ar X iv : h ep - p h / 02 10 08 8 v 1 4 O ct 2 00 2 Novel QCD phenomena in pA collisions at LHC

We discuss novel QCD phenomena in pA collisions at LHC energies such as the possibility to invesigate the behaviour of hard processes at x as low as 10 −7 , signals for onset of the black body limit in the hard processes and in the nucleon fragmentation, multijet production. 1. Physics motivations The proton-ion collisions at LHC will be qualitatively different from those at fixed target energies 1 In the soft interactions two prime effects are a strong increase of the mean number of " wounded " nucleons-from about 6 to about 20 in pA central collisions (due to increase of σ inel (N N)) and a 50 % increase of the average impact parameters in pp collisions (as manifested in the shrinkage with increase with energy of the diffractive peak in the elastic pp collisions) making it possible for a proton to interact often simultaneously with nucleons which have essentially the same longitudinal coordinate but different impact parameters. At the same time geometry of pA collisions at LHC should be more close to that in the classical mechanics because increase of cross sections combined with the suppression of the inelastic diffraction in pp scattering, leads to the suppression of fluctuations in the value of effective N N cross section (and hence the suppression of inelastic shadowing corrections in pA scattering), to a very strong suppression of inelastic diffraction in pA collisions as compared to the fixed target FNAL energies [2]. In the hard interactions the prime effect is a strong increase of the gluon densities at small x both in projectile and ion. Gluon densities can be measured at LHC up to ultra small x ∝ 10 −6÷7 where PQCD will be probed in a new domain [1]. As a consequence of blackening of the interaction of proton partons with nuclear partons at small x A one should expect disappearance in the proton fragmentation region of hadrons with small transverse momenta, and hence a strong suppression of nonperturbative QCD effects 2 Thus the high transverse momentum proton fragmentation region is a natural place to search for unusual QCD phases which should live long time because of the Lorentz dilatation of time. In spite of a small coupling constant, gluon densities of heavy ions become larger in a wide range of x and Q 2 and impact parameters than that permitted by the conservation of probability within the leading twist …