Strong decay of in light cone sum rules

In this study, we assign the tetraquark state for the resonance and investigate the mass and decay constant of in the framework of SVZ sum rules through a different calculation technique. Then, we calculate the strong coupling by considering soft-meson approximation techniques within the framework of light cone sum rules, and we use the strong coupling to obtain the width of the decay . Our prediction for the mass agrees with the experimental measurement, and that for the decay width of is within the upper limit.


I. INTRODUCTION
ψ( 4230) is the first observed Y state, which was detected through the initial-state-radiation (ISR) technique in the process by the BABAR experiment in 2005 [1] and then confirmed by CLEO [2] and Belle [3] in the same process.An accumulation of events with similar characteristics was reported in two other processes (and ) by CLEO [4] and also in the decay of (4230) by the BABAR [5] collaboration.In 2017, the BESIII collaboration announced a new precise measurement of the cross section [6], reporting updated values for the mass and width of the .Particularly, a second resonance is also presented in the mass spectrum.The values of the two observed resonances are and MeV for and and MeV for, namely, .Although Ref. [6] proposes that the structure around 4260 MeV could be read as a superposition of these two resonances, and Ref. [7] further suggests them as and respectively, this discussion has not yet been settled.Here, our study still concentrates on the resonant state instead of discussing the combined structure., which is consistent with that of a vector charmonium state.Theorists have tried to categorize it into the vector charmonium group.However, because its mass does not fit into any mass of the charmonium states in the same mass region, and mainly decays to , but the observed Y in such decay does not match the peaks in the cross sections measured by the BABAR [7,8] and Belle [9] collaborations, the Y(4230) does not look like a normal state.Furthermore, for the radially excited charmoniums, four of the S-wave states, , , , and , have already been assigned to , , and mesons, respectively, and two of the D-wave states and have been assigned to and mesons, respectively.In addition, the masses of the and states in the quark model are 4.76 and 4.52 GeV, and are thus higher than that of the .According to the above analysis, one can conclude that may not be consistent with any of the states [10−12].
To further explain the structure of , many theoretical interpretations have emerged, including that it is a tetraquark state [13−16], a compact tetraquark state [17], a hadrocharmonium state [18,19], hadronic molecule of , , or [15, 20−22], [23], [24], [25], [26], agluon hybrid [14,27,28], a charm baryonium [29], and a coupled-channel model [30,31].However, within the available experimental data, none of these theoretical interpretations can be completely accepted or excluded from the nature of .For example, in the compact tetraquark model [32], an isospin-violating process exists with a sizeable decay width, where both and can be produced from decay.Therefore, the interpretation of in the compact tetraquark model can lead to a peak in the cross section and a very prominent peak should appear in mass spectrum between the threshold [33].However, using the data from the BESIII experiment and searching for isospin-violating [34], no signal is observed.Whatever the case, the compact tetraquark model should have isospin and SU(3)-multiplet partner states.However, none of those partners for ψ(4230) has been observed in experiments so far.If ψ(4230) is a hadrocharmonium, it's structure would be formed by mixing with another hadrocharmonium.These two hadrocharmonia states contain spin 1 and spin 0 compact cores, respectively [18].However, based on BESIII data [6], the decay rate of to noncharmonium states should be suppressed [18], indicating that the above suggestion may not be consistent.If we assign the molecule to ψ(4230), the binding energy being approximately 66 MeV is rather large, though this possibility is not excluded [35].There are other candidates for ψ(4230), e.g., , , , and , whose open charm thresholds are around 4.26 GeV with .Unfortunately, besides , these candidates have widths that are too broad to make a bound state, which could not be consistent with the total decay width of [21,36].For the molecule, its mass is GeV, which should also be excluded [15].In any case, Y(4230) does not seem to be a hadronic molecule.

