Proton nucleus collisions are expected to be unique tool to probe the physics of the initial state of ultra relativistic heavy ion collisions, without the presence of thermalisation and collective evolution of high energy density QCD matter. However, many observations related to the production of light and strange hadrons at low transverse momentum (pT) in p-Pb collisions at the LHC surprisingly exhibit a qualitative similarity to measurements in nucleus-nucleus collisions and are in qualitative agreement with hydrodynamical model
8 calculations. Similar observations have been performed in high multiplicity proton-proton collisions at the LHC. On the other hand, the experimental results of high pT hadrons, jet, heavy quarks production in p-Pb collisions at the LHC can be explained as binary scaling with respect to pp collisions with a moderate shadowing effect as expected in the absence of collective phenomena. In the case of J/ψ and Υ(1S) production an additional energy loss mechanism has to be considered to explain their production. Among the hard probes, only the Ψ(2S), Υ(2S) and Υ(3S) exhibit a puzzling behaviour in proton-nucleus collisions at the LHC, also observed for the Ψ(2S) in central d-Au collisions at RHIC, and which violate the binary scaling. A priori, the observed relative suppression of theses quarkonium resonances in p-A collisions, ravels the interpretation of the experimental results obtained in A-A collisions.