LAB observables for the muon polarization in K^+ --> π^+ μ^+ μ^-

Abstract

We analyse the muon longitudinal polarization asymmetry Δ_long in the decay K^+ --> π^+ μ^+ mu^-. It is stressed that, since the muon helicities are not Lorentz-invariant quantities, the magnitude of Δ_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 --> μ^+ μ^-. We find that in both cases the asymmetry is diluted when increasing the energy of the decaying kaons.

12 pages, 2 figures, REVTeX, to be published in Phys. Lett. B