STRANGE MESONS
($\boldsymbol S$ = $\pm1$, $\boldsymbol C$ = $\boldsymbol B$ = 0)
${{\mathit K}^{+}}$ = ${\mathit {\mathit u}}$ ${\mathit {\overline{\mathit s}}}$, ${{\mathit K}^{0}}$ = ${\mathit {\mathit d}}$ ${\mathit {\overline{\mathit s}}}$, ${{\overline{\mathit K}}^{0}}$ = ${\mathit {\overline{\mathit d}}}$ ${\mathit {\mathit s}}$, ${{\mathit K}^{-}}$ = ${\mathit {\overline{\mathit u}}}$ ${\mathit {\mathit s}}$, similarly for ${{\mathit K}^{*}}$'s
INSPIRE search

${{\boldsymbol K}^{\pm}}$ $I(J^P)$ = $1/2(0^{-})$

See related reviews:
Charged Kaon Mass
Rare Kaon Decays
Dalitz Plot Parameters for ${{\mathit K}}$ $\rightarrow$ 3 ${{\mathit \pi}}$ Decays
${{\mathit K}}{}^{\pm}_{{{\mathit \ell}}3}$ and ${{\mathit K}}{}^{0}_{{{\mathit \ell}}3}$ Form Factors
${{\mathit K}^{\pm}}$ MASS   $493.677 \pm0.016$ MeV (S = 2.8)
${\mathit m}_{{{\mathit K}^{+}}}–{\mathit m}_{{{\mathit K}^{-}}}$   $-0.03 \pm0.09$ MeV 
${{\mathit K}^{\pm}}$ MEAN LIFE   $(1.2380 \pm0.0020) \times 10^{-8}$ s (S = 1.8)
$({\mathit \tau}_{{{\mathit K}^{+}}}–{\mathit \tau}_{{{\mathit K}^{-}}})/{\mathit \tau}_{\mathrm {average}}$   $0.0010 \pm0.0009$  (S = 1.2)
$\boldsymbol CPT$ VIOLATION TESTS IN ${{\boldsymbol K}^{\pm}}$ DECAYS
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \mu}^{\pm}}{{\mathit \nu}_{{\mu}}}$ ) RATE DIFFERENCE/SUM   $-0.0027 \pm0.0021$  
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}$ ) RATE DIFFERENCE/SUM   $0.004 \pm0.006$  
$\boldsymbol CP$ VIOLATION TESTS IN ${{\boldsymbol K}^{\pm}}$ DECAYS
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ ) RATE DIFFERENCE/SUM   $-0.022 \pm0.016$  
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) RATE DIFFERENCE/SUM   $0.010 \pm0.023$  
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \gamma}}$ ) RATE DIFFERENCE/SUM   $0.0000 \pm0.0012$  
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ ) RATE DIFFERENCE/SUM   $(4 \pm6) \times 10^{-4}$  
$\Delta $( ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$ ) RATE DIFFERENCE/SUM   $-0.0002 \pm0.0028$  
$\boldsymbol T$ VIOLATION TESTS IN ${{\boldsymbol K}^{+}}$ AND ${{\boldsymbol K}^{-}}$ DECAYS
$\mathit P_{T}$ in ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$   $-0.0017 \pm0.0025$  
$\mathit P_{T}$ in ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}{{\mathit \gamma}}$   $-0.006 \pm0.019$  
Im($\xi $) in ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$ DECAY (from transverse ${{\mathit \mu}}$ pol.)   $-0.006 \pm0.008$  
ENERGY DEPENDENCE OF ${{\boldsymbol K}^{\pm}}$ DALITZ PLOT
LINEAR COEFFICIENT $\mathit g$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$   $-0.21134 \pm0.00017$  
QUADRATIC COEFFICIENT $\mathit h$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$   $0.0185 \pm0.0004$  
QUADRATIC COEFFICIENT $\mathit k$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$   $-0.00463 \pm0.00014$  
($\mathit g_{+}$ $–$ $\mathit g_{-}$) $/$ ($\mathit g_{+}$ + $\mathit g_{-}$) FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$   $(-1.5 \pm2.2) \times 10^{-4}$  
LINEAR COEFFICIENT $\mathit g$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.626 \pm0.007$  
QUADRATIC COEFFICIENT $\mathit h$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.052 \pm0.008$  
QUADRATIC COEFFICIENT $\mathit k$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.0054 \pm0.0035$  (S = 2.5)
($\mathit g_{+}$ $–$ $\mathit g_{-}$) $/$ ($\mathit g_{+}$ + $\mathit g_{-}$) FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $(1.8 \pm1.8) \times 10^{-4}$  
ALTERNATIVE PARAMETRIZATIONS OF ${{\boldsymbol K}^{\pm}}$ $\rightarrow$ ${{\boldsymbol \pi}^{\pm}}{{\boldsymbol \pi}^{0}}{{\boldsymbol \pi}^{0}}$ DALITZ PLOT
LINEAR COEFFICIENT ${\mathit g}_{0}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.6525 \pm0.0034$  
QUADRATIC COEFFICIENT $\mathit h{}^{'}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $-0.0433 \pm0.0027$  
QUADRATIC COEFFICIENT ${{\mathit k}_{{0}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.0095 \pm0.0005$  
