# Discovery Potential for the Neutral Charmonium-LikeZ0(4200) by p-p Annihilation

Authors: Xiao-Yun Wang; Xu-Rong Chen
Journal: Advances in High Energy Physics
 (2015)

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## Abstract

Inspired by the observation of charmonium-likeZ(4200), we explore the discovery potential of the neutral Z0(4200) production by antiproton-proton annihilation with an effective Lagrangian approach. By investigating the p-p→J/ψπ0 process including the Z0(4200) signal and background contributions, it is found that the center of mass energy Ec.m. ≃ 4.0–4.5 GeV is the best energy window for searching the neutral Z0(4200), where the signal can be clearly distinguished from background. The relevant calculations not only are helpful to search for the neutral Z0(4200) in the future experiment but also will promote the understanding of the nature and production mechanism of neutral Z0(4200) better.


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## Body

## 1. Introduction

In the conventional constituent quark model (CQM), all mesons can be described as quark-antiquark (qq¯) states. Such as the charmonium state J/ψ, it is composed of a cc¯ quark pair. However, a series of charmonium-like states (including X(3872), Y(4260), Y(4008), Z(3900), Z(4025), Z(4020), Z(4050), Z(4250), Z(4430), and Z(4200)) have been observed in experiments [1–5], many of which do not fit into the qq¯ meson spectrum in the classical CQM. In particular, some charged charmonium-like Z states are considered to have a minimal quark content of cc¯ud¯ (Z+) or cc¯u¯d (Z-), which have attracted great interest in experimental and theoretical aspect [1–11]. These charmonium-like Z states are interpreted as a tetraquark, a hadron molecule or just a cusp effect, and so on [1, 7–11]. In addition, a series of hidden-charm baryons also have been investigated in [12–14], which were considered as the pentaquark candidates and composed of cc¯qqq. On the one hand, these studies enriched the picture of exotic states. On the other hand, it should be noted that our understanding of these exotic states is far from being sufficient. Thus, more theoretical and experimental works are needed.Recently, a new charged charmonium-likeZ+(4200) decaying to J/ψπ+ has been observed by the Belle Collaboration with a significance of 6.2σ [5]. The relevant experiment results show that the spin-parity quantum number of Z+(4200) favors 1+. Furthermore, its mass and width are measured to be [5] (1)M=4196-29-13+31+17MeV/c2,Γ=370-70-132+70+70MeV.It is noticed that the Z+(4200) has the largest width among those charmonium-like Z states which have been observed in experiment. In [7], by analyzing all the available experimental information about charmonium-like states within the framework of the color-magnetic interaction, the Z(4200) was supposed to be a very promising candidate of the lowest axial-vector hidden-charm tetraquark state and its dominant decay should be J/ψπ. In [8–10], the relevant results also support the tetraquark interpretation of Z(4200) in the frame of QCD sum rule. Moreover, with QCD sum rule approach, the Z(4200) was described as the molecule-like state in [11]. These experimental and theoretical results indicate that the Z(4200) is an ideal candidate for investigating and understanding the nature of exotic charmonium-like states.As of now, the charmonium-like states are only observed in four ways including theγγ fusion process, e+e- annihilation, B meson decay, and hidden-charm dipion decays of higher charmonia or charmonium-like states. Obviously, it is an important topic to study the production of the charmonium-like state in different processes. As mentioned above, Z(4200) was only observed in the B meson decay. It is natural to ask whether Z(4200) can be found in other processes.In addition, we note that searching for the neutral partner of the charmonium-like or bottomonium-like state also aroused the great interest in both experiment and theory. The first observation of neutralZc(3900) has been reported by CLEO-c [15]. Later, the neutral bottomonium-like Zb0(10610) and charmonium-like Zc0(4020) have been observed by Belle and BESIII [16, 17], respectively. In theory, the production of the neutral Z0(4430) in p¯p→ψ′(2s)π0 reaction was investigated in our previous work [18]. These studies not only help in confirming these charmonium-like states, but also open a window to investigate the nature and production mechanism of exotic state beyond the conventional qq¯ states.Since theZ(4200) was observed in J/ψπ channel [5] and its dominant decay mode is very likely to be J/ψπ [7], the neutral Z0(4200) should has a coupling with J/ψπ0. Besides, the tetraquark or molecule-like state can be regard as a general four-quark state [10], which means that the neutral Z0(4200) probably is composed of cc¯uu¯ or cc¯dd¯. According to the OZI rule [19–21], one can speculate that the partial decay width of Z0(4200)→p¯p may be larger than that of J/ψ→p¯p. Therefore, the p¯p→J/ψπ0 reaction is probably an ideal channel for searching and studying the neutral Z0(4200).In this work, with an effective Lagrangian approach, the production of neutralZ0(4200) in p¯p→J/ψπ0 reaction is investigated for the first time. Furthermore, in light of the situations of PANDA detector at FAIR@GSI [22–24], the feasibility of searching the neutral Z0(4200) by p¯p annihilation is discussed, which can provide valuable information to future experimental exploration of neutral Z0(4200).This paper is organized as follows. After an introduction, we present the investigated method and formalism forZ0(4200) production. In Section 3, the background contributions to the J/ψπ0 final states are discussed. The numerical results are given in Section 4. Finally, This paper ends with the discussion and conclusion.


