${{\boldsymbol \Lambda}}$ BARYONS
($\boldsymbol S$ = $-1$, $\boldsymbol I$ = 0)
${{\mathit \Lambda}^{0}}$ = ${\mathit {\mathit u}}$ ${\mathit {\mathit d}}$ ${\mathit {\mathit s}}$
INSPIRE search

${{\boldsymbol \Lambda}{(1405)}}$ $I(J^P)$ = $0(1/2^{-})$

In the 1998 Note on the ${{\mathit \Lambda}{(1405)}}$ in PDG 1998 , R.H. Dalitz discussed the S-shaped cusp behavior of the intensity at the ${{\mathit N}}-{{\overline{\mathit K}}}$ threshold observed in THOMAS 1973 and HEMINGWAY 1985 . He commented that this behavior "is characteristic of ${\mathit S}{\mathrm -wave}$ coupling; the other below threshold hyperon, the ${{\mathit \Sigma}{(1385)}}$, has no such threshold distortion because its ${{\mathit N}}-{{\overline{\mathit K}}}$ coupling is ${\mathit P}{\mathrm -wave}$. For ${{\mathit \Lambda}{(1405)}}$ this asymmetry is the sole direct evidence that $\mathit J{}^{P} = 1/2{}^{-}$." A recent measurement by the CLAS collaboration, MORIYA 2014 , definitively established the long-assumed $\mathit J{}^{P} = 1/2{}^{-}$ spin-parity assignment of the ${{\mathit \Lambda}{(1405)}}$. The experiment produced the ${{\mathit \Lambda}{(1405)}}$ spin-polarized in the photoproduction process ${{\mathit \gamma}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit \Lambda}{(1405)}}$ and measured the decay of the ${{\mathit \Lambda}{(1405)}}$ (polarized) $\rightarrow$ ${{\mathit \Sigma}^{+}}$ (polarized) ${{\mathit \pi}^{-}}$ . The observed isotropic decay of ${{\mathit \Lambda}{(1405)}}$ is consistent with spin $\mathit J = 1/2$. The polarization transfer to the ${{\mathit \Sigma}^{+}}$(polarized) direction revealed negative parity, and thus established $\mathit J{}^{P} = 1/2{}^{-}$.
See related review:
Pole Structure of the ${{\mathit \Lambda}{(1405)}}$ Region
${{\boldsymbol \Lambda}{(1405)}}$ REGION POLE POSITIONS
REAL PART
$-2{\times }$IMAGINARY PART
${{\boldsymbol \Lambda}{(1405)}}$ MASS
PRODUCTION EXPERIMENTS   $1405.1 {}^{+1.3}_{-1.0}$ MeV 
EXTRAPOLATIONS BELOW ${{\mathit N}}{{\overline{\mathit K}}}$ THRESHOLD
${{\boldsymbol \Lambda}{(1405)}}$ WIDTH
PRODUCTION EXPERIMENTS   $50.5 \pm2.0$ MeV 
EXTRAPOLATIONS BELOW ${{\mathit N}}{{\overline{\mathit K}}}$ THRESHOLD
$\Gamma_{1}$ ${{\mathit \Sigma}}{{\mathit \pi}}$ 100$\%$155
$\Gamma_{2}$ ${{\mathit \Lambda}}{{\mathit \gamma}}$ 260
$\Gamma_{3}$ ${{\mathit \Sigma}^{0}}{{\mathit \gamma}}$ 196
$\Gamma_{4}$ ${{\mathit N}}{{\overline{\mathit K}}}$ -1