We report on a precision measurement of the cross section for the reaction e+e−→π+π− in the mass range 0.30<Mππ<1.00 GeV/c2 with the initial state radiation (ISR) method, using 817 pb−1 of data at ...e+e− center-of-mass energies near 3.77 GeV and 586 pb−1 of data at e+e− center-of-mass energies near 4.17 GeV, collected with the CLEO-c detector at the CESR e+e− collider at Cornell University. The integrated cross sections in the range 0.30<Mππ<1.00 GeV/c2 for the process e+e−→π+π− are determined with a statistical uncertainty of 0.7% and a systematic uncertainty of 1.6%. The leading-order hadronic contribution to the muon anomalous magnetic moment calculated using these measured e+e−→π+π− cross sections in the range Mππ=0.30 to 1.00 GeV/c2 is calculated to be (500.6±3.6(stat)±8.0(syst))×10−10.
Using e+e− annihilation data taken at the CESR collider with the CLEO-c detector, measurements of hyperon pair production cross sections and elastic and transition electromagnetic form factors have ...been made at the charmonium resonances: ψ(2S), s=3.69 GeV, |Q2|=13.6 GeV2, L=48 pb−1; ψ(3770), s=3.77 GeV, |Q2|=14.2 GeV2, L=805 pb−1; and ψ(4170), s=4.17 GeV, |Q2|=17.4 GeV2, L=586 pb−1. Results with good statistical precision are obtained with high efficiency particle identification. Systematics of pair production cross sections, and form factors with respect to the number of strange quarks in the hyperons are studied, and evidence is presented for the effects of diquark correlations in comparative results for Λ0 and Σ0, both of which have the same uds quark content but different isospin.
The electromagnetic structure of the lightest hadrons, proton, pion, and kaon is studied by high-precision measurements of their form factors for the highest timelike momentum transfers of ...|Q2|=s=14.2 and 17.4 GeV2. Data taken with the CLEO-c detector at sqrts=3.772 and 4.170 GeV, with integrated luminosities of 805 and 586 pb(-1), respectively, have been used to study e+ e- annihilations into π+ π-, K+ K-, and pp. The dimensional counting rule prediction that at large Q2 the quantity Q2F(Q2) for pseudoscalar mesons is nearly constant, and should vary only weakly as the strong coupling constant αS(Q2) is confirmed for both pions and kaons. However, the measurements are in strong quantitative disagreement with the predictions of the existing quantum chromodynamics-based models. For protons, it is found that the timelike form factors continue to remain nearly twice as large as the corresponding spacelike form factors measured in electron elastic scattering, in significant violation of the expectation of their equality at large Q2. Further, in contrast to pions and kaons, a significant difference is observed between the values of the corresponding quantity |Q4|G(M)(|Q2|)/μ(p) for protons at |Q2|=14.2 and 17.4 GeV2. The results suggest the constancy of |Q2|G(M)(|Q2|)/μ(p), instead, at these large |Q2|.
Using e+e− annihilation data taken with the CLEO-c detector, measurements of two major Ω− branching fractions have been made using the reaction ψ(2S)→Ω−Ω¯+. The Ω− decay channels are identified ...through momentum distributions of charged particles, and systematics of the Ω− branching fractions have been studied. The results are B(Ω−→Λ0K−)=(68.9±9.5±4.3)% and B(Ω−→Ξ0π−)=(19.0±4.0±1.2)%.
Using 805 pb−1 of e+e− annihilation data taken with the CLEO-c detector at ψ(3770), s=3770 MeV, we report the first measurements of the electromagnetic form factors of the Λ0, Σ0, Σ+, Ξ0, Ξ−, and Ω− ...hyperons for the large timelike momentum transfer of |Q2|=14.2 GeV2. The form factors for the different hyperons are found to vary by nearly a factor two. It is found that |GM(Λ0)|=1.66(24)×|GM(Σ0)|. The Λ0 and Σ0 hyperons have the same uds quark content, but differ in their isospin, and therefore the spin of the ud quark pair. It is suggested that the spatial correlation implied by the singlet spin–isospin configuration in the Λ0 is an example of strong diquark correlations in the Λ0, as anticipated by Jaffe and Wilczek. Improved measurements of the branching fractions of ψ(2S)→pp¯ and hyperon–antihyperon pairs are also reported.
The data for 9.3 million Υ(2S) and 20.9 million Υ(1S) taken with the CLEO III detector have been used to study the radiative population of states identified by their decay into 26 different exclusive ...hadronic final states. In the Υ(2S) decays, an enhancement is observed at a ~5σ level at a mass of 9974.6±2.3(stat)±2.1(syst) MeV. It is attributed to η(b)(2S) and corresponds to the Υ(2S) hyperfine splitting of 48.7±2.3(stat)±2.1(syst) MeV. In the Υ(1S) decays, the identification of η(b)(1S) is confirmed at a ~3σ level with Mη(b)(1S) in agreement with its known value.
Fermilab experiment E835 has measured the cross section for the reaction p̄p→e+e− at s=11.63, 12.43, 14.40 and 18.22 GeV2. From the analysis of the 66 observed events new high-precision measurements ...of the proton magnetic form factor are obtained.
We report measurements of the photon beam asymmetry for the reactions γp → pπ0 and γp → pη from the GLUEX experiment using a 9 GeV linearly polarized, tagged photon beam incident on a liquid hydrogen ...target in Jefferson Lab’s Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous π0 measurements and are the first η measurements in this energy regime. The results are compared with theoretical predictions based on t-channel, quasiparticle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.
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