CCFM prediction on forward jets and $F_2$: parton level predictions and a new hadron level Monte Carlo generator CASCADE

TL;DR

This work demonstrates that the CCFM evolution, with careful treatment of the non-Sudakov form factor, can describe both the inclusive structure function F2 and forward jet production at small x. By developing a backward-evolution hadron-level Monte Carlo, Cascade, and calibrating it with an unintegrated gluon density obtained from forward evolution, the authors achieve consistent parton- and hadron-level predictions that align with HERA data. The study highlights the importance of kinematic constraints and scale choices in shaping forward-jet and F2 observables, and delivers a practical generator for small-x final states. Overall, the approach provides a robust alternative to DGLAP-based generators for forward physics at high-energy lepton-proton colliders.

Abstract

A solution of the CCFM equation for a description of both the structure function $F_2$ and the cross section of forward jet production as measured by the HERA experiments is obtained on the basis of the parton level Monte Carlo program SMALLX. The treatment of the non - Sudakov form factor and the so - called "consistency constraint" are discussed. Following this a backward evolution scheme according to CCFM is developed which then is used to construct an efficient hadron level Monte Carlo program CASCADE. The results from the forward evolution and the backward evolution Monte Carlos are compared and found to be consistent.

Figures (7)

• Figure 1: Kinematic variables for multi-gluon emission. The $t$-channel gluon four - vectors are given by $k_i$ and the gluons emitted in the initial state cascade have four - vectors $p_i$.
• Figure 2: The structure function $F_2(x,Q^2)$ compared to H1 data H1_F2_1996. The solid (dashed) line is the prediction of the Smmod Monte Carlo without (with) applying the "consistency constraint" (c.c.).
• Figure 3: $a.-c.$ The cross section for forward jet production as a function of $x$, for different cuts in $p_t$ compared to H1 data H1_fjets_data ($a.-b.$) and compared to ZEUS data ZEUS_fjets_data ($c.$). $d.$ The cross section for forward jet production as a function of $E^2_T/Q^2$ compared to ZEUS_fjets_pt2/q2. The solid (dashed) line is the prediction of the Smmod Monte Carlo without (with) applying the "consistency constraint" (c.c.).
• Figure 4: Comparison of the cross section obtained from the backward evolution Monte Carlo Cascade (solid line) with Smmod (dashed line) both at parton level only. The upper 3 plots show the cross section is shown as a function of the quark rapidity $\eta_q$, the quark transverse momentum $p_t$ and the transverse momentum of the quark pair $p_t^{pair}$. The lower 3 plots show the cross section as a function of the gluon transverse momentum $k_t$ and the multiplicity and transverse energy flow of the gluons from the initial state cascade are shown as a function of the rapidity $\eta$
• Figure 5: Comparison of the structure function $F_2$ obtained from the backward evolution Monte Carlo Cascade (solid line) with Smmod (dashed line).