Theoretical study of scalar meson a 0 (1710) in the η c → ¯ K 0 K + π − reaction

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I. INTRODUCTION
In 2021, the BABAR Collaboration observed the scalar resonance a 0 (1710) in the π ± η invariant mass spectrum of the process η c → ηπ + π − [1].In 2022, the BE-SIII Collaboration also found the a 0 (1710) state in the K 0 S K 0 S invariant mass spectrum of the process D + s → K 0 S K 0 S π + [2], and in the K 0 S K + invariant mass spectrum of the process D + s → K 0 S K + π 0 [3].The experimental measurements of the mass and width of a 0 (1710) are tabulated in Table I.One can see that there are some discrepancies between the measured masses.Note that in Ref. [2], BESIII did not distinguish between the a 0 (1710) and f 0 (1710) in the process D + s → K 0 S K 0 S π + , and denoted the combined state as S(1710), while in Ref. [3] the a 0 (1710) was renamed as a 0 (1817) because of the different fitted mass of this state.
It should be stressed that there have been many theoretical studies about the structure of the a 0 (1710) and its isospin partner f 0 (1710) from various perspectives [4][5][6][7][8][9][10][11][12][13][14][15][16].For the f 0 (1710), although it is a well-established state according to the Review of Particle Physics (RPP) [17], there are still different interpretations of its structure.In Ref. [12], it was shown that the f 0 (1710) wave function contains a large ss component, while in Refs.[13][14][15][16], it was regarded as a scalar glueball.In addition, the f 0 (1710) and a 0 (1710) states could be dynamically generated from the vector-vector interactions [18,19], and this picture remains essentially the same when the pseudoscalar-pseudoscalar coupled-channels were taken into account [20].In Ref. [21], one isovector scalar state a 0 with a mass of 1744 MeV is also predicted in the approach of Regge trajectories, which is roughly consistent with the experimental mass of the a 0 (1710).As shown in Table I, the mass of the a 0 (1710) is not well determined experimentally.This can complicate the understanding of the nature of the a 0 (1710).For instance, a 0 (1710) (or a 0 (1817)) and X(1812) have been explained as the 3 3 P 0 state by assuming a 0 (980) and f 0 (980) as 1 3 P 0 states [22].Indeed, X(1812) was observed in the process J/ψ → γϕω by the BESIII Collaboration [23,24], and the enhancement near the ϕω threshold, associated to X(1812), could be described by the reflection of f 0 (1710), as discussed in Ref. [8].By regarding the a 0 (1710) as a K * K * molecular state, Refs.[25][26][27][28][29] have successfully described the invariant mass distributions of the processes D + s → K 0 S K 0 S π + and D + s → K 0 S K + π 0 measured by the the BESIII Collaboration .
Since the peak positions of the a 0 (1710) in the K K invariant mass distributions of the processes D + s → K 0 S K 0 S π + , K 0 S K + π 0 observed by the BESIII Collaboration are very close to the boundary region of the K K invariant mass, it is crucial to measure the properties of the a 0 (1710) precisely in other processes with larger phase space [30].Taking into account that the dominant decay channel of the a 0 (1710) is K K in the molecular picture [18,20], we propose to search for this state in the process η c → K0 K + π − .Indeed, there have been some experimental studies of this process.In 2012, the BESIII Collaboration has measured the branching fraction with a sample of 106 million ψ(3686) events [31].In 2019, the BESIII Collaboration measured the branching fraction of this process  33,34], and the measured K 0 S K + mass spectrum shows some structure in the region of 1.7∼1.8GeV, which could hint at the existence of the a 0 (1710), as we show in this work.
Based on the BABAR data [33,34], we will investigate the process η c → K0 K + π − .In addition to the contribution from the scalar resonance a 0 (1710), we also take into account the contribution from the intermediate resonance K * 0 (1430), which plays an important role in this process according to Refs.[33,34].
The paper is organized as follows.In Sec.II, we present the theoretical formalism of the η c → K0 K + π − decay, and in Sec.III, we show our numerical results and discussions, followed by a short summary in the last section.

