Identification of pile-up using the quality factor of pulse shapes in the ATLAS Tile Calorimeter

The ATLAS experiment records data from the proton-proton collisions produced by the Large Hadron Collider (LHC). The Tile Calorimeter is the hadronic sampling calorimeter of ATLAS in the region |η| <; 1.7. It uses iron absorbers and scintillators as active material. The LHC will provide collisions every 25 ns, putting very strong requirements on the energy measurement in presence of energy deposits from different collisions in the same read out window and physical calorimeter channel (pile-up). In 2011 the LHC is running with filled bunches at 50 ns spacing and at intensities which yield up to about 8 proton-proton collisions per bunch crossing. We present a quality factor that can be computed online for each collision and for each calorimeter channel within the 10 μs latency of the ATLAS first level trigger (L1 trigger), and could allow to identify calorimeter channels presenting pile-up. In presence of a poor quality factor the data from the corresponding channel is read out with additional information to allow for an offline dedicated treatment of the signals to account for pile-up.