Bychkov M, Pocanić D, Vandevender B A, Baranov V A, Bertl W, Bystritsky Yu M, Frlez E, Kalinnikov V A, Khomutov N V, Korenchenko A S, Korenchenko S M, Korolija M, Kozlowski T, Kravchuk N P, Kuchinsky N A, Li W, Mekterović D, Mzhavia D, Ritt S, Robmann P, Rondon-Aramayo O A, Rozhdestvensky A M, Sakhelashvili T, Scheu S, Straumann U, Supek I, Tsamalaidze Z, van der Schaaf A, Velicheva E P, Volnykh V P, Wang Y, Wirtz H-P
Department of Physics, University of Virginia, Charlottesville, Virginia 22904-4714, USA.
Phys Rev Lett. 2009 Jul 31;103(5):051802. doi: 10.1103/PhysRevLett.103.051802. Epub 2009 Jul 30.
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.
我们基于使用PIBETA探测器获取的总共65460个事例，在相空间的一个宽广区域测量了π⁺→e⁺νγ的分支比。对测量的（E(e⁺)，E(γ)）能量分布进行最小χ²拟合，在固定F(V)=0.0259的情况下得到弱形状因子值F(A)=0.0119(1)。无约束拟合得到F(V)=0.0258(17)和F(A)=0.0117(17)。此外，我们测量了F(V)对q²（e⁺ν对的不变质量平方）的依赖关系的a = 0.10(6)，其参数化为F(V)(q²)=F(V)(0)(1 + aq²)。运动学区域E(γ)>10 MeV且θ(e⁺γ)>40°的分支比测量值为B(expt)=73.86(54)×10⁻⁸。我们之前在高E(γ) - 低E(e⁺)运动学区域报告的偏差无需张量项即可解决。我们还推导出了π介子极化率α(E)=2.78(10)×10⁻⁴ fm³和中性π介子寿命τ(π⁰)=(8.5±1.1)×10⁻¹⁷ s的新值。
