Using a data set corresponding to an integrated luminosity of 3 fb−1ainer hidden">, collected by the LHCb experiment in pp collisions at centre-of-mass energies of 7 and 8 TeV, the effective lifetime in the ainer hidden"> decay mode, τeffainer hidden">, is measured to be
Assuming CP conservation, τeffainer hidden"> corresponds to the lifetime of the light ainer hidden"> mass eigenstate. This is the first measurement of the effective lifetime in this decay mode.