The knee and beyond: A review of recent results

Primary cosmic rays of energy greater than ∼ 1014eV must be studied by ground based experiments measuring the particles generated in the EAS (Extensive Air Shower) development in atmosphere. These experiments are mainly limited by the systematic uncertainties on an event due to their energy calibration. I will discuss the main sources of these errors: the choice of the hadronic interaction model and of the mass of the primary particle (that cannot be measured on a event by event basis). I will then summarize some of the more recent measurements of the all particle spectrum, and I will show that, keeping into account the differences due to the energy calibration, they all agree on the spectral shape. In addition to the well known change of slope, the so called knee, two faint but significant structures have been measured by different experiments: a concavity around 1016eV and a steepening at ∼ 1017eV. Then I will describe the measurements of the spectra of light and heavy primaries, discussing the claimed spectral features. Using a simple calculation of the elemental spectra (based on the hypothesis that the knee energies follow a Peter's cycle) I will try to discuss if and how these results can be interpreted in a single picture.