QCD matrix elements + parton showers. The NLO case

Abstract

We present a process-independent technique to consistently combine nextto-leading order parton-level calculations of varying jet multiplicity and parton showers. Double counting is avoided by means of a modified truncated shower scheme. This method preserves both the fixed-order accuracy of the parton-level result and the logarithmic accuracy of the parton shower. We discuss the renormalisation and factorisation scale dependence of the approach and present results from an automated implementation in the SHERPA event generator using the test case of W -boson production at the Large Hadron Collider. We observe a dramatic reduction of theoretical uncertainties compared to existing methods which underlines the predictive power of our novel technique.

Search for pair production of gluinos decaying via stop and sbottom in events withb-jets and large missing transverse momentum inppcollisions ats=13Aad1, Abbott2, Abdallah3 et al. 2016Phys. Rev. D410331View full textAdd to dashboardCite

“…W=Z þ jets and diboson processes are simulated using the SHERPA v2.1.1 [50] generator with CT10 PDF sets. Matrix elements for these processes are calculated using the Comix [51] and OpenLoops [52] generators and merged with the SHERPA parton shower [53] using the ME þ PS@NLO prescription [54].…”

Scaling patterns for QCD jets

“…First attempts to combine the NLO/LL approaches with the tree-level merging ansatz have been reported recently [67,68]. An unprecedented level of sophistication for predicting multi-jet final states is achieved by the promotion of merging algorithms to next-to-leading order accuracy [69][70][71].…”

Controlling inclusive cross sections in parton shower + matrix element merging

“…Indeed, the role of the correction we apply here is to precisely replace the first order virtual approximation stemming from expanding the Sudakov form factor by its exact counterpart. Fulfilling this condition is at the heart of the merging efforts at NLO presented so far, [31][32][33][34]. Let us stress the importance of maintaining inclusive cross sections in the case of NLO corrections.…”

“…NLO matching has been used to improve tree level merging algorithms, [27][28][29][30], but only recently, taking full advantage of developments in both merging, matching and shower algorithms, first experience has been gained in combining NLO QCD corrections to multijet final states of different multiplicity with subsequent parton shower emissions [31][32][33][34]. The aim of such approaches is to obtain an event sample which will provide LO (NLO) precision for observables controlled by hard n-parton emission as long as exact QCD corrections are included for up to n partons at tree level (n partons at one loop and n + 1 partons at tree JHEP08(2013)114 level), while including resummation by means of shower simulation at whatever accuracy is provided by these algorithms.…”