English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We have measured the $\pi^+\to {\rm e}^+\nu\gamma$ branching ratio over awide region of phase space, based on a total of 65,460 events acquired usingthe 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 ${\rm e}^{+}\nu$ pairinvariant mass squared, parametrized as $F_V(q^2)=F_V(0)(1+a\cdot q^2)$. Thebranching ratio for the kinematic region $E_\gamma > 10 $MeV and $\theta_{{\rme^+}\gamma} > 40^\circ $ is measured to be $B^{\rm exp}=73.86(54) \times10^{-8}$. Earlier deviations we reported in the high-$E_\gamma$/low-$E_{{\rme}^+}$ kinematic region are resolved, and we find full compatibility with CVCand standard $V$$-$$A$ calculations without a tensor term. We also derive newvalues 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.

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