Figure. The mid-rapidity, pt integrated, anti-baryon/baryon ratio in Au-Au collisions at sqrt(s)=130 GeV compared to that in Pb-Pb collisions at sqrt(s)=17 GeV.

The anti-baryon to baryon ratio supplies information about the amount of baryon transfer from the colliding nuclei to the central collision region. At RHIC there is a gap of 12 rapidity units between the two beams. Therefore a baryon needs to be transported through 6 units of rapidity to be observed at mid-rapidity. The proximity of the anti-baryon/baryon ratio to unity tells us that the net baryon number is close to zero at mid-rapidity for Au-Au collisions. At RHIC the anti-proton to proton ratio at colliding energies of 130 GeV/c is measured by STAR to be 0.71, which means that 70% of the protons are produced via pair production. This is very different to the measurements made at SPS where nearly all the measured protons are derived from the colliding nuclei.

The ratios increase with increasing strange quark content of the particles and have been shown to be approximated by a multiplicative factor given by the K+/K- ratio. This trend is predicted by quark coalesence and statistical models.