XQCD 2018

Non-central relativistic heavy ion collisions break longitudinal boost invariance due to forward-backward asymmetry in local participant densities. This gives rise to the observed rapidity-odd directed flow of charged particles. We demonstrate that the heavy flavor $v_1$ due to the forward-backward asymmetric drag of the bulk is several times larger than that of the bulk $v_1$[1]. This makes the heavy flavor $v_1$ slope at mid-rapidity, a sensitive probe of the initial longitudinal distribution of the thermalized medium. Further, the electromagnetic fields of the initial stage can also give rise to rapidity-odd $v_1$ but of opposite signs for D and $\bar{D}$ mesons, unlike the tilt mechanism which provides same sign $v_1$ to both D and $\bar{D}$. We show that the measurement of the $v_1$ sum as well as difference of D and $\bar{D}$ can provide simultaneous constraint on the initial longitudinal asymmetry in matter distribution as well as the electromagnetic field produced by the passage of the heavy nuclei. [1]. S. Chatterjee and P. Bożek (2017), arXiv:1712.01189 [nucl-th]