KASCADE-Grande - Contributions to the 32nd International Cosmic Ray Conference, Beijing, August, 2011

Contributions of the KASCADE-Grande Collaboration to the 32nd International Cosmic Ray Conference, Beijing, August, 2011.

💡 Research Summary

The paper summarizes the contributions of the KASCADE‑Grande collaboration presented at the 32nd International Cosmic Ray Conference in Beijing (2011). KASCADE‑Grande is an extension of the original KASCADE experiment, adding a large “Grande” array of 37 stations covering 0.5 km² to the existing 200 × 200 m² KASCADE setup. The combined detector system measures three main components of extensive air showers: the total charged particle number (N_ch), the electron component, and the muon component (N_μ). This multi‑detector approach allows simultaneous reconstruction of the primary cosmic‑ray energy and mass in the range from 10¹⁶ eV (the knee region) up to 10¹⁸ eV (approaching the ankle).

Data acquisition reached full trigger and reconstruction efficiency at ≈10¹⁶ eV. Core position resolution is about 5 m, angular resolution ≈0.7°, and the uncertainties on N_ch and N_μ are ~15 % and ~25 %, respectively. The analysis strategy relies on using several independent reconstruction methods on the same data set to cross‑check results and to assess systematic uncertainties, especially those arising from the choice of hadronic interaction models. The primary model used for most of the results is QGSJet‑II (with FLUKA for low‑energy interactions); EPOS and other models were also tested.

The all‑particle energy spectrum reconstructed with QGSJet‑II shows a generally smooth power‑law behavior but exhibits a subtle break at about 8 × 10¹⁶ eV. Below the break the spectral index is γ = −2.95 ± 0.05; above it, γ = −3.24 ± 0.08. The statistical significance of the change exceeds 2 σ, and the feature persists when alternative interaction models are employed, indicating that it is not an artefact of the simulation.

To investigate the origin of this structure, four independent composition analyses were performed:

1. Muon‑to‑charged‑particle ratio (N_μ/N_ch) – The distribution of this ratio was fitted in bins of N_ch with a linear combination of three mass groups (light, intermediate, heavy) derived from Monte‑Carlo simulations. The widths of the measured distributions require contributions from all three groups, and the resulting spectra show that the break in the all‑particle spectrum is driven by a reduction of the heavy component.

2. Y‑cut method – The parameter Y_CIC = log N_μ / log N_ch, corrected for atmospheric attenuation using the Constant Intensity Cut (CIC) technique, serves as a mass discriminator. Simulations provide expected Y_CIC distributions for different primaries; the data are split into electron‑rich (light) and electron‑poor (heavy) subsets. The heavy subset reproduces the spectral break, while the light subset remains smooth.

3. Electron‑rich / electron‑poor sample separation – Similar to the Y‑cut, this approach classifies events based on the relative size of the electron component, yielding consistent results: the heavy‑dominated sample shows the knee‑like feature, the light‑dominated sample does not.

4. Direct muon component measurement – Using the Muon Tracking Detector (MTD), the angular distribution (pseudorapidity) of muons was measured, providing an independent mass‑sensitive observable. Again, the heavy‑mass group exhibits the spectral steepening.

All four methods converge on the same conclusion: the observed structure at ~8 × 10¹⁶ eV is caused by a decrease in the flux of heavy nuclei (primarily iron‑group). The light component (protons and helium) continues with a nearly unchanged spectral index up to at least 10¹⁷ eV.

Systematic uncertainties were quantified. The overall flux uncertainty is about 10–15 %, while the composition fractions depend more strongly on the hadronic interaction model (variations of 20–30 %). Nonetheless, the heavy‑component suppression remains robust across models, suggesting a genuine astrophysical effect rather than a simulation artefact.

These findings support the rigidity‑dependent knee scenario: heavier nuclei, having larger charge, reach their acceleration limit at higher energies, so their individual “knees” appear at proportionally higher rigidities. The observed break therefore corresponds to the iron‑group knee, marking the onset of the transition from galactic to extragalactic cosmic rays. KASCADE‑Grande’s high‑precision measurements thus provide crucial experimental evidence for the composition‑dependent structure of the cosmic‑ray spectrum in the 10¹⁶–10¹⁸ eV range, bridging the gap between the well‑studied knee (≈3 PeV) and the ankle (≈3 EeV).