New results for the neutron-deuteron analyzing power A(y)(theta) at E-n = 1.2 and 1.9 MeV and their comparison to proton-deuteron data reveal a sizeable and unexpected difference which increases with decreasing center-of-mass energy. This finding calls for the theoretical treatment of a subtle electromagnetic effect presently not incorporated in rigorous three-nucleon scattering calculations, before it is justified to invoke charge-dependent three-nucleon forces and/or other new physics. (C) 2002 Elsevier Science B.V. All rights reserved.

New results for the neutron-deuteron analyzing power Ay(θ) at En = 1.2 and 1.9 MeV and their comparison to proton-deuteron data reveal a sizeable and unexpected difference which increases with decreasing center-of-mass energy. This finding calls for the theoretical treatment of a subtle electromagnetic effect presently not incorporated in rigorous three-nucleon scattering calculations, before it is justified to invoke charge-dependent three-nucleon forces and/or other new physics.

Neutron capture by a proton at low neutron energies is one of the most important reactions in both nuclear physics and astrophysics. In nuclear physics the n-p two body system is solvable and the reaction can provide valuable information about the nucleonnucleon interaction, wave function of the deuteron and the electromagnetic properties of nucleons. Polarized neutrons of 6 and 13.4 MeV were previously used in the measurement of the analyzing power for the H(n, )H reaction and the results were di erent from theory [Sod87]. Due to experimental di culties, cross-section measurements for the H(n, )H reaction have not been well studied at low neutron energies [Nag97]. In nuclear astrophysics, all deuterium in the early universe is known to be created from the H(n, )H reaction and the production yield of this primordial element is determined from the cross sections of the reaction. At astrophysically relevant energies between 10 and 600 keV, cross sections were estimated by e ective-range theory [Fow67]. The reverse reaction H( ,p)n measured recently just above the threshold for photodisintegration (E 2:23 MeV) [Sch00] showed that the relative M1 contribution to the total cross section agrees with theoretical treatments using potential model with meson-exchange current and relativistic e ects [Are99] as well as with the e ective eld theory [Che99]. The cross section of the H(n, )H reaction was quite recently measured for the rst time at En = 550 keV [Nag97], and also previously between the energies of 10 and 80 keV [Suz95], where both experimental results agree with the theoretical calculations which include meson-exchange currents. The motivation of our work is to measure the analyzing powers of the n-p capture reaction using 300{500 keV polarized neutrons. This will test theoretical predictions of the M1/E1 ratio in n-p capture, or deuteron photo-disintegration. This energy regime is important to Big-Bang nucleosynthesis. A knowledge of the M1/E1 ratio will help determine the cross section in this region to the precision needed for accurate predictions of the abundances of elements.