Experimental study of rare charged pion decays

The combination of simple dynamics, small number of available decay channels, and extremely well controlled radiative and loop corrections, make charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for improving our understanding of pion structure and chiral dynamics. This paper reviews the current state of experimental study of the allowed rare decays of charged pions: (a) leptonic, pi(+) -> e(+)v(e), or pi(e2), (b) radiative, pi(+) -> e(+)v(e)gamma, or pi(e2 gamma), and pi(+) -> e(+)v(e)e(+)e(-), or pi(e2ee), and (c) semileptonic, pi(+) -> pi(0)e(+)v, or pi(e3). Taken together, the combined data set presents an internally consistent picture that also agrees well with standard model predictions. The internal consistency is illustrated well by the value of the pi(e2) branching ratio of (R-l/mu(pi))(PIBETA) (1.2366 +/- 0.0064) x 10(-4) PIBETA PIBETA measurement of the pi(e3) decay and the current best value for the CKM matrix element V-ud. However, even after the great progress of the recent decades, experimental precision is lagging far behind that of the theoretical description for all above processes. We review the implications of the present state of knowledge and prospects for further improvement in the near term.