A Bfkl Monte Carlo Approach to Jet Production at Hadron-hadron and Lepton-hadron Colliders

Fixed-order QCD perturbation theory fails in some asymptotic regimes where large logarithms multiply the coupling constant. In those regimes resummation of the perturbation series to all orders is necessary to describe many high-energy processes. The Balitsky-FadinKuraev-Lipatov (BFKL) equation 1 performs such a resummation for virtual and real soft gluon emissions in dijet production at large rapidity difference in hadronhadron collisions (see Figure 1(a)) and in forward jet production in lepton-hadron collisions (Figure 1(b)). In the latter case, resummation leads to the characteristic BFKL rise in the forward jet cross section, σ̂ ∼ (xj/xBj) , with λ = 4CA ln 2αs/π ≈ 0.5. Similarly, in dijet production at hadron colliders BFKL resummation gives 2 a subprocess cross section that increases with rapidity difference as σ̂ ∼ exp(λ∆), where ∆ is the rapidity difference of the two jets with comparable transverse momenta pT1 and pT2. Experimental studies of these processes have recently begun at the Tevatron pp̄ and HERA ep colliders. Tests so far have been inconclusive; the data tend to lie between fixed-order QCD and analytic BFKL predictions. However the applicability of these analytic BFKL solutions is limited by the fact that they implicitly contain integrations over arbitrary numbers of emitted gluons with arbitrarily large transverse momentum: there are no kinematic constraints included. Furthermore, the implicit sum over emitted gluons leaves only leading-order kinematics, i.e. only the momenta of the ‘external’ particles are made explicit. The absence of kinematic constraints and energy-momentum conservation cannot, of course, be reproduced in experiments. While the effects of such constraints are in principle sub-leading, it is desirable to ... ... (a) (b) pT1,-