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

We compute as a function of rapidity y the baryon number carried by quarks and antiquarks with pT > p0 ≈ 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 p0 = 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 τ = 0.1 fm will be more than the normal nuclear matter density but the baryon-to-entropy ratio is only (B − ¯ B)/S ∼ 1/5000. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.