  Using a partonic transport model based on the 3-flavor Nambu-Jona-Lasinio
model and a relativistic hadronic transport model to describe, respectively,
the evolution of the initial partonic and the final hadronic phase of heavy-ion
collisions at energies carried out in the Beam-Energy Scan program of the
Relativistic Heavy Ion Collider, we have studied the effects of both the
partonic and hadronic mean-field potentials on the elliptic flow of particles
relative to that of their antiparticles. We find that to reproduce the measured
relative elliptic flow differences between nucleons and antinucleons as well as
between kaons and antikaons requires a vector coupling constant as large as 0.5
to 1.1 times the scalar coupling constant in the Nambu-Jona-Lasinio model.
Implications of our results in understanding the QCD phase structure at finite
baryon chemical potential are discussed.
