Baryon-to-entropy ratio in very high energy nuclear collisions

Abstract

We compute as a function of rapidity $y$ the baryon number carried by quarks and antiquarks with $p_T > p_0 \approx$ 2 GeV produced in Pb+Pb collisions at TeV energies. The computation is carried out in lowest order QCD perturbation theory using structure functions compatible with HERA results. At $p_0=2$ GeV the initial gluon density is both transversally saturated and thermalised in the sense that the energy/gluon equals to that of an ideal gas with the same energy density. Even at these high energies the initial net baryon number density at $y=0$ at $\tau=0.1$ fm will be more than the normal nuclear matter density but the baryon-to-entropy ratio is only $(B-\bar B)/S\sim 1/5000$. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.