Underlying event studies at ATLAS and CDF

Improving our understanding and modeling of the underlying event in high energy collider environment is important for more precise measurements at the LHC. CDF Run II data for the underlying event associated with Drell-Yan lepton pair production and early ATLAS data measuring underlying event activity with respect to the leading transverse momentum track are presented. The data are compared with several QCD Monte Carlo models. It is seen that no current standard Monte Carlo tune adequately describes all the early ATLAS data and CDF data simultaneously. The underlying event observables presented here are particularly important for constraining the energy evolution of multiple parton interaction models. One of the goals of these analyses is to provide data that can be used to test and improve MC models for current and future physics studies at the LHC. The underlying event observables presented here are particularly important for constraining the energy evolution of multiple partonic interaction models, since the plateau heights of the underlying event profiles are highly correlated to multiple parton interaction activity. The data at 7 TeV are crucial for MC tuning, since measurements are needed with at least two energies to constrain the energy evolution of MPI activity. PYTHIA tune A and tune AW do a good job in describing the CDF data on the underlying-event observables for leading jet and Drell-Yan events, respectively, although the agreement between predictions and data is not perfect. The leading-jet data show slightly more activity in the underlying event than PYTHIA Tune A, although they are very similar - which may indicate the universality of underlying event modeling. However, all pre-LHC MC models predict less activity in the transverse region (i.e in the underlying event) than is actually observed in ATLAS leading track data, for both center-of-mass energies. There is therefore no current standard MC tune which adequately describes all the early ATLAS data. However, using diffraction-limited minimum bias distributions and the plateau of the underlying event distributions presented here, ATLAS has developed a new PYTHIA tune AMBT1 (ATLAS Minimum Bias Tune 1) and a new HERWIG+ JIMMY tune AUET1 (ATLAS Underlying Event Tune 1) which model the pT and charged multiplicity spectra significantly better than the pre-LHC tunes of those generators. It is critical to have sensible underlying event models containing our best physical knowledge and intuition, tuned to all relevant available data.