Measurement of initial-state–final-state radiation interference in the processes e^+e^− → μ^+μ^−γ and e^+e^− → π^+π^−γ

Abstract

Charge asymmetry in the processes e^+e^− → μ^+μ^−γ and e^+e− → π^+π^−γ is measured using 232 fb^(−1 of data collected with the BABAR detector at eþe− 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 (FSR). The asymmetry is determined as a function of the invariant mass of the final-state tracks from production threshold to a few GeV=c^2. It is compared to the expectation from QED for e^+e^− → μ^+μ^−γ, and from 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 f_2(1270) resonance. The inferred rate of lowest-order FSR production is consistent with the QED expectation for e^+e^− → μ^+μ^−γ, and is negligibly small for e^+e^− → π^+π^−γ.

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