High baryon densities in heavy ion collisions at energies attainable at the BNL Relativistic Heavy-Ion Collider and the CERN Large Hadron Collider

In very high-energy collisions nuclei are practically transparent to each other but produce very hot nearly baryon-free matter in the so-called central rapidity region. The energy in the central rapidity region comes from the kinetic energy of the colliding nuclei. We calculate the energy and rapidity loss of the nuclei using the color glass condensate model. This model also predicts the excitation energy of the nuclear fragments. Using a space-time picture of the collision we calculate the baryon and energy densities of the receding baryonic fireballs. For central collisions of gold nuclei at the highest energy attainable at the Relativistic Heavy-Ion Collider, for example, we find baryon densities more than ten times that of atomic nuclei over a large volume.

Figure 1

The dependence of $F_{\mathcal{A}}$ and $F_{\mathcal{B}}$ on proper time for $Q=4\mathrm{GeV}$.

Figure 2

Rapidity of the central core of a Au projectile nucleus in the center-of-momentum frame for $\sqrt{s_{NN}}=200\mathrm{GeV}$ as a function of proper time. The result is insensitive to the choice of $Q$ in the physically relevant range.

Figure 3

Excitation energy per baryon in the central core of a Au projectile nucleus in the center-of-momentum frame for $\sqrt{s_{NN}}=200\mathrm{GeV}$ as a function of proper time. The result is mildly sensitive to the choice of $Q$ in the physically relevant range.

Figure 4

The energy density and baryon density at $\tau =0.6\mathrm{fm}/\mathrm{c}$ as functions of the transverse distance for central collisions of Au nuclei at $\sqrt{s_{NN}}=200\mathrm{GeV}$.

Figure 5

Contour plot of the proper baryon density for central collisions of Au nuclei at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The numbers are in units of baryons per ${\mathrm{fm}}^3$. The horizontal axis measures the distance along the beam direction in the local rest frame. Care must be taken when interpreting this plot since the rapidity of the matter and therefore the frame of reference depend on $r_{\perp }$.

Figure 6

Contour plot of the proper baryon density at $\tau =0.6\mathrm{fm}/\mathrm{c}$ for central collisions of Au nuclei at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The units are baryons per ${\mathrm{fm}}^3$. The horizontal axis is the space-time rapidity.

Figure 7

Entropy per baryon rapidity distribution at $\tau =0.6\mathrm{fm}/\mathrm{c}$. A rapidity scan might help locate a critical point.