## 2. TheZ0(4200) Yield by the Antiproton-Proton Scattering

It will be difficult to studyZ0(4200) at quark-gluon level in the now energy range. Therefore, the effective Lagrangian method in terms of hadrons will be used in our research.


### 2.1. Feynman Diagrams and Effective Interaction Lagrangian Densities

The basic tree level Feynman diagram for the production ofZ0(4200) in p¯p→J/ψπ0 reaction through s-channel is depicted in Figure 1. For the Z(4200), its quantum number of spin-parities has been determined by Belle Collaboration to be JP=1+ [5]. Therefore, the relevant effective Lagrangian for the vertices of Zp¯p and Zψπ (for simplicity, we use Z and ψ to denote Z0(4200) and Jψ, resp.) read as [25]:(2)LZp¯p=gZp¯pϕ¯p¯γμγ5ϕpZμ,LZψπ=gZψπMZ∂μψν∂μπZν-∂μψν∂νπZμ,where Z, ψ, and ϕ denote the fields of Z(4200), J/ψ, and nucleon, respectively. The gZp¯p and gZψπ are the coupling constants. Considering the size of the hadrons, we introduce the general form factor for the intermediate Z0(4200) as used in [26–28]:(3)FZq2=ΛZ4ΛZ4+q2-MZ22,where q, MZ, and ΛZ are the 4-momentum, mass, and cut-off parameters for the intermediate Z0(4200), respectively.Figure 1
The production ofZ0(4200) through pp¯ collision.


### 2.2.  Coupling Constants and the OZI Analysis in the Process ofZ0(4200)→p¯p

With the effective Lagrangians above, the coupling constantsgZp¯p and gZψπ can be determined by the partial decay widths ΓZ0(4200)→p¯p and ΓZ0(4200)→J/ψπ0, respectively:(4)ΓZ04200→p¯p=gZp¯p223πMZ2p→Nc.m.3,ΓZ04200→J/ψπ0=gZψπMZ2p→πc.m.24πMZ2·MZ2-mψ2-mπ222+mψ2Eπ2,with(5)p→Nc.m.=λ1/2MZ2,mp¯2,mp22MZ,p→πc.m.=λ1/2MZ2,mψ2,mπ22MZ,Eπ=p→πc.m.2+mπ2,where λ is the Källen function with λ(x,y,z)=(x-y-z)2-4yz.As of now, no relevant experiment data aboutΓZ(4200)→p¯p and ΓZ(4200)→J/ψπ are available [29]. However, in [10, 11] the decay widths ΓZ(4200)→J/ψπ=87.3±47.1MeV or ΓZc(4200)→J/ψπ=24.6MeV were obtained with QCD sum rule by assuming that the Z(4200) is a tetraquark state or molecule-like state, respectively. Thus we get the coupling constants gZψπ/Mz=1.73, 0.918 GeV-1, which correspond to the partial decay width ΓZ(4200)→J/ψπ=87.3, 24.6MeV.For the partial decay width ofZ0(4200)→p¯p, we try to obtain it by analyzing and comparing with the OZI suppressed process of J/ψ→p¯p. According to constituent quark model, the traditional charmonium J/ψ is regarded as a pure cc¯ state, while there are only up and down quarks (antiquarks) in the proton (antiproton). Thus the J/ψ→p¯p decay is actually a disconnected process and at least needs three gluons to connect it (as shown in Figure 2(a)). In the frame of the Okubo-Zweig-Iizuka (OZI) rule [19–21], the incidence of J/ψ→p¯p process is greatly suppressed. With the total decay width and branch ratios of J/ψ listed in PDG [29], one get the partial decay width ΓJ/ψ→p¯p≃0.2keV, which is indeed a small value and consistent with the prediction by OZI rule [19–21]. Since the neutral Z0(4200) may have minimal quark content of (cc¯uu¯) or (cc¯dd¯), as seen in Figure 2(b), the Z0(4200)→p¯p reaction is a connected whole and it is an OZI allowed process [19–21]. Therefore, in principle, the probability of Z0(4200) decay to p¯p should be higher than J/ψ→p¯p, which may be part of reason that the total decay width of Z(4200) is 3 orders larger than the total width of J/ψ. Accordingly we speculate that the partial width ΓZ0(4200)→p¯p may well be at least three magnitudes larger than ΓJ/ψ→p¯p. Then we get gZp¯p≃0.05 by taking ΓZ0(4200)→p¯p=200keV.Figure 2
The quark level diagram depictingJ/ψ→p¯p decay (a) and Z0(4200)→p¯p decay (b).
(a)
(b)


