A Study of Missing Transverse Energy in Minimum Bias Events with In-time Pile-up at The Large Hadron Collider using The ATLAS Detector and $\sqrt{s}$=7 TeV Proton-Proton Collision Data

A sample of $\int L\;dt=3.67 ~\text{pb}^{-1}$ of minimum bias events observed using the ATLAS detector at the Large Hadron Collider at $\sqrt{s}$=7 TeV is analyzed for Missing Transverse Energy (MET) response in the presence of in-time pile-up. We find that the MET resolution ($\sigma_\text{X,Y}$) is consistent with a simple model of the detector response for minimum bias events, scaling with respect to the sum of the scalar energy ($\sum E_\text{T}$) as $\sigma_\text{X,Y}=A\sqrt{\sum E_\text{T} }$. This behavior is observed in the presence of in-time pile-up and does not vary with global calibration schemes. We find a bias in the mean ($\mu_\text{X,Y}$) of the MET that is linear with respect to $\sum E_\text{T}$, leading to an asymmetry in the $\phi_\text{X,Y}$ distribution of the MET. We propose an explanation for this problem in terms of a misalignment of the nominal center of the ATLAS detector with respect to its real center. We contrast the data with a Monte Carlo sample produced using PYTHIA. We find that the resolution, bias and asymmetry are all approximately reproduced in simulation.