For the general reader interested in particle physics or astronomy:
Most of the Standard Model’s particles have a mass [a rest mass, to be precise], excepting only the photon (the particle of light) and the gluon (found in protons and neutrons.) For reasons not understood at all, these masses stretch out over a range of a trillion or more.
If it weren’t for the three types of neutrinos, the range would be a mere 400,000, from the top quark’s mass (172 GeV/c2) to the electron’s (0.000511 GeV/c2), still puzzling large. But neutrinos make the puzzle extreme! The universe’s properties strongly suggest that the largest mass among the neutrinos can’t be more than 0.0000000001 GeV/c2 , while other experiments tell us it can’t be too much less. The masses of the other two may be similar, or possibly much smaller.
This striking situation is illustrated in Figure 1, in which
- I’ve used a “logarithmic plot”, which compresses the vertical scale; if I used a regular “linear” plot, you’d see only the heaviest few masses, with the rest crushed to the bottom;
- For later use, I’ve divided the particles into two classes: “fermions” and “bosons”.
- Also, though some of these particles have separate anti-particles, I haven’t shown them; it wouldn’t add anything, since the anti-particle of any particle type has exactly the same mass.
As you can see, the neutrinos are way down at the bottom, far from everyone else? What’s up with that? The answer isn’t known; it’s part of ongoing research. But today I’ll tell you why
- once upon a time it was thought that the Standard Model solved this puzzle;
- today we know of two simple solutions to it, but don’t know which one is right;
- each of these requires a minor modification of the Standard Model: in one case a new type of particle, in another case a new phenomenon.