In collaboration with J. Bellm, J. Gaunt, A. Kulesza, L. Lönnblad and T. Sjöstrand.
The parton shower in Herwig is the product of many years of continuous development and its main purpose is to evolve the event from the hard energy scale to an infrared cutoff scale where non-perturbative hadronisation models take over. The next step in development of the shower is to include higher orders to increase the logarithmic accuracy. This requires the implementation of splitting functions for two or more emissions which involves the organisation and bookkeeping of the singularity structures of QCD cross sections.
The MCnet short-term project aimed at implementing a complete description of spacetime coordinates and evolution during event generation in Herwig 7. The current version of Herwig, as with most other event generators, is formulated in an energy-momentum picture, with no concrete understanding of geometry and spacetime information generation. After a high-energy collision, outgoing QCD partons will undergo bremsstrahlung, lowering their energy more and more until reaching the non-perturbative, low-energy regime of QCD.
Recently in CMS, we have measured the azimuthal separation between the two leading jets in multi-jet events when these are approximately back-to-back. We have also measured the more exclusive and complementary azimuthal separation between the leading jets when the presence of at least an extra much softer jet is required in the event. We have observed a tension between the data and the theory predictions from parton shower event generators as well as from improved predictions using higher order matrix elements.