Cosmic rays were discovered at the beginning of the 20th century and their study has contributed extensively to science areas such as Astrophysics and Particle Physics. However, there are still big open questions. The sources and the acceleration mechanisms of ultra-high energy cosmic rays yet remain elusive.
To improve both descriptions, an accurate measurement of mass composition is required.
Furthermore, mass composition is closely related to the so-called \emph{muon puzzle}, which states that the current understanding of hadronic interactions is not enough to explain the results of several cosmic-ray experiments.
The main problem is that the muon content in extensive air shower simulations is significantly less than the one measured. Several exotic mechanisms have been proposed but improvements in measurements at colliders make them unlikely to be viable processes.
In hadron collisions the fraction of energy going into electromagnetic particles has a large impact on the number of muons produced in air shower cascades.
Therefore, any proposed model needs to find a way to transfer energy from the electromagnetic cascade to the hadronic one to increase the muon production.
We developed a new technique to quickly test on air showers the effect of any kind of modification of the hadronic interaction properties.
In this novel approach the secondary particle spectra in the CONEX framework are modified. There are many different ways of implementing these modifications, each representing different interesting scenarios.
As an application, we chose to test the core-corona model of heavy ion interactions on air shower development.
The basic assumption of the core-corona model is a mixture of different underlying particle production mechanisms such as a collective statistical hadronization in large density regions (core) in addition to the expected string fragmentation in low
density regions (corona).
Recent measurements at the LHC confirm features that can be linked to this model.
Since the two mechanisms present in the core-corona model imply different electromagnetic energy fractions, each mechanism impacts the muon production in extensive air showers in a different way.
Our tool allows, for the first time, to compare the energy evolution of muon-based observables accessible to experiments and to find the most adequate parameters for the core-corona model.
The simplified core-corona model based results were compatible with a significant
increase of the number of muons in simulations.
Hence, we have included direct muon measurements into our analysis and compared them with full end-to-end simulations in the context of the Pierre Auger Observatory.
As part of the ongoing upgrade of the Observatory, dubbed AugerPrime, the Underground Muon Detector (UMD) offers a unique and straight-forward opportunity to directly measure high-energy muons of extensive air showers.
Thus, the first two and a half years of data acquired with the array of the UMD have been reconstructed and analyzed.
In particular, we have built data driven average muon lateral distributions (MLDF) in bins of energy and zenith angle and we have compared them with full detector simulations in a core-corona scenario.
Thanks to a new technique providing fast and realistic air shower simulations with modified hadronic interaction properties, we showed that a model partially based on hadronization properties of a QGP applied gradually from low energy to the highest energy significantly reduces the muon deficit. Further detailed investigation is needed to determine if it can be integrated in a future hadronic interaction model
