Multiparton Interactions in Photoproduction at HERA

TL;DR

This paper investigates the potential for multiparton interactions in photoproduction at HERA due to high parton densities at small x. It develops an eikonal MI model and embeds it in the HERWIG Monte Carlo to simulate the full hadronic final state, including parton showers and hadronization. The study shows MI can significantly alter jet cross sections, energy flow, and jet profiles, complicating the extraction of photon parton densities, while providing a potential signature in high-multiplicity jet events. Preliminary comparisons to HERA data suggest MI improve agreement in certain regions, but large systematic uncertainties remain; the work also supplies a practical Monte Carlo tool for exploring MI in γp and γγ processes.

Abstract

The high energy photoproduction of jets is being observed at the ep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets per ep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult. Total rates for the production of many (i.e. > 2) jets could provide direct evidence for the presence of multiple interactions, although parton showering and hadronization significantly affect low transverse energy jets.

Figures (8)

• Figure 1: An example of a multiple scattering in a $\gamma p$ collision.
• Figure 2: a) $\sigma_H$ as a function of $s_{\gamma p}$, the squared c.m. energy of the photon-proton system. b) $d\sigma^{ep}_H/dy$ as a function of $y$. In a) and b) the dashed lines show the result with no multiple interactions, the solid lines show the result of including multiple interactions and the points indicate the cross sections actually generated by the Monte Carlo program (see text). In c) the probability of $N$ and only $N$ scatters as a function of $y$ is shown. In d) the cross section for $N$ and only $N$ scatters as a function of $y$ is shown.
• Figure 3: Inclusive jet cross sections: a) $d\sigma/d\eta$ and b) $d\sigma/dE_T$. Dijet cross sections: c) $d\sigma/d\eta$ and d) $d\sigma/dE_T$. In all cases, $E_T^{{\rm jet}} > 6$ GeV. The solid lines show the distributions when multiple scattering is included, the dashed lines show the distributions when no multiple scattering is allowed. In the $E_T^{{\rm jet}}$ plots, the statistical errors are indicated on the distribution for which multiple interactions are included. The errors on the 'no multiple interactions' distribution are similar.
• Figure 4: a) $d\sigma/dx_\gamma^{{\rm obs}}\;$ b) $d\sigma/dx_p^{{\rm obs}}$ The solid histograms show the distributions when multiple scattering is included, the broken histograms show the distributions when multiple scattering is neglected.
• Figure 5: Transverse energy flow as a function of $\Delta\eta$ integrated over $|\Delta\phi| < 1.0$ for jets with $E_T > 6$ GeV in the range $-2.0 < \eta < 2.0$ and $x_\gamma^{{\rm obs}} \ge 0.75$ (a) and $x_\gamma^{{\rm obs}} < 0.75$ (b). The solid lines show the distributions when multiple scattering is included, the dashed lines show the distributions when no multiple scattering is allowed.