Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) collaboration has searched for antihelium in the cosmic radiation with the highest sensitivity reported. BESS-Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007–2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among $8.4\times {10}^6$ $|Z|=2$ nuclei from 1.0 to 20 GV or in BESS-Polar II data among $4.0\times {10}^7$ $|Z|=2$ nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit to the possible abundance of antihelium relative to helium of $6.9\times {10}^{-8}$ was determined combining all BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies for each flight, an upper limit of $1.0\times {10}^{-7}$ from 1.6 to 14 GV was determined for the combined BESS-Polar data. Under both antihelium spectral assumptions, these are the lowest limits obtained to date.

Figure 1

Cross-sectional and side views of the BESS-Polar II Spectrometer.

Figure 2

Selection of He ($\overline{\mathrm{He}}$) in BESS-Polar II. The upper panel shows ${\beta }^{-1}$ vs absolute rigidity. The lower two panels show $dE/dx$ from the TOF vs absolute rigidity. The $|Z|=2$ particles are between the lines.

Figure 3

$R^{-1}$ distribution of $|Z|=2$ events for BESS-Polar II. The negative $1/R$ events are spillover of He due to the finite spectrometer resolution. The “bump” around $1/R\sim 1.4$ is due to differing range-energy relationships of ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$.

Figure 4

The new upper limits of $\overline{\mathrm{He}}/\mathrm{He}$ at the TOA calculated assuming the same energy spectrum for $\overline{\mathrm{He}}$ as for He with previous experimental results ([13, 20, 21, 22, 23, 24, 25, 26]). The limit calculated with no spectral assumption is about 25% higher.