Signal processing for the ATLAS liquid argon calorimeter: Studies and implementation

The ATLAS detector operated successfully at the LHC studying the proton-proton collisions produced with a center-of-mass of energy up to 8 TeV. During the period from 2009 to 2012, called the LHC Run 1, up to more than 37 independent collisions were produced every 50 ns at the LHC. The ATLAS detector has a set of calorimeters measuring the energy of different types of particles. The liquid argon calorimeters work by ionization of their active material and the free electrons are collected by electrodes at high voltages. Multiple samples of the analog signal are registered and digital signal processing techniques are used to extract its amplitude which is related to the energy deposited. The signal pulse is relatively long (up to 400 ns) and the probability of other physics events happening in the same detector area during that interval is high. A technique called Optimal Filter is used to minimize the effects of such pile-up. In the next data taking period, the luminosity will be higher and new techniques are being considered to mitigate the impact of the pile-up induced noise. Such techniques are also being envisaged for usage in the ATLAS trigger. This paper will discuss these techniques and their implementation in the ATCA standard electronics. A prototype using the processing power of FPGAs is being prepared for studies and planned to be used when the LHC returns to operation in 2015.