High precision beam momentum determination in a synchrotron using a spin resonance method

In order to measure the mass of the η meson with high accuracy using the dp → 3He η reaction, the momentum of the circulating deuteron beam in the Cooler Synchrotron COSY of the Forschungszentrum Jülich has to be determined with unprecedented precision. This has been achieved by studying the spin dynamics of the polarized deuteron beam. By depolarizing the beam through the use of an artificially induced spin resonance, it was possible to evaluate its momentum p with a precision of ∆p/p < 10−4 for a momentum of roughly 3 GeV/c.