LAB observables for the muon polarization in K+->pi(+)mu(+)mu(-)

We analyse the muon longitudinal polarization asymmetry Delta(long) in the decay K+ --> pi(+)mu(+)mu(-). It is stressed that, since the muon helicities are not Lorentz-invariant quantities, the magnitude of Delta(long) depends in general on the reference frame. We consider the muon helicities in the LAB system, and study the sensitivity of the longitudinal polarization asymmetry to the flavour-mixing parameters in the Standard Model for stopped and in-flight decaying K+. A similar analysis is carried out for the decay K-L --> mu(+)mu(-). We find that in both cases the asymmetry is diluted when increasing the energy of the decaying kaons.