Abstract
The exclusive photoproduction reaction γp→ϒ(1S)p has been studied with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 468 pb−1. The measurement covers the kinematic range 60<W<220 GeV and Q2<1 GeV2, where W is the photon–proton centre-of-mass energy and Q2 is the photon virtuality. The exponential slope, b, of the t dependence of the cross section, where t is the squared four-momentum transfer at the proton vertex, has been measured, yielding b=4.3−1.3+2.0(stat.)−0.6+0.5(syst.) GeV−2. This constitutes the first measurement of the t dependence of the γp→ϒ(1S)p cross section.
Highlights
IntroductionJ /ψ and Υ , the masses of the charm and the bottom quarks provide a hard scale and the process can be described by models based on perturbative QCD (pQCD) [1,2]
In exclusive photoproduction of heavy vector mesons (VMs), J /ψ and Υ, the masses of the charm and the bottom quarks provide a hard scale and the process can be described by models based on perturbative QCD [1,2]
The interaction may be viewed at leading order as shown in Fig. 1: the photon fluctuates into a qqstate of small transverse size, which interacts with partons in the proton through a two-gluon colour-singlet state, forming a heavy vector meson
Summary
J /ψ and Υ , the masses of the charm and the bottom quarks provide a hard scale and the process can be described by models based on perturbative QCD (pQCD) [1,2]. A characteristic feature of heavy VM photoproduction is the rapid rise of the cross section with the photon–proton centre-of-mass energy, W. This can be explained through the increasing gluon density with decreasing fractional momentum, x ∝ 1/W 2 (where the x region accessible in heavy-quark production at HERA is 10−4 < x < 10−2). Studies of the exclusive photoproduction of light and heavy vector mesons [3] have shown that the t dependence of the differential cross section may be approximated in the region of small t (|t| < 1 GeV2) with a single exponential: dσ /d|t| ∝.
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