Anomalous APD Signals in the CMS Electromagnetic Calorimeter

Anomalous large signals are observed in the CMS Electromagnetic Calorimeter during proton-proton collisions at the Large Hadron Collider. They are ascribed to direct energy deposition by particles in the Avalanche Photodiodes (APDs) used for the light readout. Laboratory and test beam studies, as well as Monte Carlo simulations, have been used to understand their origin. The methods that have been employed to reject these signals in the online trigger of CMS and in the reconstruction of physics data are also presented.