Hadron Spectroscopy at BESIII

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1 Available online at Nuclear and Particle Physics Proceedings ( Hadron Spectroscopy at BESIII Shuangshi FANG (for the BESIII collaboration Institute of High Energy Physics, Beijing, China Abstract The BESIII detector has collected the largest data samples around τ charm energy region since the upgrade was completed in 8. The hadron spectroscopy, as one of the main physics goals, was extensively studied and many important progresses were achieved these years. In this report the recent results on the study of hadron spectroscopy were presented, including the light hadron spectroscopy and the observations of the charmoniumlike Z c particles. Keywords: the BESIII detector, hadron spectroscopy, charmonium decays 1. Introduction A precise determination of the hadron spectroscopy would help us uderstand the substructure of observed hadrons and verify the theory, Quantum Chromodynamics (QCD, for describing the strong interactions. In addition, the hadron spectroscopy also provides informations for those particles beyond the quark model, e.g., glueballs, hybrids and multiquark states, which are expected from the QCD. In the last decays, considerable progresses on the hadron spectroscopy have been made, in particular the new observations from the BESII and the XYZ particles from Belle and BaBar, from the charmonium (or charm and bottomium (or B meson decays in e e annihilation decays. Since the upgrade was completed in 8, the BESIII detector [1], has collected the world s largest data samples at τcharm energy region including 1.3 billion J/ψ events, 4.5 million ψ(3686 events,.9 fb 1 at the peak of the ψ(377 resonance, and 1.8 fb 1 around the peak of the Y(46 resonance, which offers us a unique opportunity to study the hadron spectroscopy and search for the new hadrons. In this talk I present a brief review about the recent results that have been obtained from these data samples / 16 Published by Elsevier B.V.. Light hadron spectroscopy.1. The structures around 1.85 GeV observed in J/ψ radiative decays In 3 BESII reported the first observation of p p mass threshold enhancement in J/ψ γp p, which attracted both experimental and theoretical attentions to investigate its nature. Using the J/ψ events collected by the BESIII detector in 9, a Partial Wave Analysis (PWA of J/ψ γp p [] with M p p <. GeV/c was performed to determine its spinparity, mass and width. In the analysis, the final state interactions were considered by including the Julich formulation [3]. The PWA results indicate that the assignment fit for this structure is better than that for or other J PC assignments. The mass, width and product branching fraction of the X(p p are measured to be M = ± 19 (model MeV/c, Γ=13 ± ± 4 (model MeV (a total width of Γ < 76 MeV/c at the 9% C.L. and B(J/ψ γxb(x p p = ( ±.3 (model 1 5, respectively. We also observed the structures around 1.85 GeV in the other J/ψ decays, X(1835 in J/ψ γπ π η [4], X(187 in J/ψ ωπ π η [5]. To search for their new decays, the J/ψ γ3(π π [6] is analyzed using the 5.3 million J/ψ events taken at the BESIII detector in 9, and a structure, X(184, is observed. Fig. 1 (a shows the 3(π π invariant mass spectrum in J/ψ

2 195 S. Fang / Nuclear and Particle Physics Proceedings ( γ3(π π and the fit gives M= 184. ± MeV/c and Γ=83 ± 14 ± 11 MeV with a statistical significance of 7.6σ. The product branching fraction is determined to be B(J/ψ γx(184b(x(184 3(π π = (.44 ± A study of the doubly OZI suppressed decays of J/ψ γωφ [7] is performed and a strong deviation (> 3σ from threebody J/ψ γωφ phase space is observed near the ωφ mass threshold. Assuming that the enhancement is due to the presence of a resonance X(181, A PWA with a tensor covariant amplitude found that its spinparity is and the resonance parameters are determined to be: M = 1795 ± ±19(model MeV/c and Γ=95± ±75(model MeV, which are consistent with the previous work from BESII [8]. EVENTS/(1 MeV/c Width (MeV 3 (a M(3(π π (GeV/c (b X(184;J P unknown( [6] X(187; J P unknown(ref. [5] X(1835; J P = (Ref. [4] X(p p; J P = (Ref. [] X(181; J P = (Ref. [7] Mass (MeV/c Figure 1: The fit of mass spectrum of 3(π π (a and comparisons of observations at BESIII. The error bars include statistical, systematic, and, where applicable, model uncertainties (b. The comparison to the BESIII results of the masses and widths of the X(p p, X(1835, X(187, X(184 and X(181 are displayed in Fig. 1 (b. The mass of X(184 is in agreement with X(p p, while its width is significantly broader. Therefore, based on these data, one cannot determine whether X(184 is a new state or the signal of a 3(π π decay mode of X(p p. Further study, including an amplitude analysis to determine the spin and parity of the X(184, is needed to establish the relationship between these experimental observations... PWA of J/ψ γηη [9] For J/ψ γηη, the PWA results shown in Fig. indicate that the scalar contributions are mainly from f (15, f (171 and f (1, while no evident contributions from f (137 and f (179 are seen. Recently, the production rate of the pure gauge scalar glueball in J/ψ radiative decays predicted by the lattice QCD [1] was found to be compatible with the production rate of J/ψ radiative decays to f (171; this suggests that f (171 has a larger overlap with the glueball compared to other glueball candidates (e.g., f (15. In this analysis, the production rates of f (171 and f (1 are both about one order of magnitude larger than that of the f (15, which are consistent with, at least not contrary to, lattice QCD predictions. The tensor components, which are dominantly from f (155, f (181 and f (34, also have a large contribution in J/ψ γηη decays. The significant contribution from f (155 is shown as a clear peak in the ηη mass spectrum; a tensor component exists in the mass region from 1.8 GeV/c to GeV/c, although we cannot distinguish f (181 from f (191 or f (195; and the PWA requires a strong contribution from f (34, although the possibility of f (3 cannot be ruled out. Events /. GeV/c (GeV/c M ηη Figure : The invariant mass of ηη (dots with error bars and the PWA fit projections (histogram.

