Rapidity gaps and the PHOJET Monte Carlo

TL;DR

The paper presents PHOJET as a Monte Carlo implementation of the two-component Dual Parton Model to describe rapidity-gap and diffractive events in high-energy $pp$ collisions. It combines Regge-based soft interactions with LO QCD hard processes, implements a two-channel eikonal unitarization, and uses soft color reconnection to generate rapidity gaps, testing predictions against Tevatron data at $\sqrt{s}=1.8$ TeV. Key findings show that unitarization dampens diffractive cross sections and that SCR can reproduce jet-gap-jet signatures, though normalization depends on pomeron PDFs and the possible presence of a direct pomeron coupling; comparisons to diffractive dijet data indicate areas where the model may require stronger hard pomeron components to match observed rates. Overall, PHOJET provides a comprehensive framework to study both soft and hard diffraction across multiple collision systems, guiding interpretation of current data and outlining measurements capable of constraining pomeron structure and gap-survival effects.

Abstract

A model for the production of large rapidity gaps being implemented in the Monte Carlo event generator PHOJET is discussed. In this model, high-mass diffraction dissociation exhibits properties similar to hadron production in non-diffractive hadronic collisions at high energies. Hard diffraction is described using leading-order QCD matrix elements together with a parton distribution function for the pomeron and pomeron-flux factorization. Since this factorization is imposed on Born graph level only, unitarity corrections lead to a non-factorizing flux function. Rapidity gaps between jets are obtained by soft color reconnection. It was previously shown that this model is able to describe data on diffractive hadron production from the CERN-SPS collider and from the HERA lepton-proton collider. In this work we focus on the model predictions for rapidity gap events in p-p collisions at \sqrt{s} = 1800 GeV and compare to TEVATRON data.

Figures (8)

• Figure 1: Enhanced pomeron exchange graphs considered in the model: a) triple-pomeron, b) loop-pomeron, and c) double-pomeron graphs. The zig-zag lines represent pomeron propagators.
• Figure 2: (a) Single and double diffractive $p-\bar{p}$ cross sections as a function of the center of mass energy $\sqrt s$. Model results are compared to data on single diffractive cross sections Chapman74Schamberger75Albrow76Armitage82Ansorge86Robinson89Amos90aAmos93aAbe94c. In addition, some experimental estimates for the cross section on double diffraction dissociation Ansorge86Robinson89 are shown (triangles). (b) The energy dependence of the central diffraction cross section. We compare the cross section as obtained from Phojet with unitarization using a supercritical pomeron with the cross section obtained by Streng Streng86a without unitarization and with a critical pomeron. Both cross sections are for the same two kinematic cuts: $M_{\rm CD}>2$ GeV/c${}^2$ and Feynman-$x$ of the scattered hadron $x_F >0.95$ (upper curves) and $0.97$ (lower curves).
• Figure 4: Distribution of the diffractive mass in single diffraction dissociation (pomeron--proton) and central diffraction (pomeron--pomeron) at TEVATRON with $\sqrt s = 1.8$ TeV for three different cuts of the Feynman-$x$ of the diffractive nucleons.
• Figure 5: (a) Pseudorapidity distribution of jets with $E_{\perp}$ larger than 5 GeV and 15 GeV in (one side) single diffraction (Pom--p) at TEVATRON for three different cuts of the Feynman-$x$ of the diffractive nucleon. The upper curves with the same plotting symbol are generally for $E_{\perp}$ = 5 GeV, the lower curves are for $E_{\perp}$ = 15 GeV. (b) Pseudorapidity distribution of jets with $E_{\perp}$ larger than 5 GeV in central diffraction (Pom--Pom) at TEVATRON for three different cuts of the Feynman-$x$ of the diffractive nucleons.
• Figure 6: (a) $E^{\rm jet}_{\perp}$ distributions in JJ and JJg events obtained in Phojet using the CDF triggers. (b) $\phi^{{\rm jet}1}$ - $\phi^{{\rm jet}2}$ distributions in JJ and JJg events obtained with Phojet using the CDF trigger.