[Paper Review] Highlights from the Pierre Auger Observatory (ICRC17)

M. Unger|arXiv (Cornell University)|Oct 25, 2017

Astrophysics and Cosmic Phenomena2 references5 citations

TL;DR

This paper presents key results from the Pierre Auger Observatory up to 2016, including a precise measurement of the ultra-high-energy cosmic ray energy spectrum showing a break at ~5 EeV and flux suppression at ~40 EeV, evidence of mass composition changes, and large-scale anisotropy in arrival directions. The study uses hybrid SD/FD data and radio detection to probe hadronic interactions and cosmic ray origins, with the AugerPrime upgrade aiming to resolve the flux suppression origin and improve mass composition measurements via enhanced surface detectors and muon detection.

ABSTRACT

In this contribution we summarize the highlights from the Pierre Auger Observatory presented at the 35th International Cosmic Ray Conference. We discuss the update of the measurement of the energy spectrum of cosmic rays over a wide range of energy ($10^{17.5}$ to above $10^{20}$ eV), studies of the cosmic-ray mass composition with the fluorescence and surface detector of the Observatory, the discovery of a large-scale anisotropy in the arrival direction of cosmic rays above $8 imes 10^{18}$ eV and indications of anisotropy at intermediate angular scales above $4 imes 10^{19}$ eV. Moreover, we report on tests of hadronic interactions beyond LHC energies, multi-messenger analyses with neutral primaries and the progress of the upgrade of the Observatory, AugerPrime, aimed at elucidating the origin of the observed flux suppression at ultra-high energies.

Motivation & Objective

• To precisely measure the energy spectrum of ultra-high-energy cosmic rays (UHECRs) over 10^17.5 to >10^20 eV using hybrid SD and FD data.
• To determine the mass composition of UHECRs by disentangling electromagnetic and muonic shower components using surface detector upgrades.
• To investigate large-scale and intermediate-scale anisotropies in UHECR arrival directions to probe cosmic ray sources and propagation.
• To test hadronic interaction models beyond LHC energies using air shower data and constrain new physics such as Lorentz invariance violation.
• To address the origin of the flux suppression at ultra-high energies through the AugerPrime upgrade, which enhances mass and energy resolution.

Proposed method

• Combines data from the Surface Detector (SD) and Fluorescence Detector (FD) to measure shower development and energy, using a data-driven calibration method independent of hadronic model assumptions.
• Applies the method of constant intensities and footprint templates to correct shower size measurements for zenith angle dependence, enabling energy estimation.
• Uses simultaneous SD/FD events to calibrate surface detector shower size to absolute energy via fluorescence calorimetry and precise atmospheric monitoring.
• Employs the AugerPrime upgrade, adding 4 m² Surface Scintillator Detectors (SSDs) on top of existing Water-Cherenkov Detectors (WCDs) to separately measure electromagnetic and muonic shower components.
• Introduces new fast electronics and underground muon detectors to improve dynamic range and validate muon content extraction from combined WCD-SSD signals.
• Extends FD duty cycle by enabling operation during higher night sky background conditions to increase hybrid event statistics.

Experimental results

Research questions

• RQ1 What is the precise shape of the ultra-high-energy cosmic ray energy spectrum, and what causes the observed flux suppression at ~40 EeV?
• RQ2 How does the mass composition of cosmic rays evolve with energy, and what does this imply for their sources and propagation?
• RQ3 What is the origin of the large-scale anisotropy in UHECR arrival directions, and how do intermediate-scale anisotropies depend on particle rigidity?
• RQ4 Are current hadronic interaction models consistent with UHECR data, or do discrepancies point to new physics beyond the Standard Model?
• RQ5 Can the AugerPrime upgrade resolve the origin of the flux suppression by enabling event-by-event mass and energy measurements?

Key findings

• The energy spectrum shows a clear break at ~5 EeV (the ankle) and a flux suppression at ~40 EeV, consistent with the GZK cutoff or source energy limits.
• The average cosmic ray mass decreases from heavy to light composition around 10^18.3 eV, with possible hints of a composition change above 10^19.5 eV.
• A large-scale dipolar anisotropy of ~7% is observed in arrival directions above 8×10^18 eV, indicating anisotropic cosmic ray sources or propagation effects.
• Intermediate-scale anisotropies are observed above 4×10^19 eV, suggesting structure in the cosmic ray distribution at these energies.
• The proton-air cross section at √s = 57 TeV is consistent with LHC extrapolations, but surface detector data reveal inconsistencies in hadronic shower modeling.
• The AugerPrime Engineering Array has successfully demonstrated stable operation, linearity of scintillator signals, and accurate calibration, validating the design for full deployment.