First Measurement of ${\mathrm{\Lambda }}_c$ Baryon Production in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

We report on the first measurement of the charmed baryon ${\mathrm{\Lambda }}_c^{\pm }$ production at midrapidity ($|y|<1$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ collected by the STAR experiment at the Relativistic Heavy Ion Collider. The ${\mathrm{\Lambda }}_c/D^0$ [denoting $({\mathrm{\Lambda }}_c^{+}+{\mathrm{\Lambda }}_c^{-})/(D^0+{\overline{D}}^0)$] yield ratio is measured to be $1.08\pm 0.16(\mathrm{stat})\pm 0.26(\mathrm{sys})$ in the 0%–20% most central $\mathrm{Au}+\mathrm{Au}$ collisions for the transverse momentum ($p_T$) range $3<p_T<6\mathrm{GeV}/c$. This is significantly larger than the pythia model calculations for $p+p$ collisions. The measured ${\mathrm{\Lambda }}_c/D^0$ ratio, as a function of $p_T$ and collision centrality, is comparable to the baryon-to-meson ratios for light and strange hadrons in $\mathrm{Au}+\mathrm{Au}$ collisions. Model calculations including coalescence hadronization for charmed baryon and meson formation reproduce the features of our measured ${\mathrm{\Lambda }}_c/D^0$ ratio.

Figure 1

The $pK\pi$ invariant mass distributions for right-sign (solid red points) and wrong-sign (shaded histograms) combinations in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ for 0%–20% (top) and 10%–80% (bottom) centrality classes. The wrong-sign distributions are scaled by $1/3$, the ratio of the number of right-sign to wrong-sign combinations for the $pK\pi$ triplet. The error bars shown are statistical uncertainties. The solid line depicts a fit with a Gaussian function, for a ${\mathrm{\Lambda }}_c^{\pm }$ signal, and a second-order polynomial function, the shape of which is fixed by fit to the wrong-sign distribution (dashed line), for the background.

Figure 2

The measured ${\mathrm{\Lambda }}_c/D^0$ ratio at midrapidity ($|y|<1$) as a function of $p_T$ for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ in 10%–80% centrality, compared to the baryon-to-meson ratios for light and strange hadrons (top) and various model calculations (bottom). The vertical lines and shaded boxes on the ${\mathrm{\Lambda }}_c/D^0$ data points indicate statistical and systematic uncertainties, respectively. The $p_T$ integrated ${\mathrm{\Lambda }}_c/D^0$ ratio from the thermus [10] model calculation with a freeze-out temperature of $T_{\mathrm{ch}}=160\mathrm{MeV}$ is shown as a horizontal bar on the left axis of the plot.

Figure 3

The measured ${\mathrm{\Lambda }}_c/D^0$ yield ratio in $3<p_T<6\mathrm{GeV}/c$ (solid circles) as a function of collision centrality (expressed in $N_{\mathrm{part}}$) for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$. The open diamonds and squares show the baryon-to-meson ratio measured for strange and light-flavor hadrons, respectively. The vertical lines and the shaded boxes on the ${\mathrm{\Lambda }}_c/D^0$ data points indicate statistical and systematic uncertainties, respectively. The dashed curves indicate the ${\mathrm{\Lambda }}_c/D^0$ ratio calculated from a model with charm-quark coalescence, and the up and down triangles indicate the ratios from the pythia model for $p+p$ collisions without and with CR respectively, for the same $p_T$ region.