Measurement of Elastic $Υ$ Photoproduction at HERA

TL;DR

This study reports the first observation of elastic Υ photoproduction at HERA with the ZEUS detector, measuring the cross section for γp → Υ p multiplied by the Υ → μ+μ− branching fraction in the photoproduction regime. By combining Υ(1S–3S) contributions under hadroproduction-like ratios, the analysis yields σ(γp → Υ p)·B(μμ) ≈ 13 pb at ⟨W⟩ ≈ 120 GeV and derives a Υ(1S) cross section of about 375 pb, which is larger than leading pQCD predictions. The results provide a new test of perturbative QCD and the gluon density in the proton at high scales, though the measured Υ photoproduction rate is higher than the benchmark Frankfurt calculation. The analysis also demonstrates the feasibility of studying heavy vector meson diffraction in ep collisions with an extensive detector, careful background treatment, and robust systematic uncertainty assessment.

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.

Figures (3)

• Figure 1: Schematic diagrams for diffractive vector meson ($V$) electroproduction. (a) Elastic $V$ production. (b) Proton dissociative $V$ production where the proton dissociates into a hadronic system of invariant mass $M_N$. The dotted line represents a colorless exchange between the virtual photon and the proton.
• Figure 2: Mass distribution of $\mu^+ \mu^-$ pairs. The histogram represents the simulated Bethe-Heitler background. Points in the $J/\psi$ region are connected by a dotted line to guide the eye. The insert shows the signal remaining in the $\Upsilon$ region after subtraction of the non-resonant background.
• Figure 3: (a) $\sigma_{\gamma p \rightarrow \Upsilon(1S)~ p}$ and (b) the ratio $\sigma_{\gamma p \rightarrow \Upsilon(1S)~ p}/ \sigma_{\gamma p \rightarrow J/\Psi~ p}$ as a function of $W$, the centre of mass energy of the photon-proton system. Data (full squares) are compared with the predictions of Frankfurt for the GRV94(HO) (dashed line) and CTEQ(4M) (full line) parametrisations of the proton structure function. The data have been scaled taking into account the muonic branching ratio and the contributions of the $\Upsilon$(2S) and $\Upsilon$(3S), as described in the text. The inner error bars show the statistical errors, the outer bars correspond to the statistical and systematic errors added in quadrature.