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HK: Fachverband Hadronen und Kerne

HK 45: Heavy Ion Collisions and QCD phases

HK 45.1: Gruppenbericht

Mittwoch, 18. März 2009, 14:00–14:30, H-ZO 10

Saturation and geometric scaling at RHIC and LHC with polarized and unpolarized final states — •Andre Utermann¹, Daniel Boer² und Erik Wessels² — ¹Institut für Theoretische Physik, Universität Regensburg — ²Department of Physics and Astronomy, Vrije Universiteit Amsterdam

A strong rise of the gluon distribution, as predicted by linear QCD evolution, is clearly observed at small x. At some saturation scale Q_s(x), the gluons in the proton may interact with each other and non-linear corrections to the evolution equations become significant, taming the growth of the gluon distribution. Geometric scaling, i.e. the observed property that the small-x DIS cross section depends only on the combination Q²/Q_s²(x), is seen as an indication for gluon saturation, since this property is expected asymptotically x→ 0 from QCD evolution. We show that the whole range of RHIC data for hadron production in d -Au collisions is compatible with geometric scaling as well. To establish the scaling violations, expected from the non-linear evolution equation at small but finite x, a larger kinematic range in transverse momentum and rapidity would be needed. We point out that the fall-off of the p_t distribution of produced hadrons at large p_t is a sensitive probe of small-x evolution especially at the LHC. Furthermore, we show that the transverse polarization of forward Λ hyperons produced in high-energy p -A collisions is expected to display an extremum at a transverse momentum around the saturation scale. Moreover, the measurement of Λ polarization over a range of x_F values actually provides a direct probe of the x dependence of the saturation scale.