Threshold measurement of the λp → nπ+ reaction

Threshold photoproduction of pions off nucleons is one of a few low-energy phenomena for which QCD-based effective field theories can be formulated and tested. As part of a program aimed at determining the threshold amplitudes of the various γN → Nπ channels, a cross section measurement of the reaction γp→nφ+ was carried out at the Saskatchewan Accelerator Laboratory (SAL). The measurement involved the use of the SAL photon-tagging facility, a liquid hydrogen target, and a segmented liquid-scintillator neutron counter. The outgoing neutrons were detected in a forward-angle cone covering 1 degrees 8 degrees around the beam direction. Data for photon energies ranging from below threshold up to about 6 MeV above threshold were collected simultaneously in 0.25 MeV steps. The motivation and experimental details of the measurement will be outlined along with preliminary results