3 S. Fang / Nuclear and Particle Physics Proceedings ( PWA of ψ(3686 p pπ [11] To search for new excited N baryons, we performed apwaofψ(3686 p pπ and found that the dominant contributions are from 7 N intermediate resonances. Among these N resonances, two new resonances are significant, one 1/ resonance with a mass of MeV/c and width of MeV, and one 5/ resonance with a mass of MeV/c and width of 5 14 MeV. For the remaining 5 N intermediate resonances, the mass and width values from the PWA shown in Table. 1 are consistent with those from established resonances. Table 1: Summary of the PWA results for ψ p pπ. Resonance M(MeV/c Γ(MeV/c N( N( N( N( N( N( N( Events/5 MeV/c Events/5 MeV/c data Full fit J/ψ γ f (17, f (17 π π π π J/ψ γ π π π π J/ψ γ η, η π π η,η γ π π J/ψ γ η, η π π e e M π (GeV/c 6 data Full fit J/ψ γ f (17, f J/ψ γ π π π π J/ψ γ η, η π π J/ψ γ η, η γ (17 π η,η γ ω, ω π π π π π π π π π π π J/ψ γ η, η π π η, η π π π (a (b.4. Observation of η π π π ( π ( [1] and η γγπ Based on a sample of 1.3 billion J/ψ events taken with the BESIII detector, we report the first observation of η π π π ( π ( decays via J/ψ radiative decays. Fig. 3(a and Fig. 3(b show the M π π π π and M π π π ( π (, respectively, where the η peaks are clearly observed and the dominant background events are from the other η decays, but none of them contribute to the η peak. The branching fractions are determined to be B(η π π π π =(8.63±.69± and B(η π π π π = (1.8 ±.35 ± , which are consistent with the theoretical predictions based on the combination of chiral perturbation theory (ChPT and vectormeson dominance [13], but could rule out the brokensu 6 O 3 quark model. In addition we also observed the η γγπ for the first time coming from J/ψ γη. The clear η peak is seen in the γγπ mass spectrum shown in Fig. 4. MC study indicates that the peaking background events mainly come from η γω(ρ with ω(ρ γπ. The preliminary result is determined to be B(η γγπ = (6.91 ±.51 ± M π (GeV/c π π π Figure 3: The invariant mass distributions of (a π π π π and (b π π π π and the fit results. Events/(4.5MeV/c 3 1 Sum of η γω and η γρ Sum of η π π η, η 3π Other J/ψ decay Signal Data (GeV/c M π γγ and η γγ Preliminary Figure 4: The invariant mass distributions of γγπ and the fit results.

