Probing the electromagnetic fields in ultrarelativistic collisions with leptons from $Z^0$ decay and charmed mesons

Abstract

Ultrarelativistic heavy ion collisions are expected to generate a huge electromagnetic field, $eB\approx 10^{18}Gauss$, that induces a splitting of the directed flow, $v_1=⟨p_x/p_T⟩$, of charged particles and anti-particles. Such a splitting for charmed meson manifests even for neutral particle/anti-particles pairs ($D^0,{\overline{D}}^0$), hence being also a unique probe of the formation of the quark-gluon plasma phase. For the first time here we show that electromagnetic field may generate a $v_1(D^0)-v_1({\overline{D}}^0)$ as huge as the one recently observed at LHC against early expectations. Within this new research topic we point out a novel measurement: $v_1$ of leptons from $Z^0$ decay and its correlation to that of $D$ and $B$ mesons. The correlation for $\Delta v_1$ of $l^{+}-l^{-}$, $D^0-{\overline{D}}^0$ and $B^0-{\overline{B}}^0$ would provide a strong probe of the electromagnetic origin of the splitting and hence of the formation of a quark-gluon plasma phase with heavy quarks as degrees of freedom. The case of the $v_1(l^{\pm })$ presents features due to the peculiar form of the $p_T$ spectrum never appreciated before in the study of heavy ion collisions. We specifically predict a sudden change of the $\Delta v_1(p_T)$ of leptons at $p_T=45$ GeV$/c$, that can be traced back to a universal relation of $\Delta v_1$ with the slope of the $p_T$ particle distribution and the integrated effect of the Lorentz force.

Comments: 6 pages, 4 figures