Search for antihelium with the BESS-Polar spectrometer

Area of Science:

• Cosmic ray physics
• Particle astrophysics
• Antimatter research

Background:

• Cosmic rays provide a unique window into astrophysical phenomena and fundamental physics.
• The search for antimatter, such as antihelium, in cosmic radiation is crucial for testing theories of the early universe and particle physics.
• Previous experiments have set limits on antihelium abundance, but higher sensitivity is needed.

Purpose of the Study:

• To search for antihelium nuclei in cosmic radiation with unprecedented sensitivity.
• To set new upper limits on the abundance of antihelium relative to helium.
• To constrain models of cosmic ray propagation and exotic particle production.

Main Methods:

• Utilized two long-duration balloon flights over Antarctica (BESS-Polar I and II).
• Collected data on cosmic ray nuclei with |Z|=2 over extended observation periods (8.5 and 24.5 days).
• Analyzed data using a superconducting spectrometer to identify and quantify antihelium candidates.

Main Results:

• No antihelium candidates were found in either BESS-Polar I or II data.
• A 95% confidence upper limit for antihelium/helium abundance was determined to be 6.9×10⁻⁸ (combined BESS data).
• An upper limit of 1.0×10⁻⁷ was set for the combined BESS-Polar data (1.6–14 GV) without spectral assumptions.

Conclusions:

• The BESS-Polar experiment has established the most stringent upper limits to date on the abundance of antihelium in cosmic rays.
• These results place significant constraints on theories predicting antihelium production in astrophysical sources or from exotic physics.
• Further searches with enhanced sensitivity are warranted to probe lower antihelium abundances.