Measurement of Elastic $\Upsilon$ Photoproduction at HERA

TL;DR

This paper reports the first observation of Upsilon meson photoproduction at HERA, measuring cross sections that challenge existing perturbative QCD predictions, using data from the ZEUS detector.

Contribution

First measurement of elastic Upsilon photoproduction at HERA, providing new experimental data on heavy quarkonium production in ep collisions.

Findings

Observed Upsilon mesons in photoproduction for the first time.

Measured cross section of 13.3 pb with statistical and systematic uncertainties.

Cross section exceeds perturbative QCD model predictions.

Abstract

The photoproduction reaction gamma p -> mu+ mu- p has been studied in ep interactions using the ZEUS detector at HERA. The data sample corresponds to an integrated luminosity of 43.2 pb^{-1}. The Upsilon meson has been observed in photoproduction for the first time. The sum of the products of the elastic Upsilon(1S), Upsilon(2S), Upsilon(3S) photoproduction cross sections with their respective branching ratios is determined to be 13.3 +- 6.0(stat.)^{+2.7}_{-2.3}(syst.) pb at a mean photon-proton centre of mass energy of 120 GeV. The cross section is above the prediction of a perturbative QCD model.