It is once again stocky particle spring. You might recall that, one year ago, researchers at Fermilab announced the discovery of a fat wobbly muon. Now, they have another offering for us: a boson with some chonk.
Yesterday, FermiLab researchers detailed in Science that their measurements for the mass of the W boson turned up a result that was pretty surprising—it was a tenth of a percent more massive than they expected. That doesn’t sound like a lot, but when you’re dealing with particles this small, apparently a little extra mass really matters. More tests are needed, but if it’s confirmed it could herald some changes to the standard model of physics.
In honor of this heaving boson, we reconnected with Emily Conover, a writer for Science News with a Ph.D. in particle physics from the University of Chicago, to help break it down for us. You can read her coverage of the discovery here.) This interview has been condensed and edited for clarity.
Sarah Braner: To start out … what actually happened?
Emily Conover: What they’re doing is, they’re smashing together protons and antiprotons, and this is at the Tevatron [accelerator] at Fermilab. One of the properties that you want to measure about your various particles is their masses, and so the W boson mass is particularly interesting because you can predict what you expect that mass to be from the standard model, which is this theory that particle physicists have that describes all the little elementary particles that make up basically everything.
And because you can predict that with the standard model, you can then compare that prediction to the measurement. That’s what they did here. And I should say that the particle accelerator that we’re talking about actually shut down in 2011, and it’s taken them this long to finish this measurement. That’s just how complicated it is to measure the mass of the W boson. So when they measured the particle’s mass and then they compared it to this prediction from the standard model, they found that it was about 0.1 percent more massive than they expected. And that’s very surprising, extremely surprising, because the standard model… everything that we’ve measured, almost everything has been pretty consistent with the standard model and there’s a few little slightly weird things. But this measurement was so precise, and even though 0.1% doesn’t sound like a big difference, it was enough that it was very significant that it disagreed with the standard model. And it’s really hard to find anything that disagrees with this theory. It’s a very successful theory. So that’s why people were surprised.
A tenth of a percent off doesn’t seem like a lot.
This measurement was so precise that even though 0.1 percent doesn’t sound like a big difference, it was enough that it was very significant that it disagreed with the standard model. And it’s really hard to find anything that disagrees with this theory. It’s a very successful theory. So that’s why people were surprised.
Is everyone sure that this measurement is correct?
Other experiments have measured the mass of the W boson recently, including some of those experiments at the Large Hadron Collider. And also this collaboration at Fermilab, which is called CDF, they have measured it previously too. And those measurements have been closer in line with the standard model. They haven’t been a perfect match, but you don’t always expect a perfect match because you have experimental error and statistics and all this. But those previous measurements were closer in line with the standard model, and there’s enough of a difference between the new measurement and those older measurements that some people are a little bit skeptical or a little bit like, “We need another experiment to confirm that this is real, that this measurement that they’ve made is correct.”
Let’s go back to the basics a bit: What is a boson?
There’s two classes of particles. There are the fermions, which are the stuff that makes up matter. Electrons are fermions, and those are found in atoms and quarks, in protons and neutrons. And then there’s the boson, and the bosons tend to be particles that don’t make up stuff, but they transmit forces, for example. And so the W boson is the particle that transmits this force called the weak force, and the weak force is responsible for certain types of radioactive decay.
So it’s something that interacts with matter, but isn’t actually matter.
Yeah. For example, when you have one of these types of radioactive decays, you have an atom that will decay and it’ll turn into a different type of atom. And if you zoom in really closely to what’s happening, there’s the W boson involved in that radioactive decay process. If your atom has more neutrons than it’s comfortable adding, if you want to personify the atom, one of the neutrons will convert into a proton, and at the same time you’ll have an electron that shoots out. And so that process, that decay on that scale is where the W boson comes in. When the proton converts into a neutron, there’s a W boson that exists for a very brief period of time. It’s like an intermediary in the process of decay.
How does one figure out the mass of a boson?
You slam a bunch of particles together. In the case of the experiment that we’re talking about today, it was protons and antiprotons. Then, when you collide stuff together sometimes you’ll produce W bosons, and those W bosons don’t last long enough that you can actually see the W boson itself, and you can’t weigh a W boson itself, but you can look at what that decays into. You might get electrons and neutrinos, or a heavier version of the electron called a muon. You look at those particles that are produced in that, when that W boson decays, and from there, looking at their energies and stuff like this, you can then back out the W boson mass.
I’ve seen a lot of people, or a lot of chatter, saying this could really mess with the standard model. If this 0.1 percent holds true if they test it again, it’s like, “yep, still 0.1 percent off,” what ramifications could that have for understanding the standard model?
It would be pretty big. I mean, it would mean there was something wrong with the standard model, unless there’s some weird error that somebody made.
Maybe if somebody made a goof in the prediction, that could explain it. But I don’t think that’s very likely, because people have done a lot of work to do that calculation, of the theoretical W boson mass. Lots of people have thought about that, I think. If we’re really certain that the prediction and the measurement disagree, it would mean there might be some new particles out there that we don’t know about, that are maybe too heavy to have been seen so far in our particle accelerators, which would be a big deal.
Although it’s so successful, there’s stuff that the standard model doesn’t explain. It doesn’t explain dark matter, which is this matter that we know exists out there in the universe because we see its gravitational effect on galaxies and other structures in the universe. And we know that it’s out there, but we know that it can’t be any of the particles that we know exist, that there must be some other particles out there, but we have no evidence of what exactly those particles are.
Physicists are really looking for places where the standard model could be wrong. And so, if this turns out to be a place where the standard model’s wrong, it’s a big deal because it’ll open a door to trying to understand why it’s wrong and then what particles might be responsible for it being wrong.