New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay

We have measured the $\pi^+\to {\rm e}^+\nu\gamma$ branching ratio over a wide region of phase space, based on a total of 65,460 events acquired using the PIBETA detector. Minimum-$\chi^2$ fits to the measured $(E_{e^+},E_\gamma)$ energy distributions result in the weak form factor value of \(...

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Published in	arXiv.org
Main Authors	Bychkov, M, Počanić, D, VanDevender, B A, Baranov, V A, Bertl, W, Bystritsky, Yu M, Frlež, E, Kalinnikov, V A, Khomutov, N V, Korenchenko, A S, Korenchenko, S M, Korolija, M, Kozlowski, T, Kravchuk, N P, Kuchinsky, N A, W Li, 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, H -P Wirtz
Format	Paper Journal Article
Language	English
Published	Ithaca Cornell University Library, arXiv.org 01.07.2009
Subjects	Dependence Form factors Gamma emission Kinematics Mathematical analysis Physics - High Energy Physics - Experiment Physics - High Energy Physics - Phenomenology Physics - Nuclear Experiment Tensors
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Summary:	We have measured the $\pi^+\to {\rm e}^+\nu\gamma$ branching ratio over a wide region of phase space, based on a total of 65,460 events acquired using the PIBETA detector. Minimum-$\chi^2$ 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 $q^2$, the ${\rm e}^{+}\nu$ pair invariant mass squared, parametrized as $F_V(q^2)=F_V(0)(1+a\cdot q^2)$. The branching ratio for the kinematic region $E_\gamma > 10 $MeV and $\theta_{{\rm e^+}\gamma} > 40^\circ $ is measured to be $B^{\rm exp}=73.86(54) \times 10^{-8}$. Earlier deviations we reported in the high-$E_\gamma$/low-$E_{{\rm e}^+}$ kinematic region are resolved, and we find full compatibility with CVC and standard $V$$-$$A$ calculations without a tensor term. We also derive new values for the pion polarizability, $\alpha_E = \rm 2.78(10) \times 10^{-4} fm^3$, and neutral pion lifetime, $\tau_{\pi 0} = (8.5 \pm 1.1) \times 10^{-17} $s.
ISSN:	2331-8422
DOI:	10.48550/arxiv.0804.1815

Summary:	We have measured the \(\pi^+\to {\rm e}^+\nu\gamma\) branching ratio over a wide region of phase space, based on a total of 65,460 events acquired using the PIBETA detector. Minimum-\(\chi^2\) 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 \(q^2\), the \({\rm e}^{+}\nu\) pair invariant mass squared, parametrized as \(F_V(q^2)=F_V(0)(1+a\cdot q^2)\). The branching ratio for the kinematic region \(E_\gamma > 10 \)MeV and \(\theta_{{\rm e^+}\gamma} > 40^\circ \) is measured to be \(B^{\rm exp}=73.86(54) \times 10^{-8}\). Earlier deviations we reported in the high-\(E_\gamma\)/low-\(E_{{\rm e}^+}\) kinematic region are resolved, and we find full compatibility with CVC and standard $V$$-$$A\( calculations without a tensor term. We also derive new values for the pion polarizability, \)\alpha_E = \rm 2.78(10) \times 10^{-4} fm^3\(, and neutral pion lifetime, \)\tau_{\pi 0} = (8.5 \pm 1.1) \times 10^{-17} $s.
ISSN:	2331-8422
DOI:	10.48550/arxiv.0804.1815