Can baryon stopping explain the breakdown of constituent quark scaling and proposed signals of chiral magnetic waves at RHIC?

Abstract

Azimuthal emission spectra of various hadron species in ultra-relativistic heavy ion collisions at ≈ 200 GeV exhibit a curious hierarchy at intermediate pt (≈ 2 – 3 GeV). Rather than being ordered by mass, the spectra seem to be ordered by whether the species is a baryon or meson. It is seen that when the elliptic flow v2 and transverse momentum pT are both scaled by the number of quarks in each hadron, the spectra fall in line with each other. This number of constituent quark (NCQ) scaling suggests a system where the relevant degrees of freedom are colored partons as opposed to hadrons: the quark-gluon plasma (QGP). Thus, a break down of this scaling as beam energy is reduced could be indicative of the QGP threshold. However, at lower energies, there is also an increase in the number of entrance-channel partons transported to mid-rapidity due to baryon stopping, which can violate NCQ scaling even above the QGP threshold. We describe a specific pattern for the break down of the scaling that includes the observed difference in elliptic flow for positive and negative pions. We also contrast baryon stopping with the Chiral Magnetic Effect (CME)–an alternative model for π+/π− flow difference–and discuss results from tests that can distinguish between them.