Measurement of the Cosmic Ray Helium Energy Spectrum from 70 GeV to 80 TeV with the DAMPE Space Mission

The measurement of the energy spectrum of cosmic ray helium nuclei from 70 GeV to 80 TeV using 4.5 years of data recorded by the Dark Matter Particle Explorer (DAMPE) is reported in this work. A hardening of the spectrum is observed at an energy of about 1.3 TeV, similar to previous observations. In addition, a spectral softening at about 34 TeV is revealed for the first time with large statistics and well controlled systematic uncertainties, with an overall significance of $4.3\sigma$. The DAMPE spectral measurements of both cosmic protons and helium nuclei suggest a particle charge dependent softening energy, although with current uncertainties a dependence on the number of nucleons cannot be ruled out.

Figure 1

The distributions of PSD charge, defined as the minimum charge value of the two PSD layers, for events with deposited energy ranges 316–501 GeV (left), 1259–1995 GeV (middle), and 19.95–70.79 TeV (right). The flight data are shown in black points. The histograms show the distributions of the best-fit proton MC (blue), helium MC (green), and $\mathrm{proton}+\mathrm{helium}$ MC (red). The vertical dashed lines indicate the PSD charge range used to select helium candidate events.

Figure 2

Effective acceptance after all the analysis selections, as derived from the helium MC sample.

Figure 3

Helium spectrum weighted by $E^{2.6}$ (top panel) measured by DAMPE. In the bottom panel, we compare the DAMPE spectrum (converted to kinetic energy per nucleon assuming the AMS-02 measured ${}^3\mathrm{He}/{}^4\mathrm{He}$ isotope ratio [26]) with previous measurements by PAMELA [4], AMS-02 [6], CREAM-III [3], ATIC-2 [2], and NUCLEON (KLEM) [9]. Error bars of the DAMPE data show the statistical uncertainties. The inner and outer shaded bands denote the systematic uncertainties from the analysis (${\sigma }_{\mathrm{ana}}$) and the total systematic uncertainties including those from hadronic models $(\sqrt{{\sigma }_{\mathrm{ana}}^2+{\sigma }_{\mathrm{had}}^2})$. For the PAMELA and AMS-02 results, the error bars contain both the statistical and systematic uncertainties added in quadrature. For the other measurements, only the statistical uncertainties are shown.