Spin and parity measurement of the Lambda(1405) baryon

A determination of the spin and parity of the $\mathrm{\Lambda }(1405)$ is presented using photoproduction data from the CLAS detector at Jefferson Lab. The reaction $\gamma +p\to K^{+}+\mathrm{\Lambda }(1405)$ is analyzed in the decay channel $\mathrm{\Lambda }(1405)\to {\mathrm{\Sigma }}^{+}+{\pi }^{-}$, where the decay distribution to $\Sigma^+ \pi^-$andthevariationofthe$\Sigma^+$polarizationwithrespecttothe$\Lambda(1405)$polarizationdirectiondeterminestheparity\mathrm{.}The$\Lambda(1405)$isproduced,intheenergyrange$2.55 < W < 2.85$GeVandfor$0.6 < \cos \theta_{K^+} < 0.9$,withpolarization$P = 0.45 \pm 0.02 ($\text{stat}$) \pm 0.07 ($\text{syst}$)$. The analysis shows that the decays are in $S$ wave, with the ${\mathrm{\Sigma }}^{+}$ polarized such that the $\mathrm{\Lambda }(1405)$ has spin-parity $J^P=1\mathrm{/}{2}^{-}$, as expected by most theories.