Signals of phase transition in atmospheric showers
  We discuss collective effects like percolation, quark gluon plasma, or string fusion and their effect on the longitudinal development of high energy showers in air. It is shown that iron-air showers could develop more slowly than expected and produce the observed change in the slope of $X_max$ at the highest energies. 
C. Pajares, D. Sousa, and R.A. V\'azquez