Abstract
The exclusive deep inelastic electroproduction of ψ (2 S ) and J / ψ (1 S ) at an ep centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range 2 Q 2 2 , 30 W t | 2 , where Q 2 is the photon virtuality, W is the photon–proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The data for 2 Q 2 2 were taken in the HERAI running period and correspond to an integrated luminosity of 114 pb −1 . The data for 5 Q 2 2 are from both HERAI and HERAII periods and correspond to an integrated luminosity of 468 pb −1 . The decay modes analysed were μ + μ − and J / ψ (1 S ) π + π − for the ψ (2 S ) and μ + μ − for the J / ψ (1 S ). The cross-section ratio σ ψ (2 S ) / σ J / ψ (1 S ) has been measured as a function of Q 2 ; W and t . The results are compared to predictions of QCD-inspired models of exclusive vector-meson production.
Highlights
Exclusive electroproduction of vector mesons in deep inelastic scattering at high energies, ep → eV p, where V denotes a vector meson, may be described as a multi-step process
The electron emits a virtual photon, γ∗, with virtuality Q2 and γ∗ p centre-of-mass energy W, the virtual photon fluctuates into a qqpair and which subsequently interacts with the proton via a colour-neutral exchange, e.g. through a two-gluon ladder, and hadronises into the vector meson
The ψ(2S ) and the J/ψ(1S ) have the same quark content, different radial distributions of the wave functions, and their mass difference is small compared to the HERA centre-of-mass energy
Summary
Exclusive electroproduction of vector mesons in deep inelastic scattering at high energies, ep → eV p, where V denotes a vector meson, may be described as a multi-step process. The ratio of their electroproduction cross sections allows checking perturbative QCD predictions regarding their wave functions[1]. For the probability that the cc-dipole forms the vector state, its centre-of-mass wave function has to be boosted into the infinite momentum frame, which can be done using the boosted Gaussian model [8] calculations are used.
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