The proton asymmetry parameter C in neutron decay describes the correlation between neutron spin and proton momentum. In this Letter, the first measurement of this quantity is presented. The result C=-0.2377(26) agrees with the Standard Model expectation. The coefficient C provides an additional parameter for new and improved Standard Model tests. From a differential analysis of the same data (assuming the Standard Model), we obtain lambda=-1.275(16) as ratio of axial-vector and vector coupling constant.
@article{arxiv.0712.2442,
title = {Measurement of the Proton Asymmetry Parameter C in Neutron Beta Decay},
author = {M. Schumann and M. Kreuz and M. Deissenroth and F. Glueck and J. Krempel and B. Maerkisch and D. Mund and A. Petoukhov and T. Soldner and H. Abele},
journal= {arXiv preprint arXiv:0712.2442},
year = {2008}
}