Early LHC Underlying Event Data - Findings and Surprises

TL;DR

This paper evaluates PYTHIA tunes against early LHC underlying event data, finding improved descriptions with Tune Z1 but highlighting persistent challenges in modeling minimum-bias events.

Contribution

It provides a detailed comparison of PYTHIA 6.2 and 6.4 tunes against LHC data, revealing strengths and limitations in current event modeling.

Findings

PYTHIA 6.4 Tune Z1 better describes UE data than 6.2

Models struggle to accurately predict min-bias features at LHC energies

Underlying event behavior aligns roughly with expectations

Abstract

The CDF PYTHIA 6.2 Tune DW predictions of the LHC underlying event (UE) data at 900 GeV and 7 TeV are examined in detail. The behavior of the UE at the LHC is roughly what we expected. The new LHC PYTHIA 6.4 Tune Z1 does an even better job describing the UE data at 900 GeV and 7 TeV. However, the modeling of "min-bias" at the LHC (i.e. the overall inelastic cross section) is a very different story. No model describes all the features of "min-bias" collisions at 900 GeV and 7 TeV.