J/Psi K_S > We study the corrections to the determination of sin(2 beta) from
the time dependent CP asymmetry of B → J/Psi K_S which arise in
the standard model. Although a precise prediction of these corrections
is not possible we find that they are indeed extremely small, of the
order of less than a per mil of the observed value. This
means in turn that any deviation
visible at the B factories will be a clear signal for new
physics.