Two- and three-body long-range forces in argon and xenon by means of SANS

The long-range interaction in fluids is dominated by the attractive pair contribution U2; nevertheless it is well known and experimentally demonstrated that an accurate investigation requires the inclusion of the three-body contribution U3 even in the low-density regime. The characterization of both U2 and U3 is usually obtained by means of semi- empirical methods, the only exception being presently represented by SANS experiments on rare gases fluids. In this case the knowledge of S(k) in the low k-range allows a direct experimental determination of the U2 and U3 intensities, namely C6 and v. We report here the results of SANS experiments on low-density Ar and Xe performed at the PAXE diffractometer (LLB, Saclay). The effects of three-body forces are clearly separated from the pair contribution: the first experimental determination of C~, and v in those systems has been obtained, showing good agreement with the corresponding values found in literature.