Influence of electromagnetic fields in proton-nucleus collisions at relativistic energy

TL;DR

This study investigates the impact of intense electromagnetic fields on proton-gold collisions at RHIC energy, revealing significant electric and magnetic field effects on particle flow and charge asymmetries, with implications for understanding electromagnetic influences in nuclear collisions.

Contribution

It provides the first detailed analysis of electromagnetic field effects on particle flow in proton-nucleus collisions using the PHSD approach, highlighting the electric field's dominant role.

Findings

Electric fields are comparable in magnitude to magnetic fields during collisions.

Electromagnetic fields induce a charge-dependent splitting in directed flow $v_1$.

Directed flow from the deconfined phase is significant, especially for kaons.

Abstract

We study proton-gold collisions at RHIC energy $\sqrt{s_{NN}}=200$ GeV within the Parton-Hadron-String Dynamics (PHSD) off-shell transport approach, investigating the influence of the intense electromagnetic fields generated in these small systems. We show the space-time evolution of the magnetic and electric components, emphasizing the huge values of the latter one, in particular the electric field $E_x$ along the impact parameter direction whose magnitude is comparable to the magnetic field $B_y$ perpendicular to the reaction plane. We find a fair agreement of the charged particle pseudorapidity density of the high-multiplicity events with respect to the experimental result of the PHENIX Collaboration. Focusing on the most central collision, we show rapidity distributions and spectra as well as the flow coefficients $v_1$ and $v_2$ and we discuss the impact of the electromagnetic fields on identified particle observables. We compute the directed flow $v_1$ of $\pi^+$, $\pi^-$, $K^+$, $K^-$ for collisions at fixed impact parameter and predict that an electromagnetically-induced splitting in the $v_1$ of positively and negatively charged particles is generated in the Au-going side of p+Au reaction mainly driven by the huge $E_x$ component. We find that this effect is stronger for the strange mesons and increases for increasing impact parameter. Furthermore, we highlight the amount of directed flow generated in the deconfined phase, finding that it constitutes the main contribution in the central rapidity region, especially for kaons. Thus, we support the idea that the directed flow is a promising probe for the electromagnetic fields generated in relativistic nuclear collisions and show that in proton-induced reactions the electric component along the impact parameter axis is the primary origin of a charge-odd $v_1$ of pions and kaons.