Manuel Bähr is a PhD student from Karlsruhe University, working on Herwig++, on a three-month MCnet studentship at UCL.
For my Ph.D., I am working on a description of the so-called underlying event for the Event Generator Herwig++. From the experimental point of view, the underlying event contains all activity in a hadronic collision that is not related to the signal particles from the hard process, e.g. leptons or missing transverse energy. The additional particles may result from the initial- or final-state radiation of coloured particles or from multiple semi-hard or soft interactions. The perturbative, i.e. semi-hard part has been implemented into Herwig++ before my short-term studentship.
During my stay at UCL, we studied a model to incorporate soft partonic scatters in addition to the semi-hard ones. Hence, the former hard cut-off is transformed into a matching scale that separates semi-hard and soft interactions. It thereby reduces the extrapolation uncertainty that was associated with this cut-off.
In detail this model offers the possibility to establish a connection between the underlying event activity and the values of the total and elastic cross section. By imposing internal consistency on the model in comparing it to current data we constrained the allowed range of its parameters. We showed that measurements of the total proton-proton cross-section at the LHC are likely to break this internal consistency, and thus to require an extension of the model. An example of the parameter space and its constraints, for LHC enrgies, can be seen in the figure.
Likely such extensions are that hard scatters probe a denser matter distribution inside the proton in impact parameter space than soft scatters, a conclusion also supported by Tevatron data on double parton scattering, and/or that the basic parameters of the model are energy dependent. The findings obtained during my short-term studentship have been used later on to construct an improved model where the proton possesses a hot spot-like matter distribution. This model has finally been implemented into Herwig++ and is available from version 2.3 onwards as the default model for the underlying event.