Color coherence in multi-jet final states

We discuss coherent QCD radiation from space-like parton branching and its effects on multi-jet final states at high-energy hadron colliders. Coherence effects for small longitudinal momentum fractions x are not included in the branching algorithms of standard shower Monte Carlo event generators. On the other hand, they are taken into account only partially in perturbative next-to-leading-order calculations for multi-jets. Such effects are present to all orders in αs and can become logarithmically enhanced with √ s/ET , where ET is the hardest jet transverse energy. We present results of summing coherence effects to all orders by parton-branching Monte Carlo methods based on transverse-momentum dependent matrix elements. To appear in the Proceedings of the Conference QCD 08 Montpellier 7-12 July 2008 Color coherence in multi-jet final states F. Hautmann and H. Jung University of Oxford, Theoretical Physics, Oxford OX1 3NP Deutsches Elektronen Synchrotron, D-22603 Hamburg We discuss coherent QCD radiation from space-like parton branching and its effects on multi-jet final states at high-energy hadron colliders. Coherence effects for small longitudinal momentum fractions x are not included in the branching algorithms of standard shower Monte Carlo event generators. On the other hand, they are taken into account only partially in perturbative next-to-leading-order calculations for multi-jets. Such effects are present to all orders in αs and can become logarithmically enhanced with √ s/ET , where ET is the hardest jet transverse energy. We present results of summing coherence effects to all orders by parton-branching Monte Carlo methods based on transverse-momentum dependent matrix elements. Studies of final states containing multiple hadron jets at the forthcoming LHC require realistic event simulation by parton shower Monte Carlo [1], including validation and tuning of the event generators using multi-jet data from Tevatron and HERA, see e.g. [2,3,4]. Standard parton-shower generators, such as Herwig and Pythia, are based on collinear evolution of the jets developing (both “forwards” and “backwards”) from the hard event. Besides smallangle, incoherent gluon emission, the approach of these generators also incorporates coherent softgluon emission from partonic lines carrying longitudinal momentum fraction x ∼ O(1). The phenomenological relevance of this treatment has been emphasized by extensive collider data studies [5]. However at the LHC, due to the phase space opening up for large center-of-mass energies, jet production enters a new regime with a great many events characterized by multiple hard scales, in which (a) effects of emissions that are not collinearly ordered become increasingly nonnegligible, and (b) coherence effects set in from space-like partons carrying momentum fractions x ≪ 1 (Fig. 1) [6]. These effects are not included in standard shower Monte Carlo generators. It is not at all obvious that the approximations involved in Monte Carlo that have successfully served for event simulation in past collider experiments will be up to the new situation. The theoretical framework to take account of non-collinear emission and coherence in the space-like branching requires the introduction of partonic distributions unintegrated not only in the longitudinal momenta but also in the transverse momenta [7,8,9,10,11]. The corrections to