ATLAS tunes of the Pythia8 generator to measurements of ttbar production

At the LHC collider for the first time $t\bar{t}$ processes can be measured with enough accuracy so as to be used for constraining the parameters of the Monte Carlo models. We present tunes of the initial- and final-state radiation parameters of the Pythia8 generator to various measurements of $t\bar{t}$ observables, performed by the ATLAS experiment in $pp$ collisions at $\sqrt{s} = 7$ TeV. The resulting tunes are compared to tunes to the $Z$ boson transverse momentum at the LHC, and to the LEP event shapes in $Z$ boson hadronic decays. Such a comparison provides a test of the universality of the parton shower model. The tune is also applied to the next-to-leading order generators MadGraph5_aMC@NLO and Powheg, and additional parameters of these generators are tuned to the $t\bar{t}$ data. For the first time in the context of Monte Carlo tuning, the correlation of the experimental uncertainties has been used to constrain the parameters of the Monte Carlo models.