II. FORMALISM
First in Subsect.II A we present the theoretical formalism for the process η c → K0 K + π − via the K * K * and ωρ final state interactions, which dynamically generate the scalar resonance a 0 (1710) .Next, we show the formalism for the process of η c → K * 0 (1430 Finally, the formalism for the double differential widths of the process With the assumption that the η c is a singlet of SU(3), and a 0 (1710) is a vector-vector molecular state [18,19], one needs to first produce the vector-vector pairs in the η c decay.Considering that this process has a π − in the final states, we introduce one combination mode of <V V P > in the primary vertex [35,36], where V and P are the SU(3) vector and pseudoscalar matrices respectively [35][36][37][38][39], where the η −η ′ mixing is assumed according to Ref. [40].
The symbol '<>' stands for the trace of the SU(3) matrices.One could obtain the relevant contributions by isolating the terms containing π − , as follows, In the molecular picture, the a 0 (1710) is dynamically generated from the S-wave K * 0 K * + and ωρ + final-state interactions [18,19], and then decays into the final states K0 K + , as depicted in Fig. 1.The decay amplitude of Fig. 1 can be written as, where V P is the normalization factor, and t K * 0 K * + → K0 K + and t ωρ + → K0 K + are the transition amplitudes.
The loop functions G K * 0 K * + and G ωρ + are for the K * 0 K * + and ωρ + channels, respectively, and read [18,41], where with the Källen function λ(x, y, z) = x 2 + y 2 + z 2 − 2xy − 2xz − 2yz.Here, we consider the decay channels ππ and Kπ for the vector mesons ρ and K * , respectively, and neglect the contribution from the small width (Γ ω = 8.68 MeV) of ω.Taking the vector K * for example, Similarly, one can obtain m 2 1+ and m 2 1− for the ρ.The masses, widths, and spin-parities of the involved particles are taken from the RPP [17], and listed in Table II.The loop function G of Eq. ( 5) is for stable particles, and in the dimensional regularization scheme it can be written as [18], with where a µ is the subtraction constant, µ is the dimensional regularization scale, and s = M 2 K0 K + .We take a µ = −1.726and µ = 1000 MeV as used in Ref. [18].It is worth mentioning that any change in µ could be reabsorbed by a change in a µ through a µ ′ −a µ = ln(µ ′2 /µ 2 ), which implies that the loop function G is scale independent [42].
In order to show the influence of the widths of vector mesons on the loop functions, we calculate the loop function G ωρ and Gωρ as functions of the K0 K + invariant mass, and show them in Fig. 2. The blue long-dashed and red dot-dashed curves correspond to the real and imaginary parts of the loop function G considering the width of ρ, respectively.While, the green solid and purple dotted curves correspond to the real and imaginary parts of the loop function G without the contribution from the ρ width, respectively.One can see that the loop function G, considering the width of the vector meson, becomes smoother around the threshold.TABLE III.Mass, width, and coupling constants of the scalar a0(1710) [18].g K * K * , gωρ, and g K K stand for the coupling constants of a0(1710) to the K * K * , ωρ, and K K channels, respectively, while Γ K K denotes the partial decay width of the a0(1710) → K K.All values are in units of MeV.
On the other hand, the transition amplitudes t K * 0 K * + /ωρ + → K0 K + in Eq. ( 4) could be obtained from the coupled-channel approach in Ref. [10], where one state a 0 with mass around 1760 MeV could be dynamically generated from the ηπ, KK, ωρ, ϕρ, and K * K * interactions within SU(6) spin-flavor symmetry.However, the width of a 0 is about 24 MeV, much smaller than the one for the a 0 (1710) resonance as quoted in the PDG [17].On the other hand, it is customary to obtain the coupling constants and the pole position of the dynamically generated state by fitting the Breit-Wigner form to the amplitude of the coupled-channel approach around the pole position, where g i,j are the couplings to channel i (j).It implies that the amplitude of the Breit-Wigner form with the same position and couplings should give similar behavior around the pole position.Thus, we take the transition amplitude as, where M a0 and Γ a0 are the mass and width of the a 0 (1710), respectively, and we take their values from Refs.[18,43], which are tabulated in Table III.g K * K * , g ωρ , and g K K are the coupling constants of the vertices K * K * /ωρ → a 0 (1710) 1 and a 0 (1710) → K K, respectively, whose values are determined in Ref. [18].We determine the coupling g K K from the partial decay width of a 0 (1710 where ⃗ p K is the three momentum of the K or K meson in the a 0 (1710) rest frame, With the partial decay width Γ K K = 36 MeV [18], one can only obtain the absolute value of the coupling constant, but not the phase, thus we assume that g K K is real and positive in this work, as done in Refs.[25,26]. 1 The couplings of a 0 (1710) to the channels K * K * and ωρ are obtained at the pole position [18].In this work, we take the coupling to be complex, and don't consider the extra phase interference between the coupled-channels K * K * and ωρ.Gωρ/ Gωρ Real and imaginary parts of the loop functions Gωρ and Gωρ as a function of the K0 K + invariant mass.
The decay amplitude for η c → K + K * 0 (1430) − → K0 K + π − of Fig. 3 can be written as where M K0 π − is the invariant mass of the K0 π − system, and The mass and width of the K * 0 (1430) are given in Table II.
1430) 0 , followed by the decay K * 0 (1430 The coupling constants appearing in Eqs. ( 16) and ( 17) could be determined from the experimental partial decay widths of η c → KK * 0 (1430) and K * 0 (1430) → Kπ, respectively.The effective Lagrangians accounting for the vertices of η c → KK * 0 (1430) and K * 0 (1430) → Kπ are given by [44], With the above effective Lagrangians, we can express the corresponding partial decay widths as, where P is the three-momentum of the two final-state particles in the rest frame of the parent particle, which reads, and M and m 1,2 are the masses of the initial parent particle and the two final-state mesons, respectively.The masses and widths of these particles are given in Table II.
According to the RPP [17], the branching fraction of K * 0 → Kπ is B(K * 0 → Kπ) = (93 ± 10)%, and we take it to be 100% in this work.One can then easily obtain the coupling constant g K * 0 Kπ = 4721 MeV.In addition, with the branching fraction B(η c → K0 K + π − ) = (2.4 ± 0.2)% [45] and the ratio of 2)% [33] , we could estimate the branching fraction Then, we can determine the coupling constants g ηcK + K * − 0 = g ηc K0 K * 0 0 = 180 MeV.It is worth mentioning that the coupling constants appearing in Eqs. ( 16) and ( 17) are assumed to be real and positive, and the values of those coupling constants are listed in Table IV.

