The interaction of Lambda and Sigma hyperons (Y) with nucleons (N) is strongly influenced by the coupled-channel dynamics. Due to the small mass difference of the N Lambda and N Sigma systems, the sizable coupling strength of the N Sigma <-> N Lambda processes constitutes a crucial element in the determination of the N Lambda interaction. In this letter we present the most precise measurements on the interaction of p Lambda pairs, from zero relative momentum up to the opening of the N Sigma channel. The correlation function in the relative momentum space for p Lambda circle plus (p) over bar(Lambda) over bar pairs measured in high-multiplicity triggered pp collisions at root s = 13 TeV at the LHC is reported. The opening of the inelastic N Sigma channels is visible in the extracted correlation function as a cusp-like structure occurring at relative momentum k* = 289 MeV/c. This represents the first direct experimental observation of the N Sigma <-> N Lambda coupled channel in the p Lambda system. The correlation function is compared with recent chiral effective field theory calculations, based on different strengths of the N Sigma <-> N Lambda transition potential. A weaker coupling, as possibly supported by the present measurement, would require a more repulsive three-body NN Lambda interaction for a proper description of the Lambda in-medium properties, which has implications on the nuclear equation of state and for the presence of hyperons inside neutron stars.
