Automation of one-loop QCD computations

Abstract

We present the complete automation of the computation of one-loop QCD corrections, including UV renormalization, to an arbitrary scattering process in the Standard Model. This is achieved by embedding the OPP integrand reduction technique, as implemented in CutTools, into the MadGraph framework. By interfacing the tool so constructed, which we dub MadLoop, with MadFKS, the fully automatic computation of any infrared-safe observable at the next-to-leading order in QCD is attained. We demonstrate the flexibility and the reach of our method by calculating the production rates for a variety of processes at the 7 TeV LHC.

One-loop calculations in quantum field theory: From Feynman diagrams to unitarity cuts

“…It will be interesting to see how numerical methods will develop in the future, given the robust seminumerical nature of the OPP approach and recent attempts to automate it [17,124].…”

“…In the spirit of Refs. [123,124] it is possible to interface the one-loop calculation with a tool that provides the real emission corrections, the subtraction terms, and can carry out the phase space integration. Such an approach has been used recently to compute the NLO QCD corrections to five jet production in e + e − annihilation where one-loop amplitudes, calculated with generalized unitarity, were interfaced with Madevent/MadFKS [10].…”

“…In order to keep this discussion in line with the main theme of this review, we will assume that the OPP method is employed for the reduction of tensor integrals to the scalar basis, but a Passarino-Veltman reduction could also be used. We note that various implementations of diagrammatic calculations [125,126,10,124] can differ in the details -for example, in how the diagrams are grouped together before the numerical evaluation or in how the rational parts are computed, but are otherwise very similar in spirit.…”

“…For instance, one can consider replacing the one-loop result at the exceptional point with the geometric average of the two results obtained using two points in the phase-space that are close enough to the exceptional point [124].…”

Soft evolution of multi-jet final states

“…Next-to-leading order (NLO) corrections have also reached a high level of automation [7][8][9][10][11][12][13][14][15][16][17], and fully differential multi-jet cross…”