First Model-Independent Measurement of the Spin Triplet $pLambda$ Scattering Length from Final State Interaction in the $vec{p}p rightarrow pK^{+}Lambda$ Reaction

The $vec{p}p rightarrow pK^{+}Lambda$ reaction has been measured with the COSY-TOF detector at a beam momentum of $2.7,mathrm{GeV}/c$. The polarized proton beam enables the measurement of the beam analyzing power by the asymmetry of the produced kaon ($A_N^{K}$). This observable allows the $pLambda$ spin triplet scattering length to be extracted for the first time model independently from the final-state interaction in the reaction. The obtained value is $a_{t} = (-2.55 ^{+0.72}_{-1.39} {}_{textrm{stat.}} pm 0.6_{textrm{syst.}} pm 0.3_{textrm{theo.}})mathrm{fm}$. This value is compatible with theoretical predictions and results from model-dependent analyses.