Small x Physics: From HERA to LHC and beyond


We summarize the lessons learned from QCD--based studies of hard
scattering phenomena in high--energy $ep/pp$ collisions at HERA and
Tevatron, with the aim of making predictions for the next generation
 of collider experiments (LHC, EIC). Phenomena reviewed include inclusive
DIS at small x, exclusive and diffractive processes in ep and hadron--hadron
scattering (`3D structure'' of the nucleon), as well as
color transparency and nuclear shadowing effects. A unified treatment
of these processes is possible using general QCD
factorization theorems, and the correspondence between the ``partonic''
picture in the infinite--momentum frame and the ``dipole''
picture in the target rest frame, in which small color--singlet
configurations in the projectile scatter from the target.
A new dynamical effect predicted at high energies is the
black body (unitarity) limit in the interaction of a small dipole
with hadronic matter, due to the increase of the gluon density at small
x. Signs of this effect can already be seen in diffractive interactions of
gluon--gluon dipoles at maximum HERA energies. In ep scattering at
higher energies, this effect leads to the disappearance of Bjorken
scaling. In hadron--hadron scattering at LHC energies and beyond (cosmic ray
physics), this effect will be a standard feature of the dynamics, with numerous
implications for  i) hadron production at forward and central
rapidities in inelastic central pA and pp collisions, in particular
events with new heavy particle production (Higgs,  etc.),
ii) pp elastic scattering, iii) heavy--ion collisions.