A comparative study of small x Monte Carlos with and without QCD coherence effects

TL;DR

This paper compares two Monte Carlo methods for small x parton evolution, highlighting differences in azimuthal angle predictions due to QCD coherence effects, challenging the assumption of their equivalence at very small x.

Contribution

It provides a detailed comparison of BFKL and CCFM Monte Carlo implementations, revealing their differences in azimuthal angle distributions at small x.

Findings

BFKL is dominated by zero conformal spin at high energies

CCFM includes contributions from all conformal spins

Differences observed in azimuthal angle predictions at small x

Abstract

We compare two Monte Carlo implementations of resummation schemes for the description of parton evolution at small values of Bjorken x. One of them is based on the Balitsky-Fadin-Kuraev-Lipatov (BFKL) evolution equation and generates fully differential parton distributions in momentum space making use of reggeized gluons. The other one is based on the Catani-Ciafaloni-Fiorani-Marchesini (CCFM) partonic kernel where QCD coherence effects are introduced. It has been argued that both approaches agree with each other in the x -> 0 limit. We show that this is not the case for azimuthal angle dependent quantities since at high energies the BFKL approach is dominated by its zero conformal spin component while the CCFM gluon Green function receives contributions from all conformal spins even at very small x.