Spin and parity measurement of the Lambda(1405) baryon

A determination of the spin and parity of the $Λ(1405)$ is presented using photoproduction data from the CLAS detector at Jefferson Lab. The reaction $γ+ p \to K^+ + Λ(1405)$ is analyzed in the decay channel $Λ(1405) \to Σ^+ + π^-$, where the decay distribution to $Σ^+ π^-$ and the variation of the $Σ^+$ polarization with respect to the $Λ(1405)$ polarization direction determines the parity. The $Λ(1405)$ is produced, in the energy range $2.55 < W < 2.85$ GeV and for $0.6 < \cos θ_{K^+} < 0.9$, with polarization $P = 0.45 \pm 0.02 (\text{stat}) \pm 0.07 (\text{syst})$. The analysis shows that the decays are in $S$ wave, with the $Σ^+$ polarized such that the $Λ(1405)$ has spin-parity $J^P = 1/2^-$, as expected by most theories.

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