Collinearity approximations and kinematic shifts in partonic shower algorithms
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Letter
Collinearity approximations and kinematic shifts in partonic shower algorithms F. Hautmann1,a , H. Jung2 1 2
University of Oxford, Physics Department, Oxford OX1 3NP, UK Deutsches Elektronen Synchrotron, 22603 Hamburg, Germany
Received: 28 September 2012 / Revised: 5 November 2012 / Published online: 11 December 2012 © The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract We study kinematic effects due to the approximation of on-shell, collinear partons in shower Monte Carlo event generators. We observe that the collinearity approximation, combined with the requirements of energymomentum conservation, gives rise to a kinematic shift, event by event, in longitudinal momentum distributions. We present numerical results in the case of jet and heavy-flavor production processes measured at the LHC.
1 Introduction Phenomenological analyses of complex final states produced by hard processes at the Large Hadron Collider (LHC) rely on event simulation by parton shower Monte Carlo generators [1]. These are used both to supplement finite-order perturbative calculations with all-order, leadinglogarithmic QCD radiative terms and to incorporate nonperturbative effects from hadronization, multiple parton interactions, underlying events [2–4]. The shower Monte Carlo generators [2–4] treat QCD multi-parton radiation within collinear ordering approximations. These approximations have proved to be very successful for Monte Carlo simulation of final states at LEP and Tevatron. On the other hand at the LHC, unlike previous collider experiments, the phase space opens up for events with multiple hard scales and multiple jets to occur with sizeable rates, while the angular and rapidity coverage of detectors extends over a much wider range. In this case corrections to collinear ordering can significantly affect the structure of multi-jet final states [5–7]. This, for instance, will influence uncertainties [8] of NLO-matched shower calculations [9] for jet observables. a e-mail:
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In this letter we investigate effects of kinematic origin arising from collinearity approximations in the parton showering algorithms. While the dynamical corrections studied in [5–7] come from terms beyond NLO in QCD perturbation theory (possibly enhanced in certain regions of phase space), the contributions studied in this paper are not obviously suppressed by powers of the strong coupling. Rather, they correspond to approximations made by showering algorithms in the parton kinematics. They can be discussed already at the level of leading-order and next-to-leading-order [8, 9] shower calculations. We find that the collinearity approximation, combined with the requirements of energy-momentum conservation, gives rise to a kinematic shift, event by event, in longitudinal momentum distributions. The size of this shift depends on the observable and on the phase space region, but becomes in general non-negligible with increasing rapidities. In Sect. 2 we discuss the physical origin of this effect.
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