The Muon Puzzle in cosmic-ray induced air showers and its connection to the Large Hadron Collider

Abstract

High-energy cosmic rays are observed indirectly by detecting the extensive air showers initiated in Earth’s atmosphere. The interpretation of these observations relies on accurate models of air shower physics, which is a challenge and an opportunity to test QCD under extreme conditions. Air showers are hadronic cascades, which give rise to a muon component through hadron decays. The muon number is a key observable to infer the mass composition of cosmic rays. Air shower simulations with state-of-the-art QCD models show a significant muon deficit with respect to measurements; this is called the Muon Puzzle. By eliminating other possibilities, we conclude that the most plausible cause for the muon discrepancy is a deviation in the composition of secondary particles produced in high-energy hadronic interactions from current model predictions. The muon discrepancy starts at the TeV scale, which suggests that this deviation is observable at the Large Hadron Collider. An enhancement of strangeness production has been observed at the LHC in high-density events, which can potentially explain the puzzle, but the impact of the effect on forward produced hadrons needs further study, in particular with future data from oxygen beam collisions.

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

“…In the first decade of operation of Auger, the proton-air and proton-proton cross sections at energies well-beyond the reach of the Large Hadron Collider (LHC) were measured for the first time [40,41], and most top-down scenarios arising from BSM physics were strongly constrained through strict limits on the UHE photon flux [42]. However, systematic studies have confirmed earlier observations of a muon excess in the data (or a muon deficit in the EAS physics models) [19-21, 43, 44], hinting at some processes in the accelerator-based hadronic interaction models that have not been taken into account [7,8]. The quality of measurements obtained by current UHECR experiments enables narrowing down the potential root causes of the muon problem, thereby informing new investigations to be performed at accelerators.…”

“…However, in this work event-by-event mass resolution is defined solely by the second definition as it represents a significant improvement over current capabilities and therefore represents a major goal for the field. Precise mass determination is currently limited by the systematic uncertainties between hadronic model predictions and the known issues with the modeling of the EAS muon component for example [8,32]. Over the last few years, some hadronic models, such as EPOS-LHC [45] or the latest version of Sibyll [46], have integrated new accelerator data, especially from the LHC, but more heavy ion data need to be collected.…”

“…This is discussed in Ch. 8. First of all, the scientific community needs to become a model for diversity, equity, inclusion, and accessibility, in which underrepresented groups not only feel welcomed and supported, but are actively provided with opportunities to succeed.…”

“…The cosmic ray community has developed sophisticated simulation packages that integrate state-of-the-art models of electromagnetic and hadronic interactions (see e.g., Ref. [8] for a recent review). Commonly used hadronic interaction models are Sibyll [46,[281][282][283], QGSJet [284][285][286][287], EPOS [45,[288][289][290], and DPMJet [291][292][293][294][295].…”

Charged hadron production at LHCb

“…Monte-Carlo event generators are generally used for those predictions, but the lack of hadron production data at forward rapidity complicates their improvement. This has an impact in experimental high-energy physics at LHC, but also in cosmic-ray physics, where these models are fundamental to study the atmospheric evolution of hadronic cascades from high-energy cosmic rays [1].…”