First measurement of the Z+jets production cross section with the ATLAS experiment at the LHC

The measurement of the inclusive jet production cross section in association with a Z boson is one of the most important Standard Model measurements at the Large Hadron Collider (LHC). It constitutes a stringent test of perturbative Quantum ChromoDynamics (pQCD) and is sensitive to new particles decaying into Z+jets final states. Moreover, the process represents an important background for new physics searches. This document reports a first measurement of the inclusive Z+jets production cross section with the ATLAS detector in sqrt(s) = 7 TeV proton-proton collisions. The measurement is performed in the Z(->e+e-) channel with 33 1/pb of integrated luminosity collected by ATLAS in 2010. The electron and muon channel measurements are compared, and the combination of both results is performed. The cross section is measured as a function of the jet multiplicity, the kinematic properties of the jet, and the invariant mass and spatial separation between the two leading jets. The measured cross sections are compared to the pQCD Next-to-Leading Order (NLO) predictions by MCFM and BlackHat, which include non-perturbative corrections. Comparisons to Leading Order (LO) predictions supplemented by parton shower (as implemented in ALPGEN and Sherpa Monte Carlo generators) are also performed, where the predictions are normalized to the inclusive Next-to-Next-to-Leading Order (NNLO) predictions. The predictions from PYTHIA LO predictions supplemented by parton shower and normalized to the measured cross section in the 1 jet inclusive bin are considered as well. The measured cross sections are well described by NLO pQCD predictions as well as by ALPGEN and Sherpa, while PYTHIA underestimates the cross section for high jet multiplicities.