Measurement of γγ^* → π⁰ transition form factor at Belle

Sadaharu Uehara^a for the Belle Collaboration

KEK, Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, Tsukuba, Japan

^a e-mail: uehara@post.kek.jp

Abstract

We report a measurement of the process γγ^* → π⁰ with a 759 fb^-1 data sample recorded with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. The pion transition form factor, F(Q²), is measured for the kinematical region GeV² ≲ Q² ≲ 40 GeV², where −Q² is the invariant mass squared of a virtual photon. The measured values of Q²|F(Q²)| agree well with the previous measurements below Q² ≃ 9 GeV² but do not exhibit the rapid growth in the higher Q² region seen in another recent measurement, which exceeds the asymptotic QCD expectation by as much as 50%.