Measurement of gamma gamma* --> pi0 transition form factor at Belle

TL;DR

This paper presents a measurement of the pi0 transition form factor in gamma gamma* interactions over a broad Q^2 range, confirming previous results at low Q^2 and contrasting with recent findings at higher Q^2.

Contribution

First measurement of the pi0 transition form factor at high Q^2 using Belle data, providing insights into QCD predictions and previous experimental discrepancies.

Findings

Q^2|F(Q^2)| agrees with earlier data below 9 GeV^2

No rapid growth in form factor at high Q^2 as seen in some recent measurements

Results align with QCD expectations at high Q^2

Abstract

We report a measurement of the process gamma gamma* --> pi0 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(Q^2), is measured for the kinematical region 4 GeV^2 <~ Q^2 <~ 40 GeV^2, where -Q^2 is the invariant mass squared of a virtual photon. The measured values of Q^2|F(Q^2)| agree well with the previous measurements below Q^2 ~= 9 GeV^2 but do not exhibit the rapid growth in the higher Q^2 region seen in another recent measurement, which exceeds the asymptotic QCD expectation by as much as 50%.