Measurement of the proton-air cross section at √s=57 TeV with the Pierre Auger Observatory. by Abreu, P, Aglietta, M, Ahn, E, Albuquerque, I, Allard, D, Allekotte, I, Allen, J, Allison, P, Almeda, A, Alvarez Castillo, J, Alvarez-Muñiz, J, Ambrosio, M, Aminaei, A, Anchordoqui, L, Andringa, S, Antičić, T, Aramo, C, Arganda, E, Arqueros, F, Asorey, H, Assis, P, Aublin, J, Ave, M, Avenier, M, Avila, G