C. Invariant mass distributions
With the amplitudes obtained above, we can write down the total decay amplitude of η c → K0 K + π − as follows, and the double differential widths of the process Furthermore, one can easily obtain dΓ/dM K0 K + , dΓ/dM K0 π − , and dΓ/dM K + π − by integrating over each of the invariant mass variables with the limits of the Dalitz plot given in the RPP [17].For example, the upper and lower limits for M K0 K + are: , where E * K + and E * K0 are the energies of K + and K0 in the K0 π − rest frame, respectively, FIG. 5. K0 K + invariant mass distribution of the process ηc → K0 K + π − .The red-solid curve stands for the total contributions, while the blue-dashed curve, the green-dot-dashed curve, and purple-dotted curve correspond to the contribution from the a0(1710) state, the intermidiate K * 0 (1430) − , and K * 0 (1430) 0 , respectively.The BABAR data are taken from Fig. 7(a) of Ref. [33].

III. RESULTS AND DISCUSSION
It should be pointed out that the K 0 S K + invariant mass distribution of the process η c → K 0 S K + π − has been measured by the BABAR Collaboration [33].In this work, we take V p = 0.8 in order to match with the BABAR measurements of the K 0 S K + invariant mass distribution around 1.6 ∼ 2.1 GeV.In Fig. 5, we show our results of the K0 K + invariant mass distribution, where the red-solid curve stands for the total contribution from the a 0 (1710) state and the vector K * 0 meson, 0 3 6 9 12 0.9 1.4 1.9 2.4 2.9 7. K0 K + invariant mass distribution of the process ηc → K0 K + π − obtained with a phase angle ϕ = 0, π/3, 2π/3, and π, respectively.See the text for details.
while the blue-dashed curve corresponds to the contribution from the a 0 (1710) state.Moreover, the green-dotdashed and purple-dotted curves stand for the contributions from the intermediate K * 0 (1430) − and K * 0 (1430) 0 , respectively.We also show the BABAR data points in the region of 1.6 ∼ 2.1 GeV 2 , which has been multiplied by an overall normalization factor 4 × 10 −7 [33].As one can see from Fig. 5, the contributions from the K * 0 (1430) are smooth in the region of 1.4 ∼ 2.4 GeV.In particular, we note that the dip structure around 1800 MeV is in agreement with the BABAR measurement [33].This dip structure is mainly due to the interference between the contributions from the a 0 (1710) and the K * 0 (1430), and should be associated to the scalar a 0 (1710).
In order to show the dependence of our results on the parameter V p , we present the K0 K + invariant mass distribution of the process η c → K0 K + π − with the parameter V p = 0.6, 0.8, 1.0 in Fig. 6.One can see that the dip structure around 1.8 GeV persists, which is in agreement with the BABAR measurements [33], labeled as 'BABAR 2016'.It should be stressed that the BABAR Collaboration has also measured the K + K − invariant mass distribution of the process η c → K + K − π 0 , as shown by the data of 'BABAR 2014' in Fig. 6, where one dip structure also appears around 1.8 GeV [46].
However, it should be pointed out that the dip structure appearing in the K0 K + invariant mass distribution of Fig. 5 could also manifest itself as a peak structure if the interference between M a , M b , and M c are different from our naive assignments explained above.For instance, if we multiply the term M a of Eq. ( 23) by a phase 2 As pointed out in Ref. [33], for the ηc → K0 K + π − decay, some other resonances also contribute, such as the a 0 (980), a 0 (1450), a 0 (1950), and a 2 (1320).Since in this work we focus on the possible signal of the a 0 (1710), only the BABAR data in the region of 1.6 ∼ 2.1 GeV are presented in Figs. 5 and 6. factor e iϕ with ϕ = 0, π/3, 2π/3, and π, we would obtain the K0 K + invariant mass distributions shown in Fig. 7, where one can see a peak structure around 1.8 GeV for ϕ = 2π/3 and π.