### 2.3. Amplitude

Following the Feynman rules and using the above Lagrangian densities, we can obtain the invariant amplitudeMZsignal for the p¯(p1)p(p2)→J/ψ(p3)π0(p4) reaction through s-channel as shown in Figure 1:(6)MZsignal=gZp¯pgZψπFZq2Mzενp3·p3·p4gμν-p3μp4ν·GZμαqv¯p1γαγ5up2,where GZμα are the propagators of the Z0(4200), taking the Breit-Wigner form [30]:(7)GZμαq=iP1q2-MZ2+iMZΓZ.Here(8)P1=∑spinsεμ·εα∗=g~μαq=-gμα+qμqαq2is the projection operator for the state with spin-1.


## 2.1. Feynman Diagrams and Effective Interaction Lagrangian Densities

The basic tree level Feynman diagram for the production ofZ0(4200) in p¯p→J/ψπ0 reaction through s-channel is depicted in Figure 1. For the Z(4200), its quantum number of spin-parities has been determined by Belle Collaboration to be JP=1+ [5]. Therefore, the relevant effective Lagrangian for the vertices of Zp¯p and Zψπ (for simplicity, we use Z and ψ to denote Z0(4200) and Jψ, resp.) read as [25]:(2)LZp¯p=gZp¯pϕ¯p¯γμγ5ϕpZμ,LZψπ=gZψπMZ∂μψν∂μπZν-∂μψν∂νπZμ,where Z, ψ, and ϕ denote the fields of Z(4200), J/ψ, and nucleon, respectively. The gZp¯p and gZψπ are the coupling constants. Considering the size of the hadrons, we introduce the general form factor for the intermediate Z0(4200) as used in [26–28]:(3)FZq2=ΛZ4ΛZ4+q2-MZ22,where q, MZ, and ΛZ are the 4-momentum, mass, and cut-off parameters for the intermediate Z0(4200), respectively.Figure 1
The production ofZ0(4200) through pp¯ collision.


