Colour-octet contributions to exclusive charmonium decays

We investigate the theoretical uncertainties of P-wave charmonium decays into two pions, chi(cJ) --> pi(+)pi(-), pi(0) pi(0). Constraining the pion distribution-amplitude from the recent precise data on F-pi gamma(Q(2)), 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 \c(c) over barg$] of the chi(cJ) wave function is actually not power suppressed with respect to the usual colour-singlet decay arising from the dominant \<c(c)over bar>] Fock state. An explicit calculation yields an agreement with the data for a very reasonable value for the single extra non-perturbative parameter.