LIGHT UNFLAVORED MESONS($\mathit S$ = $\mathit C$ = $\mathit B$ = 0) For $\mathit I = 1$ (${{\mathit \pi}}$ , ${{\mathit b}}$ , ${{\mathit \rho}}$ , ${{\mathit a}}$ ): ${\mathit {\mathit u}}$ ${\mathit {\overline{\mathit d}}}$, ( ${\mathit {\mathit u}}$ ${\mathit {\overline{\mathit u}}}−$ ${\mathit {\mathit d}}$ ${\mathit {\overline{\mathit d}}})/\sqrt {2 }$, ${\mathit {\mathit d}}$ ${\mathit {\overline{\mathit u}}}$;for $\mathit I = 0$ (${{\mathit \eta}}$ , ${{\mathit \eta}^{\,'}}$ , ${{\mathit h}}$ , ${{\mathit h}^{\,'}}$ , ${{\mathit \omega}}$ , ${{\mathit \phi}}$ , ${{\mathit f}}$ , ${{\mathit f}^{\,'}}$ ): ${\mathit {\mathit c}}_{{\mathrm {1}}}$( ${{\mathit u}}{{\overline{\mathit u}}}$ $+$ ${{\mathit d}}{{\overline{\mathit d}}}$ ) $+$ ${\mathit {\mathit c}}_{{\mathrm {2}}}$( ${{\mathit s}}{{\overline{\mathit s}}}$ )

#### ${{\mathit \pi}^{\pm}}$

$I^G(J^{P})$ = $1^-(0^{-})$
We have omitted some results that have been superseded by later experiments. The omitted results may be found in our 1988 edition Physics Letters B204 1 (1988).
 See related review: Form Factors for Semileptonic Kaon (${{\mathit K}}$ $_{{{\mathit \ell}} 3}$), Radiative Pion (${{\mathit \pi}}$ $_{ {{\mathit \ell}}2 {{\mathit \gamma}} }$) and Kaon (${{\mathit K}}$ $_{ {{\mathit \ell}}2 {{\mathit \gamma}} }$) Decays
 ${{\mathit \pi}^{\pm}}$ MASS $139.57039 \pm0.00018$ MeV (S = 1.8)
 ${\mathit m}_{{{\mathit \pi}^{+}}}–{\mathit m}_{{{\mathit \mu}^{+}}}$
 $({\mathit m}_{{{\mathit \pi}^{+}}}–{\mathit m}_{{{\mathit \pi}^{-}}})/{\mathit m}_{\mathrm {average}}$ $(2 \pm5) \times 10^{-4}$
 ${{\mathit \pi}^{\pm}}$ MEAN LIFE $(2.6033 \pm0.0005) \times 10^{-8}$ s (S = 1.2)
 $({\mathit \tau}_{{{\mathit \pi}^{+}}}–{\mathit \tau}_{{{\mathit \pi}^{-}}})/{\mathit \tau}_{\mathrm {average}}$ $(6 \pm7) \times 10^{-4}$
 ${{\mathit \pi}}$ ELECTRIC POLARIZABILITY ${{\mathit \alpha}}$ $_{{{\mathit \pi}} }$ $(2.0 \pm0.9) \times 10^{-4}$ fm${}^{3}$
${{\mathit \pi}^{+}}$ $-$ POLARIZATION OF EMITTED ${{\mathit \mu}^{+}}$
 ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}}$
${{\mathit \pi}^{\pm}}$ FORM FACTORS
 $\mathit F_{\mathit V}$, VECTOR FORM FACTOR $0.0254 \pm0.0017$
 $\mathit F_{\mathit A}$, AXIAL-VECTOR FORM FACTOR $0.0119 \pm0.0001$
 VECTOR FORM FACTOR SLOPE PARAMETER $\mathit a$ $0.10 \pm0.06$
 $\mathit R$, SECOND AXIAL-VECTOR FORM FACTOR $0.059 {}^{+0.009}_{-0.008}$
 ${{\mathit \pi}^{\pm}}$ CHARGE RADIUS $0.659 \pm0.004$ fm
${{\mathit \pi}^{-}}$ modes are charge conjugates of the modes below.
For decay limits to particles which are not established, see the section on Searches for Axions and Other Very Light Bosons.
 $\Gamma_{1}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$ [1] $(99.98770\pm{0.00004})\%$ 30
 $\Gamma_{2}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}{{\mathit \gamma}}$ [2] $(2.00\pm{0.25})\times 10^{-4}$ 30
 $\Gamma_{3}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}$ [1] $(1.230\pm{0.004})\times 10^{-4}$ 70
 $\Gamma_{4}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit \gamma}}$ [2] $(7.39\pm{0.05})\times 10^{-7}$ 70
 $\Gamma_{5}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit \pi}^{0}}$ $(1.036\pm{0.006})\times 10^{-8}$ 4
 $\Gamma_{6}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ $(3.2\pm{0.5})\times 10^{-9}$ 70
 $\Gamma_{7}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ $<9\times 10^{-6}$ CL=90% 30
 $\Gamma_{8}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{e}}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ $<1.6\times 10^{-7}$ CL=90% 70
 FOOTNOTES