A New Approach to Mass Measurements of UHECR: Horizontal Air Showers
  The role of Horizontal Showers induced by cosmic rays is discussed in detail. A new approach to the calculation of the muon component in horizontal air showers induced by protons, heavier nuclei or photons is presented which allows a simple analytical evaluation of the muon density profiles at ground level. The results of the first application of these results to horizontal air showers detected at the Haverah Park Array by the recently started {\sl Leeds-Santiago collaboration}, leading to important restrictions on composition at ultra high energies, are reported. 
Enrique Zas (Universidad de Santiago de Compostela)