engleski

Investigation of Rare Pion Decays with the PIBETA Spectrometer

Experimental results on pion decays obtained with the PIBETA spectrometer at the Paul Scherrer Institute (PSI) are reviewed. For pion beta decay (+) (0)?(+) (), a precision measurement of relative probability yields ?() = [1.036 +/- 0.004(stat) +/- 0.004(syst) +/- 0.003((+)?(+))] x 10(-8), which implies V-ud = 0.9728(30) for the corresponding element of the Cabibbo-Kobayashi-Maskawa mixing matrix. Using a sample of 65 x 10(3) events, relative probability of the (+)?(+) radiative pion decay (RPD) in the kinematic region of E > 10 MeV and (e) > 40 degrees is measured as B-exp = 73.86(54) x 10(-8). A statistical analysis of measured Ee+ and E distributions for this decay yield the values F-V = 0.0258(17) and F-A = 0.0117(17) for the pion weak formfactors. Assuming that F-V linearly depends on the ?(+) invariant mass q(2) as F-V(q(2)) = F-V(0)(1 + aq(2)), the slope parameter is extracted as a = 0.10(6). The pion polarizability and neutral- pion lifetime are estimated as (E) = 2.78(10) x 10(-4) fm(3) and ((0)) = (8.5 +/- 1.1) x 10(-17) s, respectively. The data for decays pi(+) -> (+) and nu and mu+ -> e+nu(nu) over bar gamma have been collected and are being processed. The follow-up PEN experiment aims at reducing the uncertainty on the (+) ?(+) relative probability by almost an order of magnitude (to 5 x 10(-4)).

axial form-factor ; branching ratio ; beta-decay ; probability ; symmetry

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