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

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 $O\left(100\right)\mathrm{TeV}$. 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 $\gamma$-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 $1.013\pm 0.012_{\mathrm{stat}}\pm 0.026_{\mathrm{sys}}$ 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.