Parallel adaptive Monte Carlo integration and vector-boson scattering at the Large Hadron Collider

The Standard Model of particle physics has proved to be a reliable theory for the description of the interaction between elementary particles. With the discovery of the Higgs boson at the LHC, all predicted particles of the Standard Model have been observed. As a result, particle physics faces new challenges such as further validation of the Standard Model or the search for New Physics beyond the Standard Model. More precise and more comprehensive measurements and experimental data analyses at the LHC or other future collider will shed light on this.
Monte Carlo event generators are an indispensable tool for measurements and experimental data analyses, as well as for theoretical predictions at collider experiments. They are based on the acceptance-rejection method for event generation with a given probability distribution in configuration space. This is supplemented by the methods of Monte Carlo integration of the d -dimensional phase space with d = 3n - 4 degrees of freedom of an n-particle final state, for which the application of classical, numerical integration rules for large n > 4 are disadvantageous. In particular, Monte Carlo integration and event generation are closely linked by the so-called importance sampling. The application of iterative and adaptive Monte Carlo algorithms for numerical integration allows us to optimize the efficiency of event generation with a previous Monte Carlo integration.
We present the parallelization of the doubly-adaptive Monte Carlo algorithm Vamp using the OpenMP and MPI paradigms, under special consideration of minimizing communication needs and a method to improve the efficiency of parallelization by a static load balancer. We have written a new implementation of the doubly-adaptive algorithm, Vamp2, for adaptive Monte Carlo integration and event generation, which is available as part of the Monte Carlo event generator Whizard. We reached an overall improvement of the integration run-time of Whizard for typical use cases at collider experiments by order 10, reducing possible computation times from days or weeks to hours.
In a first application, we employ the parallelized integration in the electroweak and Higgs sector of the Standard Model for high energies at the TeV scale accessible by the LHC. For this purpose, we consider the role of transverse-polarized gauge bosons and the Higgs boson in vector boson scattering (VBS), that is, the scattering of W and Z bosons in pairs to W, Z, and Higgs.
In a bottom-up approach of the effective field theory of the Standard Model, we model anomalous contributions by possible new physics in a model-independently way with dimension-eight operators. We provide relations for the dimension-eight operators for different representations of the Higgs field to allow comparisons between our results and existing studies.
A description of the VBS processes by the effective field theory at such high energies violates the unitarity of the scattering matrix. By applying the T-matrix unitarity projection we can restore the unitarity of the theory and thus the first principles of quantum field theory. The simplified models, defined in this way, allow us to study quantitative statements about (typical) scenarios of New Physics and estimate possible event rates beyond the standard model predictions by upper limits. From this we can pinpoint the sensitivity on results for New Physics at the LHC.