The cross section for the process $e^+e^- \to K^+K^-$ was measured in the energy range $\sqrt{s}$ = 1.04--1.38 GeV in the SND experiment at VEPP-2M $e^+e^-$ collider. The measured cross section is described by the Vector Meson Dominance model with contributions from the light vector mesons $\rho$, $\omega$, $\phi$ and their lowest excitations. The mean statistical accuracy of the measurement is 4.4 %, and the systematic uncertainty is 5.2 %.
In experiments with the CMD-2 detector at the VEPP-2M electron-positron collider at Novosibirsk about 150000 $e^+e^-\to\pi^+\pi^-$ events were recorded in the center-of-mass energy range from 0.61 up to 0.96 GeV. The result of the pion form factor measurement with a 1.4% systematic error is presented. The following values of the $\rho$-meson and $\rho-\omega$ interference parameters were found: $M_\rho=(775.28\pm 0.61\pm 0.20) MeV, \Gamma_\rho=(147.70\pm 1.29 \pm 0.40) MeV, \Gamma(\rho\to e^+e^-)=(6.93\pm 0.11\pm 0.10) keV, Br(\omega\to\pi^+\pi^-) = (1.32\pm 0.23)%$.
The $e^+e^-\to K^+K^-$ cross section and charged-kaon electromagnetic form factor are measured in the $e^+e^-$ center-of-mass energy range ($E$) from 2.6 to 8.0 GeV using the initial-state radiation technique with an undetected photon. The study is performed using 469 fb$^{-1}$ of data collected with the BABAR detector at the PEP-II $e^+e^-$ collider at center-of-mass energies near 10.6 GeV. The form factor is found to decrease with energy faster than $1/E^2$, and approaches the asymptotic QCD prediction. Production of the $K^+K^-$ final state through the $J/\psi$ and $\psi(2S)$ intermediate states is observed. The results for the kaon form factor are used together with data from other experiments to perform a model-independent determination of the relative phases between single-photon and strong amplitudes in $J/\psi$ and $\psi(2S)\to K^+K^-$ decays. The values of the branching fractions measured in the reaction $e^+e^- \to K^+K^-$ are shifted relative to their true values due to interference between resonant and nonresonant amplitudes. The values of these shifts are determined to be about $\pm5\%$ for the $J/\psi$ meson and $\pm15\%$ for the $\psi(2S)$ meson.
The process $e^+e^-\to n\bar{n}$ has been studied at the VEPP-2000 $e^+e^-$ collider with the SND detector in the energy range from threshold up to 2 GeV. As a result of the experiment, the $e^+e^-\to n\bar{n}$ cross section and effective neutron form factor have been measured.
Using data samples collected with the BESIII detector at the BEPCII collider, we measure the Born cross section of $e^{+}e^{-}\rightarrow p\bar{p}$ at 12 center-of-mass energies from 2232.4 to 3671.0 MeV. The corresponding effective electromagnetic form factor of the proton is deduced under the assumption that the electric and magnetic form factors are equal $(|G_{E}|= |G_{M}|)$. In addition, the ratio of electric to magnetic form factors, $|G_{E}/G_{M}|$, and $|G_{M}|$ are extracted by fitting the polar angle distribution of the proton for the data samples with larger statistics, namely at $\sqrt{s}=$ 2232.4 and 2400.0 MeV and a combined sample at $\sqrt{s}$ = 3050.0, 3060.0 and 3080.0 MeV, respectively. The measured cross sections are in agreement with recent results from BaBar, improving the overall uncertainty by about 30\%. The $|G_{E}/G_{M}|$ ratios are close to unity and consistent with BaBar results in the same $q^{2}$ region, which indicates the data are consistent with the assumption that $|G_{E}|=|G_{M}|$ within uncertainties.
The cross section of the process e+e- -> pi+pi- has been measured using about 114000 events collected by the CMD-2 detector at the VEPP-2M e+e- collider in the center-of-mass energy range from 0.61 to 0.96 GeV. Results of the pion form factor determination with a 0.6% systematic uncertainty are presented. Implications for the hadronic contribution to the muon anomalous magnetic moment are discussed.
The cross section of the process e^+e^-\to \pi^+\pi^- was measured in the SND experiment at the VEPP-2M collider in the energy region 400<\sqrt[]{s}<1000 MeV. This measurement was based on about 12.4 \times 10^6 selected collinear events, which include 7.4\times 10^6 e^+e^-\to e^+e^-, 4.5\times 10^6 e^+e^-\to\pi^+\pi^- and 0.5\times 10^6 e^+e^-\to\mu^+\mu^- selected events. The systematic uncertainty of the cross section determination is 1.3 %. The \rho-meson parameters were determined: m_\rho=774.9\pm 0.4\pm 0.5 MeV, \Gamma_\rho=146.5\pm 0.8\pm 1.5 MeV, \sigma(\rho\to\pi^+\pi^-)=1220\pm 7\pm 16 nb as well as the parameters of the G-parity suppressed decay \omega\to\pi^+\pi^-: \sigma(\omega\to\pi^+\pi^-)=29.9\pm 1.4\pm 1.0 nb and \phi_{\rho\omega} = 113.5\pm 1.3\pm 1.7 degree.
