A test for the existence of protons in ultra-high energy cosmic rays

The determination of the composition of cosmic rays at ultra-high energies is still a challenge in astroparticle physics. Beside of the determination of the overall composition the knowledge of the general existence of single elements, specifically protons, is sufficient for certain analyses. This work presents a method which is able to reject the helium origin of single very deep air shower events. Utilizing a parametrized distribution of shower maxima, predicted by different hadronic interaction models and CONEX, it is possible to determine a probability value of a helium nucleus to induce an air shower with the observed depth of shower maximum or deeper. If this derived probability value is small enough, the hypothesis of a helium primary can be rejected. Since heavier primaries than helium tend to produce air showers with lower depths than Helium, which leads to smaller probabilities for deep events, these possible primaries can be excluded as well. After the development of the method, a check of the method’s performance is presented and a first application to the data of the Pierre Auger Observatory is performed. The result shows that this method might be able to exclude the helium origin hypothesis with good significance.