Commissioning of the ATLAS electromagnetic calorimeter with minimum bias events

This note presents the potentiality to commission the ATLAS EM calorimeter during the (very) first days of data taking, prior to be able to trig and identify correctly electrons. For this purpose, a very simple analysis using the $\phi$ symmetry of the abundant minimum bias events and computing the energy accumulated in the EM calorimeter is proposed. No input from the Monte Carlo is necessary, and only information from the calorimeter is used. To cope with the high number of events, simulation and analysis are performed using the GRID technology. Adopting a simple energy reconstruction scheme based on the cosmic muon experience should allow to spot intrinsic problems of the EM calorimeter in a few days. It should also give first hints on the $\phi$ dispersion of the Inner Detector material, if the excess is sizeable >10\% X$_0$ in a region $\Delta \eta \times \Delta \phi=0.1\times 0.1$. Ultimately, a first flavor of systematics coming from the EM calorimeter $\phi$~non-uniformity and its positionning (in the ATLAS framework or with respect to the beam) could be possible.