## Precision QCD

Contact Person: Lance Dixon

The high-energy proton collisions recorded by Large Hadron Collider (LHC) experiments at CERN provide more insight than ever into the dynamics of nature at smallest distances. But they are dominated by effects of a force that has been known since long, the strong nuclear force. Interactions between the incoming protons at the LHC may result in collimated sprays of strongly interacting particles that are called jets when they are observed in detectors. Jets are produced copiously at the LHC, and they lead to large background noises that may cover up most signals of interesting new physics. The understanding of jet production and evolution from first principles is therefore essential to extract new information from experimental data. In addition to the typically striking signatures of jets, there are more subtle effects of the strong force, which must be understood in order to interpret experimental measurements correctly.

Strong interactions are well modeled by the theory of Quantum Chromodynamics (QCD). The non-abelian, nonlinear nature of QCD leads to interesting phenomena, but it also makes theoretical calculations very cumbersome. At high energies, calculations can be performed in perturbation theory, using an expansion in the strong coupling constant *α _{s}*. The SLAC theory group contributes to the advancement of precision QCD through the development of new techniques for the computation of higher-order terms in this expansion, and through phenomenologically relevant predictions for current and future collider experiments based on the newly developed theoretical tools.

## High-Multiplicity Next-to-Leading Order Calculations

The traditional approach to perturbative QCD is to compute results from Feynman diagrams, named after iconic physicist Richard Feynman. This approach works well as long as the number of particles in the process - and therefore the number of jets in the measurement for which the calculation is performed - is small. But the number of Feynman diagrams grows factorially with the number of particles produced. For example, computation of the one-loop corrections to six-gluon production at a hadron collider such as the LHC requires about three million Feynman diagrams, where each corresponds to a mathematical expression with up to 10,000 terms. A new approach based on unitarity and complex analysis has changed the way such calculations are performed: In 1994, Bern, Dixon, Dunbar and Kosower showed that, in supersymmetric QCD, the loop diagrams could be computed by cutting the loops open to obtain tree diagrams, which can be calculated very efficiently. The idea has been extended to QCD in various ways. Dixon and his collaborators use the technique to compute predictions for multi-jet production, the production of W and Z bosons in association with multiple jets, the production of photons in association with jets, and more.