Probes of New Physics from Deep Underground

The SNO+ Experiment, over a mile underground, places new limits on grand unified theories and studies neutrinos from the Sun

Image courtesy of the SNO+ Collaboration
Fig. 1: Photograph of the upgraded Sudbury Neutrino Observatory (SNO+) Detector taken from inside the cavern.

The Science

The SNO+ experiment has made new measurements of the lifetime of the proton. It also measured how the flow of solar neutrinos changes over time as well as the energy spectrum of those neutrinos. Neutrinos are extremely light, uncharged subatomic particles. Solar neutrinos are produced in the core of the Sun through nuclear fusion reactions and are the most common type striking the Earth. SNO+ is located 2 km (~ 1.2 mi) underground at SNOLAB in Sudbury, Ontario. It can take these measurements because of its ultra-low levels of background contamination and high-precision detector.

The Impact

Scientists have long sought to understand if there is a single theory that can explain all particle interactions at all scales. Many such “Grand-Unified Theories” predict processes where baryons, such as protons or neutrons, would decay into leptons, such as electrons or neutrinos. SNO+’s measurements of the lifetime of the proton put new limits on one such process. It reveals that some decay processes that scientists have hypothesized are actually impossible. SNO+ has also made new measurements of solar neutrinos. Solar neutrino observations allow scientists to probe the interactions within the sun’s core.


A new limit on the partial lifetime of the proton through the so-called “invisible” decay modes has been set at greater than 3.6 x 1029 years by the SNO+ experiment using its initial 114 day dataset. The SNO+ detector, located 2 km (~ 1.2 mi) underground at SNOLAB in Sudbury, Ontario, consists of a 12-meter diameter spherical acrylic vessel filled with ultra-pure water. Visible light, known as Cherenkov radiation, emitted by particle interactions within the water is collected by roughly 9400 8-inch photomultiplier tubes and is used to extract details about the decay. The decay is labelled as “invisible” because the decay products are not easily observed within the detector. However, since the decay occurs within an oxygen nucleus, the subsequent nuclear de-excitation emits gamma rays in the 6-MeV range, for which the SNO+ detector has very high detection efficiency. Searches for the decay of protons and bound neutrons have been ongoing for the past few decades and the discovery of such phenomena would provide key insight into physics beyond the Standard Model.

The deep location and ultra-clean environment also enabled a highly accurate measurement of the solar neutrino flux above 5 MeV. Low backgrounds will be critical for the future program, which includes a search for neutrinoless double beta-decay from tellurium suspended in liquid scintillator. This is a rare process, possible only if the neutrino is its own antiparticle.


Gabriel D. Orebi Gann
Lawrence Berkeley National Laboratory and University of California, Berkeley


The funding for this research was provided by agencies in Canada, Germany, Mexico, Portugal, the United Kingdom, and the United States. In the United States, the funding sources include Department of Energy Office of Nuclear Physics, National Science Foundation, the University of California, Berkeley, and Department of Energy National Nuclear Security Administration through the Nuclear Science and Security Consortium, as well as computing support from Berkeley Research Computing program at the University of California, Berkeley and long-term storage by the Fermilab Scientific Computing Division. 


M. Anderson et al. (The SNO+ Collaboration), “Search for invisible modes of nucleon decay in water with the SNO+ detector”, Phys. Rev. D 99, 032008 (2019) [DOI: 10.1103/PhysRevD.99.032008]

M. Anderson et al. (The SNO+ Collaboration), “Measurement of the 8B solar neutrino flux in SNO+ with very low backgrounds”, Phys. Rev. D 99, 012012 (2019) [DOI: 10.1103/PhysRevD.99.012012]

Highlight Categories

Program: NP

Performer: University , DOE Laboratory

Additional: Collaborations , International Collaboration