure of $us$ in $Ξ^{+}_{c}$ may be a mixture of scalar and vector, namely the spin-flavor function of $Ξ^{+}_{c}$ could be ${\rm cos}θ\, [us]_{0}[c]+{\rm sin}θ\, [us]_{1}[c]$. An alternative combination $-{\rm sin}θ\,[us]_{0}[c]+{\rm cos}θ\, [us]_{1}[c]$ would correspond to $Ξ^{'+}_{c}$. Introducing the mixing mechanism the ratio $Γ(Ξ^{++}_{cc}\toΞ^{'+}π^+)/Γ(Ξ^{++}_{cc}\toΞ^{+}π^+)$ depends on the mixing angle $θ$. With the mixing scenario, the theoretical prediction on the ratio between the transition rate of $Ξ^{+}_{cc}\toΞ^{'+}_c$ and that of $Ξ^{+}_{cc}\toΞ^{+}_c$ can coincide with the data as long as $θ=16.27^\circ\pm2.30^\circ$ or $85.54^\circ\pm2.30^\circ$ is set. Definitely, more precise measurements on other decay portals of $Ξ^{+}_{cc}$ are badly needed for testing the mixing mechanism and further determining the mixing angle.

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arXiv2022

Possible Molecular States of $D^{()}D^{()}$ and $B^{()}B^{()}$ within the Bethe-Salpeter framework

Recently the LHCb collaboration reported a new exotic state $T^+_{cc}$ which possesses $cc\bar u\bar d$ flavor structure. Since its mass is very close to the threshold of $D^0D^{*+}$ (or $D^{*0}D^{+}$) and its width is very narrow, it is inclined to conjecture that $T^+_{cc}$ is a molecular state of $D^0D^{*+}$ (or $D^{*0}D^{+}$). In this paper we study the possible molecular structures of $D^{(*)}D^{(*)}$ and $B^{(*)}B^{(*)}$ within the Bethe-Salpeter (B-S) framework. We employ one boson exchange model to stand the interaction kernels in the B-S equations. With reasonable input parameters we find the isospin eigenstate $\frac{1}{\sqrt{2}}(D^0D^{*+}-D^{*0}D^{+})$ ($J^P=1^+$) constitutes a solution, which supports the ansatz of $T^+_{cc}$ being a molecular state of $D^0D^{*+}$ (or $D^{*0}D^{+}$). With the same parameters we also find that the isospin-1 state $\frac{1}{\sqrt{2}}(D^{*0}D^{*+}+D^{*0}D^{*+})$ ($J^P=0^+$) can exist. Moreover, we also study the systems of $B^{(*)}B^{(*)}$ and their counterparts exist as possible molecular states. Consistency of theoretical computations based on such states with the data of the future experiments may consolidate the molecular structure of the exotic state $T^+_{cc}$.

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arXiv2021

The weak decays of $Ξ^{(')}_{c}\toΞ$ in the light-front quark model

Without contamination from the final state interactions, the calculation of the branching ratios of semileptonic decays $Ξ^{(')}_{c}\toΞ+e^+ν_e$ may provide us more information about the inner structure of charmed baryons. Moreover, by studying those processes, one can better determine the form factors of $Ξ_c\toΞ$ which can be further applied to relevant estimates. In this work, we use the light-front quark model to carry out the computations where the three-body vertex functions for $Ξ_c$ and $Ξ$ are employed. To fit the new data of the Belle II, we re-adjust the model parameters and obtain $β_{s[sq]}=1.07$ GeV which is 2.9 times larger than $β_{s\bar s}=0.366$ GeV. This value may imply that the $ss$ pair in $Ξ$ constitutes a more compact subsystem. Furthermore, we also investigate the non-leptonic decays of $Ξ^{(')}_c\to Ξ$ which will be experimentally measured soon, so our model would be tested by consistency with the new data.

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arXiv2020

Revisiting the determining fraction of glueball component in $f_0$ mesons via radiative decays of $J/ψ$

QCD theory predicts the existence of glueballs, but so far all experimental endeavors have failed to identify any such states. To remedy this discrepancy between QCD, which has proven to be a successful theory for strong interactions, and the failure of experimental searches for glueballs, one is tempted to accept the promising interpretation that the glueballs mix with regular $q\bar q$ states of the same quantum numbers. The lattice estimate of the masses of pure $0^{++}$ glueballs ranges from 1 to 2 GeV, which is the region of the $f_0$ family. Thus many authors suggest that the $f_0$ mesonic series is an ideal place to study possible mixtures of glueballs and $q\bar q$. In this paper, following the strategy proposed by Close, Farrar and Li, we try to determine the fraction of glueball components in $f_0$ mesons using the measured mass spectra and the branching ratios of $J/ψ$ radiative decays into $f_0$ mesons. Since the pioneering papers by Close et al., more than 20 years has elapsed and more accurate measurements have been done by several experimental collaborations, so it is time to revisit this interesting topic using new data. We suppose $f_0(500)$ and $f_0(980)$ to be pure quark states, while for $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, to fit both the experimental data of $J/ψ$ radiative decay and their mass spectra, glueball components are needed. Moreover, the mass of the pure $0^{++}$ glueball is phenomenologically determined.

