On the Sigma N cusp in the pp -> pK(+)Lambda reaction

Measurements of the $pp \to pK^+\ Lambda$ reaction at $T_p$ = 2.28 GeV have been carried out at COSY-TOF. In addition to the $\ Lambdap$ FSI and $N^*$ resonance excitation effects a pronounced narrow structure is observed in the Dalitz plotand in its projection on the $p\ Lambda$- invariant mass. The structure appears at the $pp \to $N$K^+\Sigma$ threshold and is interpreted as $\Sigma$N cusp effect. The observed width of 20 MeV/$c^2$ is substantially broader than anticipated from previous inclusive measurements. Angular distributions of this cusp structure are shown to be dissimilar to those in the residual $pK^+\ Lambda$ channel, but similar to those observed in the $pK^+\Sigma^0$ channel.