Y(4230)
The may also be assumed to be a charmonium hybrid meson.However, in Ref. [37], the authors found that the masses of the hybrid states lie at GeV, heavier than the mass of the .In non-relativistic EFTs, the mass of may be consistent with one state of hybrid multiplet, but disfavors the hybrid interpretation since it decays to spin triplet charmonium while is only a spin singlet [38].In Ref. [39], the color halo picture was found to be compatible with decay properties, and LHCb and BelleII were suggested to search for charmonium-like hybrids in and final states.We should not hastily conclude that can not be the hybrid state.

Y(4230)
In summary, the structure of at this point is not yet fully settled.
In this study, we investigate the strong decay of observed in the process .Notice will decay into a , which fits the S-wave hypothesis [40].Furthermore, be- ing a member of the vector charmonium family suggests ψ(4230) a composition.We, therefore, consider to be a tetraquark state, as in Ref. [41].This differs from other ideas, i.e., in Ref. [42], the was suggested to be a bound system.We calculate the strong coupling using the light cone sum rules method, with the interpolating current taken from Ref. [15].We evaluate the mass of through a different calculation technique developed in Ref. [43], not the usual way in two-point sum rules [44].Comparing the mass prediction of with the result in PDG [45], we confirm our technique generalization is credible.We then extend it to evaluate the decay constant of , which will be used in the numerical calculation of the strong coupling .Finally, the decay width of is obtained, and further results are compared with the experimental measurement and discussed.
Our work is organized as follows: In Section II, we calculate the mass and decay constant of the state within the two-point sum rule approach developed by Shifman, Vainshtein, and Zakharov (SVZ sum rules) [46].We also calculate the strong coupling , which is derived with the light cone sum rules approach.The numerical results and discussions are shown in Section III.We present our summary in Section IV.

II. CALCULATION FRAMEWORK Y(4230)
A. The mass and the decay constant of We begin by calculating the mass and the decay constant using the two-point correlation function: where the interpolating currents are given by the following expression: As a first step, we calculate the correlation function by inserting a complete set of hadronic states into Eq.( 1): where the higher resonances and continuous states are represented by .The subtraction terms are not displayed because they would disappear following the Borel transformation.We define the decay constant according to with being the polarization vector of .After performing the polarization sum equation, we can obtain On the right side of Eq. ( 5), we begin to observe a pole.The Borel transformation can be performed on Eq. ( 5) to remove the pole, which yields Next, let us consider the correlation function in the OPE side.Following the Wick Theorem for contraction of the heavy and light quarks, we obtain S ab q (x) where represents .We accept the following expression for propagators of the u, d, and s quarks in coordinate-space [47,48]: The heavy quark propagator is given in terms of Bessel functions of the second kind as [49] Notice the heavy quark propagator here is different from the expression presented in the usual way, for example, in Ref. [44], where the heavy quark propagator is expressed in the momentum space.If we use the momentum expression of the propagator in Eq. ( 7), we have to face divergences in the double integrals such as As shown in Ref. [43], results without any divergences can be obtained by using an appropriate representation of the modified Bessel functions in the heavy quark propagator, like in Eq. ( 9).Since here, we are using the SVZ sum rules instead of the LCSR, we have to modify the calculation when the particle distribution function does not participate in Eq. (7).We showed the details of the modification in Appendix V.C.
The correlation function also has the following decomposition over the Lorentz structures: ∼ g µν and we choose to work with the term , which can be represented as the dispersion integral: where is the corresponding spectral density.
The Borel transformation and the quark-hadron duality can be applied to to obtain Next, take out the contribution from the continuum to get The strong decay of in light cone sum rules Chin.Phys.C 48, 023105 (2024) 023105-3

Y(4230)
The state mass can be determined by the sum rule: It is necessary to calculate the strong coupling first, based on the light cone sum rules (LCSR), before predicting the width of .We begin by using the two-point correlation function: where represents the scalar meson .has momentum , and p, q represent the four-momentum for and , respectively.is the interpolating current of given by [15, 50] Here, i denotes the color indexes, and C is the charge conjugation matrix.

