Search for Antihelium with the BESS-Polar Spectrometer

TL;DR

This study used the BESS-Polar spectrometer during two Antarctic balloon flights to search for antihelium in cosmic rays, setting the most stringent upper limits to date on antihelium presence.

Contribution

It provides the first high-sensitivity search for antihelium with the BESS-Polar spectrometer and establishes new upper limits on antihelium abundance in cosmic radiation.

Findings

No antihelium candidates were detected.

Established upper limits of 6.9 x 10^-8 and 1.0 x 10^-7 on antihelium abundance.

Set the most stringent limits to date on antihelium in cosmic rays.

Abstract

In two long-duration balloon flights over Antarctica, the BESS-Polar collaboration has searched for antihelium in the cosmic radiation with higher sensitivity than any reported investigation. 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 of 6.9 \times 10^-8 was determined by combining all the BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies from 1.6 to 14 GV, an upper limit of 1.0 \times 10^-7 was determined for the combined BESS-Polar data. These are the most stringent limits obtained to date.