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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 $\mathit I = 0$ (${{\mathit \eta}}$, ${{\mathit \eta}^{\,'}}$, ${{\mathit h}}$, ${{\mathit h}^{\,'}}$, ${{\mathit \omega}}$, ${{\mathit \phi}}$, ${{\mathit f}}$, ${{\mathit f}^{\,'}}$): ${\mathit {\mathit c}}_{{\mathrm {1}}}({{\mathit u}}{{\overline{\mathit u}}}$ $+$ ${{\mathit d}}{{\overline{\mathit d}}}$) $+$ ${\mathit {\mathit c}}_{{\mathrm {2}}}({{\mathit s}}{{\overline{\mathit s}}}$)

INSPIRE   JSON PDGID: S008

${{\mathit \pi}^{\pm}}$ $I^G(J^{P})$ = $1^-(0^{-})$

We have omitted some results that have been superseded by later experiments.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

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${{\mathit \pi}^{\pm}}$ MASS [1] $139.57039$ $\pm0.00017$ MeV (S = 1.6)

${\mathit m}_{{{\mathit \pi}^{+}}}-{\mathit m}_{{{\mathit \mu}^{+}}}$

$({\mathit m}_{{{\mathit \pi}^{+}}}-{\mathit m}_{{{\mathit \pi}^{-}}})/{\mathit m}_{\mathrm {average}}$ ($2$ $\pm5$) $ \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${}^{3}$

▸  exp ${{\mathit \pi}^{+}}$ $-$ POLARIZATION OF EMITTED ${{\mathit \mu}^{+}}$

${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}}$

▸  exp ${{\mathit \pi}^{\pm}}$ 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

${{\mathit \pi}^{+}}$ DECAY MODES
${{\mathit \pi}^{-}}$ 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.
Mode   Fraction ($\Gamma_i$ / $\Gamma$) Scale Factor/ Conf. Level P(MeV/c)   $\Gamma_{1}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{\mu}}}}$ [2] ($99.98770$ $\pm0.00004$ ) $\%$ 30   $\Gamma_{2}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{\mu}}}}{{\mathit \gamma}}$ [3] ($2.00$ $\pm0.25$) $ \times 10^{-4}$ 30   $\Gamma_{3}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}$ [2] ($1.230$ $\pm0.004$) $ \times 10^{-4}$ 70   $\Gamma_{4}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}{{\mathit \gamma}}$ [3] ($7.39$ $\pm0.05$) $ \times 10^{-7}$ 70   $\Gamma_{5}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}{{\mathit \pi}^{0}}$ ($1.036$ $\pm0.006$) $ \times 10^{-8}$ 4   $\Gamma_{6}$ ${{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ ($3.2$ $\pm0.5$) $ \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   ▸  exp Lepton Family number ($\mathit LF$) or Lepton number ($\mathit L$) violating modes $\Gamma_{9}$ ${{\mathit \mu}^{+}}{{\overline{\mathit \nu}}_{{{e}}}}$ L [4] $<1.5$ $\times 10^{-3}$ CL=90% 30   $\Gamma_{10}$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{e}}}}$ LF [4] $<8.0$ $\times 10^{-3}$ CL=90% 30   $\Gamma_{11}$ ${{\mathit \mu}^{-}}{{\mathit e}^{+}}{{\mathit e}^{+}}{{\mathit \nu}}$ LF $<1.6$ $\times 10^{-6}$ CL=90% 30

[1] The ${{\mathit \pi}^{\pm}}$ mass has increased by three (old) standard deviations since our 1992 edition, and the ${{\mathit \pi}^{0}}$ mass, which is determined using the mass difference (${\mathit m}_{{{\mathit \pi}^{\pm}}}–{\mathit m}_{{{\mathit \pi}^{0}}}$), has increased accordingly. See the ``Note on the Charged Pion Mass'' in these Particle Listings for a discussion. [2] Measurements of $\Gamma ({{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}})/\Gamma ({{\mathit \mu}^{+}}{{\mathit \nu}_{{{\mu}}}}$) always include decays with ${{\mathit \gamma}}$'s, and measurements of $\Gamma ({{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}{{\mathit \gamma}}$) and $\Gamma ({{\mathit \mu}^{+}}{{\mathit \nu}_{{{\mu}}}}{{\mathit \gamma}}$) never include low-energy ${{\mathit \gamma}}$'s. Therefore, since no clean separation is possible, we consider the modes with ${{\mathit \gamma}}$'s to be subreactions of the modes without them, and let $\lbrack{}\Gamma ({{\mathit e}^{+}}{{\mathit \nu}_{{{e}}}}$) $+$ $\Gamma ({{\mathit \mu}^{+}}{{\mathit \nu}_{{{\mu}}}})]/\Gamma _{{\mathrm {total}}}$ = 100$\%$. [3] See the Particle Listings below for the energy limits used in this measurement; low-energy ${{\mathit \gamma}}$'s are not included. [4] Derived from an analysis of neutrino-oscillation experiments.