First measurement of the K-n → Λπ- non-resonant transition amplitude below threshold

We present the analysis of K(-)absorption processes on He-4 leading to Lambda pi-final states, measured with the KLOE spectrometer at the DA Phi NE e(+) e-collider and extract, for the first time, the modulus of the non-resonant K(-)n ->Lambda pi-direct production amplitude about 33 MeV below the (K) over barN threshold. This analysis also allows to disentangle the K- nuclear absorption at-rest from the in-flight capture, for K- momenta of about 120 MeV. The data are interpreted with the help of a phenomenological model, and the modulus of the non-resonant K(-)n ->Lambda pi-amplitude for K- absorption at-rest is found to be vertical bar A(K-n ->Lambda pi-)vertical bar =(0.334 +/- 0.018 stat(-0.058)(+0.034)syst)fm. (C) 2018 The Author(s). Published by Elsevier B.V.