STAR helps to pin down a key phenomenon in gold collisions

The STAR collaboration at the Brookhaven National Laboratory (BNL) has published new evidence indicative of a “chiral magnetic wave” rippling through the quark–gluon plasma created in high-energy gold–gold collisions at the Relativistic Heavy Ion Collider (RHIC).

Heavy-ion collisions at RHIC and the LHC involve many spectators – nucleons that are not involved in any direct collision. The charged spectators – protons – have an important influence because they can produce a magnetic field of some 10¹⁴ T. In principle, this can lead to a collective excitation in the hot dense matter produced, the chiral magnetic wave. It results from the separation both of electric charge and of chiral charge, that is, right or left “handedness”, but only in a chirally symmetric phase. The phenomenon is predicted to manifest itself as an electric quadrupole moment of the collision system, where the “poles” and “equator” of the system acquire, respectively, additional positive and negative particles. This in turn influences differently the elliptic flow of positive and negative particles, decreasing the former and increasing the latter.

To look for this effect, STAR measured the elliptic flow, v₂, of π⁺ and π^– produced in gold–gold collisions at mid-rapidity, as a function of the event-by-event charge asymmetry, A_CH, over a range of energies. The team found that v₂ increased linearly with A_CH for π^–, but decreased for π⁺. At the highest energy, √s_NN = 200 GeV, the slope of the difference in v₂ between the π⁺ and π^– as a function of A_CH depends on the centrality of the collision in a manner consistent with calculations that incorporate the chiral magnetic wave. The team also found a similar result for energies down to √s_NN = 27 GeV, with no obvious dependence on beam energy. The researchers note that none of the conventional models they have considered appear to explain the observations.