Abstract
Exclusive J/ψ and ψ(2S) vector meson production has been observed in the dimuon channel using the LHCb detector. The cross-section times branching fractions to two muons with pseudorapidities between 2.0 and 4.5 are measured to bewhere the first uncertainties are statistical and the second are systematic. The measurements are found to be in good agreement with results from previous experiments and theoretical predictions. The J/ψ photoproduction cross-section has been measured as a function of the photon-proton centre-of-mass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction cross-section.
Highlights
Exclusive vector meson production through photoproduction, γ p → V p, has attracted much interest both experimentally and theoretically as it provides a rich testing ground for QCD
The cross-section for exclusive J/ψ production has been observed to have a strong power law dependence on W [5, 6]. This feature is described in perturbative QCD (pQCD) by a hard pomeron and the fact that the gluon density in the proton increases rapidly with decreasing x, the fractional momentum of the proton carried by the parton [1]
The data are consistent with a power law dependence of the form σ = a(W/1 GeV)δ with a = 3 nb and δ = 0.72 [9]
Summary
Exclusive vector meson production through photoproduction, γ p → V p, has attracted much interest both experimentally and theoretically as it provides a rich testing ground for QCD. The cross-section for exclusive J/ψ production has been observed to have a strong power law dependence on W [5, 6] This feature is described in pQCD by a hard pomeron (or two gluon exchange) and the fact that the gluon density in the proton increases rapidly with decreasing x, the fractional momentum of the proton carried by the parton [1]. At a sufficiently low value of x, the strong growth of the gluon density with decreasing x is expected to be restrained by gluon recombination [8] This saturation effect could be visible as a reduction in the photoproduction cross-section with respect to the power law behaviour at W values accessible at the LHC [9]. This model predicts a cross-section which is higher than that observed at HERA
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