Light mesons at BESIII

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1 EPJ Web of Conferences 96, 134 ( 15 DOI: 1.151/ epjconf/ C Owned by the authors, published by EDP Sciences, 15 Light mesons at BESIII Xiaobin Ji 1, a (for the BESIII Collaboration 1 Institute of High Energy Physics, Chinese Academy of Sciences Abstract. BESIII had accumulated J/ψ data samples. Selected results of the light mesons study based on part or full J/ψ samples are presented, including several observed structures around 1.85 GeV/c, partial wave analysis of J/ψ γηη, measurements of η and η anomalous decays, and a (98 in J/ψ p pa (98. 1 Introduction Hadron spectroscopy would help us understand the substructure of observed hadrons and verify the theory. Quantum Chromodynamics (QCD describes the strong interactions of colored quarks and gluons, it predicts the existence of new types of hadrons except the ordinary meson and baryon in the quark model, such as glueballs, hybirds and multiquark states. Experimental search of new hadrons and measure the new decay modes of known hadron would provide valuable information for the further understanding the strong interactions. The BESIII detector [1] at BEPC II (the upgraded Beijing Electron and Positron Collider, has collected the world s largest data samples at τcharm energy region since 9, including 1.3 billion J/ψ events,.5 billion ψ(3686 events,.9 fb 1 at the peak of the ψ(377 resonance, and lots of data samples above 4. GeV/c, which offers us a unique opportunity to study the hadron spectroscopy and search for the new hadrons. The article reviews several structures around 1.85 GeV/c found or confirmed by BESIII, the particle wave analysis results of J/ψ γηη, the study of η and η anomalous decays, and the analysis of J/ψ p pa (98. Structures around 1.85 GeV/c in J/ψ decay Results in this section are based on 5.3 million J/ψ collected in 9 at BESIII..1 X(186 and X(1835 An anomalous enhancement near the p p mass threshold in the process J/ψ γp p was first observed by BE SII experiment []. It was confirmed by BESIII [3] and CLEOc experiment [4]. In order to determine the parameters of the p p mass threshold structure, a partial wave analysis (PWA of a. jixb@ihep.ac.cn J/ψ γp p with M p p <. GeV/c was performed [5]. In the fit, the signal amplitudes is described by the relativistic covariant tensor amplitude method [6] and the final states interaction (FSI is included using Julich formulation [7]. The spinparity of the p p mass threshold structure was determined to be, and the mass, width and product BR for the X(p p were measured to be: M = (stat (syst. ± 19 (model MeV/c, Γ= 13 ± 39 (stat (syst. ± 4 (model MeV (a total width of Γ < 76 MeV/c at the 9% C.L and B(J/ψ γxb(x p p = ( (stat (syst. ±.3 (model 1 5, respectively. APWAonψ γp p was also performed by fixing the mass, width and spinparity of X(p p to results from J/ψ. The measured BRs is B(ψ γx B(X p p = (4.57±.36 (stat (syst.±1.8 (model 1 6 and the production ratio of the X(p p between J/ψ and ψ radiative decays is R = ( (stat (syst. ±.1 (model%. No p p threshold enhancement was found in the J/ψ ωp p [8], which means pure FSI is disfavored. The X(1835 was observed from the invariant mass of η in J/ψ γ η decay with a statistical significance of 7.7σ by the BESII experiment [9]. The same process was studied at BESIII. X(1835 was confirmed with the statistical significance larger than σ [1]. The angular distribution analysis shows that it is consistent with expectation for pseudoscalar state. Furthermore, two new structure named X(1 and X(37 were observed with the statistical significance larger than 7.σ and 6.4σ respectively.. X(187 in J/ψ ωη To further understand the nature of p p mass threshold structure (X(186 and/or X(1835, the decay of J/ψ ω η was studied [11]. The process of J/ψ ωx(187, X(187 a ± (98 was first observed with the signal significance estimated to be 7.σ. The f 1 (185 and η(145 were also clearly observed in the η mass This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4., which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at or

