Angular correlations of dijets in ultra-peripheral Pb+Pb collisions at sqrt(s[NN])=5.02 TeV

Abstract

One of the most remarkable predictions of the physics of strong interactions, and quantum chromodynamics (QCD) in particular, is gluon saturation. It is observed that the gluon density in hadrons, such as the proton, grows with energy, or equivalently with decreasing Bjorken-$x$ (the fraction of the hadron momentum carried by the parton). At some point this growth exceeds the unitarity limit and some new phenomena such as non-linear effects must set in. Data on the proton structure function, and on exclusive vector-meson photoproduction from the electron-proton collider HERA, which ended operations in 2007, have been inconclusive on whether or not gluon saturation has been observed. The search for non-linear QCD effects such as gluon saturation in both the proton and the nucleus is one of the main lines of research in high energy nuclear physics today. In nuclei the quantum fluctuations ought to be stronger than in protons. Recently, it has been found that the Quark Gluon Plasma (QGP) created in nucleus-nucleus collisions at RHIC and LHC expands with very little dissipation. The quantum fluctuations of the initial state described by the overlap of two highly Lorentz-contracted nuclei traveling on light-cone trajectories are probably imprinted upon the distribution of particles created in the QGP. Without assessing these quantum fluctuations in nuclei, fundamental properties of the QGP such as its viscosity-to-entropy ratio cannot be determined to a high precision. In this thesis we have studied, for the first time, the angular correlations of photoproduced dijets in ultra-peripheral Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV. This process has been suggested as a way to extract information of the nuclear gluon density in the Pb target, and thus provide information about the initial state of high energy nucleus-nucleus collisions.