Abstract
We apply the $k_T$-factorization approach to the production of $\phi$ meson in deep-inelastic scattering. The helicity-conserving $\gamma^*(T,L) \to \phi$ impact factor is calculated for longitudinal and transverse photon polarization using $\phi$ meson distribution amplitudes. Different unintegrated gluon distributions are used in the calculations. The formalism for massless quarks/antiquarks gives too large transverse and longitudinal cross sections for photon virtualities below $Q^2\sim 8 \, \rm{GeV}^2$. We suggest how to improve the description of the HERA data by introducing effective strange quark masses into the formalism. We derive the corresponding massive impact factor by comparing to the light-cone wave function representation used in previous $k_T$-factorization calculations and the color-dipole approaches. As a byproduct we present expressions for higher twist-amplitudes as weighted integrals over the light-cone wave function. The quark mass $m_q \approx 0.5$ GeV allows to improve the description of both longitudinal and transverse cross section down to $Q^2 \sim$ 4 GeV$^2$. We present also the polarized cross section ratio $\sigma_L/\sigma_T$ and the behavior of the total cross section $\sigma_{tot} = \sigma_{L} + \sigma_{T}$ as a function of photon virtuality.
Highlights
The diffractive electroproduction of vector mesons, γÃp → Vp, has attracted much attention at the HERA collider and it is expected to be an important subject in future experiments e.g., at an electron-ion collider (EIC) [2]
We suggest how to improve the description of the HERA data by introducing effective strange-quark masses into the formalism
We start by calculating the longitudinal cross section using the formalism described in the previous section
Summary
The diffractive electroproduction of vector mesons, γÃp → Vp, has attracted much attention at the HERA collider (for a review see e.g., Ref. [1]) and it is expected to be an important subject in future experiments e.g., at an electron-ion collider (EIC) [2]. [1] includes, besides the transverse momentum of gluons, the transverse momentum of the quark and antiquark in the vector meson as encoded in the light-cone wave function of the meson. The recent analysis of helicity amplitudes for ρ0 meson production [8] showed that this approach may be useful in testing UGDs. In this paper, in order to test the formalism further, we wish to focus on and investigate the exclusive photoproduction of φ mesons: γÃp → φp: Corresponding experimental data were obtained by the H1 [14,15] and ZEUS [16] collaborations at HERA.
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