Precise momentum determination of the external COSY proton beam near 1930 MeV/c

We present a method to determine precisely the absolute momentum of the external proton beam from the Julich Cooler Synchrotron COSY near 1930 MeV/c. In the pp→dπ+ reaction at 1930.477 MeV/c incident beam momentum, the forward going pions (θc.m. = 0°) and the backward going deuterons (θc.m. = 180°) have the same laboratory momentum. Such coincident pion-deuteron events are detected in the focal plane of the magnetic spectrometer BIG KARL located at θlab = 0°. Using the nearly linear dependence of the difference between the measured pion and deuteron momenta as a function of the proton beam momentum, the absolute momentum of the external proton beam from COSY near 1930 MeV/c was determined with a precision of 5.2×10-5.