Jan. 11, 2017
The Super-Kamiokande collaboration has published the new results on search for proton decay on January 7th 2017.
The missing pierce of the Standard Model, Higgs boson has been found and the Standard Model has been fully established. However, the Standard Model doesn't have answer to some fundamental questions like why leptons and quarks are separated, or why three interactions exist. Therefore, the Standard Model is regarded as a part of larger theoretical frame, the Grand Unified Theory. This ultimate theory predicts that protons, which have been believed as indestructible, will decay in someday. Materials are made of atoms, an atom comes from a nuclear and electrons, and a nuclear consists of protons and neutrons. So if protons decay, it means all materials in the world will be destroyed and vanished.
The Super-Kamiokande experiment group has been trying to find out proton decay events in 50,000-ton water Cherenkov detector. There are many proton decay modes expected, but many GUT models predict a proton decaying into positron and a neutral pion is the most dominant mode. The neutral pion decays into two gamma rays immediately, thus, involving positron, we can observe three Cherenkov rings in the detector.Atmospheric neutrino interactions could be background of the proton decay searches. A new technique has been employed to the analysis method to suppress the background. Neutrons often accompany with the atmospheric neutrino interactions and a neutron is slowing down in water, and is captured by hydrogen, making deuteron and gamma ray. We succeeded to reduce the atmospheric neutrino background by half by tagging neutrons with the delayed gamma rays. Also we developed another new technique to improve sensitivity for the proton decay. There are 10 protons in a water molecular, 8 in oxygen and 2 for hydrogen. Protons in nucleus have fermi momentum, thus proton decay in oxygen has relatively larger background than hydrogen case. We introduced new analysis method which separates proton decay in oxygen and hydrogen by total momentum. It improves discovery potential of the proton decay.
As results with about 300 kiloton・year data of Super-Kamiokande, unfortunately, we could not find any candidate events for proton decaying into positron and neutral pion, and we set lower life time limit of proton as > 1.6x1034 years. The alternative mode in which positron is replaced by muon have two candidates but it is consistent with expected background.
Then, it seems the world end happens in very far future. You feel relieved?
This article was published from Physical Review D and selected as "Editor's Suggestion".