Probing forward hadron production in proton-air interactions at √s ~ 100 TeV using data from the Pierre Auger Observatory

Ultra-high-energy cosmic rays (UHECRs) provide a unique opportunity to probe hadronic interactions at center-of-mass energies around 100 TeV.  Their interactions with the atmosphere initiate extensive air showers, which are driven by particle production in the forward kinematic region of hadron-nucleus collisions. The lack of an ab initio description of these interactions makes air-shower simulations reliant on phenomenological hadronic interaction models, which must extrapolate accelerator measurements into poorly constrained regions of energy–momentum phase space. These uncertainties contribute to longstanding tensions between air-shower observations and model predictions, most notably the so-called Muon Puzzle.

In this seminar, I will present a nearly universal relation between the average depth of shower maximum, ⟨Xmax⟩, and moments of the secondary-particle energy distributions in the first proton–air interaction, which persists despite substantial differences between hadronic interaction models. I will show how this universality allows us to interpret recent ⟨Xmax⟩ measurements from the Pierre Auger Observatory, the world's largest cosmic-ray observatory, in terms of a larger fraction of energy retained in hadronically interacting particles and a shift in hadronic energy flow towards higher rapidities. This interpretation is also consistent with the Muon Puzzle.