The QCD Black Disk Limit in Cosmic Ray Air Showers
  We discuss particle production in the high energy limit of QCD. Due to a large gluon density, the interaction reaches the black disk limit and the projectile is resolved into its partonic structure at the saturation scale. This leads to suppression of forward particle production and hereby to a faster absorption of cosmic ray air showers. This property is most suitable for the distinction of evolution scenarios for the saturation scale, e.g. fixed and running coupling BFKL, the latter of which is favored by air shower measurements. 
H.J. Drescher