Bychkov M, Pocanić D, Vandevender BA, Baranov VA, Bertl W, Bystritsky YM, Frlez E, Kalinnikov VA, Khomutov NV, Korenchenko AS, Korenchenko SM, Korolija M, Kozlowski T, Kravchuk NP, Kuchinsky NA, Li W, Mekterović D, Mzhavia D, Ritt S, Robmann P, Rondon-Aramayo OA, Rozhdestvensky AM, Sakhelashvili T, Scheu S, Straumann U, Supek I, Tsamalaidze Z, van der Schaaf A, Velicheva EP, Volnykh VP, Wang Y, Wirtz HP. New precise measurement of the pion weak form factors in pi+-->e+ nugamma decay.
PHYSICAL REVIEW LETTERS 2009;
103:051802. [PMID:
19792486 DOI:
10.1103/physrevlett.103.051802]
[Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/15/2008] [Revised: 06/10/2009] [Indexed: 05/28/2023]
Abstract
We have measured the pi+-->e+ nugamma branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-chi2 fits to the measured (E(e+), E(gamma) energy distributions result in the weak form factor value of F(A)=0.0119(1) with a fixed value of F(V)=0.0259. An unconstrained fit yields F(V)=0.0258(17) and F(A)=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of F(V) on q2, the e+ nu pair invariant mass squared, parametrized as F(V)(q2)=F(V)(0)(1+aq(2)). The branching ratio for the kinematic region E(gamma)>10 MeV and theta(e(+)gamma)>40 degrees is measured to be B(expt)=73.86(54)x10(-8). Earlier deviations we reported in the high-E(gamma)-low-E(e+) kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability alpha(E)=2.78(10)x10(-4) fm3 and neutral pion lifetime tau(pi0)=(8.5+/-1.1)x10(-17) s.
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