Automation of next-to-leading-order predictions for physics beyond the Standard Model at the LHC

• Applicant: Professor Dr. Stefan Dittmaier
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2017 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 337250025

• Final Report Year: 2020

Final Report Abstract

After the successful completion of Runs 1 and 2, the Large Hadron Collider (LHC) has explored the fundamental interactions of elementary particles up to energies in the range of a few TeV, thereby finding good agreement with all theoretical predictions that are based on the Standard Model (SM) of particle physics. In the absence of spectacular signals of new particles, discovery via precision seems to be the only way the significantly extend the reach in the analysis of LHC data in order to find any trace of deviations from SM predictions that could point to new phenomena. After all, we know that the SM is not able to explain some crucial facts observed in our universe at microscopic and macroscopic scales, such as the existence of Dark Matter, the matter–antimatter asymmetry, the flavour structure of matter, etc.. Many potential signals of physics beyond the Standard Model (BSM) hide in small deviations from SM predictions or subtle effects in specific tails of kinematical distributions. Fully exploiting LHC data with precision—typically at the level of percent, in detail depending on the observables—is a formidable task, since the number of different processes is huge, the processes themselves are complicated and often involve multi-particle states, and the model candidates continuously increases over time. In order to test the validity of the SM and to explore whether new-physics models are needed to describe data, precise predictions are required both within and beyond the SM. The only way to cope with this situation is via the automated calculation of transition matrix elements and cross sections, where next-to-leading-order (NLO) radiative corrections, i.e. loop calculations typically at the one-loop level, have to be supported. Within the SM, tremendous progress towards this direction has been seen in the previous 20 years. Within quantum chromodynamics (the theory of strong interaction) the problem is considered solved at NLO, and for standard electroweak interaction the process is well advanced. On the other hand, the automation of BSM physics is still subject of current research and pursued by several groups. At the beginning of the project, the major player in view of BSM automation were the developers of FeynArts/FormCalc, MadGraph/MadLoop, and Recola 2. The goal of this research project was to extend the functionality of the one-loop matrix element generator OpenLoops, which is distinguished by excellent speed and stability in SM NLO calculations, to BSM physics. In detail, the research project targeted on the following objectives: 1. Technical developments in OpenLoops. Two major objectives were addressed in this part of the project: First, the development of a BSM model generator to drastically simplify the incorporation of new physics models in OpenLoops. As a starting point we chose to use model files in the UFO format as generated by FeynRules from a given Lagrangian density. Second, developing a new Feynman diagram rsp. amplitude generator to replace FeynArts for diagram generation, increase the performance and reduce memory usage so that processes of even higher complexity can be calculated. While these two components have been implemented in the course of this project, facing major compatibility issues with the existing OpenLoops implementation, we decided to abandon the idea of extending the existing code and instead create an entirely new matrix element generator. This new generator is still under development. In particular, the components for the numerical evaluation of loop amplitudes are still under construction. 2. Field-theoretical preparation of BSM models and program validation. Promoting new-physics models from its leading-order formulation to the NLO level is straightforward formally, but nontrivial in practice. The renormalization procedure has to be formulated in such a way that the models are parametrized in terms of a phenomenologically appropriate set of input parameters and corrections are perturbatively stable in the whole BSM parameter space. On the other hand, properties like gauge independence, symmetries, and process independence should be maintained. Part of the research project was also to continue and complete groundwork in this direction that had already begun before the funding period. Specifically, different types of renormalization schemes (MS, on-shell, and symmetry-inspired schemes) were formulated for Two-Higgs-Doublet Models and a simple Higgs Singlet Extension of the SM and applied in NLO calculations for Higgs-boson production and decay processes. These applications serve as important independent test results to validate the new OpenLoops generator.

Publications

• “Precision calculations for h → W W/ZZ → 4 fermions in a Singlet Extension of the Standard Model with Prophecy4f,” JHEP 04 (2018), 062
  L. Altenkamp, M. Boggia and S. Dittmaier
  
• “Precision calculations for h → W W/ZZ → 4 fermions in the Two-Higgs-Doublet Model with Prophecy4f,” JHEP 03 (2018), 110
  L. Altenkamp, S. Dittmaier and H. Rzehak
  
• “Renormalization of mixing angles,” JHEP 11 (2018), 104
  A. Denner, S. Dittmaier and J. N. Lang
  
• Les Houches 2019: Physics at TeV Colliders: Standard Model Working Group Report
  S. Amoroso et al.
  
• “OpenLoops 2,” Eur. Phys. J. C 79 (2019) no.10, 866
  F. Buccioni, J. N. Lang, J. M. Lindert, P. Maierhöfer, S. Pozzorini, H. Zhang and M. F. Zoller
  
• “QCD and electroweak corrections to WZ scattering at the LHC,” JHEP 06 (2019), 067
  A. Denner, S. Dittmaier, P. Maierhöfer, M. Pellen and C. Schwan
  
• “Electroweak Radiative Corrections for Collider Physics,” Phys. Rept. 864 (2020), 1-163
  A. Denner and S. Dittmaier
  
• “Next-to-leading-order QCD and electroweak corrections to triple-W production with leptonic decays at the LHC,” JHEP 02 (2020), 003
  S. Dittmaier, G. Knippen and C. Schwan
  
• “PROPHECY4F 3.0: A Monte Carlo program for Higgsboson decays into four-fermion final states in and beyond the Standard Model,” Comput. Phys. Commun. 254 (2020), 107336
  A. Denner, S. Dittmaier and A. Mück