We discuss the theoretical bases that underpin the automation of the computations of tree-level and next-to-leading order cross sections, of their matching to parton shower simulations, and of the merging of matched samples that differ by light-parton multiplicities. We present a computer program, MadGraph5 aMC@NLO, capable of handling all these computations — parton-level fixed order, shower-matched, merged — in a unified framework whose defining features are flexibility, high level of parallelisation, and human intervention limited to input physics quantities. We demonstrate the potential of the program by presenting selected phenomenological applications relevant to the LHC and to a 1-TeV e + e − collider. While next-to-leading order results are restricted to QCD corrections to SM processes in the first public version, we show that from the user viewpoint no changes have to be expected in the case of corrections due to any given renormalisable Lagrangian, and that the implementation of these are well under way.

/pdf/the-automated-computation-of-tree-level-and-next-to-leading-4etm0g6w46.pdf

The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations

https://npac2013.lal.in2p3.fr/2010-2011/Cours/Exam-IF/Altarelli-Parisi.pdf

Asymptotic Freedom in Parton Language

http://cds.cern.ch/record/413720/files/CM-P00060268.pdf

A planar diagram theory for strong interactions

The Renormalization group and the epsilon expansion

QCD and resonance physics. theoretical foundations

An explicit calculation shows perturbation theory to be arbitrarily good for the deep Euclidean Green's functions of any Yang-Mills theory and of many Yang-Mills theories with fermions. Under the hypothesis that spontaneous symmetry breakdown is of dynamical origin, these symmetric Green's functions are the asymptotic forms of the physically significant spontaneously broken solution, whose coupling could be strong.

Reliable Perturbative Results for Strong Interactions

Asymptotic freedom: an approach to strong interactions

The effects of new, heavy quarks are examined in a colored quark-gluon model. The e+e- total cross section scales for energies far above any quark mass. However, it is much greater than the scaling prediction in a domain about the nominal two-heavy-quark threshold, despite σe+e- being a weak-coupling problem above 2 GeV. We expect spikes at the low end of this domain and a broad enhancement at the upper end.

/pdf/heavy-quarks-and-e-e-annihilation-3uosinzvth.pdf

Heavy Quarks and e+ e− Annihilation

The effects of new, heavy quarks are examined in a colored quark-gluon model. The ${e}^{+}{e}^{\ensuremath{-}}$ total cross section scales for energies far above any quark mass. However, it is much greater than the scaling prediction in a domain about the nominal two-heavy-quark threshold, despite ${\ensuremath{\sigma}}_{{e}^{+}{e}^{\ensuremath{-}}}$ being a weak-coupling problem above 2 GeV. We expect spikes at the low end of this domain and a broad enhancement at the upper end.

/pdf/orthocharmonium-and-e-e-annihilation-rm9fb2qm8g.pdf

Orthocharmonium and e+ e- Annihilation

We discuss the effects of quark and target masses in inclusive lepton-hadron scattering, assuming a color SU(3) gauge theory of strong interactions. Our tools are the renormalization group, suitably extended to include quark masses, and the operator-product expansion. We argue that the best renormalization procedure is to specify all parameters in the Lagrangian (coupling constants and masses) as well as wave-function normalizations at Euclidean momenta of scale M. We use the renormalization group to develop an understanding of the relation between current-algebra and constituent-quark masses. In the operator-product expansion, we use the equations of motion to eliminate operators with D, D2, etc. acting on a quark field. We order the expansion as a power series in the gauge coupling constant as it occurs in the coefficient functions. We predict approximate scaling in a new variable ξ≠x, which depends on Q2 and the quark and target masses. We also discuss the distribution of antiquarks and heavy quarks in the nucleon.

/pdf/freedom-at-moderate-energies-masses-in-color-dynamics-koyg0rro1p.pdf

H. David Politzer

Papers

Reliable Perturbative Results for Strong Interactions

Asymptotic freedom: an approach to strong interactions

Heavy Quarks and e+ e− Annihilation

Orthocharmonium and e+ e- Annihilation

Freedom at Moderate Energies: Masses in Color Dynamics