Speaker
Description
Directed flow $v_1$ has been used to probe early dynamics in high-energy nuclear collisions. The vector meson $\phi(s \overline{s})$, with a mass comparable to that of light baryons, exhibits a small interaction cross section with other hadrons. Therefore, the measurement of $\phi$-meson directed flow $v_1$ provides clean access to the early collision dynamics and the production mechanisms of the vector-mesons.
In this talk, we report the measurement of $\phi$-meson directed flow ($v_1$) from Au+Au collisions at center-of-mass energies of 3.0, 3.2, 3.5, 3.9 and 4.5 GeV, using data collected by the STAR experiment as part of the RHIC Beam Energy Scan program. In the high-baryon-density region, the observed $\phi$-meson $v_1$ values are all positive and comparable to those of baryons (protons and $\Lambda$), while the $v_1$ values of lighter mesons, such as pions and kaons, are much smaller than those of $\phi$ mesons. The new results will be compared within the framework of hadronic transport model calculations (UrQMD and JAM), and the role of vector meson-baryon coupling in $\phi$-meson production will be discussed.