We present a measurement of the pion form factor based on e+e- annihilation data from the CMD-2 detector in the energy range 0.6<sqrt(s)<1.0 GeV with a systematic uncertainty of 0.8%. A data sample is five times larger than that used in our previous measurement.
We have measured the cross section $\sigma(e^+e^-\to \pi^+\pi^- \gamma)$ at an energy $W=m_\phi=1.02$ GeV with the KLOE detector at the electron-positron collider DA$\Phi$NE. From the dependence of the cross section on the invariant mass of the two-pion system, we extract $\sigma(e^+e^-\to \pi^+\pi^-)$ for the mass range $0.35<s<0.95$ GeV$^2$. From this result, we calculate the pion form factor and the hadronic contribution to the muon anomaly, $a_\mu$.
Cross sections for e^+e^- -> ppbar have been measured at 10 center-of-mass energies from 2.0 to 3.07 GeV by the BESII experiment at the BEPC, and proton electromagnetic form factors in the time-like region have been determined.
The cross section of the process e+e- -> pi+pi- has been measured at the CMD-2 detector in the 370-520 MeV center-of-mass (c.m.) energy range. A systematic uncertainty of the measurement is 0.7 %. Using all CMD-2 data on the pion form factor, the pion electromagnetic radius was calculated. The cross section of muon pair production was also determined.
The e+e- -> p anti-p cross section is determined over a range of p anti-p masses, from threshold to 4.5 GeV/c^2, by studying the e+e- -> p anti-p gamma process. The data set corresponds to an integrated luminosity of 232 fb^-1, collected with the BABAR detector at the PEP-II storage ring, at an e+e- center-of-mass energy of 10.6 GeV. The mass dependence of the ratio of electric and magnetic form factors, |G_E/G_M|, is measured for p anti-p masses below 3 GeV/c^2: its value is found to be significantly larger than 1 for masses up to 2.2 GeV/c^2. We also measure J/psi -> p anti-p and psi(2S) -> p anti-p branching fractions and set an upper limit on Y(4260) -> p anti-p production and decay.
The electromagnetic form factors of the neutron in the time-like region have been measured for the first time, from the threshold up to q 2 ⋟ 6 GeV 2 . The neutron magnetic form factor turns out to be larger than the proton one; the angular distribution suggests that for the neutron, at variance with the proton case, electric and magnetic form factors could be different. Further measurements are also reported, concerning the proton form factors and the Σ Σ production, together with the multihadronic cross section and the J / Γ branching ratio into n n .
We have measured the cross section $\sigma(e^+e^-\to\pi^+\pi^-\gamma(\gamma))$ at DA$\Phi$NE, the Frascati \phi-factory, using events with initial state radiation photons emitted at small angle and inclusive of final state radiation. We present the analysis of a new data set corresponding to an integrated luminosity of 240 pb$^{-1}$. We have achieved a reduced systematic uncertainty with respect to previously published KLOE results. From the cross section we obtain the pion form factor and the contribution to the muon magnetic anomaly from two pion states in the mass range $0.592 < M_{\pi\pi} < 0.975$ GeV. For the latter we find $\Delta a^{\pi\pi}_\mu = (387.2\pm0.5_{\rm stat}\pm2.4_{\rm exp}\pm2.3_{\rm th})\times 10^{-10}$
The pion electromagnetic form factor has been measured at the VEPP-2M collider in the c.m. energy range 360 MeV–1400 MeV with the detectors OLYA and CMD. On the basis of all available data for the pion form factor collected in the timelike region, the following values for ρ-meson parameters were obtained: m ρ = 775.9 ± 1.1 MeV, σ ρ = 150.5 ± 3.0 MeV. The ω-meson branching ratio into π + π − pair, electromagnetic radius of the pion, ππ scattering length in the P-wave and the strong interaction contribution to the muon ( g − 2) value were found to be B ωππ = (2.3 ± 0.4)%, 〈 r π 2 〉 = 0.422 ± 0.013 fm 2 , a 1 1 = 0.033 ± 0.033m π −3 , a H = (68.4 ± 1.1) × 10 −9 .
In Phys. Lett. B 753, 629-638 (2016) [arXiv:1507.08188] the BESIII collaboration published a cross section measurement of the process $e^+e^-\to \pi^+ \pi^-$ in the energy range between 600 and 900 MeV. In this erratum we report a corrected evaluation of the statistical errors in terms of a fully propagated covariance matrix. The correction also yields a reduced statistical uncertainty for the hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon, which now reads as $a_\mu^{\pi\pi\mathrm{, LO}}(600 - 900\,\mathrm{MeV}) = (368.2 \pm 1.5_{\rm stat} \pm 3.3_{\rm syst})\times 10^{-10}$. The central values of the cross section measurement and of $a_\mu^{\pi\pi\mathrm{, LO}}$, as well as the systematic uncertainties remain unchanged.
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