Using kaon regeneration to probe the quark mixing parameter cos2beta in B --> psiK decays

We suggest a novel method to determine the sign of cos2beta in the decays B --> psiK, by creating interference between K(L) and K(S) final states via "regeneration," that is propagation through a matter target region to convert some K(L) to K(S). The determination of this quantity resolves an ambiguity between beta and 90 degrees -beta that remains after the standard measurements of sin2beta and may turn out to be important in resolving whether the result is in agreement with standard model predictions or indicates the presence of new physics. We find the measurement is feasible at a B factory, but requires several years of high-luminosity running with a regeneration target affecting a significant fraction of the detector.