The beam energy measurement system (BEMS) for the upgraded Beijing electron-positron collider BEPC-II is described. The system is based on measuring the energies of Compton back-scattered photons. ...The relative systematic uncertainty of the electron and positron beam energy determination is estimated as 2⋅10−5.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A new high precision measurement of the J/ψ- and ψ′-meson masses has been performed at the VEPP-4M collider using the KEDR detector. The resonant depolarization method has been employed for the ...absolute calibration of the beam energy. The following mass values have been obtained: MJ/ψ=3096.917±0.010±0.007 MeV,Mψ′=3686.111±0.025±0.009 MeV. The relative measurement accuracy has reached 4×10−6 for J/ψ and 7×10−6 for ψ′, approximately 3 times better than in the previous precise experiments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Phys. Rev. Lett. 114, 062005 (2015) The ratio of the elastic $e^+ p$ to $e^- p$ scattering cross sections has
been measured precisely, allowing the determination of the two-photon exchange
...contribution to these processes. This neglected contribution is believed to be
the cause of the discrepancy between the Rosenbluth and polarization transfer
methods of measuring the proton electromagnetic form factors. The experiment
was performed at the VEPP-3 storage ring at beam energies of 1.6 and 1.0 GeV
and at lepton scattering angles between $15^\circ$ and $105^\circ$. The data
obtained show evidence of a significant two-photon exchange effect. The results
are compared with several theoretical predictions.
The process \(e^+e^- \to \pi^0\gamma\) is studied with the SND detector at the VEPP-2000 \(e^+e^-\) collider. Basing on data with an integrated luminosity of 41 pb\(^{-1}\) recorded in 2010-2012 we ...measure the \(e^+e^- \to \pi^0\gamma\) cross section in the center-of-mass energy range from 1.075 up to 2 GeV. In the range 1.4-2.0 GeV the process \(e^+e^- \to \pi^0\gamma\) is studied for the first time.
A search for the rare decay \(\eta \rightarrow e^+e^-\) is performed using the inverse process \(e^+e^- \rightarrow \eta\) in the decay mode \(\eta\to\pi^0\pi^0\pi^0 \). We analyze data with an ...integrated luminosity of 654 nb\(^{-1} \) accumulated at the VEPP-2000 \(e^+e^-\) collider with the SND detector at the center-of-mass energy \(E=m_\eta c ^2\approx 548 \) MeV, and set the upper limit \( {\cal B} (\eta \rightarrow e^+ e^-) < 7 \times 10^{-7} \) at the 90\% confidence level.
Phys. Lett. B 738 (2014), 391-396 Using the inclusive photon spectrum based on a data sample collected at the
$J/\psi$ peak with the KEDR detector at the VEPP-4M $e^+e^-$ collider, we
measured the ...rate of the radiative decay $J/\psi\to\gamma\eta_{\rm c}$ as well
as $\eta_{\rm c}$ mass and width. Taking into account an asymmetric photon
lineshape we obtained $\Gamma^0_{\gamma\eta_{\rm c}}=2.98\pm0.18
\phantom{|}^{+0.15}_{-0.33}$ keV, $M_{\eta_{\rm c}} = 2983.5 \pm 1.4
\phantom{|}^{+1.6}_{-3.6}$ MeV/$c^2$, $\Gamma_{\eta_{\rm c}} = 27.2 \pm 3.1
\phantom{|}^{+5.4}_{-2.6}$ MeV.
The product of the electronic width of the \(\psi(2S)\) meson and the branching fraction of its decay to the muon pair was measured in the \(e^{+}e^{-} \to \psi(2S) \to \mu^{+}\mu^{-}\) process using ...nine data sets corresponding to an integrated luminosity of about 6.5 pb\(^{-1}\) collected with the KEDR detector at the VEPP-4M electron-positron collider: \ \Gamma_{ee}\times\mathcal{B}_{\mu\mu} = 19.3 \pm 0.3 \pm 0.5 ~\text{eV}. \ Adding the previous KEDR results on hadronic and leptonic channels, the values of the \(\psi(2S)\) electronic width were obtained under two assumptions: either with the assumption of lepton universality \ \Gamma_{ee} = 2.279 \pm 0.015 \pm 0.042 ~\text{keV} \ or without it, summing up hadronic and three independent leptonic channels: \ \Gamma_{ee} = 2.282 \pm 0.015 \pm 0.042 ~\text{keV}. \
The product of the electronic width of the \(J/\psi\) meson and the branching fractions of its decay to hadrons and electrons has been measured using the KEDR detector at the VEPP-4M \(e^+e^-\) ...collider. The obtained values are: \(\Gamma_{ee}(J/\psi) = 5.550 \pm 0.056 \pm 0.089 \, \text{keV}, \) \(\Gamma_{ee}(J/\psi) \cdot \mathcal{B}_\text{hadrons}(J/\psi) = 4.884 \pm 0.048 \pm 0.078 \, \text{keV}, \) \(\Gamma_{ee}(J/\psi) \cdot \mathcal{B}_{ee}(J/\psi) = 0.3331 \pm 0.0066 \pm 0.0040 \, \text{keV}.\) The uncertainties shown are statistical and systematic, respectively. Using the result presented and the world-average value of the electronic branching fraction, one obtains the total width of the \(J/\psi\) meson: \(\Gamma = 92.94 \pm 1.83 \, \text{keV}.\) These results are consistent with the previous experiments.
Phys. Lett. B731, 227 (2014) The ratio of the electron and muon widths of the $J/\psi$ meson has been
measured using direct $J/\psi$ decays in the KEDR experiment at the VEPP-4M
electron-positron ...collider. The result
$\Gamma_{ee}(J/\psi)/\Gamma_{\mu\mu}(J/\psi)=1.0022\pm0.0044\pm0.0048\
(0.65\%)$ is in good agreement with the lepton universality. The experience
collected during this analysis will be used for a $J/\psi$ lepton width
determination with up to 1% accuracy.
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