## 2.2.  Coupling Constants and the OZI Analysis in the Process ofZ0(4200)→p¯p

With the effective Lagrangians above, the coupling constantsgZp¯p and gZψπ can be determined by the partial decay widths ΓZ0(4200)→p¯p and ΓZ0(4200)→J/ψπ0, respectively:(4)ΓZ04200→p¯p=gZp¯p223πMZ2p→Nc.m.3,ΓZ04200→J/ψπ0=gZψπMZ2p→πc.m.24πMZ2·MZ2-mψ2-mπ222+mψ2Eπ2,with(5)p→Nc.m.=λ1/2MZ2,mp¯2,mp22MZ,p→πc.m.=λ1/2MZ2,mψ2,mπ22MZ,Eπ=p→πc.m.2+mπ2,where λ is the Källen function with λ(x,y,z)=(x-y-z)2-4yz.As of now, no relevant experiment data aboutΓZ(4200)→p¯p and ΓZ(4200)→J/ψπ are available [29]. However, in [10, 11] the decay widths ΓZ(4200)→J/ψπ=87.3±47.1MeV or ΓZc(4200)→J/ψπ=24.6MeV were obtained with QCD sum rule by assuming that the Z(4200) is a tetraquark state or molecule-like state, respectively. Thus we get the coupling constants gZψπ/Mz=1.73, 0.918 GeV-1, which correspond to the partial decay width ΓZ(4200)→J/ψπ=87.3, 24.6MeV.For the partial decay width ofZ0(4200)→p¯p, we try to obtain it by analyzing and comparing with the OZI suppressed process of J/ψ→p¯p. According to constituent quark model, the traditional charmonium J/ψ is regarded as a pure cc¯ state, while there are only up and down quarks (antiquarks) in the proton (antiproton). Thus the J/ψ→p¯p decay is actually a disconnected process and at least needs three gluons to connect it (as shown in Figure 2(a)). In the frame of the Okubo-Zweig-Iizuka (OZI) rule [19–21], the incidence of J/ψ→p¯p process is greatly suppressed. With the total decay width and branch ratios of J/ψ listed in PDG [29], one get the partial decay width ΓJ/ψ→p¯p≃0.2keV, which is indeed a small value and consistent with the prediction by OZI rule [19–21]. Since the neutral Z0(4200) may have minimal quark content of (cc¯uu¯) or (cc¯dd¯), as seen in Figure 2(b), the Z0(4200)→p¯p reaction is a connected whole and it is an OZI allowed process [19–21]. Therefore, in principle, the probability of Z0(4200) decay to p¯p should be higher than J/ψ→p¯p, which may be part of reason that the total decay width of Z(4200) is 3 orders larger than the total width of J/ψ. Accordingly we speculate that the partial width ΓZ0(4200)→p¯p may well be at least three magnitudes larger than ΓJ/ψ→p¯p. Then we get gZp¯p≃0.05 by taking ΓZ0(4200)→p¯p=200keV.Figure 2
The quark level diagram depictingJ/ψ→p¯p decay (a) and Z0(4200)→p¯p decay (b).
(a)
(b)


## 2.3. Amplitude

Following the Feynman rules and using the above Lagrangian densities, we can obtain the invariant amplitudeMZsignal for the p¯(p1)p(p2)→J/ψ(p3)π0(p4) reaction through s-channel as shown in Figure 1:(6)MZsignal=gZp¯pgZψπFZq2Mzενp3·p3·p4gμν-p3μp4ν·GZμαqv¯p1γαγ5up2,where GZμα are the propagators of the Z0(4200), taking the Breit-Wigner form [30]:(7)GZμαq=iP1q2-MZ2+iMZΓZ.Here(8)P1=∑spinsεμ·εα∗=g~μαq=-gμα+qμqαq2is the projection operator for the state with spin-1.


## 3. The Background Analysis and Cross Section

Figure3 shows the p¯(p1)p(p2)→J/ψ(p3)π0(p4) process through t-channel (a) and u-channel (b) by exchanging a proton, which can be regarded as the main background contributions for the production of Z0(4200) as described in Figure 1.Figure 3
Feynman diagrams for the nucleon exchange inp¯p→J/ψπ0 reaction.
(a)
(b)The Lagrangian densities for the vertices ofp¯pπ0 and J/ψp¯p read as [31](9)Lp¯pπ0=-igp¯pπϕ¯p¯γ5τ·πϕp,Lp¯pψ=-gp¯pψϕ¯p¯γμϕpψμ,where ψ and ϕ denote the fields of J/ψ and nucleon, respectively, while τ is Pauli matrix.The coupling constantgp¯pπ=13.5 is adopted [32], while coupling constant gp¯pψ is determined by partial decay widths:(10)Γψ→p¯p=gp¯pψ2p→c.m.6πmψ2mψ2+2mp2with(11)p→c.m.=mψ2-4mp22.Thus we get gp¯pψ≃1.6×10-3, which is calculated by the measured branching fractions and total widths of J/ψ (mψ=3096.916MeV and Γψ=92.9keV) [29].The monopole form factors for thep¯pπ0 and J/ψp¯p vertices are introduced as the same as Bonn potential model [33]:(12)Fqi2=ΛN2-mp2ΛN2-qi2,i=t,u,where ΛN, mp, and qi (qt=(p3-p1) and qu=(p4-p1)) are the cut-off parameter, mass, and four momentums of the exchanged proton, respectively.According to the Feynman rules and the above equations, the full invariant amplitudeMN=MNt+MNu for the background as depicted in Figure 3 can be obtained:(13)MN=MNt+MNu=gp¯pπgp¯pψv¯p¯p1·γμεμp3qt+mpqt2-mp2γ5F2qt2+γ5qu+mpqu2-mp2γμεμp3F2qu2upp2.With the amplitudes listed in (6) and (13), we get the square of the total invariant amplitude (in principle, the interference between amplitudes for the signal and the nonresonant background should be considered; since we do not now have experimental data, we take the relative phase between different amplitudes as zero in the present work; thus the total cross section for the p¯p→J/ψπ0 process obtained by us is an upper estimate):(14)M2=∑MZsignal+MN2.We define s=q2=(p1+p2)2, and then the unpolarized differential cross section for the reaction p¯(p1)p(p2)→J/ψ(p3)π0(p4) at the center of mass (c.m.) frame is as follows:(15)dσdcos⁡θ=132πsp→3c.m.p→1c.m.14∑spinsM2,where p→1c.m. and p→3c.m. are the three momentums of initial antiproton and final J/ψ, while θ denotes the angle of the outgoing J/ψ meson relative to the antiproton beam direction in the c.m. frame. The total cross section can be easily obtained by integrating the above equation.