In Fig. 9, we present the Dalitz plots for the process η c → K0 K + π − with the parameter V p = 0.8.From Figs. 9(a) and 9(b), we can clearly see that there is a vertical blue band around M K0 K + = 1.8 GeV, which should be associated with the signal of the scalar a 0 (1710), and we also find a yellow band around M K0 π − /K + π − = 1.43 GeV, corresponding to the signal of the K * 0 (1430) state.From Fig. 9(c), can see that most events of the process η c → K0 K + π − will appear in the region around M K0 π − = 1.43 GeV and M K + π − = 1.43 GeV, which is in agreement with the BABAR measurements (see Fig. 4 of Ref. [33]).
Finally, we predict the branching fractions of the processes η c → K * 0 K * + π − and η c → ωρ + π − , which have not yet been measured.Without the contributions from intermediate resonances, based on Eq. ( 3) the amplitudes for the processes η c → K * 0 K * + π − and η c → ωρ + π − are, where ⃗ ϵ i is the polarization of the vector meson, and [47].With the parameter V p = 0.8, we could estimate the branching fractions of these two processes, where the formalism of the differential width of the threebody decay could be found in the RPP [17].We note that our prediction for B(η c → K * 0 )% [17], which seem reasonable.
The BESIII Collaboration has collected 10 billion J/ψ events and 3 billion ψ(3686) events, and the available η c events via the decays of J/ψ → γη c and ψ(3686) → γη c are recently proposed to precisely measure the η c decay modes [45], which could be helpful to search for the possible signal of the a 0 (1710), and test our theoretical predictions.The η c → K0 K + π − reaction could be a good platform to investigate the a 0 (1710), especially its mass.
It should be stressed that one can not exclude the other interpretations based the present experimental information.In Ref. [48], the authors have studied the coupledchannels influence on the a 0 (1710) line shape by assuming it as four-quark state in the MIT bag model, and found that the strong couplings of a 0 to V V channel can narrow the a 0 peak in the P P mass spectra, and the a 0 width could be 150 ∼ 300 MeV in the absence of K K and πη channels.It is suggested to detect the a 0 (1710) → V V decay directly to test their results in Ref. [48].

IV. SUMMARY
Assuming the a 0 (1710) as a K * K * molecular state, we have investigated the process η c → K0 K + π − taking into account the contribution from the S-wave ωρ + and K * 0 K * + interactions, as well as the contribution from the intermediate resonance K * 0 (1430).We predicted one dip structure around 1.8 GeV in the K0 K + invariant mass distribution, which is in agreement with the BABAR measurements [33].It should be pointed out that a similar dip structure also appears around 1.8 GeV in the K + K − invariant mass distribution of the process η c → K + K − π 0 of the BABAR measurements [46].Furthermore, we predicted the K + π − and K0 π − invariant mass distributions of the process η c → K0 K + π − , and found clear peaks of the resonance K * 0 (1430) 0,− , consistent with the BABAR measurements [33].In addition, we have also plotted the Dalitz plots of the process η c → K0 K + π − , and shown the possible signals of the a 0 (1710) and K * 0 (1430).Finally, we have estimated the branching fractions B(η c → K * 0 K * + π − ) = 5.5×10 −3 and B(η c → ωρ + π − ) = 7.9 × 10 −3 , which are reasonable by comparing with the experimental data.Our theoretical predictions could be tested by the BESIII and Belle II experiments in the future, and the precise measurements of the process η c → K0 K + π − could shed light on the nature of the scalar a 0 (1710).

TABLE I .
Experimental measurements on the mass and width of the scalar state a0(1710).The first error is statistical and the second one is systematic.All values are in units of MeV.

TABLE II .
Masses, widths, and spin-parities of the involved particles in this work.All values are in units of MeV.