Phenomenological side calculation
Next, we must build a relationship between the correlation function and the strong coupling .By adding two complete sets of hadronic states to Eq. ( 16), we can construct the phenomenological expression of the correlation function: where represents the contributions of the continuum states and higher resonances.The lowest continuum state thresholds are indicated by the symbols and .
By parameterizing the hadronic matrix element and performing the polarization sum, we can easily show that where and are the masses of and respectively.ε and denote the polarization vectors of the and , respectively.is the invariant constant parameterizing the hadronic matrix element.
In this study, we choose to proceed with a structure that is proportional to where we define .The correlation function in Eq. ( 21) can be transformed into the equation below by applying the Borel transformations to the variables and , Thus, we have the following formula for a general dispersion relation: g µν where the subtraction terms and single dispersion integrals are not provided because they would all vanish when the double Borel transformation is applied to Eq. (23).By choosing to proceed with a structure that is proportional to , we can represent the OPE result for the correla- tion function as where After performing the Borel transformations, we can derive This is accomplished by applying the quark-hadron duality, which allows the integral of the hadronic spectral density to equal that of the OPE spectral density in a certain region: After equating Eqs. ( 22) and ( 26) and substituting with Eq. ( 27), we get the following equation for the strong coupling: As we can see from Eq. ( 16), since the interpolating currents of and are located at points x and , respectively, there will still be a quark element after the and c quark fields are contracted.This is because disappears and reduces to normalization factors when .This situation can be replaced by the kinematical limit , which is called soft-meson approximation [51].Such approximation leads to the following hadronic representation: and the Borel transformation on the variable applied to this correlation function yields Following [51,52], we apply the operator on both sides of the sum rules expression to remove unsuppressed contributions to obtain ŝ which depends only on owing to the soft-meson approximation.

OPE side calculation g XJ/ψϕ
Considering has a relationship to the OPE part of the correlation function, we will calculate it.Using the Wick Theorem, we can derive For the heavy quark propagator on the light cone, we employ its expression in terms of [53] S ab c (x) =i where we adopt the notation Substituting the summation and the expansion Y( 4230)→J/ψ f 0 (980) The strong decay of in light cone sum rules Chin.Phys.C 48, 023105 (2024) 023105-5 into Eq.( 33), we can obtain where After substituting the propagator, using the particle distribution amplitudes (DAs) of in Appendix V.A and contracting the color index by the SU(N) algebra, we will encounter four-dimensional integrals, for example, In Appendix V.B., we provide the main steps to calculate some four-dimensional integrals like in (40).By choosing the term proportional to , we can derive where and are the mass and decay constant of , respectively.The strong coupling is then evaluated using Eq.(32).Besides, we can derive the decay width of as [54] Γ(Y(4230 where In this section, we present the mass and decay constant of Y(4230), and analyze the numerical results for the decay width of .We use the following parameters for the numerical calculation.The current charm-quark mass, GeV, the meson mass

III. NUMERICAL CALCULATION A. Input parameters
MeV and the (980) mass MeV from the Particla Data Group (PDG) [45].The and (980) decay constants are taken as = GeV [55] and = GeV [56], respectively.The current-quark-mass for the squark is MeV from the PDG.In addition, we also need to know the values of the non-perturbative vacuum condensates.The related parameters are [15,57,58] ⟨ qq⟩ = −(0.24± 0.01) 3 GeV 3  , The sum rule predictions depend on two parameters: continuum threshold and borel mass .
is correlated with the first of the excited states of .However, the experimental results show that there is no resonance activity associated with the states of .We can naturally choose , because the mass gap between the ground state and the first excited state is regularly around GeV in charmonia and bottomonia (Table 1).
Additionally, Table 2 contains experimental data taken from the PDG that supports the majority of the computations in Table 1.
Moreover, we can refer to the QCD sum rule calculations listed in Table 3.There is a mass difference of The borel mass can be determined based on two principles: 10 1.The high dimension condensates make up not more than % of the total contribution to the OPE: where the ellipsis represents higher dimension contributions.2. The pole contribution (PC) in Eq. ( 5) should exceed % As seen in Fig. 1, the red dot indicates the point at which CVG becomes %, where the maximum achievable can be attained.We can select the minimum from the black dot where PC converges with %.Therefore, we require the region of to be The outcomes of the mass and the decay constant as functions of the parameters are shown in Fig. 2. The orange shape in the first picture of Fig. 2 corresponds to the measurements taken by the Belle collaboration [6].The other curves show our prediction at a fixed .Our prediction is consistent with the measurement.At a fixed point of , our result for the mass reads

