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Probing hadronic interactions at the 100 TeV scale with the Pierre Auger Observatory

Eva dos Santos

TL;DR

Ultra-high-energy cosmic rays provide a unique probe of hadronic interactions at $\sqrt{s} \gtrsim 100\,\mathrm{TeV}$, where forward-QCD uncertainties and extrapolations challenge interpretation. The Pierre Auger Observatory, with its hybrid detection approach and the AugerPrime upgrade, enables multi-faceted tests of hadronic and electromagnetic cascades using observables like $X_{ m max}$ and $S(1000)$. Key results include a data-driven update of the $pp$ cross-section at high energies and indications that reconciling muon-content with shower depth may require deeper $X_{ m max}$ shifts and heavier inferred mass compositions, along with first neutron-content studies from AugerPrime. Overall, the work highlights residual tensions in hadronic-interaction modeling and points to AugerPrime as a path toward tighter constraints on hadronic- and EM-cascade physics at the 100 TeV scale.

Abstract

Extensive air showers produced by the interaction of ultra-high-energy cosmic rays ($E > 10^{18}$ eV) in the Earth's atmosphere provide a challenging yet unique channel to probe hadronic interactions at the 100 TeV center-of-mass energy scale. Over more than 20 years of operation, the Pierre Auger Observatory has delivered invaluable insights into the modeling of hadronic interactions at energies beyond human-made particle accelerators. Notably, predictions from current models of hadronic interactions yield a muon deficit that becomes more pronounced with energy when compared to measurements. Presently, the interpretation of the nuclear mass composition estimated from the muon content is in tension with that from direct measurements of the depth of the maximum of electromagnetic profiles. Yet, the measured fluctuations of the muon content of air showers are in agreement with model predictions. These findings hint at small deviations in hadronic models that accumulate throughout the whole shower development rather than at large errors in the calculation of the first hadronic interactions. Also, in an independent, data-driven analysis, we show that the muon deficit can be alleviated if we allow a shift of the predicted depth of the maximum of air-shower profiles by 30 -50 g cm$^{-2}$ towards a heavier mass composition. More recently, we have also provided an updated measurement of the proton-proton cross-section at a center-of-mass energy of 57 TeV and the first estimates of the neutron content of air showers by exploiting late-time signals from the surface scintillator detectors of AugerPrime, the present upgrade of the Observatory. With the advent of AugerPrime, we expect to deliver breakthrough results on the 100 TeV-scale hadronic interactions in the next decade.

Probing hadronic interactions at the 100 TeV scale with the Pierre Auger Observatory

TL;DR

Ultra-high-energy cosmic rays provide a unique probe of hadronic interactions at , where forward-QCD uncertainties and extrapolations challenge interpretation. The Pierre Auger Observatory, with its hybrid detection approach and the AugerPrime upgrade, enables multi-faceted tests of hadronic and electromagnetic cascades using observables like and . Key results include a data-driven update of the cross-section at high energies and indications that reconciling muon-content with shower depth may require deeper shifts and heavier inferred mass compositions, along with first neutron-content studies from AugerPrime. Overall, the work highlights residual tensions in hadronic-interaction modeling and points to AugerPrime as a path toward tighter constraints on hadronic- and EM-cascade physics at the 100 TeV scale.

Abstract

Extensive air showers produced by the interaction of ultra-high-energy cosmic rays ( eV) in the Earth's atmosphere provide a challenging yet unique channel to probe hadronic interactions at the 100 TeV center-of-mass energy scale. Over more than 20 years of operation, the Pierre Auger Observatory has delivered invaluable insights into the modeling of hadronic interactions at energies beyond human-made particle accelerators. Notably, predictions from current models of hadronic interactions yield a muon deficit that becomes more pronounced with energy when compared to measurements. Presently, the interpretation of the nuclear mass composition estimated from the muon content is in tension with that from direct measurements of the depth of the maximum of electromagnetic profiles. Yet, the measured fluctuations of the muon content of air showers are in agreement with model predictions. These findings hint at small deviations in hadronic models that accumulate throughout the whole shower development rather than at large errors in the calculation of the first hadronic interactions. Also, in an independent, data-driven analysis, we show that the muon deficit can be alleviated if we allow a shift of the predicted depth of the maximum of air-shower profiles by 30 -50 g cm towards a heavier mass composition. More recently, we have also provided an updated measurement of the proton-proton cross-section at a center-of-mass energy of 57 TeV and the first estimates of the neutron content of air showers by exploiting late-time signals from the surface scintillator detectors of AugerPrime, the present upgrade of the Observatory. With the advent of AugerPrime, we expect to deliver breakthrough results on the 100 TeV-scale hadronic interactions in the next decade.

Paper Structure

This paper contains 8 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Pierre Auger Observatory
  3. Exploring the 100 TeV energy scale with the Pierre Auger Observatory
  4. Inelastic proton-proton cross-section:
  5. Testing the electromagnetic and hadronic model predictions:
  6. Estimating the muon content at high-zenith angles:
  7. Summary and conclusions
  8. Acknowledgments:

Figures (2)

  • Figure 1: Inelastic proton-proton cross-section measurements for the current (orange circles) and previous (black star) methods. Predictions from several hadronic model predictions and accelerator data are also shown. The darker (light) shaded bands denote the statistical (total uncertainty) Tkachenko:2025coj.
  • Figure 2: Average (left) and fluctuations (middle) of the muon content as a function of the shower energy for hybrid events, PierreAuger:2014uczPierreAuger:2021qsd. Right: Same as the left panel, but replacing the FD by AERA, PierreAuger:2025kym.