Measurement of Initial-State-Final-State Radiation Interference in the Processes e⁺e⁻ → μ⁺μ⁻γ and e⁺e⁻ → π⁺π⁻γ


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 e⁺e⁻ 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/c². 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 f₂(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.



Charge Asymmetry, Atomic mass, Final-state radiation, Initial-state radiation, Photons

This work is supported by the US Department of Energy and National Science Foundation, the Natural Sciences and Engineering Research Council (Canada), the Commissariat à l’Energie Atomique and Institut National de Physique Nucléaire et de Physique des Particules (France), the Bundesministerium für Bildung und Forschung and Deutsche Forschungsgemeinschaft (Germany), the Istituto Nazionale di Fisica Nucleare (Italy), the Foundation for Fundamental Research on Matter (The Netherlands), the Research Council of Norway, the Ministry of Education and Science of the Russian Federation, Ministerio de Economía y Competitividad (Spain), the Science and Technology Facilities Council (United Kingdom), and the Binational Science Foundation (U.S.-Israel). Individuals have received support from the Marie-Curie IEF program (European Union) and the A. P. Sloan Foundation (USA).


©2015 American Physical Society