Search For Baryonic Decay Modes Of Charmonium χ_cJ And η_c With The BESⅢdetector At BEPCⅡ

In this thesis, I report my work carried out at Beijing Spectrometer (BESIII) ex-periment. The BESIII is the only experiment of Beijing Electron and Positron Collider (BEPCII), which is a double-ring collider covering the center-of-mass energy from2GeV to4.6GeV and provides a highest luminosity of7.0x1032cm-2s-1at energy point3770MeV, and a typical flavor factory for probing the T-charm physics. This energy regime is full of attracting features:rich of resonances, such as charmnoium and charm me-son; mass threshold for producing TT and D)(*)D(*) pairs; a variety of exotic hadrons; interplay of perturbative QCD and nonperurbative QCD. BESIII has accumulated world largest data samples at the peak positions of J/ψ andψ(3686) in the year of2009, and at the center-of-mass energies ranging from3.80GeV to4.42GeV in the year of2012and2013. Based on these samples, my thesis work mainly focus on searches for baryonic decay modes of χcJ and ηcJ and searches for exotic states above open charm t.I Search for baryonic decay modes of charmonium χcJ and ηcJ.1. The first measurements ofχcJ decaying into ppK+K-final statesFirst measurements of the decays of the three χcJ (J=0,1,2) states to ppK+K-final states are presented by using samples of χcJ mesons produced via radiative tran-sitions from106x106ψ(3686) mesons collected in the BESIII detector. Intermediate φâ†'K+K-and A(1520)â†'pK-resonance states are observed, and branching frac-tions for χcJâ†'pK+ΛA(1520), Λ(1520)Λ(1520), and φpp are reported. We also measure branching fractions for direct χcJâ†'ppK+K-decays. The measured branching frac-tions are listed in the following table, where the first error are statistical and the second systematic.These are first observations of χcJ decays to unstable baryon resonances. This new measurement to search for excited baryon pairs in the decays of P-wave charmonia can provide more information to understand the earlier contradiction between χcJâ†'PP and χcJâ†'ΛΛ when applying the Color-Octet Mechanism (COM). COM is a good approach to compensate the insufficiency of Color-Singlet Mechanism for the P-wave decays de-scription.2. The first observation ofηc decay into Σ+Σ-and (?)-(?)+final statesUsing a data sample of2.25x108J/ψ events collected with the BESIII detector, first observations of the decays of r\c mesons to Σ+Σ-î—„and (?)-(?)+are presented, where the ηc is produced via J/ψâ†'γηc,Σ is reconstructed with pÏ€0, and (?) withΛπ。 The branching fractions are measured to be where the first uncertainties are statistical, the second systematic, and the third uncertain-ties are from the precision of the intermediate branching fractions.ηcâ†'baryonpair violates the helicity selection rule that is the direct conclusion of QCD asymptotic behavor, and Intermediate charm Meson Loop mechanism (IML), a kind of long-distance effect, is introduced to account for the unexpected large branching fractions of them. The experimental results are quite well consistent with ML predic-tions. At present, the branching fraction of J/ψâ†'γηchas-20%uncertainty which is the dominant error of the final values. Better comparisons and understandings of the long-distance effect can be done by measuring the higher mass baryon pairs.3. Measurement of ψ(3686)â†'pK+Σ0and χcJâ†'pK+ΛUsing a sample of106x106ψV(3686) mesons collected with the BESIII detector and χcJ mesons produced via radiative transitions from theψ(3686), we performed the first observation forψ(3686)â†'pK+Σ0, as well as improved measurements for theχ cJ hyperon decays χcJâ†'pK+A. The Σ0is reconstructed by Σ0â†'γΛSâ†'γΛ with the branching fraction almost100%and the A is tagged by its dominant decay Λâ†'pÏ€.The branching fractions are measured to be where the first error is statistical, and the second is systematic.The nucleon-antinucleon (NN) bound state is always the interesting topic in high energy physics, because it is the exotic state predicted under the QCD frame but has yet to be confirmed in experiment. The bound NN system is named as "baryonium" in analogy to positronium and sensitive to threshold effect due to easily bound via strong interaction. Based on this idea, we try to search for it with p and A final state. In the decays of XcJâ†'pK+Λ, an anomalous enhancement near threshold is observed in the invariant mass distribution of pΛ, which cannot be explained by phase space.II Search for exotic states above open charm1. The first observation of a charged charmonium-like structure in e+e-â†'(D*D*)±π(?) at center-of-mass energy4.26GeVe+e-â†'(D*D)±π(?) at a center-of-mass energy of4.26GeV is studied using a827pb"1data sample. This is motivated by the observation of a charged four-quark candidate Zc(3900) at BESIII and searches for its potential excited partners with strongly coupling to D*D*final state. Based on a partial reconstruction technique, the Born cross section is measured to be137±9±15pb. A structure near the (D*D*)±threshold in the Ï€(?) recoil mass spectrum is observed, which is denoted as the Z±(4025). The measured mass and width of the structure areIts production ratio is determined to beThe first uncertainties are statistical and the second are systematic.2. Cross section measurement of e+e-â†'ηJ/ψand Ï€0J/ψUsing the13data samples covering center-of-mass energy from3.80GeV to4.42GeV collected with BESIII detector, cross section of e+e-â†'ηJ/ψ,Ï€0J/ψ are measured with the decay modes of ηâ†'γγ,Ï€0â†'γγ, and J/ψâ†'e+e-/μ+μ-. Significant ηJ/ψ are observed at several energy points and the corresponding cross section line shape is obtained. The majority of our measurements are more precise than BELLE’s results. Due to the isospin violation, we only give the upper limits of cross section of Ï€0J/ψ at90%confidence level with the world largest data samples at center-of-mass energies4.23GeV and4.26GeV, where samples of1047pb-1and827pb-1are accumulated, respectively.The cross sections of e+e-â†'ηJ/ψ and Ï€0J/ψ, which have been evaluated theo-retically in the framework of nonrelativistic QCD and predicted to be strongly affected by open charm effect, is helpful to the understanding of the nature of Y(4260) which has peculiar features and is quite different from general Jpc=1--cc charmonium.