Measurement of the proton-air cross section at 's POT. 1/2' = 57 TeV with the Pierre Auger Observatory - Abreu, P.; Albuquerque, Ivone Freire da Mota e; Souza, Vitor de; Gouffon, Philippe; Carvalho, W. Rodrigues de; Peixoto, C J Todero; Tridapalli, D B (Physical Review Letters, 2012)