QCD Matrix Elements + Parton Showers

Abstract

We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in e + e − annihilation. The matrix element and parton shower domains are separated at some value y ini of the jet resolution, defined according to the k T -clustering algorithm. The matrix elements are modified by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on y ini to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present first results of an approximate implementation using the event generator APACIC++.

Establishing jet scaling patterns with a photon

“…A major obstacle to overcome is that exclusive jet measurements have no straightforward interpretation in fixed order perturbation theory and exclusive quantities are typically plagued with large theoretical uncertainties. On the other hand, multi-jet merging, for example using the Ckkw scheme [34,35], allows us to study n jets distributions including a free choice of kinematic cuts and has proven successful in various experimental analyses so far.…”

“…Matching of a hard QCD matrix element with a collinear parton shower [30][31][32][33][34][35][36][37] allows us to reliably simulate and study jet scaling patterns from perturbative QCD. It does not require a fundamental re-organization of QCD perturbation theory [38][39][40] but simply relies on the proper phase space simulation of collinear logarithms.…”

“…This way it does not only include the radiation of one or two hard jets, as correctly described by fixed-order QCD calculations, but any number of radiated jets including high multiplicities obviously well described by the parton shower. We use Sherpa [41][42][43] with its Ckkw [34,35] matching scheme to describe radiation of up to seven jets with high precision. Typically, we check an additional two more jets for unexpected features, but with correspondingly reduced statistics.…”

Soft evolution of multi-jet final states

“…They can be merged with LO predictions for multi-jet events [31][32][33][34] and matched to NLO calculations [35][36][37] for over a decade. More recently, methods for combining next-to-leading order matched predictions of varying jet multiplicity have been devised [38][39][40], as well as matching methods at next-to-next-to leading order (NNLO) accuracy [41][42][43].…”

Lectures on LHC Physics

“…The original CKKW paper beautifully explains the general idea for final state radiation, and I follow their analysis [42]. For other approaches there is a very concise discussion included with the comparison of the different models [57].…”