High-ET Dijet Photoproduction at HERA

The cross section for high-𝐸𝑇 dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8  pb−1. The events were required to have a virtuality of the incoming photon, 𝑄2, of less than 1  GeV2 and a photon-proton center-of-mass energy in the range 142 <𝑊𝛾⁢𝑝 <293  GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of 𝐸jet⁢1 𝑇 >20  GeV and 𝐸jet⁢2 𝑇 >15  GeV and pseudorapidity (with respect to the proton beam direction) requirements of −1 <𝜂jet⁢1,2 <3, with at least one of the jets satisfying −1 <𝜂jet <2.5. The measurements show sensitivity to the parton istributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.