Measurement of the Reaction $\gamma^* p \to \phi p$ in Deep Inelastic $e^+p$ Scattering at HERA

TL;DR

This study measures the production of phi mesons in deep inelastic electron-proton scattering at HERA, revealing a strong energy dependence consistent with perturbative QCD and showing the ratio of phi to rho production approaches theoretical predictions at high Q^2.

Contribution

First measurement of phi meson production cross section in deep inelastic scattering at HERA, confirming QCD predictions and analyzing the phi/rho ratio at high Q^2.

Findings

Cross section rises strongly with energy W.

Ratio of sigma(phi) to sigma(rho^0) increases with Q^2.

Results align with perturbative QCD expectations.

Abstract

The production of $\phi$ mesons in the reaction $e^{+}p \rightarrow e^{+} \phi p$ ($\phi \rightarrow K^{+}K^{-}$), for $7 < Q^2 < 25$ GeV$^2$ and for virtual photon-proton centre of mass energies ($W$) in the range 42-134 GeV, has been studied with the ZEUS detector at HERA. When compared to lower energy data at similar $Q^2$, the results show that the $\gamma^*p \rightarrow \phid p$ cross section rises strongly with $W$. This behaviour is similar to that previously found for the $\gamma^*p \rightarrow \rho^0 p$ cross section. This strong dependence cannot be explained by production through soft pomeron exchange. It is, however, consistent with perturbative QCD expectations, where it reflects the rise of the gluon momentum density in the proton at small $x$. The ratio of $\sigma (\phi) / \sigma (\rho^0)$, which has previously been determined by ZEUS to be 0.065 $\pm$ 0.013 (stat.) in photoproduction at a mean $W$ of 70 GeV, is measured to be 0.18 $\pm $ 0.05 (stat.) $\pm$ 0.03 (syst.) at a mean $Q^2$ of 12.3 GeV$^2$ and mean $W$ of $\approx$ 100 GeV and is thus approaching at large $Q^2$ the value of 2/9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism.