Isotropy of cosmic rays beyond $10^{20}$ eV favors their heavy mass composition

Abstract: We report an estimation of the injected mass composition of ultra-high energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extra-galactic magnetic fields the results are consistent with a relatively heavy injected composition at E ~ 10 EeV that becomes lighter up to E ~ 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extra-galactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

• The paper demonstrates that isotropic arrival patterns in UHECRs reveal a transition to a heavy composition above 100 EeV.
• It employs a novel test-statistic model with a 14-year Telescope Array dataset to analyze cosmic ray mass by correlating with local universe structures.
• The findings challenge conventional proton dominance, suggesting a significant presence of iron nuclei at the highest energies.

Isotropy of Cosmic Rays Beyond 10^20 eV: Insights into Heavy Mass Composition

The paper under discussion presents an empirical investigation into the mass composition of ultra-high energy cosmic rays (UHECRs) with energies exceeding 10 EeV, with an emphasis on detecting isotropic patterns in cosmic ray arrival directions. The study utilizes data collected from the Telescope Array (TA) surface detector, comparing these observations to the large-scale structure (LSS) of the local universe to draw inferences about the mass composition of these cosmic rays.

The analysis indicates a transition from relatively heavy mass composition at intermediate energies around 10 EeV to predominantly lighter elements up to 100 EeV, reversing to a very heavy composition beyond 100 EeV. This result holds under the assumption of negligible extra-galactic magnetic fields (EGMF), even when considering the largest experimentally allowed EGMF values. The findings suggest a possible dominant presence of iron nuclei at the highest energies, challenging conventional expectations of proton dominance in UHECRs beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off.

Methodologically, this study leverages a novel approach where the isotropy of UHECR arrival directions is utilized as a proxy for mass composition analysis. The research relies on a statistically robust test-statistic (TS) model, which quantifies isotropic patterns vis-à-vis LSS and cross-references these data against various injected composition models to ascertain compatibility.

The Telescope Array's unique position as the largest cosmic-ray observatory in the Northern Hemisphere lends credibility to these findings due to its comprehensive 14-year dataset, covering a significant event energy spectrum. The experimental setup accounts for the synchronization of surface detectors, precise angular reconstruction, and calibration into calorimetric energy scales, ensuring analytical rigor in energy deflection estimations due to galactic and extra-galactic magnetic fields.

The study highlights certain tensions between different cosmic ray mass measurement models and the implications of these findings in understanding cosmic ray origins. The critical discussion on magnetic field uncertainties, both in the GMF and EGMF, underscores the complexity of these interpretations. Notably, while variations in GMF assumptions are shown to have minor repercussions on model compatibility, the potential influence of hypothetical strong EGMF could reconcile the isotropy results across wider energy scales with lighter composition models.

The implications of these findings are multifaceted. Practically, a heavier composition has consequences for cosmic ray detector designs and astrophysical source-modeling, impacting how these extremely high energy events are traced and analyzed. Theoretically, these outcomes necessitate a reevaluation of cosmic ray acceleration and propagation models, particularly concerning source distribution in the local universe.

In terms of future research directions, this study opens pathways for refining cosmic magnetic field models, enhancing UHECR detection mechanisms, and proceeding with more granular composition discrimination techniques. There is also the potential for leveraging cross-experimental collaborations between facilities such as Pierre Auger Observatory to corroborate these findings and explore the nuanced variance in northern versus southern hemisphere data.

In conclusion, this paper marks a pivotal contribution to understanding UHECR mass compositions, pivoting attention to the isotropy in cosmic ray arrival directions and substantiating the case for heavier nuclei presence at the highest recorded energies. As theoretical models evolve, these findings will serve as a benchmark for future cosmic ray physics discourse and experimentation.