The trigger system of the Pierre Auger Surface Detector: operation,   efficiency and stability
  The trigger system of the Surface Detector (SD) of the Pierre Auger Observatory is described, from the identification of candidate showers ($E>1$ EeV) at the level of a single station, among a huge background (mainly single muons), up to the selection of real events and the rejection of random coincidences at a higher central trigger level (including the reconstruction accuracy). Efficiency of a single station trigger is evaluated using real data , and the high performance of event selection hierarchy will be demonstrated. 
D.Allard, E. Armengaud, I. Allekotte, P. Allison, J. Aublin, M. Ave,   P. Bauleo, J. Beatty, T. Beau, X. Bertou, P. Billoir,...