Mitigation of Anomalous APD Signals in the CMS Electromagnetic Calorimeter

Anomalous, large signals are observed in the barrel region of the CMS electromagnetic calorimeter during pp collisions at the LHC. Laboratory and beam-test studies, as well as Monte Carlo simulations, have been used to understand their origin. They are ascribed to direct energy depositions by particles in the Avalanche Photo-Diodes used for the scintillation light readout. Their properties and rates are summarized. The methods employed to reject these signals in the online trigger selection and in the offline event reconstruction are presented.