Coalescence sum rule and the electric charge- and strangeness-dependences of directed flow in heavy ion collisions

Abstract

The rapidity-odd directed flows ($v_1$) of identified hadrons are expected to follow the coalescence sum rule when the created matter is initially in parton degrees of freedom and then hadronizes through quark coalescence. A recent study has considered the $v_1$ of produced hadrons that do not contain $u$ or $d$ constituent quarks. It has constructed multiple hadron sets with a small mass difference but given difference in electric charge $\Delta q$ and strangeness $\Delta S$ between the two sides, where a nonzero and increasing $\Delta v_1$ with $\Delta q$ has been proposed to be a consequence of electromagnetic fields. In this study, we examine the consequence of coalescence sum rule on the $\Delta v_1$ of the hadron sets in the absence of electromagnetic fields. We find that in general $\Delta v_1\ne 0$ for a hadron set with nonzero $\Delta q$ and/or $\Delta S$ due to potential $v_1$ differences between $\bar{u}$ and $\bar{d}$ and between $s$ and $\bar{s}$ quarks. We further propose methods to extract the coefficients for the $\Delta q$- and $\Delta S$-dependences of the direct flow difference, where a nonzero constant term would indicate the breaking of the coalescence sum rule. The extraction methods are then demonstrated with transport model results.

Comments: 7 pages, 4 figures