New precise measurement of the pion weak form factors in pi+-->e+ nugamma decay.
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.
Department of Physics, University of Virginia, Charlottesville, Virginia 22904-4714, USA. mab3ed@virginia.edu
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Bychkov
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Pocanić
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Vandevender
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Baranov
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Bertl
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Bystritsky
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Frlez
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Kalinnikov
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Khomutov
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Korenchenko
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Korenchenko
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Korolija
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Kozlowski
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Kravchuk
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Kuchinsky
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Li
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Mekterović
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Mzhavia
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Ritt
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Robmann
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Rondon-Aramayo
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Rozhdestvensky
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Sakhelashvili
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Scheu
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Straumann
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Supek
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Tsamalaidze
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van der Schaaf
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Velicheva
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Volnykh
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Wang
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Wirtz