Figure. Dependence of the antiproton fit parameters and yields on the midrapidity negative hadron pseudorapidity density (dNh-/deta). (a)Fitted parameters from three functional forms: sigmapT from the Gaussian(I), TmT from the exponential (II), and TB (III) from the Boltzmann functions. (b) Fiducial yield of inclusive antiprotons in 0.25 < pT < 0.95 GeV/c (points) and the estimated total yield using the three functional forms for extrapolation (solid, dashed, and dotted lines). (c) Fiducial yield of net-protons (points) and the estimated total net-proton yield using pT-Gaussian for extrapolation.

The measurement of the pbar/p ratio of 0.73 at midrapidity in minimum bias Au+Au collisions at RHIC indicates that a finite net-baryon number is present at midrapidity. This implies that a finite baryon number has been transported over 5 units of rapidity in these collisions. The transport of incoming baryon number over several units of rapidity most likely occurs very early in the collisions and affects the subsequent evolution of the system. The net-proton multiplicity density at midrapidity can be extracted from the inclusive aniproton measurement and the pbar/p ratio.

The inclusive antiproton production at midrapidity (|y| < 0.1) in the interval 0.25 < p_T < 0.95 GeV/c was measured at RHIC by STAR in Au+Au collisions at sqrt(s_nn) = 130 GeV. The measured antiproton yields include secondary products of weak decays. In the measured p_T range, the antiproton transverse mass (m_T) distributions are found to fall less steeply in more central collisions (see panel (a) in the Figure). The m_T distribution for the most central collisions is significantly flatter than in Pb+Pb collisions at the SPS. The inclusive antiproton production rate is characterized by the fiducial rapidity density (dN/dy) in the measured p_T range by summing up the data points in the m_T spectrum for each centrality bin (see panel (b) in the Figure). In order to estimate the total antiproton rapidity density, the measurement is extrapolated to all p_T (see panel (b) in the Figure). The centrality dependence of the relative antiproton production is qualitatively different from what has been observed at lower energies at the AGS and the SPS, where the antiproton production relative to pions decrease from midcentral to central collisions. Hadronic model calculations show that this decrease is a result of a strong absorption of antiprotons in the collision zone, but the initial production of antiprotons relative to pions increases with centrality. Combining the pbar/p ratio and the inclusive antiproton measurements, the midrapidity net-proton (p - pbar) fiducial density within 0.25 < p_T < 0.95 GeV/c is extracted (see panel (c) in the Figure). For the most central collisions, 9.5 +- 1.0 antiprotons per unity of rapidity are measured. As a result, 6.2 +- 1.8 protons per unity of rapidity are in excess of antiprotons taking into account the pbar/p ratio measurement. From the extrapolation of the antiproton yields and the assumption that the pbar/p ratio is constant over all p_T, approximately 14 net-protons are found per unit of rapidity at midrapidity.

Measurement of inclusive antiprotons from Au+Au collisions at sqrt(s_nn) = 130 GeV
Phys. Rev. Lett. 87 (2001) 262302
e-Print Archives (nucl-ex/0110009): Abstract | PS | PDF
Journal article: Phys. Rev. Lett. server

Midrapidity Antiproton-to-Proton Ratio from Au+Au sqrt(s_nn) = 130 GeV
Phys. Rev. Lett. 86 (2001) 4778
e-Print Archives (nucl-ex/0104022): Abstract | PS | PDF
Journal article: Phys. Rev. Lett. server

Erratum: Midrapidity Antiproton-to-Proton Ratio from Au+Au sqrt(s_nn) = 130 GeV [Phys. Rev. Lett. 86 (2001) 4778]
Phys. Rev. Lett. 90 (2003) 119903(E)
Journal article: Phys. Rev. Lett. server