Small–x Physics: From HERA to LHC and beyond

We summarize the lessons learned from studies of hard scattering processes in high–energy electron–proton collisions at HERA and antiproton–proton collisions at the Tevatron, with the aim of predicting new strong interaction phenomena observable in next–generation experiments at the Large Hadron Collider (LHC). Processes reviewed include inclusive deep–inelastic scattering (DIS) at small x, exclusive and diffractive processes in DIS and hadron–hadron scattering, as well as color transparency and nuclear shadowing effects. A unified treatment of these processes is outlined, based on factorization theorems of quantum chromodynamics, and using the correspondence between the “parton” picture in the infinite–momentum frame and the “dipole” picture of high–energy processes in the target rest frame. The crucial role of the three–dimensional quark and gluon structure of the nucleon is emphasized. A new dynamical effect predicted at high energies is the unitarity, or black disk, limit (BDL) in the interaction of small dipoles with hadronic matter, due to the increase of the gluon density at small x. This effect is marginally visible in diffractive DIS at HERA and will lead to the complete disappearance of Bjorken scaling at higher energies. In hadron–hadron scattering at LHC energies and beyond (cosmic ray physics), the BDL will be a standard feature of the dynamics, with implications for (a) hadron production at forward and central rapidities in central proton–proton and proton–nucleus collisions, in particular events with heavy particle production (Higgs), (b) proton–proton elastic scattering, (c) heavy–ion collisions. We also outline the possibilities for studies of diffractive processes and photon–induced reactions (ultraperipheral collisions) at LHC, as well as possible measurements with a future electron–ion collider. PACS numbers: 11.80.La, 12.40.Gg, 12.40.Pp, 25.40.Ve, 27.75.+r