Spectator-induced electromagnetic effect on directed flow in heavy ion collisions

Abstract

We estimate the electromagnetic effect of the spectator charge on azimuthal anisotropies observed in heavy ion collisions. For peripheral Pb+Pb reactions at the top energy of the CERN Super Proton Synchrotron, √ s N N = 17.3 GeV, we predict this effect to bring very large distortions to the observed directed flow, v 1 , of positive and negative pions emitted close to beam rapidity. The overall magnitude of this effect is comparable to values of v 1 reported by the WA98 experiment. We argue that also at lower rapidities, the spectator induced electromagnetic effect may result in the splitting of values of v 1 observed for positive and negative pions. Such a splitting is visible in the data reported by the STAR Collaboration from the RHIC Beam Energy Scan. Both effects are sensitive to the space-time scenario assumed for pion emission. Therefore, they bring new information on the collision dynamics. 25.75Dw,25.75.Ld

New results on Coulomb effects in meson production in relativistic heavy ion collisions

“…Our analysis of charged pion directed flow in Pb+Pb collisions [4] demonstrated that the spectator-induced electromagnetic effect results in very large absolute values of v 1 for positive pions at target and beam rapidities, in agreement with data from the WA98 experiment [5]. This suggests the emission site of these pions to be placed relatively close to the corresponding spectator system, at a longitudinal distance d E below 1 fm.…”

“…Putting the result of this simplified study (d E ≈ 3 fm) in the context of our earlier analysis of pion directed flow at target and beam rapidities (where a value of d E below 1 fm could be deduced from the data [4]), we note that the obtained effective value of d E decreases with increasing c.m.s. rapidity of the pion.…”

“…From our earlier studies [4] we know that v π + ,EM We now turn towards the numerical simulation of the spectator-induced electromagnetic interaction. For this purpose we use a very simple model, succesfully applied before for the description of spectator-induced electromagnetic distortions of π + /π − ratios in Pb+Pb collisions [2].…”

“…For this purpose we use a very simple model, succesfully applied before for the description of spectator-induced electromagnetic distortions of π + /π − ratios in Pb+Pb collisions [2]. An in-depth description of this model can be found in [1,4]. We approximate the two spectator systems as homogenously charged spheres moving at the proper relativistic velocity.…”

Dynamical evolution of spectator systems produced in ultrarelativistic heavy-ion collisions

“…Non-central collisions unambiguously lead to azimuthal asymmetries in the pion trajectory [1], which may be linked to the electromagnetic field generated by these fast-moving charged remnants. The influence of this field on charged pions was discussed in [2,3].…”