  Midrapidity nucleon elliptic flow is studied within the Boltzmann-equation
simulations of symmetric heavy-ion collisions. The simulations follow a lattice
Hamiltonian extended to relativistic transport. It is demonstrated that in the
peripheral heavy-ion collisions the high-momentum elliptic flow is strongly
sensitive to the momentum dependence of mean field at supranormal densities.
The high transverse-momentum particles are directly and exclusively emitted
from the high-density zone in the collisions, while remaining particles
primarily continue along the beam axis. The elliptic flow was measured by the
KaoS Collaboration as a function of the transverse momentum at a number of
impact parameters in Bi + Bi collisions at 400, 700, and 1000 MeV/nucleon. The
observed elliptic anisotropies in peripheral collisions, which quickly rise
with momentum, can only be explained in simulations when assuming a strong
momentum dependence of nucleonic mean field. This momentum dependence must
strengthen with the rise of density above normal. The mean-field
parametrizations, which describe the data in simulations with various success,
are confronted with mean fields from microscopic nuclear-matter calculations.
Two of the microscopic potentials in the comparisons have unacceptably weak
momentum-dependencies at supranormal densities. The optical potentials from the
Dirac-Brueckner-Hartree-Fock calculations, on the other hand, together with the
UV14 + TNI potential from variational calculations, agree rather well within
the region of sensitivity with the parametrized potentials that best describe
the data.
