Speaker
Description
The search for origin of the most energetic cosmic rays has been mainly obscured by uncertainties in their mass composition arising from the modelling of hadronic interactions in the air showers that these particles induce. For some time now, discrepancies between the model predictions and measured air-shower data have been complicating efforts to find a consistent mass-composition scenario of ultra-high-energy cosmic rays (UHECR, above $10^{18}$ eV). A deficit of the simulated signal relative to the measured signal in ground detectors is an inconsistency that is usually interpreted as a deficit of the muon signal induced by the hadronic component of a simulated shower. Recent advances in testing models of hadronic interactions revealed a new problem in the predicted scale of the depth of shower maximum ($X_\text{max}$), which is usually considered as the best estimator of the cosmic-ray mass. The $X_\text{max}$ scale seems to be significantly underestimated in most of the hadronic interaction models.
I will briefly review tests of models of hadronic interactions using UHECR data implying that the predicted $X_\text{max}$ scale could be at a level that is consistent with a very heavy mass composition at the highest energies. I will also introduce the MOdified Characteristics of Hadronic Interactions (MOCHI) project, which aims to explore the phase space of combinations of modifications in the cross section, multiplicity and elasticity of hadronic interactions to find qualitative directions of modifications that could explain the discrepancies between the UHECR measurements and model predictions. Finally, I will introduce an extreme Heavy-metal scenario of UHECR assuming pure iron nuclei in the flux-suppression region (above $10^{19.6}$ eV). Such a scenario might be a viable option in the nature given the recent advances in the air-shower modelling.