The results of an updated search for proton decay, as predicted by SUSY-GUTs, have been published in Physical Review D.
All materials in this world consist of atoms whose nuclei are made of protons and neutrons. In the field of elementary particle physics, heavy particles decays into lighter ones. Since the proton is a rather heavy particle a natural question arises: Will protons decay someday? Though the Standard Model of particle physics has now been completely established with the discovery of the Higgs boson and does not predict proton decay, extensions of this model known as Grand Unified Theories (GUTs), answer this question with a resounding “Yes!”
There are several GUTs and the simplest one predicts that a proton will decay into a positron and a neutral pion. However, Super-Kamiokande, the most sensitive proton decay detector in the world, has not yet observed this phenomenon and has thereby set a lower limit on the proton lifetime for this decay mode at 1034 years, long enough to reject the simplest GUT. On the other hand, GUT models in conjunction with the theory of Super-symmetry (SUSY) predict other decay modes, such as proton decay into a charged kaon and an anti-neutrino. Using 12 years of data the Super-Kamiokande collaboration has published a paper describing the search for a signal of this proton decay mode in Physical Review D. Particles predicted by SUSY models have been the target of intense searches by experiments at the LHC, a large collider in Europe, and this paper has accordingly been highlighted as an important result in the study of these theories.
If a proton decays into a charged kaon and an anti-neutrino, the kaon will not have enough momentum to emit Cherenkov light, the principal means in which Super-Kamiokande observes particles. As a result the decay products of the charged kaon (such as muons and pions) are used to search for the proton decay. There are two equivalent ways of searching for proton decay: Observe a single particle for a long time to see if it decays or observe many particles over a shorter time and see how many decay. Since there 7x1033 protons in Super-Kamiokande’s huge volume of water, the latter strategy is adopted. After 12 years of observation there has been no sign of this type of proton decay and a lower limit on the proton lifetime for this mode is set at 5.9x1033 years. This is the most stringent constraint on SUSY-GUT models to date.
The Super-Kamiokande collaboration will continue observations and the search for proton decay. In addition, the next generation water Cherenkov detector, Hyper-Kamiokande, will have 20 times more protons than Super-Kamiokande and is therefore expected to have dramatically improved sensitivity to proton decay.
- Paper submitted to Physical Review;
- The paper has been highlighted with Synopsis by editors.