Determination of the absolute energy scale of the DAMPE calorimeter with the geomagnetic rigidity cutoff method

TL;DR

The paper addresses calibrating DAMPE's BGO calorimeter energy scale by exploiting the geomagnetic rigidity cutoff of CREs. It uses a backtracing approach in the geomagnetic field across four McIlwain $L$ bins to compare flight-data CRE spectral cutoffs with full on-orbit simulations, fitting $\Phi(E)$ with a cutoff model to extract $E_c$. The main finding is a scale factor of $1.013 \pm 0.012_{\rm stat} \pm 0.026_{\rm sys}$ in the $7$–$16$ GeV range, with consistent results across $L$ bins; the dominant systematics arise from the IGRF model and backtracing. The corrected energy scale has practical impact on inter-detector CRE flux comparisons and cross-validation with other measurements (e.g., AMS-02, Fe spectrum), contributing to a more accurate interpretation of DAMPE CRE data.

Abstract

The Dark Matter Particle Explorer (DAMPE) is a satellite-borne detector designed to detect high-energy cosmic ray particles with its core component being a BGO calorimeter capable of measuring energies from $\sim$GeV to $O(100)$ TeV. The 32 radiation lengths thickness of the calorimeter is designed to ensure full containment of showers produced by cosmic ray electrons and positrons (CREs) and $γ$-rays at energies below tens of TeV, providing high resolution in energy measurements. The absolute energy scale therefore becomes a crucial parameter for precise measurements of the CRE energy spectrum. The geomagnetic field induces a rapid drop in the low energy spectrum of electrons and positrons, a phenomenon that provides a method to determine the calorimeter's absolute energy scale. By comparing the cutoff energies of the measured spectra of CREs with those expected from the International Geomagnetic Reference Field model across 4 McIlwain $L$ bins - which cover most regions of the DAMPE orbit - we find that the calorimeter's absolute energy scale exceeds the calibration based on Geant4 simulation by $1.013\pm0.012_{\rm stat}\pm0.026_{\rm sys}$ for energies between 7 GeV and 16 GeV. The absolute energy scale should be taken into account when comparing the absolute CREs fluxes among different detectors.

Figures (5)

• Figure 1: The scatter plot of the PID parameter and reconstructed energy of the flight data. The horizontal dotted line represents ${\rm PID}=2$, which is used to select CREs and reject hadrons.
• Figure 2: (a) Monte Carlo template fitting to the PID distributions in the energy range for $11.2<E/{\rm GeV}<12.2$. Flight data (black dots) are fitted with CREs (red dotted line) and hadronic (green dotted line) templates. (b) Energy dependence of the hadronic background fraction (purple dots) and electron selection efficiency (red squares) for the $L$ bin $1.00<L<1.14$
• Figure 3: Left: the zenith distributions of flight data and full on-orbit simulation data. Zenith distribution of flight data is well described by full on-orbit simulation data. Middle: the azimuth distributions of primaries and secondaries in a typical energy bin near GRC. The East-West effect is well exhibited. Right: secondary fraction versus energy in one $L$ bin of $(1.00,1.14)$. As expected, the secondary fraction near the rigidity cutoff (indicated by the vertical line) is $\sim10\%$. With the decrease of energy, the secondary fraction becomes higher. Situations in the other 3 $L$ bins are similar with the results shown here. The secondary fraction at high-energy part does not drop to zero. The reason is that the hadronic background is not taken into account here.
• Figure 4: Primary CREs fluxes of flight data and full on-orbit simulation in 4 $L$ bins after subtracting hadronic and secondary backgrounds. Red (blue) dots denote the fluxes from flight data (full on-orbit simulation), the red solid line and the blue dashed line represent the best-fit curves for flight data and full on-orbit simulation, respectively.
• Figure 5: Ratios of GRCs of the flight data to the full on-orbit simulation expectation. The errors of GRC are given by spectral fitting, and the red error bars of ratios show the statistical uncertainties only, while the green error bars represent the quadratic sum of the systematic errors and statistical errors. The horizontal dashed line indicates the averaged ratio of $1.013\pm0.012(stat)\pm0.026(sys)$.