Measurement Of Proton Form Factor And Baryon Pairs Production In E~+e~-Annihilation At BESâ…¢

Study of the internal structure of the nucleon is of high significance to particle physics. The nucleons are not point-like particles, and the most direct evidence is the observed anomalous magnetic moment of nucleons (μp=2.79μN, μn=-1.91μN), while theoretically, the magnetic moment of point-like proton and neutron is μN and0, respectively. Another evidence is from elastic scattering of electrons on nucleon-s. The differential cross section of the elastic scattering is different from point-like Dirac scattering, which brings the definition of nucleon form factors (FF). The FFs are semi-empirical formula in effective quantum field which help describe the spatial dis-tributions of electric charge and current. Besides, the FFs constitute a rigorous test of QCD as well as of phenomenological models. The FFs can be measured in space-like region (four-momentum transfer q2<0) and time-like region (q2>0). In the last forty years, lots of experiments were performed to extract the space-like FFs, relatively few to extract time-like nucleon FFs. There are still many mysteries on the shapes of pro-ton FFs, such as the very steep rise towards threshold, two rapid decreases of the FFs and the poor precision of electromagnetic FF ratio (|GE/GM|).Moreover, the knowl-edge on neutron FFs and other baryon FFs are very poor and far from being understood. Therefore, systematic study on FFs and precision measurement of FFs are mandatory.BEPCII is a double-ring e+e-collider running in2.0-4.6GeV center-of-mass energies. The designed luminosity is1.0x1033cm-2s-1at the optimized c.m. energy,(?)s=3770MeV. BESIII is the only detector operating at BEPCII. In this thesis, by us-ing data samples collected in continuum region with the BESIII detector, we measured the Born cross section of e+e-â†'pp at14c.m. energies from2232.4to3671.0MeV. Identification of proton/antiproton has been achieved mostly by means of the combined information of dE/dx and TOF, and after the requirements on momentum and back-to-back angle, the signal is selected with large signal-to-noise ratio. The measured cross sections are in agreement with recent results from BaBar, improving the overall uncer-tainty by about30%. The corresponding effective electromagnetic FF of the proton is deduced by assuming (as it is the definition of effective FF) the electric and magnet-ic FFs to be equal{|GE|=|GM|). Moreover, the ratio of electric to magnetic FFs,|GE/GM|, and|GM|are extracted by fitting the distribution of the polar angle of the proton for the data samples with larger statistics, namely at (?)s=2232.4and2400.0MeV and a combined sample at (?)s=3050.0,3060.0and3080.0MeV, respectively. For these energies the|GE/Gm|ratios are close to unity and consistent with BaBar re-sults at the same q2region. The precision of|GE/GM|is limited by statistics, being between25%and50%. Therefore the data at these energies are consistent with the assumption that|GE|=|GM|, within the aforementioned uncertainties.In addition to the proton FF, we also studied the process of electron positron annihi-lation into AA pair and measured its production cross section as well as the effective FF of A. With the data collected at2232.4MeV with the BESIII, that is only1.0MeV above the∧∧threshold, we measured the Born cross section of e+eâ†'∧∧by two methods, namely i) reconstructing charged decay channel of∧/∧(∧/∧â†'pÏ€-/pÏ€+). Since the momentum of the final states are less than200MeV, the pions are circling in MDC, and the track of proton/antiproton can not be reconstructed in MDC. Therefore, the signals are extracted by fitting the vertex of secondary particles produced from interaction-s between antiproton and beampipe. ii) by reconstructing neutral decay channel of A (∧â†'nÏ€0). Good events are identified through multiple variable analysis since antineu-tron leaves information in EMC, and the extraction of the signal is achieved by fitting the momentum of the neutral pion, since A is almost at rest. The measured Born cross section of this two methods are consistent, and the combined result is319.5±57.6pb. It is the first measurement of e+e-â†'∧∧near threshold. It contradicts the standard theoretical prejudice, which is that the cross section should vanish at2232.4MeV, since the phase space factor β=(?)1-4mB2/s is close to0. This result strongly suggests that something more is at play here beyond the expected phase space behavior. Besides, with the data collected at2400.0,2800.0,3080.0MeV, we measured the cross section of e+e-â†'AA and extracted the corresponding effective FF. The precision is between22%and33%, limited by statistics. Moreover, since the|GE/GM|ratio of A was not measured due to the statistics limitation, the uncertainty from the A angular distribution becomes an important source in the systematic error.At low energy region, because of the growing of the running QCD coupling con-stant and the associated confinement of quarks and gluons, it is meaningless to apply perturbative QCD. BEPCII is a machine operating in the energy region connecting non-pQCD to pQCD, The experimental results at BESIII is an important input for various QCD-based theoretical models.In this thesis, by using2.25×108J/φ events collected with BESIII, we for the first time observed the process J/φâ†'ppa0(980), a0(980)â†'Ï€0η, with a significance of6.5σ (3.2σ including systematic uncertainties). The product branching fraction of J/φâ†'ppa0(980)â†'ppÏ€0η is measured to be (6.8±1.2±1.3)×10-5. This mea-surement provides information on the a0production near threshold coupling to pp and improves the understanding of the dynamics of J/φ decays to four body processes. The effective field theory, Chiral Perturbation Theory (ChPT) predicts the amplitude of J/φâ†'ppiÏ€0η with a0(980) meson generated through final state interaction with some free coefficients. The experimental result will provide a quantitative comparison with the chiral unitary approach and helps settle these coefficients.