A new angle on an old problem: helicity approach to neutron beta decay in the Standard Model

We elaborate on the dichotomy between the description of the semileptonic decays of heavy hadrons on the one hand and the semileptonic decays of light hadrons such as neutron beta decays on the other hand. For example, almost without exception the semileptonic decays of heavy baryons are described in cascade fashion as a sequence of two two-body decays B-1 -> B-2 + Woff-shell and Woff-shell -> l + upsilon(l) whereas neutron beta decays are analyzed as true threebody decays n -> p+ e(-) + (nu) over bar (e). Within the cascade approach it is possible to define a set of seven angular observables for polarized neutron beta decays as well as the longitudinal, transverse and normal polarization of the decay electron. We determine the dependence of the observables on the usual vector and axial vector form factors. In order to be able to assess the importance of recoil corrections we expand the rate and the q(2) averages of the observables up to NLO and NNLO in the recoil parameter delta = ( M-n-M-p)/(M-n+M-p) = 0.689.10(-3). Remarkably, we find that the rate and three of the four parity conserving polarization observables that we analyze are protected from NLO recoil corrections when the second class current contributions are set to zero.