# $\boldsymbol F_{\boldsymbol A}$, AXIAL-VECTOR FORM FACTOR INSPIRE search

VALUE EVTS DOCUMENT ID TECN  COMMENT
$0.0119$ $\pm0.0001$ 65k 1, 2
 2009
PIBE ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$ at rest
• • • We do not use the following data for averages, fits, limits, etc. • • •
$0.0115$ $\pm0.0004$ 41k 1, 3
 2004
PIBE ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$ at rest
$0.0106$ $\pm0.0060$ 4, 1
 1990 B
SPEC 17 GeV ${{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\overline{\mathit \nu}}_{{e}}}{{\mathit \gamma}}$
$0.021$ ${}^{+0.011}_{-0.013}$ 98
 1989
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
$0.0135$ $\pm0.0016$ 4, 1
 1986
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$
$0.006$ $\pm0.003$ 4, 1
 1986
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$
$0.011$ $\pm0.003$ 5, 4, 1
 1978
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$
1  These values come from fixing the vector form factor at the CVC prediction, ${{\mathit F}_{{V}}}$ = $0.0259$ $\pm0.0005$.
2  When $\mathit F_{V}$ is released, the BYCHKOV 2009 $\mathit F_{A}$ is $0.0117$ $\pm0.0017$, and $\mathit F_{A}$ and $\mathit F_{V}$ results are highly (anti-)correlated: $\mathit F_{A}$ + 1.0286 $\mathit F_{V}$ = $0.03853$ $\pm0.00014$.
3  The sign of ${{\mathit \gamma}}$ = ${{\mathit F}_{{A}}}$ /${{\mathit F}_{{V}}}$ is determined to be positive.
4  Only the absolute value of $\mathit F_{\mathit A}$ is determined.
5  The result of STETZ 1978 has a two-fold ambiguity. We take the solution compatible with later determinations.
References:
 BYCHKOV 2009
PRL 103 051802 New Precise Measurement of the Pion Weak Form Factors in ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$ Decay
 FRLEZ 2004
PRL 93 181804 Precise Measurement of the Pion Axial Form Factor in the ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$ Decay
 BOLOTOV 1990B
PL B243 308 The Experimental Study of the ${{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\overline{\mathit \nu}}_{{e}}}{{\mathit \gamma}}$ Decay in Flight
 EGLI 1989
PL B222 533 Measurement of the Decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and Search for a Light Higgs Boson
 BAY 1986
PL B174 445 Measurement of the Pion Axial Formfactor from Radiative Decay
 PIILONEN 1986
PRL 57 1402 Unique Determination of the Formfactor Ratio in Radiative Pion Decay
 STETZ 1978
NP B138 285 Determination of the Axial-Vector Formfactor in the Radiative Decay of the Pion