Rapidity gaps between jets inpp¯collisions at √s=1.8 TeV

First experimental results are presented from a search for events with a rapidity gapbetween jets. The DO detector was used to examine events produced by the Fermilab Tevatron pp collider at fi = 1.8 TeV. The fraction of events with an observed rapidity gapbetween the two highest transverse energy (ET) jets is measured as a function of the pseudorapidityseparation between the jet edges (Aq). An upper limit at the 95% confidence level of 1.1 x 10e2 is obtained on the fraction of events with no particles between the jets, for events with Aqc>3 and jet ET greater than 30 GeV. PACS numbers: 13.87.-a, 12.38.Qk, 13.85.-t, 13.9O.+i Typeset using REVTEX Rapidity gaps, which are regions of rapiditycontaining no particles, have typically been associated with low transverse momentum processes such as elastic and diffractive scattering. However, rapidity gapsare also expected to occur in high transverse momentum processes when a color singlet is exchanged between interacting partons [1,2]. These gapsoccur between the final state jets due to the absence of radiation from the color singlet and the resulting destructive interference between initialand final state radiation[3]. Hadrons are produced only between the outgoing jets and spectator partons, resulting in an empty region of phase space between the jets. Figure 1 depicts the distribution of particles in a two-jet event with a rapidity gapof size Aqcr where Aqc is the pseudorapidityseparation between the edges of the jet cones. The exchange of a photon, W, or 2 is expected to give such an event topology. In addition, a hard Pomeron, which has been shown to be associated with jet production [4], is a color singlet which is expected to produce rapidity gaps. Although QCD interactions typically produce particles between jets due to the exchange of color via a quark or gluon (color octetexchange), rapidity gapscan also arise from fluctuations in the particle multiplicity. A rapidity gapwill not be observed in the final state, however, if spectator interactions produce particles between the jets. While both the cross section for producing a rapidity gapfrom the hard scattering (asap) and the probability of the gapsurviving spectator interactions (S) are of theoretical interest, experiments are only directly sensitive to the product of these factors. An experimentally accessible quantity is the fraction of events with a rapidity gapbetween the two leading (highest transverse energy) jets, defined as f@Tlc) = Q&W . SW) 4A%) (1) where c(Aqc) is the cross section for producing jets with Ar,rc separation between the edges of the jet cones. For small Aqc, a large fraction of events are expected to have a rapidity gap. These gapsoccur in color octetexchange events due to fluctuations in the particle multiplicity between jets. The gapfraction decreases sharply with increasing Aq= because the rising