Characteristics of the different Monte Carlo generators

On this web page several changes of Monte Carlo versions are shown.

All the analyses are based on the HepMCAnalysis tool. Here is a documentation of the used release 3.2.
Back to overview of summerstudent project (2010).

Herwig++:

The transverse mass of the generated W-bosons is illustrated in the left figure. As expected the distribution peaks at a value around the W-boson mass. Nevertheless there are W-bosons which are not on their mass shell and peak around zero. This behaviour occurs for all tested versions. The W-bosons are unphysical and are produced in electroweak interactions with a &tau -lepton. These W-bosons cause a further needle of the neutrino momentum at pT &asymp 0 GeV/c, see right figure. In order to avoid the massless W-bosons, a mass cut has to be applied.

Transverse momentum of the &tau -lepton:

A bug in the Herwig++ version 2.4.1 occured. The transverse momentum of the &tau -leptons was shifted to higher momenta. This bug was caused by a misspelling of the pdg identification number for some particles which should be parents/ancestors of the &tau -leptons. In version 2.4.2 the bug was removed.

Pythia6 and Pythia8:

The charged particle multiplicity:

The number of particles per event has been shifted to higher numbers from version 6.4.21.2 to version 6.4.23.2 for the generator Pythia6. This error occured due to a bug in PYSIGH concerning the multiple-interaction framework. This bug causes an increase of < 10% in the average charged multiplicity. On the contrary the number of charged particle has been shifted due to smaller numbers for the generator Pythia8, see right figure.

Herwig:

All histograms have also been produced with the Monte Carlo generator Herwig and are available on this webpage. The results showed no special characteristics.