The lowest-radiation environments in the Solar System: new opportunities for underground rare-event searches

TL;DR

This paper identifies low-radiation environments within the Solar System, especially underground lunar sites, as promising locations for advanced underground physics experiments due to significantly reduced cosmic-ray and natural radiation fluxes.

Contribution

It provides semi-analytical estimates and simulations showing how certain extraterrestrial underground sites offer drastically lower background radiation for fundamental physics searches.

Findings

Cosmic-ray-induced neutrino and muon fluxes are reduced by up to 10^3 in low-atmosphere environments.

Neutrino suppression increases with distance from the Sun, proportional to the square of the distance.

Proposes measuring muon and gamma-ray fluxes at lunar sites to test models and explore physics opportunities.

Abstract

We study neutrino, muon, and gamma-ray fluxes in extraterrestrial environments in our Solar System via semi-analytical estimates and Monte Carlo simulations. In sites with negligible atmosphere, we find a strong reduction in the cosmic-ray-induced neutrino and muon fluxes relative to their intensities on Earth. Neutrinos with energies between 50 MeV and 100 TeV show particularly strong suppression, by as much as 10$^3$, even at shallow depths. The solar neutrino suppression increases as the square of the site's distance from the Sun. Natural radiation due to nuclear decay is also expected to be lower in many of these locations and may be reduced to effectively negligible levels in the liquid water environments. The sites satisfying these characteristics represent an opportunity for greatly extending the physics reach of underground searches in fundamental physics, such as searches for WIMP Dark Matter, neutrinoless double-beta decay, the diffuse supernova neutrinos, and neutrinos from nearby supernova. As a potential near-term target, we propose a measurement of muon and gamma-ray fluxes in an accessible underground lunar site such as the Mare Tranquillitatis Pit to perform a first measurement of the prompt component in cosmic-ray-induced particle production, and to constrain lunar evolution models.