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arXiv2020

A possible interpretation for $X(6900)$ observed in four-muon final state by LHCb -- A light Higgs-like boson?

A peak structure of $J/ψ$ pair production around $6.9~{\rm GeV}$ was observed and analyzed by the LHCb collaboration using the Run I and II data of LHC. How to understand this peak arouses enthusiastic discussions among both theorists and experimentalists of high energy physics, because this discovery might hint something new. Overwhelming works on this topic tend to attribute the peak as a four-quark state: tetraquark or molecule. Instead, we suggest that this peak is corresponding to a fundamental Higgs-like boson with mass about $6.9~{\rm GeV}$ which is advocated by a BSM effective theory. We present a detailed analysis on both signal and SM background, including integrated cross sections and invariant mass distributions of the final-state $J/ψ$ pair. Our numerical results are well in coincidence with the experimental data, as postulating the resonance observed by LHCb to be a BSM $0^{++}$ scalar. Therefore, the peak at $M_{\text{di-}J/ψ} \sim 6.9~{\rm GeV}$ might be a hint of new physics beyond the SM whose scale is not as large as mostly expected by high energy physicists. More further works are urgently needed in both experimental and theoretical aspects to validate or negate this assumption.

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arXiv2020

Hunting for possible Higgs-like boson beyond the Standard Model

A recent preliminary investigation based on Durgut's report at the American Physical Society site shows a structure at $18.4~ {\rm GeV}$ exists in the invariant mass distribution of $Υl^+l^- ~ (l = e,\, μ)$ at the LHC center-of-mass energy of $7$ and $8~ {\rm TeV}$. Its appearance attracts attention of theorists and experimentalists of high energy physics, because it might be a Higgs-like boson of $18.4~ {\rm GeV}$ which would serve as a signal of the new physics beyond the Standard Model. We have carried out computations on the corresponding quantities (production and decay rates) based on quantum field theory and compared the results with experimental data. Our numerical results do not support the assertion that the $18.4~ {\rm GeV}$ peak corresponds to a neutral $0^{++}$ boson which decays into $Υl^+l^-$. Much further works (both experimental and theoretical) are badly needed.

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arXiv2020

A natural interpretation on the data of $Λ_c\toΣπ$

Even though the Standard Model (SM) has achieved great success, its application to the field of low energies still lacks solid foundation due to our limited knowledge on non-perturbative QCD. Practically, all theoretical calculations of the hadronic transition matrix elements are based various phenomenological models. There indeed exist some anomalies in the field which are waiting for interpretations. The goal of this work is trying to solve one of the anomalies: the discrepancy between the theoretical prediction on the sign of the up-down asymmetry parameter of $Λ_c\toΣπ$ and the experimental measurement. In the literatures several authors calculated the rate and determined the asymmetry parameter within various schemes, but there exist obvious loopholes in those adopted scenarios. To solve the discrepancy between theory and data, we suggest that not only the direct transition process contributes to the observed $Λ_c\toΣπ$, but also other portals such as $Λ_c\to Λρ$ also play a substantial role via an isospin-conserving re-scattering $Λρ\toΣπ$. Taking into account of the effects induced by the final state interaction, we re-evaluate the relevant quantities. Our numerical results indicate that the new theoretical prediction based on this scenario involving an interference between the direct transition of $Λ_c\toΣπ$ and the portal $Λ_c\toΛρ\toΣπ$ can make both the decay rate and sign of the asymmetry parameter to be consistent with data.

