Abstract
The exclusive deep inelastic electroproduction of ψ(2S) and J/ψ(1S) at an ep centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range 2<Q2<80 GeV2, 30<W<210 GeV and |t|<1 GeV2, where Q2 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<Q2<5 GeV2 were taken in the HERA I running period and correspond to an integrated luminosity of 114 pb−1. The data for 5<Q2<80 GeV2 are from both HERA I and HERA II periods and correspond to an integrated luminosity of 468 pb−1. The decay modes analysed were μ+μ− and J/ψ(1S)π+π− for the ψ(2S) and μ+μ− for the J/ψ(1S). The cross-section ratio σψ(2S)/σJ/ψ(1S) has been measured as a function of Q2,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 (DIS) at high energies is usually described as a multi-step process, as illustrated in Fig. 1: the electron emits a virtual photon, γ ∗, with virtuality, Q2; the γ ∗ fluctuates in leading-order QCD into a qqpair with a lifetime which, at large values of the γ ∗p centre-of-mass energy, W, is long compared to the interaction time; and the qqpair interacts with the proton with momentum transfer squared, t, via a colour-neutral exchange, e.g. through a two-gluon ladder, and hadronises into the vector meson, V
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
The luminosity was measured using the Bethe–Heitler reaction ep → eγp by a luminosity detector which consisted of a lead–scintillator calorimeter [14,15,16] and, in HERA II, a magnetic spectrometer [17]
Summary
Exclusive electroproduction of vector mesons in deep inelastic scattering (DIS) at high energies is usually described as a multi-step process, as illustrated in Fig. 1: the electron emits a virtual photon, γ ∗, with virtuality, Q2; the γ ∗ fluctuates in leading-order QCD into a qqpair with a lifetime which, at large values of the γ ∗p centre-of-mass energy, W , is long compared to the interaction time; and the qqpair interacts with the proton with momentum transfer squared, t , via a colour-neutral exchange, e.g. through a two-gluon ladder, and hadronises into the vector meson, V. A measurement of the ratio of the cross sections of the reactions γ ∗p →. 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. This measurement allows QCD predictions of the wave function dependence of the cc-proton cross section to be tested. The H1 Collaboration has measured the quasi-elastic component, γ ∗p → ψ(2S) + Y , in DIS [2] and photoproduction [3], as well as the ratio of the ψ(2S) to J /ψ(1S) production cross sections. The decay channels used were J /ψ(1S) → μ+μ−, ψ(2S) → μ+μ− and ψ(2S) → J /ψ(1S) π +π − with the subsequent decay J /ψ(1S) → μ+μ−
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