Midrapidity $\Lambda$ and $\overline{\Lambda }$ Production in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$

We report the first measurement of strange ($\Lambda$) and antistrange ($\overline{\Lambda }$) baryon production from $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at the Relativistic Heavy Ion Collider (RHIC). Rapidity density and transverse mass distributions at midrapidity are presented as a function of centrality. The yield of $\Lambda$ and $\overline{\Lambda }$ hyperons is found to be approximately proportional to the number of negative hadrons. The production of $\overline{\Lambda }$ hyperons relative to negative hadrons increases very rapidly with transverse momentum. The magnitude of the increase cannot be described by existing hadronic string fragmentation models alone.

Figure 1

Invariant mass distribution of $\Lambda$ (solid circles) and $\overline{\Lambda }$ (open circles) candidates from the data sample analyzed including both central and minimum-bias collisions: (a) $0.4<p_T<0.6\mathrm{G}\mathrm{e}\mathrm{V}/c$; (b) $1.4<p_T<1.6\mathrm{G}\mathrm{e}\mathrm{V}/c$.

Figure 2

Transverse mass distributions of $\Lambda$ (left) and $\overline{\Lambda }$ (right) at midrapidity ($|y|<0.5$) for selected centrality bins. The dashed lines are Boltzmann fits. Note that multiplicative factors have been applied to data from the two most central data sets for display.

Figure 3

The midrapidity $\overline{\Lambda }$ ($|y|<0.5$) transverse momentum distribution from the top $5\%$ most central collisions. For comparison the distributions for negative hadrons [$d^{2}N/(2\pi p_T)d{p}_{T}d\eta$, $|\eta |<0.1$] and antiprotons ($|y|<0.1$) for the similar centrality bin are included. Statistical errors are less than the size of the data points.

Figure 4

$\Lambda$ and $\overline{\Lambda }$ rapidity density as a function of negative hadron multiplicity at midrapidity. The open symbols in the figure are data points from an independent analysis of minimum-bias data set using event-mixing techniques. Errors shown are statistical only.