Improved limit on the branching fraction of the rare decay .

A search for the decay is performed, based on a data sample of proton-proton collisions corresponding to an integrated luminosity of , collected by the LHCb experiment at centre-of-mass energies of 7 and 8 . The observed yield is consistent with the background-only hypothesis, yielding a limit on the branching fraction of at confidence level. This result improves the previous upper limit on the branching fraction by an order of magnitude.