The exotic μ−→e− conversion rates by explicit renormalized quasiparticle RPA calculations.

We investigate the μ−e conversion matrix elements by explicit calculations within the renormalized quasiparticle random phase approximation (RQRPA). In this approach, by promoting the quasi-Boson approximation mostly used in ordinary QRPA, the Pauli-principle is restored to a large extent and the nucleon-nucleon ground state correlations are reliably taken into account. We have pointed out that the usual QRPA includes quite strong ground state correlations and it, therefore, overestimates their effect on the μ−e conversion rates and especially on the coherent mode, which is the most important μ−e conversion channel. The rate of this channel is currently measured in the SINDRUM II experiment at PSI and is expected to be extracted in the designed MECO experiment at Brookhaven. By combining our nuclear calculations with the existing experimental limits on the branching ratio Rμe− for 208Pb and 48Ti we put constraints on the flavour violating parameters entering the μ−e conversion effective currents in modern gauge theories.