How Sure are we of the Higgs?

“`It’s quite hard to destroy the Earth.’
Does that statement make anyone else nervous? I mean, does that sound like experience talking?”
from the comments on the LHC at slashdot

Last week, I started an open thread, giving you the chance to ask about how certain we were about the validity of certain physical theories. (The thread is still open, FYI.) Here’s the ranking scale I’m using, along with a few examples of where certain ideas currently fall.

And some of the most intriguing questions that came up were about the Higgs: what is it, how does it give mass to things, and is it even a certainty? Let’s take a deeper look, starting at the basics.

These are all of the different types of particles that make up the known Universe, and I’ll give you a basic introduction to them below. There are four different types:

  • The Quarks: In the upper left-hand corner, the quarks are tiny, massive subatomic particles. They make up protons, neutrons, and the nuclei of all the elements in the Universe. The also make up fun, weird little oddities (like mesons and heavy baryons) that we’ve discovered thanks to particle accelerators and cosmic rays. Each of the six quarks comes in three different colors (red, white, and blue), two different spins (+½ and -½), and in both matter and anti-matter varieties, for a total of 72 possible states. Ultimately, the quarks are responsible for the majority of the matter we know of in the Universe. (Not including dark matter.)
  • The Leptons: In the lower right-hand corner, the leptons are much lighter than their quark counterparts. The most famous, the electron, is responsible for electricity and for turning those protons, neutrons, and nuclei into atoms. The six leptons can all exist in matter and anti-matter varieties, but while the electron, muon, and tau can have two different spins (+½ and -½), the three neutrinos don’t get a choice; the “matter” neutrinos are all spin -½ and the “antimatter” ones are all spin +½. They all have masses, including neutrinos, which shouldn’t. (Indicating that there is something beyond the standard model.) All told, there are 15 different types of leptons.
  • The Force Carriers: On the right, these are the particles responsible for the forces between the quarks and leptons. There’s the photon, responsible for the electromagnetic force, which you know best in its role as light. It doesn’t come in matter and anti-matter varieties, though, as it’s its own antiparticle, and the only two states it can have are from its different spins (+1 and -1; 0 is forbidden because it’s massless). There are the W’s (electrically charged + and -, with spins +1, 0, and -1) and the Z (neutral, with spins +1, 0, and -1), the particles responsible for the weak force and for radioactive decay. And then there are the gluons, the massless “glue” that’s responsible for the strong nuclear force. It holds protons, neutrons, and nuclei together. Gluons also have two different spins (+1 and -1), eight different color combinations, and no electric charge. All told, this means there are 27 different states for the force carriers, which nails down all of the known forces except gravity.
  • The Higgs: Without this, the standard model doesn’t make sense. Yet, it’s the last part of it that hasn’t been discovered. If it exists, it would interact with (or “couple to”) the quarks, the charged leptons, and the weak force carriers, giving mass to all of them, but not with the photons or gluons, leaving them massless.

Now, those are the particles that live in the standard model’s zoo.

The quarks, leptons, and force carriers have all been discovered, and they’ve all been tested with extraordinary rigor. The last remaining mystery? Where does mass come from?

According to theory, the Higgs couples to quarks, leptons, and the W’s and Z’s, giving them mass, but not to photons or gluons. In theory, it also couples to itself, meaning that it, itself, has mass.

And what is that mass? Well, without having successfully made and detected one, we’re not sure. But, based on the standard model, we can place limits on its mass.

Before the LHC, there was LEP, which was the same underground, 27 km-long tunnel used for the LHC today. Only, instead of protons, they collided electrons and positrons. Based on measurements from LEP, the Higgs would have been discovered if it had a mass below 114 GeV, so we can be confident that if the Higgs exists, it’s heavier than this. (A GeV, by the way, is a unit of energy. A proton, for comparison, has a rest-mass energy of just less than 1 GeV, or 0.938 GeV, to be more precise.)

It also can’t be too heavy. Precise measurements of the electromagnetic and weak interactions tell us, that if the standard model is valid, the Higgs can’t be too heavy, either. From limits from multiple colliders, the Higgs must be lighter than 186 GeV as well. So there’s only a narrow range left where we can expect to find the Higgs. And it’s a question of who’ll get there first.

You may have seen, in the news this week, the Tevatron (the giant accelerator at Fermilab, in Batavia, Illinois) has ruled out part of that mass range! If the Higgs exists, it’s either lighter than 158 GeV or heavier than 175 GeV. The Tevatron will continue to both run and look at the data from its current run, and will either extend those limits or — hopefully — find the Higgs! But if not…

The LHC — the giant collider on the border between France and Switzerland — ought to find it, or be able to rule out the Standard Model’s Higgs entirely. Either way is incredibly interesting.

Although it’s still speculative, most of us expect that the Higgs will be found. Most of the theoretical particle physicists I know, in fact, expect that when it is found, it will weigh in somewhere around 140 GeV, although that’s based on some speculative assumptions. It’s hard to imagine that the standard model, good for all of these billions of collisions we’ve observed, would suddenly fail us now. But we have good imaginations.

  • Maybe the Higgs, the only fundamental scalar (completely spinless) particle in the standard model, is forbidden? We’ve never observed a fundamental scalar before, after all.
  • Maybe some of the standard model “particles” aren’t fundamental particles at all! Although these models are restricted, they’re certainly possible.
  • Or maybe, perhaps, there’s some really wild stuff going on, and that we don’t understand the origin of mass and the nature of matter nearly as well as we think we do.

What do I think?

I think that the Higgs, until we discover it directly, is still a speculative idea, but I think it’s a likely one. (The indirect evidence for it and against the Higgs-less models is pretty strong.) If I were a betting man, I’d say the odds were better than 50-50 that the Higgs exists, and that it exists in the mass range where we expect it.

But nature is often full of surprises, and one of the most interesting (theoretically, anyway) things that could happen would be for the Tevatron and the LHC to both find no Higgs.

So it’s speculative right now, but even if we find it, there’s still one more question that’s going to arise.

Why are the masses of these standard model particles so different from one another?

The electron weighs in at only half of an MeV (0.000511 GeV for those of you keeping score), while the top quark tips the scales at more than 170 GeV (173.1 GeV, give or take about 1.3 GeV). The Higgs must have different coupling constants to each of these particles, but the standard model offers no explanation for either why or how. Plus, even though this gives particles a “rest mass” (what some people term inertia), it doesn’t explain how things with mass gravitationally attract or interact. So even if we find the God particle, we still have a bunch left to learn.

Hope this has helped shed some light on the mysterious Higgs boson; what follow-up questions do you have?