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Register a new userMeasurement of $eta^primerightarrow pi^+pi^- e^+e^-$ and $eta^primerightarrow pi^+pi^-mu^+mu^-$
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Based on a sample of 225.3 million J/psi events accumulated with the BESIII detector at the BEPCII, the decays of eta to pi+pi-l+l- are studied via J/psi to gammaeta. A clear eta signal is observed in the pi+pi-e+e- mass spectrum, and the branching fraction is measured to be BR(eta to pi+pi-e+e-) = (2.11pm0.12 (stat.)pm0.15 (syst.))times10^{-3}, which is in good agreement with theoretical predictions and the previous measurement, but is determined with much higher precision. No eta signal is found in the pi+ pi- mu+ mu- mass spectrum, and the upper limit is determined to be BR(eta to pi+ pi- mu+ mu-)<2.9times10^{-5} at the 90% confidence level.
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The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.
Using $(1310.6 pm 7.0) times 10^{6}~J/psi$ events acquired with the BESIII detector at the BEPCII storage rings, the decay $eta^primerightarrowpi^+pi^-mu^+mu^-$ is observed for the first time with a significance of 8$sigma$ via the process $J/psirightarrowgammaeta$. We measure the branching fraction of $eta^primerightarrowpi^+pi^-mu^+mu^-$ to be $mathcal{B}(eta^primerightarrowpi^+pi^-mu^+mu^-)$=(1.97$pm$0.33(stat.)$pm$0.18(syst.))$times10^{-5}$, where the first and second uncertainties are statistical and systematic, respectively.
Charge asymmetry in processes e+ e- --> mu+ mu- gamma and e+ e- --> pi+ pi- gamma is measured using 232 fb-1 of data collected with the BABAR detector at center-of-mass energies near 10.58 GeV. An observable is introduced and shown to be very robust against detector asymmetries while keeping a large sensitivity to the physical charge asymmetry that results from the interference between initial and final state radiation. The asymmetry is determined as afunction of the invariant mass of the final-state tracks from production threshold to a few GeV/c2. It is compared to the expectation from QED for e+ e- --> mu+ mu- gamma and from theoretical models for e+ e- --> pi+ pi- gamma. A clear interference pattern is observed in e+ e- --> pi+ pi- gamma, particularly in the vicinity of the f_2(1270) resonance. The inferred rate of lowest order FSR production is consistent with the QED expectation for e+ e- --> mu+ mu- gamma, and is negligibly small for e+ e- --> pi+ pi- gamma.
We study the processes $e^+ e^-to 2(pi^+pi^-)pi^0gamma$, $2(pi^+pi^-)etagamma$, $K^+ K^-pi^+pi^-pi^0gamma$ and $K^+ K^-pi^+pi^-etagamma$ with the hard photon radiated from the initial state. About 20000, 4300, 5500 and 375 fully reconstructed events, respectively, are selected from 232 fb$^{-1}$ of BaBar data. The invariant mass of the hadronic final state defines the effective $e^+ e^-$ center-of-mass energy, so that the obtained cross sections from the threshold to about 5 GeV can be compared with corresponding direct epem measurements, currently available only for the $etapi^+pi^-$ and $omegapi^+pi^-$ submodes of the $e^+ e^-to 2(pi^+pi^-)pi^0$ channel. Studying the structure of these events, we find contributions from a number of intermediate states, and we extract their cross sections where possible. In particular, we isolate the contribution from $e^+ e^-toomega(782)pi^+pi^-$ and study the $omega(1420)$ and $omega(1650)$ resonances. In the charmonium region, we observe the $J/psi$ in all these final states and several intermediate states, as well as the $psi(2S)$ in some modes, and we measure the corresponding branching fractions.
The ratio R_{eta}=Gamma(eta -> pi^+pi^-gamma)/Gamma(eta -> pi^+pi^-pi^0) has been measured by analyzing 22 million phi to eta gamma decays collected by the KLOE experiment at DAPhiNE, corresponding to an integrated luminosity of 558 pb^{-1}. The eta to pi^+pi^-gamma proceeds both via the rho resonant contribution, and possibly a non-resonant direct term, connected to the box anomaly. Our result, R_{eta}= 0.1856pm 0.0005_{stat} pm 0.0028_{syst}, points out a sizable contribution of the direct term to the total width. The di-pion invariant mass for the eta -> pi^+pi^-gamma decay could be described in a model-independent approach in terms of a single free parameter, alpha. The determined value of the parameter alpha is alpha = (1.32 pm 0.08_{stat} +0.10/-0.09_{syst}pm 0.02_{theo}) GeV^{-2}
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