2 EPJ Web of Conferences spectrum. Figure 1 shows results of fit to the η mass spectrum. Events / MeV/c M η (GeV/c Figure 1. Results of fits to the M(η mass distribution for events with either the η or η in the a (98 mass window in J/ψ ωη. The dotted curve shows the contribution of nonω and/or nona (98 background, the dashed line also includes the contribution from J/ψ b 1 (135a (98, and the dotdashed curve indicates the total background with the nonresonant J/ψ ωa ± (98 included. χ /d.o. f is 1.7 for this fit..3 X(184 in J/ψ γ3( Since the X(1835 was confirmed to be a pseudoscalar particle [1] and it may have properties in common with the η c. Six charged pions is a known decay mode of the η c. Therefore, J/ψ radiative decays to 3( may be a favorable channel to search for the X states in the GeV/c region. Fig. shows the 3( invariant mass spectrum in J/ψ γ3(. A structure at 1.84 GeV/c was observed with a statistical significance of 7.6σ. The mass and width were measured to be M = 184. ± MeV/c and Γ=83 ± 14 ± 11 MeV. The product branching fraction was determined to be B(J/ψ γx(184 B(X(184 3( = (.44 ± PWA of J/ψ γωφ A study of the double OZI suppressed decays of J/ψ γωφ [1] was performed. A strong deviation (> 3σ from threebody phase space for J/ψ γωφ near the ωφ invariantmass threshold was observed (see Fig. 3. Assuming the enhancement is due to the influence of a resonance, the X(181, a partial wave analysis with a tensor covariant amplitude determined that the spinparity of the X(181 is. The mass and width of the X(181 were determined to be M = 1795 ± 7(stat 13 5 (syst±19(mod MeV/c and Γ=95 ± 1(stat 1 34 (syst±75(mod MeV/c and the product branching fraction was measured to be B(J/ψ γx(181 B(X(181 ωφ = (. ±.8(stat (syst ±1.3(mod 1 4, where the first error indicates the statistical error and the second is the systematical error. These results are consistent within errors with those from the BESII experiment [13]. EVENTS/(1 MeV/c M(3( (GeV/c Figure. The fit of mass spectrum of 3( in J/ψ γ3(. The dots with error bars are data; the solid line is the fit result. The dashed line represents all the backgrounds, including the background events from J/ψ 3( (dashdotted line, fixed in the fit and a thirdorder polynomial representing other backgrounds. Event/(.4GeV/c Data Projection X(181 f ( f (195 η(5 Phasespace Background.5 3 M(K K (GeV/c Figure 3. Comparisons between data and PWA fit projections in the invariant mass spectrum of K K in J/ψ γωφ. The comparison to the mentioned BESIII results of the masses and widths are shown in Fig. 4. 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( 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. 134p.

3 Dark Matter, Hadron Physics and Fusion Physics Width (MeV X(184;J P unknown( [14] X(187; J P unknown(ref. [11] X(1835; J P = (Ref. [1] X(p p; J P = (Ref. [5] X(181; J P = (Ref. [1] Mass (MeV/c Events /. GeV/c Figure 4. Comparisons of observations at BESIII. The error bars include statistical, systematic, and, where applicable, model uncertainties M ηη (GeV/c 3 PWA ofj/ψ γηη Radiative J/ψ decay is a gluonrich process and has long been regarded as one of the most promising hunting grounds for glueballs. In particular, for a J/ψ radiative decay to two pseudoscalar mesons, it offers a very clean laboratory to search for scalar and tensor glueballs because only intermediate states with J PC = even are possible. Using 5 million J/ψ events collected with the BE SIII detector, a PWA of J/ψ γηη has been performed. Fig. 5 shows the projection of PWA in the invariant mass spectrum of ηη. 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 [15] 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 (eg. f (15. In this analysis, the production rate of f (171 and f (1 are both about one order of magnitude larger than that of the f (15 and no clear evidence was found for f (137, which are both 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. 