Monte Carlo tuning

Monte Carlo event generators generate perturbative processes, but also they also provide higher order QCD (and QED) corrections in terms of parton showers for the initial and final state partons. In addition, in high energy hardon-hadron scattering, the parton densities can become large and the probability to have more than one partonic interaction per proton-proton collision can become large. This is called in general the underlying event or multi-parton interactions (MPI).
In addition to the hard processes including parton showers, MPI, the partons will transform into observable hadrons (hadronization).

The group is active in investigating the performance of the current simulation of soft and semi-hard collisions at the LHC. Many components add a contribution to the hard scattering in proton-proton collisions and a detailed study and tuning is needed to describe the available measurements.

Since hadronization and in general MPI cannot be calculated from first principles, they are described by models, which are effective parameterzations of the physics process, but these parameterizations depend on free parameters, which need to be determined by experiment (tuning of parameters in Monte Carlo event generators).

New sets of parameters (“tunes”) for the underlying-event modelling of available Monte Carlo event generators have been constructed and they are widely used in the CMS Collaboration. They provide an improved description of a large set of measurements at different collision energies and they are currently used to model the pile-up contribution of any measurement performed with the CMS detector.

Ongoing projects are:
    - improvement of the simulation of diffractive components based on the
      new dn/deta measurements for different event selections at 13 TeV
    - study of different models of color reconnection among partons coming
      from the hard scattering and multiple parton interactions
    - investigation of the dependence of the tunes on the parton distribution function