${{\mathit \pi}^{\pm}}$ FORM FACTORS

$\mathit F_{\mathit A}$, AXIAL-VECTOR FORM FACTOR

INSPIRE   PDGID:
S008FA
VALUE EVTS DOCUMENT ID TECN  COMMENT
$0.0119$ $\pm0.0001$ 65k 1, 2
BYCHKOV
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
FRLEZ
2004
PIBE ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$ at rest
$0.0106$ $\pm0.0060$ 4, 1
BOLOTOV
1990B
 SPEC 17 GeV ${{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\overline{\mathit \nu}}_{{e}}}{{\mathit \gamma}}$
$0.021$ ${}^{+0.011}_{-0.013}$ 98
EGLI
1989
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
$0.0135$ $\pm0.0016$ 4, 1
BAY
1986
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$
$0.006$ $\pm0.003$ 4, 1
PIILONEN
1986
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit \gamma}}$
$0.011$ $\pm0.003$ 5, 4, 1
STETZ
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
Also
PL B175 97 First Observation of the Decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and a Determination of the Formfactors F$_{V}$, F$_{A}$, R
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