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arXiv2020

Study on the possible molecular state composed of $D^*_s\bar D_{s1} $ within the Bethe-Salpeter framework

Recently a vector charmonium-like state $Y(4626)$ was observed in the portal of $D^+_sD_{s1}(2536)^-$. It intrigues an active discussion on the structure of the resonance because it has obvious significance for gaining a better understanding on its hadronic structure with suitable inner constituents. It indeed concerns the general theoretical framework about possible structures of exotic states. Since the mass of $Y(4626)$ is slightly above the production threshold of $D^+_s\bar D_{s1}(2536)^-$ whereas below that of $D^*_s\bar D_{s1}(2536)$ with the same quark contents as that of $D^+_s\bar D_{s1}(2536)^-$, it is natural to conjecture $Y(4626)$ to be a molecular state of $D^{*}_s\bar D_{s1}(2536)$, as suggested in literature. Confirming or negating this allegation would shed light on the goal we concern. We calculate the mass spectrum of a system composed of a vector meson and an axial vector i.e. $D^*_s\bar D_{s1}(2536)$ within the framework of the Bethe-Salpeter equations. Our numerical results show that the dimensionless parameter $λ$ in the form factor which is phenomenologically introduced to every vertex, is far beyond the reasonable range for inducing an even very small binding energy $ΔE$. It implies that the $D^*_s\bar D_{s1}(2536)$ system cannot exist in the nature as a hadronic molecule in this model, so that we may not think the resonance $Y(4626)$ to be a bound state of $D^*_s\bar D_{s1}(2536)$, but something else, for example a tetraquark and etc.

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arXiv2020

Study on possible molecular states composed of $Λ_c\bar D$ ($Λ_b B$) and $Σ_c\bar D$ ($Σ_b B$) within the Bethe-Salpeter framework

$P_c(4312)$ observed by the LHCb collaboration is confirmed as a pentaquark and its structure, production, and decay behaviors attract great attention from theorists and experimentalists. Since its mass is very close to sum of $Σ_c$ and $\bar D$ masses, it is naturally tempted to be considered as a molecular state composed of $Σ_c$ and $\bar D$. Moreover, $P_c(4312)$ is observed in the channel with $J/ψp$ final state, requiring that isospin conservation $P_c(4312)$ is an isospin-1/2 eigenstate. In literature, several groups used various models to estimate its spectrum. We systematically study the pentaquarks within the framework of the Bethe-Salpeter equation; thus $P_c(4312)$ is an excellent target because of the available data. We calculate the spectrum of $P_c(4312)$ in terms of the Bethe-Salpter equations and further study its decay modes. Some predictions on other possible pentaquark states that can be tested in future experiments are made.

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arXiv2019

Study on $Ξ_{cc}\toΞ_c$ and $Ξ_{cc}\toΞ'_c$ weak decays in the light-front quark model

In this work we study the weak decays of $Ξ_{cc}\toΞ_c$ and $Ξ_{cc}\toΞ'_c$ in the light-front quark model. Generally, a naive, but reasonable conjecture suggests that the $cc$ subsystem in $Ξ_{cc}$ ( $us$ pair in $Ξ^{(')}_c$) stands as a diquark with definite spin and color assignments. During the concerned processes, the diquark of the initial state is not a spectator, and must be broken. A Racah transformation would decompose the original $(cc)q$ into a combination of $c(cq)$ components. Thus we may deal with the decaying $c$ quark alone while keeping the $(cq)$ subsystem as a spectator. With the re-arrangement of the inner structure we calculate the form factors numerically and then obtain the rates of semi-leptonic decays and non-leptonic decays, which will be measured in the future.

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arXiv2019

$Σ_{b}\toΣ_c^*$ weak decays in the light-front quark model with two schemes to deal with the polarization of diquark

Thanks to the remarkable achievements of LHC, a large database on baryons has been accumulated, so it is believed that the time for precisely studying baryons especially heavy baryons, has come. By analyzing the data, the quark-diquark structure which has been under intensive discussions, can be tested. In this work the decay widths of weak transitions $Σ_b\to Σ^*_c+X$ are calculated in terms of the light front quark model (LFQM). To carry out the calculations, the quark-diquark picture is employed where an axial-vector diquark composed of two light quarks serves as a spectator in the concerned processes. The first step of this work is to construct the vertex functions for $Σ^{(*)}_c$ and $Σ_b$, then the relevant form factors are derived. It is shown that under the heavy quark limit the Isgur-Wise functions for the transition are re-deduced. Indeed, how to properly depict the polarization ($ε_μ$) of the diquark is slightly tricky. In this work, we apply two schemes to explicitly determine the momentum-dependence of the diquark. The corresponding numerical results are presented which will be testified by the future experiments.

