Measurement of the ISR-FSR interference in the e+e−→μ+μ−γ and π+π−γ processes at BaBar

The charge asymmetry in the e+e−→μ+μ−γ and π+π−γ processes is measured using of data collected with the BaBar detector at e+e− center-of-mass energies near 10.58 GeV. The asymmetry as a function of the final state mass from production threshold to a few GeV, is compared to the values expected from QED for e+e−→μ+μ−γ, and theoretical models for e+e−→π+π−γ. A clear interference pattern is observed in e+e−→π+π−γ, particularly in the vicinity of the f2(1270) resonance. The inferred rate of lowest order FSR production is consistent with the QED expectation for e+e−→μ+μ−γ, and very small for e+e−→π+π−γ.