4 η and η physics The η/η system provides a unique stage for understanding the distinct symmetrybreaking mechanisms. Furthermore, the η/η decays play an important role to explore the effective theory of QCD at low energy, especially Figure 5. The invariant mass of ηη (dots with error bars and the PWA fit projections (histogram. for the Chiral Perturbation Theory (χpth. Its main decay modes, including hadronic and radiative decays, have been well measured, but the study of anomalous decays is still an open field. BESIII had collected 1.3 billion J/ψ events, one can obtain large η/η samples ( 1 6 from processes J/ψ γη/η or J/ψ φη/η. It is a good place to study the anomalous decays of η/η. BESIII had published results of η/η invisible decays [16], weak decay of η/η e ν c.c. [17]. Here we present other three studies briefly. 4.1 Measurement of η l l η e e was first reported by CLEO recently [18]. Theoretically this decay is expected to proceed via a virtual photon intermediate state, η γ e e, and provides a more stringent test of the theories since it involves offshell photons. Based on 5 M J/ψ events, the decays of η l l were studied via J/ψ γη at BESIII [19]. A clear η signal was observed in the e e invariant mass spectrum, and the branching fraction was measured to be B(η e e = (.11 ±.1 ± , which is in good agreement with theoretical predictions and the previous measurement, but was determined with much higher precision. No η signal was found in the μ μ mass spectrum, and the upper limit was determined to be B(η μ μ < at the 9% confidence level. 4. Observation of η ( The strong decays η ( ( are not suppressed by approximate symmetries. They are expected 134p.3

4 EPJ Web of Conferences to be mediated by chiral anomalies, since an odd number (five of pseudoscalar particles are involved. In particular, a contribution from a new type of anomaly, the pentagon anomaly, might show up. There should be also a significant contribution from the intermediate state with two ρ mesons. Based on a sample of 1.3 billion J/ψ events taken with the BESIII detector, we observe the decay modes η and η with a statistical significance of 18σ and 5σ, respectively (See Fig.6. The branching fractions were determined to be B(η = (8.53 ±.69 ± and B(η = (1.8±.35± , which are consistent with the theoretical predictions based on a combination of chiral perturbation theory and vectormeson dominance [], but not with the brokensu 6 O 3 quark model [1]. coupling of photons to the neutral and η. The first sizable contribution comes at O(p 6 []. This decay provides a unique opportunity to test directly the correctness of the calculations of third order χpth. With a sample of 1.3 billion J/ψ events in the BE SIII, the rare, doubly radiative decays η/η γγ have been studied. η can be seen clearly in the invariant mass spectrum of γγ (Fig. 7, the peaking background events mainly come from η γω(ρ with ω(ρ γ. The branching fraction of η γγ is measured for the first time to be B(η γγ = (6.91±.51± The measured value is much lower than that of the theoretical predictions [3, 4] but consistent with the upper limit in PDG. No evidence for the decay of η γγ is found. Events/5 MeV/c 1 5 data Full fit J/ψ γ f (17, f (17 J/ψ γ J/ψ γ η, η η,η γ J/ ψ γ η, η e e (a M (GeV/c Events/5 MeV/c 6 data Full fit J/ψ γ f (17, f J/ψ γ 4 (17 J/ψ γ η, η η,η γ J/ψ γ η, η γ ω, ω ψ γ η, η η, η J/ (b M (GeV/c Figure 6. Results of the fits to (a M and (b M, where the background contributions are displayed as the hatched histograms. 4.3 η/η γγ (Preliminary The decay η γγ, in the frame of χpth, both at O(p and O(p 4 is forbidden because there is no direct Figure 7. The M γγ distribution with fit results superimposed for the η γγ decay. 