Can NRQCD explain the γγ* → ηc transition form factor data?

الأصل · EN

Unlike the bewildering situation in the γγ*→ π form factor, a widespread view is that perturbative QCD can decently account for the recent BaBar measurement of γγ*→ ηc transition form factor. The next-to-next-to-leading order (NNLO) perturbative correction to the γγ*→ ηc,b form factor, is investigated in the NRQCD factorization framework for the first time. As a byproduct, we obtain by far the most precise order-αₛ² NRQCD matching coefficient for the ηc,b→ γγ process. After including the substantial negative order-αₛ² correction, the good agreement between NRQCD prediction and the measured γγ*→ ηc form factor is completely ruined over a wide range of momentum transfer squared. This eminent discrepancy casts some doubts on the applicability of NRQCD approach to hard exclusive reactions involving charmonium.