Mass Dependence of Directed Collective Flow.

M. J. Huang,1 R. C. Lemmon,1 F. Daffin,1 W. G. Lynch,1 C. Schwarz,1,4 M. B. Tsang,1 C. Williams,1 P. Danielewicz,1 K. Haglin,1 W. Bauer,1 N. Carlin,6 R. J. Charity,3 R. T. de Souza,5 C. K. Gelbke,1 W. C. Hsi,1,5 G. J. Kunde,1,4 M.-C. Lemaire,2 M. A. Lisa,1 U. Lynen,4 G. F. Peaslee,1 J. Pochodzalla,4 H. Sann,4 L. G. Sobotka,3 S. R. Souza,2 and W. Trautmann4 1National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 2Laboratoire National SATURNE, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France 3Department of Chemistry, Washington University, St. Louis, Missouri 63130 4Gesellschaft für Schwerionenforschung, D-6100 Darmstadt 11, Germany 5Indiana University Cyclotron Facility and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 6Instituto de Fisica, Universidade de São Paulo, CEP 01498, São Paulo, Brazil (Received 12 February 1996) Sidewards directed fragment flow has been extracted for 84Kr 1 197Au collisions at EyA ­ 200 MeV, using techniques that are free of reaction plane dispersion. The fragment flow per nucleon increases with mass, following a thermal or coalescencelike behavior, and attains roughly constant limiting values at 4 # A # 12. Comparisons of the impact parameter dependences of the measured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field. [S0031-9007(96)01441-X]