New ATLAS Event Generator Tunes to 2010 pp Collision Data at 7 TeV Centre-of-Mass Energy Nicholas Styles, on behalf of the ATLAS Collaboration

Using data recorded during 2010, the ATLAS experiment has been able to mea-sure many QCD observables at previously inaccessible energy scales, by using the 7 TeV centre-of-mass energy proton-proton collisions provided by the LHC. These mea-surements have been used to produce new tunes of the general-purpose Monte Carlo (MC) generators PYTHIA 6 and HERWIG/JIMMY, to provide improved predictions for a wide range of QCD obervables up to jets of pT ∼ 2 TeV [1]. A comprehensive tuning of PYTHIA 6, using the pT-ordered parton shower al-gorithm, was performed as described in Ref. [1], using measurements from e+e−, Tevatron and ATLAS data. Aspects of the procedure where ATLAS 7 TeV data plays an important role will be focused on here. The tuning of initial state shower parameters and primordial kT was based on the strategy used for the Perugia 2010 tune. Figure 1(a) shows jet shape variables in data and MC for a 30-40 GeV jet pT slice. As a result of this tuning, these variables in MC, which had shown discrepancies from data of up to 15 % using the previous ABMT1 tune, now agree with data at the level of approximately 5%