as large as 0.5 for gaugino and higgsino mass parameters of order 100 GeV. The resultingcorrection can, thus, be as large as

|!RSUSYe/µ | <

! 1.3 ! 10"3 ln

!

m2L̃1

m2L̃2

"

. (120)

A numerical scan over MSSM parameters performed by the authors of Ref. [95] is consistentwith this bound. Given that the new TRIUMF and PSI experiments hope to reduce theexperimental uncertainty to the level of the present SM theory uncertainty (" 5!10"4), onecould expect to see a departure from the SM expectations of several standard deviations ifthe masses of the selectron and smuon di!er by more than a factor of " 2 and the mass ofthe lightest chargino is less than 300 GeV. Moreover, in the case of such non-degeneracyamong sleptons, the sign of !RSUSY

e/µ provides an indication of whether the lightest first orsecond generation slepton is heavier.

IVB.2 Neutron and Nuclear " Decay: General Features

Studies of nuclear and neutron "-decay have yielded both important information aboutparameters of the SM as well as constraints on physics beyond it (for comprehensive re-views, see Refs. [62, 66, 67]). In particular, the most precisely-known element of theCabibbo-Kobayashi-Maskawa (CKM) matrix that characterizes the misalignment of quarkweak interaction and mass eigenstates in CC interactions has been determined from “su-perallowed” nuclear "-decays. The latter involve transitions between spin-parity J! = 0+

nuclear states that are mediated solely by the vector charge component of the CC weakcurrent. To the extent that the initial and final 0+ nuclear states are states of pure isospinand the energy transfer to the outgoing lepton pair is negligible compared to typical nuclearscales, the transition matrix element is independent of nuclear structure. In contrast, therates for nuclear decays involving initial and/or final states with non-zero spin – includingthe decay of the neutron – depend more strongly on the details of hadronic and nuclearstructure. In these cases, the extraction of precise information on the electroweak sectorof the SM or on new physics generally requires measurement of both the decay rate andone or more decay correlation coe"cients.

In all cases, the overall decay rate in the SM is characterized by the the reduced halflife or “ft” value, given by

ft =K

(G"V )2M2

F + (G"A)2M2

GT

(121)

K = !(2#3 ln 2)(!c)6/(mec2)5 ,

where t is the half-life; f is a factor that takes into account the outgoing "-particle wave-function in the presence of the nuclear Coulomb field; the Fermi matrix element MF

is the nuclear transition matrix element of the vector current charge operator J†0(x) =

42