Light hadron spectroscopy at BESIII

Abstract. Light hadron spectroscopy is one of the most important physics goals of high statistics of charmonium decays collected at BESIII which provide an excellent place for hunting gluonic excitations and studying strangeonium. In this proceeding, we will introduce two analyses for gluonic states study on η(1405)/η(1475) puzzle, X(1835) and exotic states and two analyses for strangeonium(like) states study on Y(2175) and Zs state.


Introduction
Quantum chromodynamics(QCD) is one of the fundamental theories in modern high energy physics. Light hadron spectroscopy plays a crucial role in understanding and examining the QCD theory in non-perturbative energy region. BESIII has collected 1.31×10 9 J/ψ events and 4.48×10 8 ψ(3686) events since 2009 which gives us a good opportunity to study light hadron spectroscopy.
The X(1835) was first observed by the BESII experiment in the π + π − η [7] invariant mass spectrum and was confirmed with higher statistical significance by the BESIII [8]. Its J pc was measured to be 0 −+ in the K 0 S K 0 S η invariant mass spectrum by BESIII [9]. Furthermore, a recent BESIII result observes an anomalous line shape of the X(1835) near the pp threshold in the decay J/ψ → γπ + π − η [10]. Until now, the nature of the X(1835) is still an open question.
As a flavor filter, J/ψ → γγφ was analyzed with 1.311 billion J/ψ data sample collected since 2009. Two structures around 1.47 and 1.85 GeV/c 2 were observed in γφ invariant mass.
The fitting results are shown in Fig. 1 (a), (b). A fit on the polar angle distribution of the radiative photon shown in Fig. 1 (c), (d) favors J pc = 0 −+ assignment for the two resonances. The obtained mass, width and J pc supports the two observed resonances are η(1475) and X(1835), respectively. The observation of η(1475) and X(1835) decaying into γφ suggest these two states contain a sizable ss component. For the η(1405)/η(1475) puzzle, in the case of one state assumption, the branching fraction ratio of (Br η(1475/1475)→γρ : Br η(1475/1475)→γφ ) is calculated to be (11.10 ± 3.50 : 1) for the case of destructive interference and (7.53 ± 2.49 : 1) for constructive interference. The prediction of partial width ratio is 3.8 : 1 in Ref. [11]. For the case of two state assumption, the observation of the η(1475) decaying into γφ final state suggests that the η(1475) should be the radial excitation of the η [12].

Amplitude Analysis of
Several candidates with J pc = 1 −+ , decaying into different final states, such as ηπ, η π, f 1 (1270)π and so on, have been reported by various experiments [13]. The lightest exotic meson π 1 (1400) is only reported in the ηπ final state and the most promising J pc = 1 −+ π 1 (1600) is studied by CLEO-c which could also couple to the ηπ [14].

Observation of e + e − → ηY(2175) at center-of-mass energies above 3.7 GeV
The Y(2175) was first observed by the BaBar collaboration [16] via the initial-state-radiation (ISR) process e + e − → γ ISR φ f 0 (980) and later confirmed by the Belle collaboration [17], BESII and BESIII collaborations [18,19]. Ref. [20] predicts the existence of a sharp peaking structure (Z s1 ) close to the KK * threshold and a broad structure (Z s2 ) close to the K * K * threshold in the πφ mass spectrum. We can search Z s → π ± φ process analogied to Z c (3900) → π ± J/ψ process. In this analysis, e + e − → ηY(2175) was studied at higher energy up to 4.6 GeV accumulated at BESIII [21]. The Y(2175) is observed with a joint statistical significance lager than 10 σ using a simultaneous fit in the invariant mass spectrum of φ f 0 (980) for the seven data samples. We also searched for the Z s in the φπ mass spectrum. No significant Z s signal can be seen at the expected positions.

Search for strangeonium-like states Z s decaying into φπ at 2.125 GeV
Using a data sample of e + e − collision data corresponding to an integrated luminosity of 108 pb −1 collected with the BESIII detector at a center-of-mass energy of 2.125 GeV, we study the process e + e − → φππ, and search for a strangeonium-like structure Z s decaying into φπ. No signal is observed in the φπ mass spectrum. A PWA analysis is performed and upper limits on the cross sections for Z s production at the 90% confidence level are determined by introducing an additional Z s component in the amplitude analysis. We performed the alternative amplitude analysis by assuming J P = 1 + and J P = 1 − for Z s to explore its contribution to the data. The upper limits on the differential cross sections of Z s production as a function of the assumed mass of Z s with different width scenario are estimated at 90% C.L. which are displayed in Fig. 3.

Summary
With the largest J/ψ and ψ(3686) data sample in the world, the BESIII collaboration has contributed a lot in light hadron spectroscopy. We conducted a series of study on the gluonic states and strangeonium(like) states. Several recent results of light hadron spectroscopy are reported. With the high statistics data accumulated at the BESIII detector, more interesting results are expected in the future.