## 4. Numerical Results and Discussion

With the formalisms and equations determined above, we calculate the total and differential cross section including both signal and background contributions as presented in Figures4–7.Figure 4
The energy dependence of cross section for the production ofZ0(4200) through s-channel with the different typical cut-off ΛZ. Here, the partial decay width is taken as ΓZ(4200)→J/ψπ=87.3MeV.Figure 5
The contribution of background from the proton exchange forp¯p→J/ψπ0 reaction though t-channel and u-channel with different typical cut-off ΛN. The data are taken from [34–36].Figure 6
The energy dependence of the total cross sections for the process ofp¯p→J/ψπ0 with two typical values of Γi, where Γi denotes the decay width of Z(4200)→J/ψ. Here, the σTotal and σSignal are the total cross section and the cross section of signal, respectively.Figure 7
The differential cross sections for the process ofp¯p→Jψπ0 at different center of mass energy Ec.m.=4.0, 4.2, 4.5, and 5 GeV, where the “Total” denotes the differential cross section including both signal and background contributions. Here, the partial decay width is taken as ΓZ(4200)→J/ψπ=87.3MeV.
(a)
(b)
(c)
(d)In these calculations, we note that the cut-off parameter related to the form factor is the only free parameter. Therefore, first we need to discuss the effect of cut-off parameter on cross section of signal and background.We present the variation of the cross section from thes-channel signal contribution for p¯p→J/ψπ0 reaction with different cut-off parameters ΛZ as shown in Figure 4, where ΛZ is taken as 1.0–3.0 GeV with the step of 1.0 GeV.One notices that there is an obvious peak structure at center-mass energyEc.m.≃4.2 GeV which is near the threshold of Z(4200). Moreover, the cross section of signal increases with the increasing of cut-off parameter ΛZ, but at a modest rate. In particular, in the range of 4.0GeV≲Ec.m.≲4.5GeV, it is found that the cross sections from signal contributions are not sensitive to the cut-off parameter ΛZ. We take typical value ΛZ=1.0 GeV in the following, which can ensure the cross sections of signal are limited to a smaller value.In Figure5, we illustrate the proton exchange contributions with different cut-off parameters ΛN, which is obvious that the cross sections from background contributions are sensitive to the values of the cut-off ΛN. Fortunately, the reaction p¯p→J/ψπ0 has been measured by the E760 and E835 experiment at low energy [34–36], which can help us to constrain the cut-off parameter ΛN. From Figure 5 it can be found that the numerical results from the proton exchange contributions are consistent with the E760 and E835 data by taking ΛN=1.9 and 3.0 GeV, respectively. Based on the consideration of seeking a larger limit for the cross section of background, we take ΛN=3.0 GeV in the next calculation. Besides, we notice that the amplitude estimate of p¯p→J/ψπ0 is about 0.3 nb at Ec.m.=3.5–3.6 GeV in [37]. This value is closer to the E835 data [35, 36] if we consider its uncertainty. Thus we taking ΛN=3.0 GeV in our calculation should be reasonable.Figure6 shows the total cross sections for p¯p→J/ψπ0 reaction including both signal and background contributions by taking ΛZ=1.0 GeV and ΛN=3.0 GeV. We notice that the cross section of Z0(4200) production goes up very rapidly and has a peak around Ec.m.≃4.2 GeV. Besides, it is found that the contributions from the signal are dominant in the region of 4.0 GeV ≲Ec.m.≲4.5 GeV. Naturally, we can conclude that 4.0 GeV ≲Ec.m.≲4.5 GeV is the best energy window for searching the neutral charmonium-like Z0(4200) in experiment, in which the signal can be clearly distinguished from background. Around the center of mass Ec.m.≃4.2 GeV, the total cross section from signal and background contributions is on the order of 0.14 μb and 0.04 μb, which correspond to the decay width ΓZ0(44200)→J/ψπ0=87.3MeV and ΓZ0(44200)→J/ψπ0=24.6MeV, respectively.As mentioned above, the PANDA detector at FAIR [22–25] is an ideal platform searching for the Z0(4200) by p¯p collision. With a p¯ beam of 15 GeV/c [22–25] one has Ec.m.=5.47 GeV, which allows one to observe charmonium-like Z0(4200) state in J/ψπ0 production up to a mass MZ≃4.2 GeV. Assuming the integrated luminosity of PANDA can reach up to 1.5 fb−1 per year [22–25], taking σtotal≈0.04–0.14μb, one can expect about 6×107–2.1×108 events per year for the production of J/ψπ0 at Ec.m.≃4.2 GeV, which are enough to meet the requirement of the experiment. At present, except for the Z(4200), other charmonium-like Z states (such as Z(3900), Z(4025), and Z(4430)) were also observed in the J/ψπ invariant mass. Since all of them have probably the identical quantum numbers JP=1+, the interference of each other is possible. The above situation indicates that the contributions from other Z0 may be significant for the p¯p→J/ψπ0 process. However, in this work, we focus only on the Z0(4200) state and do not consider the contributions from other Z0 states.Figure7 shows the differential cross section including both signal and background contributions at the center of mass energy Ec.m.=4.0, 4.2, 4.5, and 5 GeV. We notice that the line shape of total differential cross section is less affected by background and almost coincident with the line shape of signal differential cross section at Ec.m.=4.0–4.5 GeV, which are consistent with the calculations as presented in Figure 6. In comparison, it is found that the shapes of total angular distributions are different from the shapes of signal angular distributions at Ec.m.=5 GeV, which due to the background has a strong effect on the total cross section at Ec.m.=5 GeV. These predictions can be checked by the future experiment.


