Pile-up corrections for jets from proton-proton collisions at $\sqrt{s} = 7$ \TeV{} in \ATLAS{} in 2011

The high luminosity and high total proton-proton interaction cross section at $\sqrt{s} = 7$~TeV at the LHC lead to multiple proton collisions per bunch crossing. With instantaneous luminosities reaching up to $3.6 \times 10^{33}$~cm$^{-2}$s$^{-1}$ in 2011, the number of these pile-up interactions increased significantly with respect to 2010. Particles from these interactions are scattered into the \ATLAS{} calorimeters independently of the particle flow from the (triggered) hard-scatter interaction, and add approximately $370(850)$~MeV per reconstructed primary vertex to the transverse momentum (\pT{}) of a jet reconstructed with the \antikt{} algorithm with $R = 0.4(0.6)$ in the central region of the detector. In addition to the effect of pile-up interactions in the same bunch crossing as the hard scatter, in 2011 the $50$ ns proton bunch spacing within LHC bunch trains introduced a sensitivity of the calorimeter signals to the energy flow in past collisions, due to the \ATLAS{} calorimeter signal shapes. The resulting increase of the reconstructed jet \pT{} per additional interaction in the central region is approximately $60(210)$~MeV, while in the forward region a decrease of approximately $350(470)$~MeV is observed. Monte Carlo simulation-based algorithms have been developed to address both effects on the jet signal individually and independently. Results from in-situ validation techniques in 2011 data indicate that the modeling of these effects is not perfect, and the resulting systematic bias in the jet \pT{} measurement is at most $3\%$ for $\pT > 40$~ GeV at the highest pile-up activities experienced in 2011.