Muon puzzle in inclined muon bundles detected by NEVOD-DECOR

TL;DR

This study analyzes inclined muon bundles from cosmic rays at energies around 1 EeV, revealing a muon excess that challenges existing models and providing new muon energy estimates to address the muon puzzle.

Contribution

It presents the first experimental estimates of muon energies in inclined air showers at energies from 10 to 1000 PeV, offering new insights into the muon puzzle.

Findings

Muon bundle intensity aligns with models only if primary cosmic rays are extremely heavy.

Discrepancy exists between muon data and fluorescence measurements on Xmax.

First estimates of muon energies in inclined air showers at high energies are provided.

Abstract

The data of cosmic ray NEVOD-DECOR experiment on the investigation of inclined muon bundles for a long time period (May 2012 - March 2021) are presented. The analysis showed that the observed intensity of muon bundles at primary cosmic ray energies of about 1 EeV and higher can be compatible with the expectation in frame of widely used hadron interaction models only under the assumption of an extremely heavy mass composition. This conclusion is consistent with data of several experiments on investigations of muon content in air showers, but contradicts the available fluorescence data on Xmax which favor a light mass composition at these energies. In order to clarify the nature of the "muon puzzle", investigations of the muon bundle energy deposit in the detector material were carried out. For the first time, experimental estimates of the average energy of muons in the bundles of inclined air showers initiated by primary particles with energies from 10 to 1000 PeV have been obtained.