[Paper Review] Variations of the cosmic ray composition at energy above 0.1 EeV as observed by muon detectors of Yakutsk array

A.V. Glushkov, A. Sabourov|arXiv (Cornell University)|Nov 6, 2013

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study analyzes muon lateral distributions in extensive air showers (EAS) from the Yakutsk array (1987–2013) to infer cosmic ray (CR) composition above 0.1 EeV. Using muon detectors with 1.0×secθ GeV threshold, it finds that CR composition shifted from lighter to heavier nuclei after 1996, with QGSJETII-04 providing the best agreement with data, indicating a transition toward lighter nuclei at 1–20×10¹⁷ eV and a possible galactic-to-extragalactic transition.

ABSTRACT

The lateral distribution of muons with ~1.0x\\sec(\ heta) GeV in extensive air showers within ~10^{17}-10^{19} eV energy region obtained during different observational periods from November 1987 to June 2013 has been analyzed. Experimental data have been compared to predictions of various hadron interaction models. The best agreement is observed with QGSJETII-04. Until 1996, the mass composition of cosmic rays with energy below 2x10^{18} eV was significantly lighter than in later periods.

Motivation & Objective

• To determine the energy-dependent mass composition of ultra-high-energy cosmic rays (UHECR) above 0.1 EeV using muon data from the Yakutsk array.
• To investigate temporal variations in CR composition, particularly the shift observed after 1996 toward heavier nuclei.
• To evaluate the performance of hadronic interaction models (QGSJET01D, QGSJETII-04) in reproducing observed muon lateral distributions.
• To assess the implications of changing CR composition for the origin and propagation of UHECR, including possible galactic explosions or transitions to extragalactic sources.

Proposed method

• Measured lateral distributions of muons with energy threshold ~1.0×secθ GeV using underground muon detectors at the Yakutsk EAS array.
• Used charged particle density at 600 m (ρs,600) and Molier radius (RM) to estimate primary energy via calibrated relations (1)–(3), incorporating atmospheric corrections.
• Applied modified Linsley approximation (4) with temperature- and pressure-dependent Molier radius (5) to model lateral distribution functions (LDFs) of charged particles.
• Compared experimental muon densities (ρμ,300) with simulations from QGSJET01D and QGSJETII-04 models to infer mean atomic number ⟨lnA⟩ via weighting functions (10)–(11).
• Calibrated detector response and control systems using established procedures from prior Yakutsk studies (JETPl2012).
• Analyzed data in three observational periods: 1987–1996 (pre-1996), 1996–2000 (transition), and 2011–2013 (current state), to track temporal evolution.

Experimental results

Research questions

• RQ1 How has the cosmic ray mass composition evolved above 0.1 EeV from 1987 to 2013, as revealed by muon detection at Yakutsk?
• RQ2 Which hadronic interaction model—QGSJET01D or QGSJETII-04—best reproduces the observed muon lateral distributions in extensive air showers?
• RQ3 What evidence exists for a temporal shift in CR composition, particularly the transition from lighter to heavier nuclei after 1996?
• RQ4 How does the inferred mean atomic number ⟨lnA⟩ vary with primary energy in different observational periods?
• RQ5 What does the observed evolution in composition suggest about the origin and propagation of ultra-high-energy cosmic rays?

Key findings

• The QGSJETII-04 model shows the best agreement with experimental muon lateral distributions across the 10¹⁷–10¹⁹ eV energy range.
• From 1987 to 1996, the mean atomic number ⟨lnA⟩ was light, with ⟨lnA⟩ = 1.0 ± 0.4 at 10¹⁷ eV.
• After 1996, the composition became significantly heavier, with ⟨lnA⟩ = 3.0 ± 0.4 at 10¹⁷ eV during 2009–2013.
• Between 2009 and 2013, ⟨lnA⟩ decreased from 3.0 ± 0.4 at 10¹⁷ eV to 0.4 ± 0.4 at 2×10¹⁸ eV, indicating a shift toward lighter nuclei with increasing energy.
• At energies ≥2×10¹⁸ eV, the composition shows no significant change, with a slight trend toward heavier nuclei, consistent with global data sets.
• The observed temporal evolution supports a hypothesis of a past galactic event injecting heavy nuclei, followed by a transition to a lighter, possibly extragalactic, component at higher energies.