The SuperK collaboration has released a new proton decay search, looking for protons decaying to a neutrino and a kaon. SuperK (Super-Kamiokande) is a large water Cherenkov detector in the Kamioka mine near Toyama, Japan that has already been running for many years. In addition to results like this, SuperK is also used as the far detector for T2K.
Proton decay is not allowed by the Standard Model due to conservation of baryon number. Many extensions of the Standard Model, such as Grand Unified Theories (GUTs) that combine electroweak interactions with strong interactions at very high energy scales do have proton decay. GUTs typically propose that the new unified force have a larger symmetry group that breaks down through some process into the SU(3)xSU(2)xU(1) symmetry of the Standard Model. This is similar to how the SU(2) (weak isospin) x U(1) (hypercharge) symmetry group of the Standard Model is broken into two clearly distinct forces at energies much less than the electroweak scale of around 100 GeV. The W and Z bosons acquire masses (80.4 and 91.2 GeV, respectively) via the Higgs mechanism, while the choice of the Higgs vacuum expectation value (vev) and mixing angle between the weak isospin and hypercharge fields are chosen to give us a massless photon, a massive neutral Higgs, and massive W and Z bosons with different masses and couplings to other particles. The fact that the W and Z have mass and the photon does not ensures that below the electroweak scale, the weak force is much weaker than electromagnetism.
One of the simplest GUTs is an SU(5) symmetry, although this has been ruled out for some time. This theory has multiplets that include both leptons and quarks, and interactions conserve the difference between lepton number and baryon number rather than both separately. This allows for a proton to decay to modes including leptons. Other GUTs similarly include various channels for proton decay.
This result looks for a kaon in the final state and finds a lower limit on the proton lifetime through this channel of 5.9×1033 years: many orders of magnitude longer than the age of the universe.