Automation of NLO QCD Corrections and the Application to N-Jet Processes at Lepton Colliders

Abstract

Despite strong indications for physics beyond the Standard Model (BSM), no definite direct detection of new physics has been made at the Large Hadron Collider (LHC) yet. Therefore, high-precision particle physics becomes ever more important in order to probe the Standard Model (SM) parameters for any deviation from theoretical predictions. This requires increasing experimental precision and theoretical accuracy. On the experimental side – besides improving hardware, luminosities, etc. – lepton colliders such as the planned International Linear Collider (ILC) provide very clean colorless initial states within a triggerless detection mode and eliminate uncertainties from non-perturbative hadronic parton distribution functions. Theoretical accuracy can be improved by computing higher-order corrections in the perturbation series beyond leading order (LO). Monte Carlo (MC) event generators automate these computations by the implementation of subtraction schemes to treat infrared divergences that occur at next-to-leading order (NLO) and beyond. The multi-purpose MC event generator WHIZARD has a wide feature set specific to lepton colliders and is well-established in the community. In this thesis, we present our work on generalizing its NLO quantum chromodynamics (QCD) capabilities for arbitrary processes at fixed order using the Frixione-Kunszt-Signer (FKS) subtraction scheme. This work is based on a prior FKS-subtraction implementation which was limited to a narrow range of processes. First, we give a thorough review of FKS subtraction, followed by a detailed description of its implementation in WHIZARD. We then validate our fixed-NLO (fNLO) QCD implementation by carrying out many self-consistency checks and by performing an intensive comparison of WHIZARD results with two other event generators at the level of total cross sections and differential distributions. As far as we know, we compute the first result for the total cross section of 6-jet production at fNLO QCD at a lepton collider with $\sigma_{\text{NLO}}^{e^+e^-\to jjjjjj}=4.46(4)\ \text{fb}$.