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arXiv2019

Revisiting $Λ_{b}\toΛ_{c}$ and $Σ_{b}\toΣ_{c}$ weak decays in the light-front quark model

In this work, we study $Λ_{b}\toΛ_{c}$ and $Σ_{b}\toΣ_{c}$ weak decays in the light-front quark model. As is well known, the key point for such calculations is properly evaluating the hadronic transition matrix elements which are dominated by the non-perturbative QCD effect. In our calculation, we employ the light-front quark model and rather than the traditional diquark picture, we account the two spectator light quarks as individual ones. Namely during the transition, they retain their color indices, momenta and spin polarizations unchanged. Definitely, the subsystem composed of the two light quarks is still in a color-anti-triplet and possesses a definite spin, but we do not priori assume the two light quarks to be in a bound system-diquark. Our purpose is probing the diquark picture, via comparing the results with the available data, we test the validity and applicability of the diquark structure which turns a three-body problem into a two-body one, so greatly simplifies the calculation. It is indicated that the two approaches (diquark and a subsystem within which the two light quarks are free) lead to similar numerical results even though the model parameters in the two schemes might deviate slightly. Thus, the diquark approach seems sufficiently reasonable.

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arXiv2018

Study on the strong decays of $ϕ(2170)$ and a grand expectation for the future charm-tau factory

The present data imply that $ϕ(2170)$ may not be an excited state of $ϕ$, but is a four quark state with $ss\bar s \bar s$ constituents. Furthermore, there are no two mesons of $s\bar s$ available to form a molecule which fits the mass spectrum of $ϕ(2170)$, thus we suggest it should be an $ss\bar s \bar s$ tetraquark state. In this scenario, we estimate its decay rates through the fall-apart mechanism. Our theoretical estimates indicate that its main decay modes should be $ϕ(2170)$ into $ϕf_0(980)$, $ h_1η$, $ h_1η'$, $K_1(1270)K$ and $K_1(1400)K$. Under this hypothesis the modes $ϕ(2170)\to K^*(890)^0\bar K^*(890)^0$, $K^+K^-$ and $K^0_LK^0_S$ should be relatively suppressed. Since the width of $h_1$ is rather large, at present it is hard to gain precise data on $BR(ϕ(2170)\to h_1η)$ and $BR(ϕ(2170)\to h_1η')$ whose measurements may be crucial for drawing a definite conclusion about the inner assignment of $ϕ(2170)$. We lay our expectation to the proposed charm-tau factory which will have much larger luminosity and better capacities.

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arXiv2018

Can we expect an excess of cosmological neutrinos during detection of gravitational waves?

As is well recognized, the supernova explosions produce a great amount of neutrinos, thus we have a strong reason to believe that all violent cosmological events, such as supernova explosions, gamma bursts, black hole merging or neutron star merging would cause remarkable neutrino sprays into the open space. Recently, a very high-energy neutrino captured by the IceCube detector is believed to be radiated from a blazar. However, as the LIGO collaboration first observed gravitational wave caused by spiral approach and merging of two giant back holes, the IceCube did not see an excess of cosmological neutrinos. The reason may be due to that the source is too far away or the neutrino spectrum is not suitable for the IceCube facility detection or just missing by chance. In this work, following Hawking's picture, we suppose that neutrinos would be ejected from black holes as strong gravitational effects lower the potential barriers to enhance neutrino escape rate through quantum tunneling effects. We use the Dirac equation in the curved spacetime to describe the neutrino status and estimate the rejection rate. Then, we also adopt a simplified version where the effects of curved spacetime is ignored to clarify the physical picture. Our conclusion is drawn based on the numerical results and a discussion on the phenomenological consequence is presented.

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arXiv2018

How can $X^{\pm}(5568)$ escape detection?

Multi-quark states were predicted by Gell-Mann when the quark model was first formulated. Recently, numerous exotic states that are considered to be multi-quark states have been experimentally confirmed (four-quark mesons and five-quark baryons). Theoretical research indicates that the four-quark state might comprise molecular and/or tetraquark structures. We consider that the meson containing four different flavors $su\bar b\bar d$ should exist and decay via the $X(5568)\to B_sπ$ channel. However, except for the D0 collaboration, all other experimental collaborations have reported negative observations for $X(5568)$ in this golden portal. This contradiction has stimulated the interest of both theorists and experimentalists. To address this discrepancy, we propose that the assumed $X(5568)$ is a mixture of a molecular state and tetraquark, which contributes destructively to $X(5568)\to B_sπ$. The cancellation may be accidental and it should be incomplete. In this scenario, there should be two physical states with the same flavor ingredients, with spectra of $5344\pm307$ and $6318\pm315$. $X(5568)$ lies in the error range of the first state. We predict the width of the second state (designated as $S_2$) as $Γ(X_{S_2}\to B_sπ)=224\pm97$ MeV. We strongly suggest searching for it in future experiments.

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