Colour-octet contributions to exclusive charmonium decays

We investigate the theoretical uncertainties of P-wave charmonium decays into two pions, χcJ → π+π−, π0π0. Constraining the pion distribution-amplitude from the recent precise data on Fπγ(Q2), we find an order-of-magnitude discrepancy between data and prediction. The disagreement persists even after inclusion of transverse degrees of freedom and Sudakov suppressions. We propose the colour-octet mechanism as the solution to the puzzle. The colour-octet decay contribution arising from the higher Fock component |ccg〉 of the χcJ wave function is actually not power suppressed with respect to the usual colour-singlet decay arising from the dominant |cc〉 Fock state. An explicit calculation yields an agreement with the data for a very reasonable value for the single extra non-perturbative parameter.