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QCD at small x and nucleus–nucleus collisions

At large collision energy √s and relatively low momentum transfer Q, one expects a new regime of quantum chromo-dynamics (QCD) known as 'saturation'. This kinematical range is characterized by a very large occupation number for gluons inside hadrons and nuclei; this is the region where higher twist contributions are as large as the leading twist contributions incorporated in collinear factorization. In this talk, I discuss the onset of and dynamics in the saturation regime, some of its experimental signatures and its implications for the early stages of heavy ion collisions.