Strongly interacting color-singlet exchange and photon exchange in high-energy proton-proton collisions

Abstract

The CERN Large Hadron Collider (LHC) offers a unique opportunity to test the Standard Model (SM) of particle physics, the quantum field theory of elementary interactions, at the shortest distances probed in a laboratory. In this context, we want to identify and understand some of the emergent properties of the strong interaction, described by quantum chromodynamics (QCD). At the same time, we strive to find signatures of physics beyond the SM, in the hope that we can thereby develop an improved theory of fundamental interactions. In this vein, the work presented in this thesis is two-fold. First, we present the measurement of events where the two highest transverse momentum (pT) jets are separated by a pseudorapidity interval devoid of particle activity, known as “jet-gap-jet” events, in proton-proton (pp) collisions at √s = 13 TeV with the CMS experiment. The signature is expected from hard color-singlet exchange, which corresponds to t-channel two-gluon exchange in perturbative QCD (pQCD), known as perturbative pomeron exchange. The fraction of events produced via color-singlet exchange, fCSE, is measured as a function of dijet kinematic variables of interest. We obtain the most precise experimental value of fCSE in an unexplored region of phase-space. The results are compared with previous measurements and with predictions from pQCD. In addition, we present the first study of jet-gap-jet events produced in association with an intact proton detected in the Roman pot (RP) detectors of the TOTEM experiment (effectively a “p-gap-jet-gap-jet” topology) in a joint CMS-TOTEM analysis. The fraction fCSE is larger in this subsample of events. Furthermore, an analysis of diffractive dijet events at √s = 13 TeV, where at least one of the protons exchanges a pomeron, is presented. The study can help better elucidate the diffractive structure function of the proton in a larger kinematic domain in the fraction of the beam momentum carried by the pomeron and the four-momentum transfer squared at the proton vertex. Also, the first investigation of double-pomeron exchange dijet events with two intact protons is presented (“p-gap-jet-jet-gap-p”). The second aspect of this thesis has to do with the phenomenology of new physics manifestations in photon-photon interactions. We consider central exclusive production reactions, pp → pX p, where X is a hard-scale system produced by two-photon fusion and the forward protons are detected in RPs. We focus on projections for massive axion-like particles (ALPs) coupled to the electromagnetic field, which induces anomalous contributions to the scattering of light-by-light (γγ →γγ). We make projections for the ALP–photon coupling for ALPs masses at the TeV scale. We also present a phenomenology analysis of anomalous γγ →γZ scattering using the proton tagging technique. Our projections for the anomalous coupling strength associated to dimension-eight γγγZ operators surpass the existing and projected constraints from the measurement of the branching fraction of Z →γγγ by about three orders of magnitude. Finally, a phenomenology study that targets the analysis of the SM and anomalous γγ →WW scattering in the hadronic and semi-leptonic channels is presented. Updated projections on the coupling strengths for anomalous dimension-six γγWW operators are presented.