LINEAR COEFFICIENT ${{\mathit g}_{{BB}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $0.6219 \pm0.0034$  
QUADRATIC COEFFICIENT ${{\mathit h}_{{BB}}^{\,'}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$   $-0.0520 \pm0.0028$  
${{\boldsymbol K}_{{{{\boldsymbol \ell}}3}}^{\pm}}$ FORM FACTORS
$\lambda _{+}$ (LINEAR ENERGY DEPENDENCE OF $\mathit f_{+}$ IN ${{\mathit K}_{{e3}}^{\pm}}$ DECAY)   $0.0297 \pm0.0005$  
$\lambda _{+}$ (LINEAR ENERGY DEPENDENCE OF $\mathit f_{+}$ IN ${{\mathit K}_{{\mu3}}^{\pm}}$ DECAY)   $0.0296 \pm0.0017$  
$\lambda _{0}$ (LINEAR ENERGY DEPENDENCE OF $\mathit f_{0}$ IN ${{\mathit K}_{{\mu3}}^{\pm}}$ DECAY)   $0.0196 \pm0.0013$  
$\lambda $'$_{+}$ (LINEAR ${{\mathit K}_{{e3}}^{\pm}}$ FORM FACTOR FROM QUADRATIC FIT)   $0.0249 \pm0.0017$  
$\lambda $'$_{+}$(QUADRATIC ${{\mathit K}_{{e3}}^{\pm}}$ FORM FACTOR)   $0.0019 \pm0.0009$  
$\vert \mathit f_{\mathit S}/\mathit f_{+}\vert $ FOR ${{\mathit K}_{{e3}}^{\pm}}$ DECAY   $-0.003 {}^{+0.008}_{-0.007}$  
$\vert \mathit f_{\mathit T}/\mathit f_{+}\vert $ FOR ${{\mathit K}_{{e3}}^{\pm}}$ DECAY   $-0.012 \pm0.023$  
$\mathit f_{\mathit S}/\mathit f_{+}$ FOR ${{\mathit K}_{{\mu3}}^{\pm}}$ DECAY   $0.002 \pm0.006$  
$\mathit f_{\mathit T}/\mathit f_{+}$ FOR ${{\mathit K}_{{\mu3}}^{\pm}}$ DECAY   $-0.001 \pm0.007$  
${{\boldsymbol K}_{{{{\boldsymbol \ell}}4}}^{\pm}}$ FORM FACTORS
${{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $5.712 \pm0.032$  
${{\mathit f}_{{s}}^{\,'}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $0.152 \pm0.009$  
${{\mathit f}_{{s}}^{''}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $-0.073 \pm0.009$  
${{\mathit f}_{{e}}^{\,'}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $0.068 \pm0.009$  
${{\mathit f}_{{p}}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $-0.048 \pm0.005$  
${{\mathit g}_{{p}}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $0.868 \pm0.014$  
${{\mathit g}_{{p}}^{\,'}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $0.089 \pm0.021$  
${{\mathit h}_{{p}}}/{{\mathit f}_{{s}}}$ FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{\pm}}{{\mathit \nu}}$ DECAY   $-0.398 \pm0.017$  
DECAY FORM FACTOR FOR ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit e}^{\pm}}{{\mathit \nu}}$
${{\boldsymbol K}^{\pm}}$ $\rightarrow$ ${{\boldsymbol \ell}^{\pm}}{{\boldsymbol \nu}}{{\boldsymbol \gamma}}$ FORM FACTORS
$\mathit F_{\mathit A}$ $+$ $\mathit F_{\mathit V}$, SUM OF AXIAL-VECTOR AND VECTOR FORM FACTOR FOR ${{\mathit K}}$ $\rightarrow$ ${{\mathit e}}{{\mathit \nu}_{{e}}}{{\mathit \gamma}}$   $0.133 \pm0.008$  (S = 1.3)
$\mathit F_{\mathit A}$ $+$ $\mathit F_{\mathit V}$, SUM OF AXIAL-VECTOR AND VECTOR FORM FACTOR FOR ${{\mathit K}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{\mu}}}{{\mathit \gamma}}$   $0.165 \pm0.013$  
$\mathit F_{\mathit A}$ $−$ $\mathit F_{\mathit V}$, DIFFERENCE OF AXIAL-VECTOR AND VECTOR FORM FACTOR FOR ${{\mathit K}}$ $\rightarrow$ ${{\mathit e}}{{\mathit \nu}_{{e}}}{{\mathit \gamma}}$   $<0.49$   CL=90.0%
$\mathit F_{\mathit A}$ $−$ $\mathit F_{\mathit V}$, DIFFERENCE OF AXIAL-VECTOR AND VECTOR FORM FACTOR FOR ${{\mathit K}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{\mu}}}{{\mathit \gamma}}$   $-0.21 \pm0.06$  
${{\mathit K}^{\pm}}$ CHARGE RADIUS   $0.560 \pm0.031$ fm 
${{\mathit K}^{+}}$ LONGITUDINAL POLARIZATION OF EMITTED ${{\mathit \mu}^{+}}$   $<-0.990$   CL=90.0%
FORWARD-BACKWARD ASYMMETRY IN ${{\boldsymbol K}^{\pm}}$ DECAYS
A$_{FB}({{\mathit K}}{}^{\pm{}}_{{{\mathit \pi}} {{\mathit \mu}} {{\mathit \mu}} }$) = ${\Gamma\mathrm {(cos({{\mathit \theta}}_{ {{\mathit K}} {{\mathit \mu}} })>0)}−\Gamma\mathrm {(cos({{\mathit \theta}}_{ {{\mathit K}} {{\mathit \mu}} })<0)}\over \Gamma\mathrm {(cos({{\mathit \theta}}_{ {{\mathit K}} {{\mathit \mu}} })>0)}+\Gamma\mathrm {(cos({{\mathit \theta}}_{ {{\mathit K}} {{\mathit \mu}} })<0)}}$   $<0.023$   CL=90.0%
${{\mathit K}^{-}}$ modes are charge conjugates of the modes below.
    constrained fit information