Energy Flow and Rapidity Gaps Between Jets in Photoproduction

Data from the H1 detector at HERA have been used to study photoproduction events which have two or more jets. The data were taken in 1996 and correspond to an integrated luminosity of 6.63 pb−1. Events were studied in the photon-proton centre of mass energy range 165 GeV < W < 243 GeV and with Q < 0.01 GeV. Jets were defined using the longitudinally invariant kt algorithm. Events selected had one jet with PT > 6 GeV and a second with PT > 5 GeV; these jets were required to have a pseudorapidity separation of 2.5 < Δη < 4.0. The cross section was measured differentially in Δη; x p , the fraction of the proton’s momentum entering the hard scatter; x γ , the fraction of the photon’s momentum entering the hard scatter; and E T , the sum of the transverse energy between the two highest PT jets. Rapidity gap events were defined as events in which E gap T < E cut T , where the parameter E T was varied in the range 0.5 < E cut T < 2.0 GeV. The cross section for rapidity gap events and the gap fraction, defined as the fraction of dijet events with a rapidity gap, were measured differentially in Δη, x p , and x jets γ . An excess of events with a rapidity gap is observed over that expected from standard photoproduction processes. This excess can be explained as a result of the exchange of a strongly interacting colour singlet object between the jets. There are large theoretical uncertainties in the predictions from the photoproduction models, and the data currently show little sensitivity to the underlying dynamics of the exchange. However, the data can be described by the exchange of the leading logarithmic approximation of the BFKL pomeron, with a choice of αs = 0.18. This is consistent with the gap fraction observed at the Tevatron in pp̄ collisions at 1800 GeV.