General-purpose event generators for LHC physics

Abstract

We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hard-scattering matrix elements for processes of interest, at both leading and next-to-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diffractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the ARIADNE, Herwig++, PYTHIA 8 and SHERPA generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.Comment: 226 pages, 30 figures, submitted to Physics Report

Measurement of the underlying event activity at the LHC with $ \sqrt {s} = 7 $ TeV and comparison with $ \sqrt {s} = 0.9 $ TeV

“…This confirms observations reported in ref. [27]. The predictions of the Z2 tune (not shown in this paper) reveal similar trends as for pythia8 4C with, however, a more uniform and more limited underestimate ( 10%) of the average p T .…”

Efficient matrix-element matching with sector showers

“…The main distinguishing feature of such showers is that only a single radiation antenna contributes to each phase-space point, as compared to a sum over all radiators in traditional "global" showers [16,18,34,36]. The coefficients of the single poles of gluon antennae are modified to reflect this reorganization.…”

“…The latter is extensively discussed in [17,18] and will not be repeated here. For a general introduction to shower Monte Carlos including use of the veto algorithm and related topics, see [16]. Here, we focus exclusively on the modifications to the showering algorithm that occur when going from the global to the sector case.…”

“…Here, we shall focus on the matching of parton showers to LO matrix elements with large numbers of additional legs. For this problem, there are essentially two dominant approaches, called MLM (see [10] for a description) and (L)-CKKW [11][12][13][14][15], see [16] for a recent pedagogical review. Two main limiting factors in these approaches are that the computational speed falls off steeply with the number of matched partons, and that both approaches only apply matching above a certain "matching scale", below which the pure shower is used for all multiplicities.…”

Underlying Event measurements in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV with the ALICE experiment at the LHC

“…Many Monte Carlo (MC) generators for the simulation of pp collisions are available; see [9] for a recent review discussing for example Pythia [10], Phojet [11], Sherpa [9] and Herwig [12]. These provide different descriptions of the UE associated with high energy hadron collisions.…”