Subproject A5: Automated NLO/NLL Monte Carlo Programs for the LHC

Project Leader:
Prof. Michal Czakon, Institute of Theoretical Physics and Cosmology, RWTH Aachen
Prof. Dr.Michael Kraemer, Institute of Theoretical Physics and Cosmology, RWTH Aachen

Summary:

Theoretical calculations within fixed-order perturbation theory allow for accurate predictions of inclusive observables like total cross sections. The analysis and interpretation of experimental signatures at the LHC, however, require theoretical predictions for exclusive final states, i.e. predictions for differential distributions or cross sections with cuts on kinematic variables. Higher-order calculations for such exclusive final states involve in general large corrections from soft or collinear parton emission, which need to be summed to all orders. An efficient way to achieve such a summation is through a parton shower. Parton showers include the leading logarithmic contributions from soft and collinear gluon or quark emission to all orders in perturbation theory. They form a central part of Monte Carlo event generators and are thus essential to connect theoretical models with realistic experimental signatures. On the other hand, standard parton shower event generators rely on leading-order expressions for the hard scattering processes and can therefore not predict inclusive cross sections accurately.

The goal of the present project is the automated calculation of next-to-leading (NLO) LHC cross sections including the summation of large corrections from multiple quark and gluon emission through parton showers. Such a calculation combines the accuracy of NLO predictions for inclusive cross sections with the power of parton shower Monte Carlo programs to reliably describe differential distributions and cross sections with cuts on kinematic variables. A naive combination of NLO calculations with parton showers would, however, lead to double counting of higher-order contributions that are included in both the NLO cross section and the parton shower. These contributions have to be identified and subtracted from the calculation by means of a matching procedure. To substantially improve on existing NLO plus parton shower implementations, quantum interference contributions, spin correlations and sub-leading colour effects in the parton shower will be included systematically. The ultimate goal of the project is the automation of calculations of LHC processes at NLO plus NLL accuracy, i.e. the combination of NLO calculations with parton showers at the next-to-leading logarithmic level. To achieve such an accuracy requires a very careful treatment of the momentum mapping in the parton shower evolution and a systematic comparison of results with dedicated NLL resummation calculations for specific processes.

Last Change: 29^th August 2011