Measurements, with the BRAHMS detector, of the antiproton-to-proton ratio at midrapidities and forward rapidities, are presented for $\mathrm{Au}+\mathrm{Au}$ reactions at $\sqrt{s_{\mathrm{NN}}}=130\mathrm{}\mathrm{GeV}$, and for three different collision centralities. For collisions in the $0\%–40\%$ centrality range, we find $N\left(\overline{p}\right)/N\left(p\right)=0.64\mathrm{}\pm {0.04}_{\left(\mathrm{stat}\right)}\pm {0.06}_{\left(\mathrm{syst}\right)}$ at $y\approx 0$, $0.66\pm 0.03\pm 0.06$ at $y\approx 0.7$, and $0.41\pm 0.04\pm 0.06$ at $y\approx 2$. The ratios are found to be nearly independent of collision centrality and transverse momentum. The antiproton and proton rapidity densities vary differently with rapidity, and indicate a significant degree of collision transparency, although a net-baryon free midrapidity plateau (Bjorken limit) is not yet reached.

• Received 28 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.112305