No one has observed any evidence for proton decay. That might be disappointing professionally for physicists, but it's good news for the universe. If it turns out to be possible, proton decay could be the beginning of the end of everything. Here's why.
How do we start with protons and end with the end of the universe? We begin with what's in those protons. Inside protons are quarks. Quarks are one of the two most basic particles we can find. Quarks are subject to the strong force, the force that keeps a nucleus together. Each quark has essentially been assigned a baryon number of one third. The most famous baryons are protons and neutrons which have three quarks each, amounting to a baryon number of one. (Also famous are the antiprotons, which have a negative baryon number. So if a proton and antiproton were simultaneously created, the overall baryon number of the system is zero.) Because the charge of the quarks in protons and neutrons is a little different, the two particles have different charges. They also have slightly different masses. The neutron is a little chunkier, which means it can be involved in a change that involves the other fundamental piece of matter in the universe.
Leptons are separate from quarks. They are things like the electron, the neutrino, and their counterparts the antineutrino and antielectron. None of them are affected by the strong force. They have lepton numbers, and their anti-counterparts have negative lepton numbers.
Lepton numbers and baryon numbers seem meaningless, until you know that no reaction in the universe has ever been observed that changed the overall baryon or lepton number. This lead to the laws of conservation of baryon number and lepton number. Think of them like you would the conservation of energy and conservation of mass. A sudden change in lepton number would be like an apple just disappearing into nothing, or a burst of energy coming from nowhere.
Which is why scientists were so puzzled, at first, by the interaction of leptons and quarks in the decay of the neutron. When a neutron decays, it turns into a proton, and sheds an electron. Since a proton is positive, and an electron is negative, charge was conserved, but it seemed to scientists like lepton number had completely changed. Later, they realized that this decay involved the emission of an anti-neutrino (specifically, an anti-electron neutrino, which is a neutrino associated with electron interactions.) Since the electron had a lepton number of +1, and the anti-electron neutrino had a number of -1, the number was conserved, as was mass, as was charge. The decay entirely involved the weak force, which meant that the strong force wasn't messing around with any leptons. And all was well.
Protons are the lightest of the baryons. They can't shed anything else, unless their quarks dissolve into lighter particles. But this would subtract baryons and add leptons from out of nowhere. It was decided that it couldn't happen.
Then along came a little thing called the Grand Unified Theory. This is an unrealized theory that holds that all the forces can reach some level of equivalence, and can be explained with one unifying and quantifiable idea. It's very aesthetically pleasing. The problem is, if the strong and the weak force are equivalent, then leptons and baryons are equivalent as well. Remember the conservation of mass and the conservation of energy? This would be like Einstein's realization that E = mc^2, and mass and energy are equivalent - that one can be subbed in for another. Suddenly an apple could disappear, and a sudden burst of energy could appear. Matter could be converted into energy. Under the Grand Unified Theory, baryons could be converted into leptons. Baryon number and lepton number are no longer conserved.
Protons could then break down into positrons and pions. Although there are various mechanisms of proton decay, scientists think that protons have a life of about 10^25 to 10^33 years. Which is a pity, since at this point the universe will have plenty of problems of its own. By 10^30 years, the universe's stars will have first receded out of view of each other, and burned out until they went dark. Energy is what organizes atoms - gravitational energy that brings particles together to form stars and planets, solar energy that heats up planets gives live a chance. By then, the most intense bursts of energy will come from chunks matter falling into black holes. That might be the only way to wring energy from the universe. And it won't work, because the matter itself will simply dissolve. Once the baryons have flat-out fizzled into leptons, there's no way of getting them back without the input of a lot of energy. Proton decay means that any civilization - any matter at all - that makes it that long will literally dissolve as even hydrogen dissolves into smaller particles.
Not a cheery picture is it?
Top Image: NASA and The Hubble Heritage Team (STScI)
Second Image: NASA, ESA and The Hubble Heritage Team (STScI/AURA)
Third Image: NASA, JPL-Caltech, J. Rho (SSC/Caltech)