Reconstruction of DC1 $J/\psi \to e^+ e^-$ decays and use for the low energy calibration of the ATLAS electromagnetic calorimeter

The reconstruction of J/psi decays to e+e- pairs is investigated with DC1 data. The low energy electrons produced are reconstructed by a dedicated algorithm. A rejection of pion tracks of \sim 450 is obtained for an electron identification efficiency of 90%. The background to the J/psi events is estimated from b{\bar b}->mu X events. The J/psi reconstruction efficiency is found to be \sim 20%, and the signal to background ratio around 2. The quality of the J/psi mass reconstruction using the Inner Detector only is compared with previous studies. The emphasis of this analysis is on the use of J/psi -> e+e- decays to calibrate the electromagnetic calorimeter. For this purpose, the mass reconstruction uses the energy taken from the electromagnetic calorimeter. Results obtained show that the intercalibration of the 448 motherboard regions (\Delta\eta*\Delta\phi = 0.2*0.4) can be obtained with a statistical precision of \sim 0.4 after a year of data taking at low luminosity.