5 J/ψ p pa (98 As one of the lowlying scalars, the state a (98 has turned out to be mysterious in the quark model scenario. Its production near threshold allows tests of various hypotheses for its structure. The measurement of J/ψ p pa (98 is an additional observable constraining any phenomenological models trying to understand the nature of the a (98. A chiral unitary coupled channels approach of the χpth is applied in investigation of the fourbody decays J/ψ N NMM process [5] where the N stands for a baryon and the M for a meson. The experiment input is needed for further progress in understanding of the dynamics of the fourbody decay processes taking the FSI of mesons into account. Based on 5 million J/ψ events at BESIII, J/ψ p pa (98, a (98 η was observed for the fist time with a statistical significance of 3.σ [6]. Fig.8 shows the fit to the invariant mass spectrum of η. Without considering the interference between the signal channel and the 134p.4

5 Dark Matter, Hadron Physics and Fusion Physics same final states with intermediate N states, the branching fraction was measured to be B(J/ψ p pa (98 p p η = (6.8 ± 1. ± Taking the branching fraction of J/ψ p p from PDG, the coefficient r 4 in the chiral unitary approach [5] shows preference to r 4 =. instead of.7. Events/(. MeV/c M η (GeV/c Figure 8. The results of fitting the mass spectrum for η. Dots with error bars are data and the solid line is the fitted spectrum. The dashdotted line shows the nona (98 background described by a thirdorder Cheybechev polynomial. The dashed line shows the signal described by an efficiencyweighted Flatté formula convoluted with a resolution function. 6 Summary BESIII had collected 1.3 billion J/ψ data samples. Based 5 million or full J/ψ samples, some progresses on the light meson study are reported. p p threshold enhancement in J/ψ γp p, X(1835 in J/ψ γη, X(181 in J/ψ γωφ were confirmed. Two new structures, X(184 in J/ψ γ3( and X(187 in J/ψ ωη were observed. Furthermore, various studies on the light meson were shown, including PWA of J/ψ γηη, η/η anomalous decays, a (98 in J/ψ p pη. It turns out that BESIII is an excellent place to study the hadron spectroscopy. With the high statistics data accumulated at the BESIII, more interesting results are expected to be coming soon. References [1] M. Ablikim et al. (BESIII Collaboration, Nucl. Instrum. Meth. A614, 345 (1, [] J. Bai et al. (BES Collaboration, Phys. Rev. Lett. 91, 1 (3, hepex/336 [3] M. Ablikim et al. (BESIII Collaboration, Chin. Phys. C34 (1, [4] J. Alexander et al. (CLEO Collaboration, Phys. Rev. D8, 9 (1, [5] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. Lett. 18, 113 (1, [6] S. Dulat, B.S. Zou, Eur. Phys. J. A6, 15 (5, hepph/5887 [7] A. Sibirtsev, J. Haidenbauer, S. Krewald, U.G. Meissner, A.W. Thomas, Phys. Rev. D71, 541 (5, hepph/ [8] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D87, 114 (13, [9] M. Ablikim et al. (BES Collaboration, Phys. Rev. Lett. 95, 61 (5, hepex/585 [1] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. Lett. 16, 7 (11, [11] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. Lett. 17, 181 (11, [1] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D87, 38 (13, [13] M. Ablikim et al. (BES Collaboration, Phys. Rev. Lett. 96, 16 (6, hepex/631 [14] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D88, 915 (13, [15] L.C. Gui, Y. Chen, G. Li, C. Liu, Y.B. Liu et al., Phys. Rev. Lett. 11, 161 (13, [16] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D87, 19 (13, [17] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D87, 36 (13, [18] P. Naik et al. (CLEO Collaboration, Phys. Rev. Lett. 1, 6181 (9, [19] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D87, 911 (13, [] F.K. Guo, B. Kubis, A. Wirzba, Phys. Rev. D85, 1414 (1, [1] D. Parashar, Phys. Rev. D19, 68 (1979 [] L. Ametller, J. Bijnens, A. Bramon, F. Cornet, Phys. Lett. B76, 185 (199 [3] R. Escribano, PoS CD1, 35 (13 [4] R. Jora, Nucl. Phys. Proc. Suppl. 78, 4 (1 [5] C.b. Li, E. Oset, M. Vicente Vacas, Phys. Rev. C69, 151 (4, nuclth/3541 [6] M. Ablikim et al. (BESIII Collaboration, Phys. Rev. D9, 59 (14, p.5