Light mesons at BESIII

BESIII had accumulated 1.3×109 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/c2, partial wave analysis of J/ψ→ γηη, measurements of η and η′ anomalous decays, and a0(980) in J/ψ→ pp̄a0(980).

No pp threshold enhancement was found in the J/ψ → ωpp [8], which means pure FSI is disfavored.
The X(1835) was observed from the invariant mass of π + π − η in J/ψ → γπ + π − 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 20σ [10]. The angular distribution analysis shows that it is consistent with expectation for pseudoscalar state. Furthermore, two new structure named X(2120) and X(2370) were observed with the statistical significance larger than 7.2σ and 6.4σ respectively.
Events / 20 MeV/c 2 Figure 1. Results of fits to the M(ηπ + π − ) mass distribution for events with either the ηπ + or ηπ − in the a 0 (980) mass window in J/ψ → ωηππ. The dotted curve shows the contribution of non-ω and/or non-a 0 (980) background, the dashed line also includes the contribution from J/ψ → b 1 (1235)a 0 (980), and the dot-dashed curve indicates the total background with the nonresonant J/ψ → ωa ± 0 (980)π ∓ included. χ 2 /d.o. f is 1.27 for this fit.
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 → π 0 3(π + π − ) (dash-dotted line, fixed in the fit) and a third-order polynomial representing other backgrounds.
The comparison to the mentioned BESIII results of the masses and widths are shown in Fig. 4. The mass of X(1840) is in agreement with X(pp), while its width is significantly broader. Therefore, based on these data, one cannot determine whether X(1840) is a new state or the signal of a 3(π + π − ) decay mode of X(pp). Further study, including an amplitude analysis to determine the spin and parity of the X(1840), is needed to establish the relationship between these experimental observations.

PWA of J/ψ → γηη
Radiative J/ψ decay is a gluon-rich 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 225 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 0 (1500), f 0 (1710) and f 0 (2100), while no evident contributions from f 0 (1370) and f 0 (1790) 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 0 (1710); this suggests that f 0 (1710) has a larger overlap with the glueball compared to other glueball candidates (eg. f 0 (1500)). In this analysis, the production rate of f 0 (1710) and f 0 (2100) are both about one order of magnitude larger than that of the f 0 (1500) and no clear evidence was found for f 0 (1370), which are both consistent with, at least not contrary to, lattice QCD predictions.
The tensor components, which are dominantly from f 2 (1525), f 2 (1810) and f 2 (2340), also have a large contribution in J/ψ → γηη decays. The significant contribution from f 2 (1525) is shown as a clear peak in the ηη mass spectrum; a tensor component exists in the mass region from 1.8 GeV/c 2 to 2 GeV/c 2 , although we cannot distinguish f 2 (1810) from f 2 (1910) or f 2 (1950); and the PWA requires a strong contribution from f 2 (2340), although the possibility of f 2 (2300) cannot be ruled out.

η and η physics
The η/η system provides a unique stage for understanding the distinct symmetry-breaking mechanisms. Furthermore, the η/η decays play an important role to explore the effective theory of QCD at low energy, especially 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 ( ∼ 10 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 → π − e + ν + c.c. [17]. Here we present other three studies briefly.

η/η → γγπ 0 (Preliminary)
The decay η → π 0 γγ, in the frame of χPTh, both at O(p 2 ) and O(p 4 ) is forbidden because there is no direct coupling of photons to the neutral π 0 and η. The first sizable contribution comes at O(p 6 ) [22]. 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 → γγπ 0 have been studied. η can be seen clearly in the invariant mass spectrum of γγπ 0 (Fig. 7), the peaking background events mainly come from η → γω(ρ) with ω(ρ) → γπ 0 . The branching fraction of η → γγπ 0 is measured for the first time to be B(η → γγπ 0 ) = (6.91±0.51±0.58)×10 −4 . The measured value is much lower than that of the theoretical predictions [23,24] but consistent with the upper limit in PDG. No evidence for the decay of η → γγπ 0 is found.

J/ψ → ppa 0 (980))
As one of the low-lying scalars, the state a 0 (980) 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 → ppa 0 (980) is an additional observable constraining any phenomenological models trying to understand the nature of the a 0 (980). A chiral unitary coupled channels approach of the χPTh is applied in investigation of the four-body decays J/ψ → NNMM process [25] 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 four-body decay processes taking the FSI of mesons into account.
Based on 225 million J/ψ events at BESIII, J/ψ → ppa 0 (980), a 0 (980) → π 0 η was observed for the fist time with a statistical significance of 3.2σ [26]. Fig.8 shows the fit to the invariant mass spectrum of π 0 η. Without considering the interference between the signal channel and the same final states with intermediate N * states, the branching fraction was measured to be B(J/ψ → ppa 0 (980) → ppπ 0 η) = (6.8 ± 1.2 ± 1.3) × 10 −5 . Taking the branching fraction of J/ψ → ppπ + π − from PDG, the coefficient r 4 in the chiral unitary approach [25] shows preference to r 4 = 0.2 instead of −0.27.  Figure 8. The results of fitting the mass spectrum for π 0 η. Dots with error bars are data and the solid line is the fitted spectrum. The dash-dotted line shows the non-a 0 (980) background described by a third-order Cheybechev polynomial. The dashed line shows the signal described by an efficiency-weighted Flatté formula convoluted with a resolution function.

Summary
BESIII had collected 1.3 billion J/ψ data samples. Based 225 million or full J/ψ samples, some progresses on the light meson study are reported. pp threshold enhancement in J/ψ → γpp, X(1835) in J/ψ → γη π + π − , X(1810) in J/ψ → γωφ were confirmed. Two new structures, X(1840) in J/ψ → γ3(π + π − ) and X(1870) in J/ψ → ωηππ were observed. Furthermore, various studies on the light meson were shown, including PWA of J/ψ → γηη, η/η anomalous decays, a 0 (980) in J/ψ → ppηπ. 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.