An invariant distribution in static granular media T. Aste, T. Di Matteo, M. Saadatfar, T. J. Senden, Matthias Schröter and Harry L. Swinney EPL, 79 2 (2007) 24003 Published online: 03 July 2007 DOI: 10.1209/0295-5075/79/24003