Review of results on forward physics and diffraction by CMS

TL;DR

This paper reviews CMS results on forward physics, diffraction, and small-x QCD phenomena, including jet correlations, vector meson production, and underlying event activity, providing insights into gluon saturation and QCD dynamics.

Contribution

It compiles and analyzes recent CMS measurements related to forward physics and diffraction, highlighting their implications for small-x QCD and saturation effects.

Findings

Jet azimuthal decorrelations sensitive to BFKL effects

Observation of vector meson production in proton-lead collisions

Insights into gluon saturation and QCD dynamics at small-x

Abstract

Results by the CMS Collaboration on forward physics, diffraction, and physics in the small-$x$ limit of quantum chromodynamics (QCD), are presented. In particular, results on azimuthal angle decorrelations between two jets in events where two outermost jets are separated by a large rapidity interval are discussed. In addition, results based on the production of two jets separated by a large rapidity gap (interval void of radiation) are presented. These dijet production processes are expected to be sensitive to Balitsky-Fadin-Kuraev-Lipatov (BFKL) evolution effects. We highlight results on inclusive forward jet production and on exclusive vector meson production in proton-lead collisions, which access gluon densities in the small-$x$ and $Q^2$ where saturation effects may play a role. A summary of results on underlying event activity studies based on inclusive $Z$ boson production, charged particle spectra in minimum bias events, and energy density in forward pseudorapidities is presented. These studies provide valuable inputs for Monte Carlo event generator tuning, and test predictions based on perturbative and non-perturbative QCD techniques.