4 195 S. Fang / Nuclear and Particle Physics Proceedings ( Charmoniumlike Z c particles 3.1. Observation of Z c (39 [14] Using 55 pb 1 data sample taken at a centerofmass energy of 4.6 GeV, we reported the observation of a prominent resonancelike charged structure, Z c (39, in the π ± J/ψ invariant mass distribution for e e π π J/ψ events. With a Swave BreitWigner (BW function, a fit to the distribution of M max (π ± J/ψ [Fig. 5], the larger one of the two mass combinations M(π J/ψ and M(π J/ψ in each event, yields M = ± 3.6 ± 4.9 MeV/c and Γ=46 ± 1 ± MeV. Shortly after the BESIII announcement, it was confirmed by the Belle [15] and CLEO [16]. This structure couples to charmonium and has an electric charge, which implies a state containing more quarks than just a charm and anticharm quark. Events /.1 GeV/c Data Total fit M max (π ± J/ψ (GeV/c Figure 5: Fit to the M max (π ± J/ψ. Background fit PHSP MC Sideband To investigate its isopsin, the e e π π J/ψ was performed to search for its neutral partner. Evidence for a neutral Z c (39 was observed in e e π π J/ψ using the data taken at the center of mass energies of 4.3 GeV, 4.6 GeV and 4.36 GeV, respectively. A neutral structure, the Zc (39 is seen in the π J/ψ mass spectra, take M π J/ψ for 4.3 GeV data shown in Fig. 6 for example. By combining all data samples, the significance is greater than 1σ. The mass and width are obtained to be M = ±.3 MeV/c and Γ=9.6 ± 8. MeV, where the errors are statistical only. The preliminary results are in good agreement with those of the Z c ± obtained from e e π π J/ψ. 3.. Observation of Z c (3885 [17] The Z c (39 mass is MeV/c above the D D mass threshold, which is suggestive of a virtual D D Preliminary Figure 6: Fit to π J/ψ mass spectrum. moleculelike structure, a charmedsector analog of the Z b (161. Therefore, it is important to measure the rate for Z c (39 decays to D D and compare it to that of the πj/ψ. We performed a study of the process e e π ± (D D at s = 4.6 GeV with a 55 pb 1 data sample collected with the BESIII detector. The π (D D final states are selected by means of a partial reconstruction technique in which only the bachelor π and one finalstate D meson are detected, and the presence of the D is inferred from energymomentum conservation. The D mesons are reconstructed in the D K π and D K π π decay channels. A distinct charged structure, denoted as Z c (3885, is observed in the (D D invariant mass distribution shown in Fig. 7 (a and Fig. 7 (b. When fitted to a Breit Wigner lineshape, the pole mass and width are determined to be M pole = ± 1.5 ± 4. MeV/c and Γ pole = 4.8 ± 3.3 ± 11. MeV. The mass and width of Z c (3885 are σ and 1σ, respectively, below those of the Z c (39. To investigate its spinparity, we analyzed the angular distribution of cos θ π and found it favours a J P = 1 quantum number assignment Observation of Z c (4 [18, 19] To search for the new decay modes of Z c (39, we performed the study of e e π π h c using the data samples taken at centerofmass energies from 3.9 GeV to 4.4 GeV. The h c γη c is reconstructed with η c s 16 exclusive decay modes. The M π ± h c distribution summed over the 16 η c decay modes is shown in Fig. 8, where a narrow structure, referred to as Z c (4, is clearly seen, but no significant Z c (39 signal is observed. Similar to Z c (39, the Z c (4 also carries

5 S. Fang / Nuclear and Particle Physics Proceedings ( Events / 4 MeV/c Events / 4 MeV/c 9 (a M(D D* (GeV/c 1 (b M(D D* (GeV/c Figure 7: Fit to the M(D D (a and M(D D (b. an electric charge and couples to charmonium. A fit to the π ± h c invariant mass spectrum gives M = (4.9 ±.8 ±.7 MeV/c and Γ=(7.9 ±.7 ±.6 MeV, respectively, with a statistical significance of greater than 8.9σ. Table : Energies ( s and born cross section ratios R πzc(4. s (GeV RπZc ( ±.31 ± ±.5 ± ±.48 ±.3 Most recently we reported the evidence of a netural Z c (4 [19] in the process e e π π h c. A fit to the π h c invariant mass spectrum shown in Fig. 9 with the width of the Z c ± (4 fixed to that of its charged isospin partner and possible interferences with nonz c ± (4 amplitudes neglected, gives a mass of (43.9 ±. ± 3.8 MeV/c for the Zc (4. The ratios of Born cross section for e e πz c (4 ππh c between neutral and charged modes, R πzc (4 = σ(e e π Zc (4 π π h c σ(e e π ± Zc π ± π h c, at three centerofmass energies are listed in Table. The measured Born cross sections are about half of those for e e π π h c, and agree with expectations based on isospin symmetry within systematic uncertainties, which indicate that there is no anomalously large isospin