Energy calibration of LHAASO-KM2A using the cosmic ray Moon shadow

TL;DR

The study leverages the Moon shadow effect as a physics-based anchor to calibrate the LHAASO-KM2A energy response in the $20$–$260\,\mathrm{TeV}$ range. By combining a detailed MC framework (CORSIKA with QGSJETII-04 and EPOS-LHC) and a robust ray-tracing approach through the geomagnetic field, the authors translate Moon-shadow shifts into an absolute energy scale, yielding a best-fit parameter $\epsilon = 0.015 \pm 0.08$ with a 95% CI of $[-14\%, +17\%]$. The analysis demonstrates strong agreement between data and KM2A–MC, establishes precise pointing and angular-resolution metrics (pointing accuracy $0.0002^\circ \pm 0.009^\circ$ and PSF width $\sigma_{\mathrm{PSF}} = 0.275^\circ \pm 0.009^\circ$), and confirms the energy estimator $N_{e\mu}$ as nearly composition-independent across the calibrated range. This method provides a robust, model-insensitive calibration for KM2A, minimizes MC-driven biases, and supports high-precision spectral and composition studies in the TeV regime.

Abstract

We present a precise measurement of the westward, rigidity-dependent shift of the Moon's shadow using three and a half years of cosmic-ray data collected by the Kilometer Square Array (KM2A) of the Large High Altitude Air Shower Observatory (LHAASO). These measurements enable us to calibrate the detector energy response in the range 20-260 TeV, with results showing excellent agreement with the response derived from Monte Carlo (MC) simulations of the KM2A detector. We also measure a best-fit parameter $ε= 0.015 \pm 0.08$, corresponding to a 95% confidence interval of [-14%, +17%] for the energy-scale estimation. This result establishes the exceptional accuracy of the KM2A-MC in simulating the detector's response within this energy range.

Figures (13)

• Figure 1: Layout of the LHAASO. The central square shows the WCDA (three pools). Surrounding it is the KM2A, consisting of 5,216 EDs; red dots and 1,188 underground MDs; blue circles. The WFCTA; black sectors are placed around the array and point toward the sky. The concentric black circles demonstrates only events with reconstructed shower core inside this region are retained for our analysis.
• Figure 2: Bin-wise aperture ($A_{\textrm{eff}}$) of the KM2A full-array for cosmic ray showers vs. the primary energy of each mass group, calculated with the EPOS-LHC interaction model. "All" represents the weighted values of all components according to the Gaisser model. The black horizontal line shows the calculated aperture $A_{0} = 0.16\, \text{km}^2\text{sr}$. The vertical black line marks the energy above which the detector is fully efficient for "All".
• Figure 3: Left: Significance map of the Moon region for events satisfying the event selection criteria in \ref{['subsec:event selection']}, observed by the LHAASO-KM2A for the period July 21-Jan 25 (1290) days. Middle: The shift along the Right-Ascension (RA) caused by the Moon shadow. Right: The shift along the Declination (DEC) direction.
• Figure 4: Bin-wise pointing accuracy of the detector.
• Figure 5: The effect of folding different contributions to the Moon signal. Left: effect of the GMF on an ideal detector without PSF. Middle: effect of the detector PSF with the GMF. Right: effect of smoothing procedure after applying GMF. The color scale represents the number of showers lying on the single pixel of the figure.