## 5. Discussion and Conclusion

In this work, we investigate the neutralZ0(4200) production in p¯p→J/ψπ0 reaction with an effective Lagrangian approach. Our numerical result indicates that the p¯p→J/ψπ0 is very likely an ideal channel to study and search for the neutral hidden-charm Z0(4200). Furthermore, it is found that the center of mass energy Ec.m.≃4.0–4.5 GeV is the best energy window for searching the neutral Z0(4200), in which the signal can be easily distinguished from background. Moreover, according to our estimation, enough Z0(4200) events near Ec.m.≃4.2 GeV can be produced at PANDA, which indicate that searching for the neutral Z0(4200) by p¯p annihilation at PANDA is feasible. Besides, since our calculations are carried out in the premise of assuming that the Z0(4200) has a coupling with p¯p and J/ψπ0, the near future experiments at LHC and BelleII will be able to check our predictions on the respective coupling strengths of the Z0(4200).It should be mentioned that the value of coupling constantgZp¯p is determined by analyzing and comparing the degree of OZI suppressed in the process of Z0(4200)→p¯p and J/ψ→p¯p, which is based on the assumption of the neutral Z0(4200) which may be composed of cc¯uu¯ or cc¯dd¯. According our estimate, even if taking ΓZ0(4200)→p¯p=20keV (this value is one order smaller than that which we used in the above calculations) and ΓZ0(44200)→J/ψπ0=24.6MeV, the cross section from signal Z0(4200) contributions by p¯p annihilation at Ec.m.≃4.2 GeV could still reach up to the level of 4 nb which is at least one order higher than that of background. This means that an obvious bump at Ec.m.≃4.2 GeV can be expected to appear in the p¯p→J/ψπ0 process. Thus, we strongly suggest that the relevant experiment can be carried out, which will not only be conducive to verify the existence of Z(4200) but also enable us to have a more comprehensive understanding of the nature of exotic states and OZI rule.



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