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. At this energy scale and below, phenomenological models are used to attempt to describe hadronization, the process by which individual coloured partons coalesce into composite colourless hadrons. One piece of hadronization is colour reconnection, which switches around the colour connections between partons, aiming to reduce some kinematic measure of the system, which in Herwig is invariant mass.

A new model aims to generate and propagate spacetime coordinates during the contributing stages of the event, namely the multiple parton interactions and the parton shower. Multiple parton interactions occur in proton-proton collisions due to the composite nature of protons. Since protons are finitely sized objects, when they collide the constituent partons which undergo scattering processes may occur at transversely separated points in spacetime. These transverse separations can be of the size of the proton, potentially reducing the likelihood of causal connection. By using spacetime coordinates to perform colour reconnection, we reflect this causal separation. Our new model generates these transverse coordinates using the protons’ individual form factors and the overlap between them.

While showering, they are expected to travel a finite distance, analogous to the Heisenberg Uncertainty Principle. In our model, the partons with higher energy (i.e. earlier emissions) should travel a shorter distance with respect to the lower energy partons (i.e. later emissions). The showering process smears the initial transverse separation of the multiple parton interactions, allowing nearby systems to come into causal contact, and changing the geometry of the system. This setup is perfect for allowing colour reconnection to change the colour topology of the event based on the distance between individual partons or entire multiple parton interaction systems.

Cody Duncan
April, 2018 to October, 2018