A new observable in extensive air showers
  We find that the ratio r_{mu e} of the muon to the electromagnetic component of an extended air shower at the ground level provides an indirect measure of the depth X_{max} of the shower maximum. This result, obtained with the air-shower code AIRES, is independent of the hadronic model used in the simulation. We show that the value of r_{mu e} in a particular shower discriminates its proton or iron nature with a 98% efficiency. We also show that the eventual production of forward heavy quarks inside the shower may introduce anomalous values of r_{mu e} in isolated events. 
C.A. Garc\'ia Canal, J.I. Illana, M. Masip, S.J. Sciutto
astro-ph.HE hep-ph