The process e+e- --> pi+ pi- pi0 gamma has been studied at a center-of-mass energy near the Y(4S) resonance using a 89.3 fb-1 data sample collected with the BaBar detector at the PEP-II collider. From the measured 3pi mass spectrum we have obtained the products of branching fractions for the omega and phi mesons, B(omega --> e+e-)B(omega --> 3pi)=(6.70 +/- 0.06 +/- 0.27)10-5 and B(phi --> e+e-)B(phi --> 3pi)=(4.30 +/- 0.08 +/- 0.21)10-5, and evaluated the e+e- --> pi+ pi- pi0 cross section for the e+e- center-of-mass energy range 1.05 to 3.00 GeV. About 900 e+e- --> J/psi gamma --> pi+ pi- pi0 gamma events have been selected and the branching fraction B(J/psi --> pi+ pi- pi0)=(2.18 +/- 0.19)% has been measured.
The cross section of the process e+e−→π+π−π0 has been measured in the c.m. energy range 984–1060 MeV with the CMD-2 detector at the VEPP-2M collider. The obtained value of Br(ϕ→e+e−)Br(ϕ→π+π−π0)=(4.51±0.16±0.11)×10−5 is in good agreement with the previous measurements and has the best accuracy. Analysis of the Dalitz plot was performed. The contributions of the dominant ϕ→ρπ mechanism as well as of a small direct ϕ→3π amplitude were determined.
Radiative decays of the $\phi$ meson have been studied using a data sample of about 19 million $\phi$ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From selected $e^+e^-\to\pi^{0}\pi^{0}\gamma$ and $e^+e^-\to\eta\pi^{0}\gamma$ events the following model independent results have been obtained: \par $Br(\phi\to\pi^{0}\pi^{0}\gamma) = (0.92\pm 0.08\pm0.06)\times10^{-4}$ for $M_{\pi^{0}\pi^{0}}>700$ MeV, \par $Br(\phi\to\eta\pi^{0}\gamma) = (0.90\pm 0.24\pm 0.10)\times10^{-4}$. It is shown that the intermediate mechanism $f_{0}(980)\gamma$ dominates in the $\phi\to\pi^{0}\pi^{0}\gamma$ decay and the corresponding branching ratio is \par $Br(\phi\to f_{0}(980)\gamma)=(2.90\pm 0.21\pm1.54)\times10^{-4}$. The systematic error is dominated by the possible model uncertainty. \par Using the same data sample the upper limit has been obtained for the P- and CP-violating decay of $\eta$ at 90% CL: \par $Br(\eta\to\pi^{0}\pi^{0}) < 4.3\times10^{-4}$ >.
Using 11.6 pb^{-1} of data collected in the energy range 0.984--1.06 GeV by CMD-2 at VEPP-2M, the cross section of the reaction e+e- to pi+pi-pi+pi- has been studied. For the first time an interference pattern was observed in the energy dependence of the cross section near the phi meson. The branching ratio of the phi to pi+pi-pi+pi- decay double suppressed by the G-parity and OZI rule is measured Br(phi to pi+pi-pi+pi-) = (3.93 +- 1.74 +- 2.14) \cdot 10^{-6}. The upper limits have been placed for the decays phi to pi+pi-pi+pi-pi0 and phi to eta pi+pi- Br(phi to pi+pi-pi+pi-pi0) < 4.6 \cdot 10^{-6} 90% CL, Br(phi to eta pi+pi-) < 1.8 \cdot 10^{-5} 90% CL.
Using 3.07 ${pb}^{-1}$ of data collected in the energy range 0.60-0.97 GeV by CMD-2, about 150 events of the process $\epm \to \pch$ have been selected. The energy dependence of the cross section agrees with the assumption of the $a_1(1260) \pi$ intermediate state which is dominant above 1 GeV. For the first time \fourpi events are observed at the $\rho$ meson energy. Under the assumption that all these events come from the $\rho$ meson decay, the value of the cross section at the $\rho$ meson peak corresponds to the following decay width: \Gamma(\rho^0 \to \fourpi) = (2.8 \pm 1.4 \pm 0.5) {keV} or to the branching ratio B(\rho^0 \to \fourpi) = (1.8 \pm 0.9 \pm 0.3) \cdot 10 ^{-5}.
First results of the study of the process e+e- \to 4\pi by the CMD-2 collaboration at VEPP-2M are presented for the energy range 1.05--1.38 GeV. Using an integrated luminosity of 5.8 pb^{-1}, energy dependence of the processes e+e- \to \pi^+\pi^- 2\pi^0 and e+e- \to 2\pi^+ 2\pi^- has been measured. Analysis of the differential distributions demonstrates the dominance of the a_1\pi and \omega\pi intermediate states. Upper limits for the contributions of other alternative mechanisms are also placed.
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.
We determine the top quark mass m_t using t-tbar pairs produced in the D0 detector by \sqrt{s} = 1.8 TeV p-pbar collisions in a 125 pb^-1 exposure at the Fermilab Tevatron. We make a two constraint fit to m_t in t-tbar -> b W^+bbar W^- final states with one W boson decaying to q-qbar and the other to e-nu or mu-nu. Likelihood fits to the data yield m_t(l+jets) = 173.3 +- 5.6 (stat) +- 5.5 (syst) GeV/c^2. When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m_t = 172.1 +- 5.2 (stat) +- 4.9 (syst) GeV/c^2. An alternate analysis, using three constraint fits to fixed top quark masses, gives m_t(l+jets) = 176.0 +- 7.9 (stat) +- 4.8 (syst) GeV/C^2, consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented.
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.
A test of the QED process e+e- -> gamma gamma (gamma) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb^{-1}. The differential and total cross-sections for the process e+e- -> gamma gamma were measured, and found to be in agreement with the QED prediction. 95% Confidence Level (C.L.) lower limits on the QED cut-off parameters of Lambda+ > 330 GeV and Lambda- > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c^2 (for lambda_gamma = 1) was obtained. s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M_S: M_S > 713 GeV/c^2 (lambda = 1) and M_S > 691 GeV/c^2 (lambda = -1).
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