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The COMPASS collaboration at CERN has investigated the pi^- gamma -> pi^- pi^- pi^+ reaction at center-of-momentum energy below five pion masses, sqrt(s) < 5 m(pi), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t < 0.001 (GeV/c)^2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3pi-invariant mass m(3pi) = sqrt(s) were extracted. The absolute cross section was determined in seven bins of $sqrt{s}$ with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.
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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}
We report the measurement of the ratio $Gamma(eta to pi^+pi^-gamma)/Gamma(eta to pi^+pi^-pi^0)$ analyzing a large sample of $phi to eta gamma$ decays recorded with the KLOE experiment at the DA$Phi$NE $e^+ e^-$ collider, corresponding to an integrated luminosity of 558 pb$^{-1}$. The $eta to pi^+pi^-gamma$ process is supposed to proceed both via a resonant contribution, mediated by the $rho$ meson, and a non resonant direct term, connected to the box anomaly. The presence of the direct term affects the partial width value. Our result $R_{eta}=Gamma(eta to pi^+ pi^- gamma)/Gamma(eta to pi^+ pi^- pi^0)= 0.1838pm 0.0005_{stat} pm 0.0030_{syst}$ is in agreement with a recent CLEO measurement, which differs by more 3 $sigma$ from the average of previous results.
The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like $pi^+pi^-$ atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96~mm and got broken up in the 2.1~textmu{}m thick platinum foil. Analysing the observed number of atomic pairs, $n_A^L= left.436^{+157}_{-61}right|_mathrm{tot}$, the lifetime of the 2$p$ state is found to be ${tau_{2p}=(left.0.45^{+1.08}_{-0.30}right|_mathrm{tot}) cdot10^{-11}}$s, not contradicting the corresponding QED $2p$ state lifetime ${tau_{2p}^mathrm{QED}=1.17 cdot 10^{-11}}$s. This lifetime value is three orders of magnitude larger than our previously measured value of the $pi^+pi^-$ atom ground state lifetime $tau=(left.3.15^{+0.28}_{-0.26}right|_mathrm{tot})cdot 10^{-15}$s. Further studies of long-lived $pi^+pi^-$ atoms will allow to measure energy differences between $p$ and $s$ atomic states and so to determine $pipi$ scattering lengths with the aim to check QCD predictions.
We study the process e+e- -> pi+pi-pi+pi-gamma, with a photon emitted from the initial-state electron or positron, using 454.3 fb^-1 of data collected with the BABAR detector at SLAC, corresponding to approximately 260,000 signal events. We use these data to extract the non-radiative sigma(e+e- ->pi+pi-pi+pi-) cross section in the energy range from 0.6 to 4.5 Gev. The total uncertainty of the cross section measurement in the peak region is less than 3%, higher in precision than the corresponding results obtained from energy scan data.
An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.
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