Gluon saturation in relativistic collisions
The test of gluon saturation in relativistic heavy-ion collisions is an important aim of the forthcoming Pb + Pb experiments at the LHC. At these energies gluons dominate the dynamical evolution of the system, which is driven by a single hard scale, the saturation scale Q_s>>Lambda_QCD. Whereas most theoretical investigations concentrate on charged-hadron production from inclusive gluon interactions, the valence-quark scattering off the gluon condensate as an observable in net-baryon distributions is expected to provide interesting new information on gluon saturation, and on geometric scaling.
Here the most promising effects arise at very forward angles, or correspondingly large values of the rapidity y = 5 - 8 at LHC energies of sqrt (s_NN) = 5.5 TeV for Pb + Pb, with a beam rapidity of 8.68. For symmetric systems, two symmetric fragmentation peaks are expected to be present in the net-baryon distributions at forward/backward rapidities. In particular, we have shown that it is in principle possible to determine the growth of the saturation-scale exponent, lambda = dln(Q_s)/dy_b, with the beam rapidity y_b from the position of the fragmentation peak in rapidity space.
This requires, however, data for identified particles (in particular, protons and antiprotons) in central heavy-ion collisions at very forward rapidities, which will be inaccessible for several years to come. We therefore concentrate the investigation on the central rapidity region y < 2 where ALICE has particle identification capabilities, to check the overall consistency of the gluon saturation picture.
Y. Mehtar-Tani and G. Wolschin, Phys. Rev. C 80, 054905 (2009); Phys. Rev. Lett. 102, 182301 (2009).