Rapidity Gaps between Jets in Photoproduction at HERA

TL;DR

This ZEUS study analyzes rapidity gaps between jets in photoproduction at HERA to test for color-singlet exchange. By measuring the gap-fraction as a function of jet separation Δη in dijet events, the analysis finds an exponential suppression at small to moderate Δη but a persistent flat region at large Δη, indicating a color-singlet contribution beyond standard colour-exchange processes. The corrected results, supported by statistical and fit-based arguments, favor hard diffractive scattering via a colour-singlet object, with implications for pomeron-like exchange and survival probabilities in γp interactions. These findings extend the understanding of diffractive dynamics in electron-proton collisions and complement similar gap studies at hadron colliders.

Abstract

Photoproduction events which have two or more jets have been studied in the $W_{γp}$ range 135~GeV $< W_{γp} <$ 280~GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals ($Δη$) of up to four units and have transverse energies greater than 6~GeV. A gap is defined as the absence between the jets of particles with transverse energy greater than 300~MeV. The fraction of events containing a gap is measured as a function of $Δη$. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of $Δη\sim 3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet object.

Figures (3)

• Figure 1: Resolved photoproduction via (a) colour singlet exchange and (b) colour non-singlet exchange. The rapidity gap event morphology is shown in (c) where black dots represent final state hadrons and the boundary illustrates the limit of the ZEUS acceptance. Two jets of radius $R$ are shown, which are back to back in azimuth and separated by a pseudorapidity interval $\Delta\eta$. An expectation for the behaviour of the gap fraction is shown in (d)(solid line). The non-singlet contribution is shown as the dotted line and the colour singlet contribution as the dashed line.
• Figure 2: Uncorrected data compared with the predictions from PYTHIA events which have been passed through a detailed simulation of the ZEUS detector and of the sample selection criteria. The errors shown are statistical only. The transverse energy flow with respect to the jet axis is shown in (a) where the data are shown as black dots and the PYTHIA non-singlet sample is shown as a solid line. In (b), (c) and (d) the data are again shown as black dots. The PYTHIA non-singlet sample is shown as open circles and the PYTHIA mixed sample (which contains 10% of colour singlet exchange events) is shown as stars. The number of events versus $\Delta\eta$ is shown in (b). The number of gap events versus $\Delta\eta$ is shown in (c) and the gap-fraction is shown in (d). In (d) the points are drawn at the mean $\Delta\eta$ of the inclusive distribution in the corresponding bin.
• Figure 3: ZEUS data (black circles) corrected for detector effects. The inner error bars represent the statistical errors from the data and Monte Carlo samples, and the outer error bars include the systematic uncertainty, added in quadrature. In (a), (b) and (c) the PYTHIA prediction for non-singlet exchange events is shown as open circles. The inclusive cross section is shown in (a). The cross section for gap events is shown in (b) and the gap-fraction is shown in (c). The gap-fraction is redisplayed in (d) and compared with the result of a fit to an exponential plus a constant. In (c) and (d) the points are drawn at the mean $\Delta\eta$ of the inclusive distribution in the corresponding bin.