Measurement of Jet Shapes in Photoproduction at HERA

TL;DR

This paper measures jet shapes in photon-proton collisions at HERA, revealing how jet broadening depends on pseudorapidity and transverse energy, and compares results with Monte Carlo predictions.

Contribution

First detailed measurement of jet shapes in photoproduction at HERA, distinguishing resolved and direct processes, and testing QCD-based Monte Carlo models.

Findings

Jet shape broadens with increasing pseudorapidity.

Jet shape narrows as transverse energy increases.

Monte Carlo models generally agree with measurements except at high pseudorapidity and low transverse energy.

Abstract

The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range $134-277$ GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the $\eta - \phi$ plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies $E^{jet}_T>14$ GeV are presented. The jet shape broadens as the jet pseudorapidity ($\eta^{jet}$) increases and narrows as $E^{jet}_T$ increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high $\eta^{jet}$ and low $E^{jet}_T$. The observed broadening of the jet shape as $\eta^{jet}$ increases is consistent with the predicted increase in the fraction of final state gluon jets.