Y(4230)
Our mass prediction shows that the generalization of our method is valid.We then extend the method to evaluate the decay constant of .The result at the same typical point reads → J/ψ f 0 (980) The mass and decay constant are input parameters to calculate the decay width of ψ(4230) .The ψ(4230) branching ratios from PDG [45] show that We can estimate the upper limit of , by assuming that is the only decay process of .With the width of MeV, we can obtain Table 2. Masses of experimentally observed states in the Particle Data Group listings [36].Also, from the PDG, the (980) branching ratios give and the partial width gives , and are the main decay processes of (980).From Eq. ( 54), we obtain the partial width .So, we estimate Then, we can finally conclude that As shown in Fig. 4, the blue, green, and black curves show a clear dependence of our prediction on and .For and , we use the same values as in the mass analysis.The results are shown in Fig. 3 and Fig. 4. By choosing appropriate parameters, our prediction for is Taking the average result of , the width of this decay can be obtained using Eq. ( 32): which is less than the upper limit of the ψ(4230) decay width.Combining this result with the predicted mass result, we may conclude that Y(4230) could be a tetraquark state.However, owing to the lack of In this research, we designate Y(4230) as a vector tetraquark state to concurrently analyze 's mass, decay constant, and decay into .The mass of is evaluated through a different calculation technique developed in two-point sum rules, and the result is in agreement with the mass of in PDG.Then, we extend the technique to calculate the decay constant of .Using the light cone sum rules method, we calculate the coupling constant and discover the result for the decay width.Then, we assume that is the most significant channel, overwhelming all the other channels.Therefore, we can consider the width of as the width of .Since we know the branching ratios of from PDG, we can estimate the upper limit of the channel.The decay width of is less than the upper limit.Our prediction of the mass of is in agreement with that of in PDG, and the decay width of does not exceed its theoretical limits.There is a possibility that Y(4230) could be a tetraquark.In the future, experiments will be more helpful in determining whether or not this structure of is appropriate.

APPENDIX A. Particle distribution amplitudes
The matrix elements of the can be expanded in terms of the corresponding distribution amplitudes.Below, we provide expressions for [56]: where the LCDA represents twist-2 light-cone distribution amplitudes of , and the other two are twist-3 distribution amplitudes.Meanwhile, we use the following normalization B. The formula for LCSR When calculating the OPE part of the correlation function, we encountered various four-dimensional integrals in the momentum spaces.Before performing the integration, it is common to use the Feynman's parametric integral formula: In general, Feynman integrals contain This integral can be reduced to To obtain a formula in proportion to such as we can differentiate equation Eq. ( 62) with momentum q one time.The higher tensors in the integrand come form higher differentiations.Now, when the above equation encounters a pole in the Gamma function, where dimension , i.e., , we can use the equation to eliminate the Gamma function and perform the replacement To obtain the final expression of the correlation function, we need the imaginary part of the results and the integra- tion over the Feynman parameters.
C. The formula for mass and decay constants Here, we present the calculation details of the integral.When dealing with Eq. ( 7), we need to consider a general integral Using the integral representation of the Bessel function we have leads to the equation Then, substituting , , we obtain where .
Introducing new variables, , we have where and .

2 Fig. 3 .Fig. 2 .
Fig. 3. (color online) Strong coupling as a function of the threshold parameter at different fixed values of .

Fig. 4 .
Fig. 4. (color online) Strong coupling as a function of the Borel parameter at different fixed values of .

Table 1 .
[59]k model masses calculated for the first three levels of charmonia and bottomonia[59].