Study Of J/ψ�'p（?）φ Decay And A₀⁰（980）-f₀（980）Mixing At BESⅢ

At present, standard model is the most successful theory to explain the composition and interaction of the matter, and Quantum Chromo Dynamics (QCD) is the elemen-tary theory of strong interaction in the framework of standard model. The predictions of the QCD in the high energy regime have been extensively approved. However, QCD becomes invalid in the low energy regime due to the perturbative theory does not work in the case of the large effective coupling. Therefore, it is more important to study the hadron physics experimentally in the low energy regime, which is one of the main reasons that the Beijing Electron-Positron Collider (BEPCⅡ) operates at the cernter-of-mass energy within 2.0～4.6 GeV. According to the current knowledge, a meson consists of quark and antiquark pair, and a baryon consists of three quarks. In addi-tion, QCD also predicts the existence of the exotic states, such as the glueball, hybrid, multiquark and molecule states and so on. The Beijing Spectrometer (BESⅢ) is not only a general purpose magnetic spectrometer operating at BEPCⅡ, but also a world-wide unique detector running at the energy region of 2.0-4.6 GeⅤ. Since successfully constructed in 2008, BESⅢ has accumulated the largest samples of J/ψ and ψ(3686) in the world. The decays of J/ψ and ψ(3686) can provide not only the unprecedented opportunity for studying the hadron spectroscopy, but also the excellent laboratory for searching the exotic states.In 2003, a strong enhancement near the pp mass threshold, known as the X(pp-), was first observed by the BESII experiment in the radiative decay J/ψ �' γpp-. It was later confirmed by the CLEO and BESIII experiments. In theory, X(pp-) is assumed to be a baryonium, a multiquark state or an effect due to pure final-state interaction (FSI), its nature has not been clarified to date. In addition, long time ago it was proposed that pφ production is well suited to search the exotic pentaquark candidate with hidden strangeness. Just in 2015, LHCb reported the observation of two charmonium pen-taquark structures with the quark content ccuud in the pJ/ψ mass spectrum. Therefore, it is vary important to search the pentaquark candidate with the quark content ssuud in the pφ or p-φ mass spectrum.Using the data sample of 1.31 x 109 J/ψ events accumulated with the BESⅢ de-tector, the decay J/ψ�'pp-φ is studied in detail via two decay modes φ�' K0sK0L and φ�' K+K-, which can not only improve the precision but also check the mea-surements to each other. The average branching fraction of J/ψ�' pp-φ is measured to be B(J/ψ�' pp-φ)= (5.23 ± 0.06 (stat) ± 0.33(syst)) × 10-5, which agrees well with a previously published measurement, but with a significantly improved preci- sion. No evident enhancement near the pp mass threshold is observed, and the up-per limit on the branching fraction of J/ψ�' X{pp-}φ�' ppφ is determined to be B(J/v�' X{pp-)0�'( ppφ)< 2.1 x 107 at a 90% confidence level.In addition, since the discovery of ao(980) and f0(980), the nature of these two light scalar mesons has been controversial for several decades. Although they are well-established, these two states, with similar masses but different decay modes and isospins, are difficult to accommodate in the traditional quark-antiquark model. Theo-retically, a00(980) and f0(980) are proposed to be tetra-quarks, KK molecule, or quark-antiquark gluon hybrid and so on. The mixing mechanism in the system of a00(980)-f0(980) was firstly proposed theoretically in the late 1970s. At present, the mixing mechanism has been accepted extensively and is thought to be of essential help to clar-ify the nature of these two mesons.In 2011, based on the samples of 2.25 x 108 J/ψ events and 1.06 x 108ψ(3686) events, a direct measurement on the a00(980)-f0(980) mixing was carried out by the isospin violated decays J/ψ�'φa00(980)�'φηπ0 and Xcl�'π0f0(980)�' π0π+π-at BESⅢ. The statistical significance of the signals of the f0(980)�' a00(980) and a00(980)�'f0(980) transitions are only 3.4σ and 1.9σ, and the corresponding mixing intensities are determined to be ξfa= (0.60 ± 0.20 (stat) ±0.12 (syst)±0.26 (para))% andξaf= (0.31 ± 0.16 (stat) ± 0.14 (syst) ± 0.03(para))%, respectively.At present, based on the samples of 1.31 x 109 J/ψ events and 4.48 × 108 ψ(3686) events accumulated with BESⅢ detector, we comprehensively studied the a00(980)-f0(980) mixing via the decays of J/ψ�'φa00(980)�' φηπ0 (η�'γγ and η�' π+π-π0) and Xcl�' π0 f0(980)�'π0π+π-. We fit the mass spectra without and with considering the interference between the mixing signal and the electromagnetic (EM) processes, respectively. The signals of f0(980)�'a00(980) and a00(980)�'f0(980) are observed with the statistical significance of 7.3Bσ and 5.5a, respectively. The branching fractions and corresponding mixing intensities are precisely measured.Various models give different predictions for the central masses and coupling con-stants of a00(980) and f0(980), which have also been measured by several experiments. Using the same method, different fits are performed with the corresponding center masses and coupling constants from different models and experiments, and the cor-responding intensities of the a00(980) ←�' f0o(980) mixing are calculated.Finally, using the inclusion-exclusion principle, we also developed the sideband-background-analysis method, which could generalize the two-dimensional sideband to three-dimensional or multi-dimensional.