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Non-linear Low-x Effects

In the long term, the acceleration of nuclei in HERA may appear to be a powerful way to address two issues raised by the present data at low x : Unitary constraints on the rise of F2(x) or G(x), and the nature of the pomeron. Up to now unitarity limits have not been reached in lepton-nucleon collisions. However, the lifetime of the hadronic content of the exchanged virtual photon is large ($\sim 1/x$) and exceeds the size of a nucleus (while its transverse extension, $\sim 1/Q$, is very small). It sees gluons, radiated by different nucleons in the nucleus, which may fuse, leading to non linear effects in the evolution equations at larger values of x than in the free nucleon case. Speculations indicate that this should occur in nuclei in the range x=10-3 to 10-2 accessible at HERA. If the origin of large rapidity gap diffractive events were the exchange of many soft gluons, the vicinity of other nucleons will certainly modify significantly the free nucleon cross-section. The acceleration of nuclei in HERA is a way to approach confinement in a regime where many partons are present in a small interaction volume. This is complementary to the study of exclusive reactions at large momentum transfer, which select the simplest partonic systems, but which require luminosities unreachable in the colliding mode. 
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