A study of optimal parameter setting for MADGRAPH5_AMC@NLO + PYTHIA 8 matched setup

This note presents a study to arrive at the optimum tune of PYTHIA 8 generator's parton shower and multiple parton interaction parameters for MADGRAPH5_AMC@NLO + PYTHIA 8 matched setup. The study is based on published distributions from ATLAS Run 1 data at $\sqrt{s}$ = 7 TeV for three different processes, inclusive jet, $Z$~boson and $t\bar{t}$ production . Two tunes, A15-MG5aMCNLO-TTBAR, applicable only for $t$$\bar{t}$ processes, and A15-MG5aMCNLO, applicable in general for all processes, have been developed. The resulting PYTHIA 8 parameters of the NLO+PS matched setup are in good agreement with the standalone PYTHIA 8 tunes.