Measurement of exclusive omega electroproduction at HERA

TL;DR

This study presents the first high-$Q^2$ measurement of exclusive $\omega$ electroproduction in $ep$ collisions, alongside $\phi$ production, using the decay $\omega\to\pi^+\pi^-\pi^0$ in the ZEUS detector at HERA. Cross sections for $ep\to eVp$ and the corresponding $\gamma^*p$ cross sections are extracted in $3< Q^2<20$ GeV$^2$, $40< W<120$ GeV, $|t|<0.6$ GeV$^2$, with the ratio $\omega/\phi$ found consistent with SU(4) flavour symmetry and no significant $Q^2$ dependence of $\omega/\rho^0$. The results, together with existing $\rho^0$, $\phi$, and $J/\psi$ measurements, indicate a transition to flavor-independent production at higher $Q^2$ and support a QCD-based picture where small-sized $q\bar{q}$ fluctuations interact with the proton. The analysis combines detailed event selection, $\pi^0$ reconstruction, MC acceptance corrections, and careful background subtraction to quantify exclusive vector-meson electroproduction at high energies.

Abstract

The exclusive electroproduction of omega mesons, ep -> e omega p, has been studied in the kinematic range 3<Q^2<20 GeV^2, 40<W<120 GeV and |t|<0.6 GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb^{-1}. The omega mesons were identified via the decay omega -> pi^+pi^-pi^0. The exclusive (ep -> e omega p) cross section in the above kinematic region is 0.108 +- 0.014(stat.) +- 0.026(syst.) nb. The reaction ep -> e phi p, phi -> pi^+pi^-pi^0, has also been measured. The cross sections, as well as the cross-section ratios omega/rho and omega/phi, are presented as a function of W and Q^2. Thus, for the first time, the properties of omega electroproduction can be compared to those of rho^0, phi and J/psi electroproduction at high W.

Figures (4)

• Figure 1: Schematic diagram of exclusive vector-meson electroproduction in $e^+p$ interactions, $ep \rightarrow eVp$. In this analysis, $V$ is either an $\omega$ or $\phi$ meson.
• Figure 2: a) The ratio $(E_{\gamma 1} - E_{\gamma 2})/(E_{\gamma 1} + E_{\gamma 2})$ for $\pi^0$ candidates (solid points), where $\gamma 1$ is the more energetic photon candidate. The histogram corresponds to the MC simulation described in the text. The vertical bars on the histogram correspond to the statistical uncertainties of the MC. The statistical uncertainties of the data are smaller than the size of the solid points. b) The invariant-mass spectrum for two photons (solid points). The line represents a fit to the sum of a Gaussian and a second-order polynomial as described in the text.
• Figure 3: a) The $\pi^+\pi^-\pi^0$ invariant-mass spectrum (solid points). The two peaks correspond to the $\omega$ and $\phi$ mesons. The full line shows the result of the fit explained in the text. The dashed line shows the fitted background. b) The dependence of $\sigma_{\gamma^*p \rightarrow \omega p}$ on $W$ at $Q^2$ = 7 $\mathrm{{\rm Ge}{\rm V}}^2$. The full circles are the results from this analysis, and the open circles are from the ZEUS $\rho^0$ data epj_6_603. c) The dependence of $\sigma_{\gamma^*p \rightarrow \omega p}$ on $Q^2$ at $W$ = 70 Ge V. The results from this analysis are shown as full circles, while the full star is the photoproduction result zfp_73_73. The open circles and star are ZEUS results on the $\rho^0$ cross sections at $W = 50$ Ge V epj_2_247epj_6_603.
• Figure 4: The ratios of the $\omega$, $\phi$ and $J/\psi$ exclusive cross sections to those for the $\rho^0$ meson as a function of $Q^2$. The $\omega/\rho^0$ and $\phi / \rho^0$ results from this analysis are shown with full symbols. The $\omega/\rho^0$ photoproduction point (open circle) was calculated using the measured cross sections from previous ZEUS publications zfp_73_73 and epj_2_247. Open squares at $Q^2 \simeq 0$ GeV$^2$pl_377_259 and $Q^2= 12.3$ GeV$^2$pl_380_220 represent the ZEUS $\phi$/$\rho^0$ cross-section ratios, while the others represent the H1 $\phi$/$\rho^0$ ratios dr_00_070; for both the ZEUS and H1 measurements, the $\phi \rightarrow K^+K^-$ channel was used. Triangles represent the $(J/\psi)/\rho^0$ cross-section ratios zfp_75_215epj_6_603. The horizontal dashed lines correspond to the SU(4) expectations.