The Unsinkable Standard Model

The big physics news of the week last week came while I was in transit on Wednesday: The MiniBooNE (the odd capitalization is because it’s sort of an acronym) neutrino experiment released their first results on the neutrino oscillation studies they’ve been doing, and found, well, nothing new. In contrast to a previous experiment that hinted at the possible existence of a fourth type of neutrinos, the MiniBooNE results were entirely consistent with having only the three previously known types. There’s a news article here, and one of the MiniBooNE experimenters did a excellent guest post explaining the results at Cosmic Variance.

I don’t have much to say about the results themselves because, well, I’m not actually a neutrino physicist. One thing about this that’s really interesting, though, is that it’s yet another “success” for the Standard Model of particle physics.

It occurs to me that the Standard Model is in kind of a unique position among scinetific theories. I know of lots of examples of theories that everybody thought were right that turned out to be wrong, and there are plenty of examples of theories that at least some people think are right but that they can’t prove right. The Standard Model is the only theory I can think of that everybody knows is wrong, but nobody can prove is wrong.

OK, “wrong” may be a little too strong– “incomplete” is probably a better word. The Standard Model consists of a set of twelve material particles: six quarks (up, down, strange, charm, top, and bottom) and six leptons (electron, muon, tau, and electron, muon and tau neutrinos) with their associated antiparticles. It also includes four forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force, plus their associated force carriers. Taken together, these particles and forces describe everything about the structure and organization of ordinary matter.

The problem is, they don’t explain everything. Most of the universe is made up of “dark matter” that we see only indirectly through its gravitational interactions with stars and galaxies. For various reasons, we know that this matter, whatever it is, can’t be made up of quarks, but beyond that, we have no idea what it is. There are lots of proposals of different sorts of particles not included in the Standard Model that could account for this extra mass, but nobody has ever conclusively seen one.

There’s also the question of mass: The Standard Model enumerates the particles and their masses, but doesn’t say why they have those masses. There’s a proposed mechanism by which fundamental particles could acquire their masses from interactions with another sort of particle– the interaction is called the “Higgs mechanism” and the particles are “Higgs bosons,” and there ought to be one for every type of material particle. Nobody has ever seen conclusive evidence of a Higgs boson, though, despite many active searches for them.

This is really a strange and awkward position to be in. The Standard Model works extremely well for those things that it describes, but we know it can’t be the whole story. And yet, every attempt to find physics beyond the Standard Model has come up empty. Nobody has yet found a particle or force that isn’t accounted for in the theory, despite a couple of decades’ worth of searching. In a certain sense, it’s a theory that works too well. We’ve got excellent indirect evidence that says it can’t be the whole story, but we can’t find any direct evidence of anything that doesn’t fit the theory.

It’s sort of like being in the early stages of one of those old Infocom text adventure games. We’ve explored all the obvious rooms, and picked up all the obvious items, but we haven’t really gotten anywhere. There’s got to be more to the game, because it takes up a lot of disk space, but we can’t find any way to get into any of the other rooms even though we know they have to be there…

I’m not sure what the particle physics analogue of being eaten by a grue is, though– loss of funding, maybe?