Search for gluon saturation in proton-lead collisions at $\sqrt{s_{NN}}=5.02$ TeV with the very forward CASTOR calorimeter at the CMS experiment

An experimental search for signals of gluon saturation in proton-lead collisions at the CMS experiment is pursued. First, the theory behind the concept of gluon saturation is introduced. The sensitivity of a measurement of forward jet production in proton-lead collisions is motivated. Thereafter, the experimental instrumentation for producing and studying proton-lead collisions is discussed, including an explanation of the CERN LHC and a description of the CMS detector. Subsequently, the CASTOR calorimeter is introduced, which is the key detector of this analysis. Next, the data analysis is discussed, as well as the strategy to interpret the data. Jet spectra are presented in the laboratory pseudorapidity range $-6.6<\eta<-5.2$, with asymmetric beam energies of 4 TeV for protons and 1.58 TeV per nucleon for Pb nuclei, corresponding to a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.02$ TeV. Measurements are presented for beam configurations with either the proton (p+Pb) or the ion (Pb+p) traveling towards the negative $\eta$ hemisphere. In addition, the cross section ratio of p+Pb and Pb+p data is presented. The cross sections are unfolded to stable-particle level and interpreted using Monte Carlo models, with the emphasis on the interpretation in terms of the saturation of gluon densities. One of these models suggests that the p+Pb spectra are highly sensitive to effects of gluon saturation; predictions for the p+Pb spectrum with and without saturation effects differ by an order of magnitude at low energies. None of the models under consideration describe all data over the full jet energy range and for all beam configurations, and discrepancies of more than two orders of magnitude are observed.