violations in ππh c and πz c (4 system. Events/(.5 GeV/c Events/(.5 GeV/c (GeV/c M π h c M π ± (GeV/c Figure 8: Fit to M π ± h c, where the inset shows the sum of the simultaneous fit to the M π h c distributions at 4.3 GeV and 4.6 GeV with Z c (39 and Z c (4. Events/(.1GeV/c recoil Mπ (GeV/c max Figure 9: Fit to π recoil mass spectrum Observation of Z c (45 [] The narrow Z c (4 observed in e e π π h c is very close to D D mass threshold. Therefore, a search of Z c candidates via their direct decays into D D pairs is strongly motivated. Based on a 87 pb 1 data sample taken at a centerofmass energy of 4.6 GeV, we presented the study of the process e e (D D ± π. A structure near the (D D ± threshold, denoted as the Z c (45, is seen in the mass spectrum recoiling against π [Fig. 1]. An unbinned maximum likelihood fit to the spectrum of RM(π results in a mass h c

6 1954 S. Fang / Nuclear and Particle Physics Proceedings ( of (46.3 ±.6 ± 3.7 MeV/c and a width of (4.8 ± 5.6 ± 7.7 MeV. After taking into account the systematic uncertainties, the statistical significance is larger than 1σ. Events / (.5 MeV/c 8 data (b total fit Z c (45 comb. BKG PHSP signal WS RM(π (GeV/c Figure 1: Fit to the π recoil mass spectrum. Table. 3 gives a summary of the charmoniumlike Z c states observed at BESIII. For both Z c ± (39 and Z c ± (4, their neutral partners are evident, which indicates they are the isospin triplets and shows a hint of a new hadron spectroscopy; Z c (39 and Z c (4 are close to the DD and D D mass threshold, respectively. Of great interest is the similar structures are also seen in the DD and D D mass spectrum though the mass and width are not consistent well with those by fitting the mass spectrum of πj/ψ and πh c, which inspired many speculations on their natures. However, based on these data, one cannot determine whether Z c (39 and Z c (3885 (Z c (4 and Z c (45 are the same state or not. Further study, including an amplitude analysis to determine the spin and parity of them, is needed to establish the relationship between different experimental observations in this mass region and determine the nature of these structures. Table 3: Summary of the charmoniumlike Z c states at BESIII. Z c M(MeV/c Γ(MeV π ± J/ψ ± 3.6 ± ± 1 ± π J/ψ ±.3 (Prel. 9.6 ± 8. (Prel. (DD ± ± 1.5 ± ± 3.3 ± 11. π ± h c 4.9 ±.8 ± ±.7 ±.6 π h c 43.9 ±. ± 3.8 (D D ± 46.3 ±.6 ± ± 5.6 ± Summary Based on the data samples taken at the peak of J/ψ, ψ(3686 and around Y(46, the recent progresses on the hadron spectroscopy are reported in this talk. For the light hadron spectroscopy, besides the confirmation of the p p mass threshold enhancement, X(1835 and X(181 in J/ψ radiative decays, another structure around 1.84 GeV, X(184, is observed in J/ψ γ3(π π ; the two new excited nucleon states, N(3 and N(57, are found in ψ(3686 p pπ ; the new decay modes of η π π π ( π ( and η γγπ are observed for the first time. For the study of charmoniumlike states, four interesting structures, Z c (39, Z c (3885, Z c (4 and Z c (45, are observed, which inspired many theoretical interpretations. However, further Further study on these structures with large statistics of data is strongly needed to determine their properties and then clarify their natures. The above interesting results not only underline the importance of the study of hadron spectroscopy, but also demonstrate the effectiveness of a programmatic approach to study hadron spectroscopy at BESIII. With the high statistics data accumulated at the BESIII detector, more interesting results are expected to be coming soon. References [1] M. Ablikim et al., Nucl. Instrum. Meth. A 614, 345 (1. [] M. Ablikim et al., Phys.Rev.Lett. 18,113 (1. [3] A. Sirbirtsen et al., Phys. Rev. D71, 541 (5. [4] M. Ablikim et al., Phys. Rev. Lett. 16, 7 (11. [5] M. Ablikim et al., Phys. Rev. Lett. 17, 181 (11. [6] M. Ablikim et al., Phys. Rev. D88, 915 (13. [7] M. Ablikim et al., Phys.Rev. D87, 38 (13. [8] M. Ablikim et al., Phys. Rev. Lett. 96, 16 (6. [9] M. Ablikim et al., Phys. Rev. D87, 99 (13. [1] L. C. Gui et al., Phys.Rev.Lett. 11, 161 (13. [11] M. Ablikim et al., Phys. Rev. Lett (13. [1] M. Ablikim et al., Phys. Rev. Lett. 11, 5181 (14. [13] F. K. Guo, B. kubis and A. Wirzba, Phys. Rev. D 85, 1414, (1. [14] M. Ablikim et al., Phys. Rev. Lett. 11, 51 (13. [15] Z. Q. Liu et al., Phys. Rev. Lett. 11, 5 (13. [16] T. Xiao et al., Phys.Lett. B77, 366 (13. [17] M. Ablikim et al., Phys. Rev. Lett. 11, 1 (14. [18] M. Ablikim et al., Phys. Rev. Lett. 111, 41 (13. [19] M. Ablikim et al., arxiv:149:6577. [] M. Ablikim et al., Phys. Rev. Lett. 11,131 (14.