Possible evidence for color transparency from dijet production with large rapidity gaps in γp scattering at HERA and how to test it in γp , γA scattering

We argue that the probability of gap survival in dijet production in γp scattering as measured by ZEUS may be due to the color transparency phenomenon and suggest ways to test this hypothesis in the future γp and γA processes. The interaction of spatially small systems with a hadron has been the subject of discussions for a long time now (for the long and somewhat contradictory history of the theoretical and experimental investigations of this phenomenon see ref [1]). One expects that small color singlets interact weakly if energies are not extremely high-color transparency (CT). The current HERA data are in the kinematic region where the coherent length l c = 1/2m N x significantly exceeds the nucleon radius. In this kinematic range color coherent effects should reveal themselves most clearly. Here we explain a practical idea how to search for CT in high p t dijet production at HERA both in γp and γA collisions. 1 Gap survival for γp case In order to study soft interactions which accompany a hard scattering, Bjorken [3] suggested to investigate the ratio of the cross sections of the high p t dijet production with a large rapidity gap (LRG) to that of dijets without a rapidity gap: f ac = σ(a + c → (jet(p t) + X) + LRG + (jet(−p t) + Y)) σ(a + c → jet(p t) + jet(−p t) + Z) = κP LRG (1) Here c can be a proton or a nuclear target. To account for the difference between scales of hard and soft processes quantify the role of soft physics Bjorken evaluated f ac as the product of 2 factors: f ac ≡ κP RGS. (2) Factor κ is the probability of producing a rapidity gap in hard subprocess, while P RGS characterizes probability of gap survival due to soft interactions of constituents which do not participate in the hard collision. Natural mechanism for the colorless hard collision is the exchange by 2 gluons. At first sight this contribution should be 0. Really it follows from the QCD factorization theorem that the exchange by an extra gluon between the partons involved in a hard collision is canceled out for the total cross section of dijet production. However for diffractive processes the presence of the LRG trigger in the final state destroys the cancelation between different terms, leading to the factorization theorem breaking[7]. In perturbative …