Light Meson Decays at BESIII

. At the precision frontier of particle physics, light mesons decays play important roles in understanding strong interactions, quark Model, CP violations, and so on. They provide valuable laboratories for the testing Chiral Perturbative Theory predictions, studying the EM form factors and fundamental sysmmetries, and last but not least, probing new physics beyond the Standard Model. In this talk, we present the new results from BESIII about the η (cid:30) and η decays based on large J /ψ datasets


Introduction
As the neutral members of the ground state pseudoscalar nonet, both η and η play an important role in understanding low energy quantum chromodynamics (QCD).In particular the η meson, much heavier than the Goldstone bosons of broken chiral symmetry, plays a special role as the predominant singlet state arising from the strong axial U(1) anomaly.In addition, the decays of both mesons are used to search for processes beyond any considered extension of the Standard Model (SM) and to test fundamental discrete symmetries [1].
There are several experiments in the world which can provide source of η or η events, including CLAS and GlueX at Jefferson Lab, Crystal Ball at BNL, WASA at COSY, KLOE-2 at INFN, and BESIII at IHEP.Among them, the BESIII is the one that had accumulated largest datasets of η and η produced from J/ψ and ψ decays, and will keep collecting data for more years.For example, with 10 billion J/ψ events, 1 × 10 7 and 4 × 10 6 of η mesons are produced from J/ψ → γη and J/ψ → φη, 5.2 × 10 7 and 2.5 × 10 6 of η meson are produced, respectively.
The BESIII detector is installed on BEPCII accelerator and located in Beijing.It consistes of a helium-gas based drift chamber (MDC), a CsI(Tl) crystal calorimeter, a Time-of-Flight system, a 9-layer PRC-based muon chamber system, and a super-conducting solenoid megnet with a central field of 1.0 Tesla [2].BESIII has performed a series of measurements on η and η decays since data collection.In this talk, some new results on the light hadron decays from BESIII are presented.
The decays η → π + π − l + l − with (l = e or µ) are especially interesting since these two decays may involve the box anomaly contribution [3].Theoretically, these decays have been investigated with different models, including the effective meson theory, the chiral unitary approach, and the hidden gauge model.In previous analyses, the CLEO Collaboration [4] and BESIII analysis [5] have not observed η → π + π − µ + µ − decays due to the virtual photon conversion to dimuon is significantly suppressed.The first experimental evidence of the decay η → π + π − e + e − was obtained by the CLEO experiment [4].Later, BESIII publised more precise result using 225 million J/ψ events [5].
Events / 0.01 The decay η → π + π − e + e − allows for a test of CP-violation due to the interference between the dominating, CP-converving, magnetic transition, and a possible CP-violating electric dipole type transition [10].Such an interference term is proportional to sin 2ϕ, where ϕ is the angle between the decay planes of the e + e − -pair and the π + π − -pair, as shown in Fig. 1 (a).Experimentally, the electric dipole transition will manifest itself as an asymmetry A ϕ = N(sin 2ϕ>0)−N(sin 2ϕ<0) N(sin 2ϕ>0)+N(sin 2ϕ<0) , where N(x) represents the acceptance-corrected number of events in the corresponding angular region.Based on the 1.3 billion J/ψ events, BESIII measured this observable as A ϕ = (2.9 ± 3.7 stat.± 1.1 syst.)% [7].The distribution of sin 2ϕ from data is shown in Fig. 1 (b).This result is consistent with the SM prediction, and no CP violation is observed.
The η decays to four leptons are of great interest for understanding the pseudoscalar transition form factor (TFF) and the interactions between pseudoscalars and virtual photons.The TFF is necessary input to the pseudoscalar-meson-pole contributions to the hadronic lightby-light scattering, which causes the second largest uncertainty for g − 2 of muon.By means of a data-driven approach based on the use of rational approximants applied to π 0 , η and η transition form factor experimental data in the space-like region, the branching ratios of double Dalitz decays are predicted [11].However, there is no concrete experimental evidence yet.
Based on 10 billion J/ψ events, the double Dalitz decay η → e + e − e + e − is observed for the first time, with a significance of 5.7σ with systematic uncertainties taken into consideration.Its branching fraction is determined to be B(η → e + e − e + e − ) = (4.5 ± 1.0 stat.± 0.5 syst. ) × 10 −5 , which is in reasonable agreement with theoretical predictions.This result provides new information for the studies about η Timelike-Form-Factor (TFF) and the interactions between η and virtual photons.
In ππ interaction, one of the prominent features is the loop contribution to the ππ scattering: the S-wave charge-exchange rescattering π + π − → π 0 π 0 causes a prominent cusp at the center of mass energy corresponding to the summed mass of two charged pions.The cusp effect can shed light on the fundamental properties of QCD at low energies, by determining the strength of the S-wave ππ interaction [13][14][15][16][17].This effect was firstly observed in K + → π 0 π 0 π + in 2006 by NA48/2 experiment.While not been seen in η → 3π 0 or η → ηπ 0 π 0 channels even there are theoretical predictions.

Figure 1 .
Figure 1.(a) Illustration of the decay plane angle ϕ in η → π + π − e + e − .(b) Distribution of sin 2ϕ.Dots with error bars represent data, the dotted (red) histrogram represents the signal MC sample generated with a symmetric sin 2ϕ distribution, the dashed (green) histrogram represents the remaining background contribution.