In the First-Person Science series, scientists describe how they made significant discoveries over years of research. Chris Polly is a physicist at the Department of Energy’s Fermilab and co-spokesperson for the Muon g-2 project.
You might think that it’s possible for a particle to be alone in the world. You might think the deepest, darkest reaches of outer space are a very lonely environment indeed for particles.
But in fact, it’s not lonely at all. Because of the quantum world, we know every particle is surrounded by an entourage of other particles. These other particles don’t exist all the time. They blink in and out of existence.
We’ve discovered many of these particles, like the Higgs Boson, through physics experiments. But there may be ones out there that we haven’t discovered yet. Because these particles appear momentarily and disappear, they’re very hard to study. So we’re using a particle called a muon to better understand them.
Muons are 207 times heavier than electrons and 40,000 times better at helping us find new types of particles. They vanish quickly after they are produced in a high energy collision, leaving two neutrinos and an electron or positron in their place. In nature, muons come from cosmic rays striking our upper atmosphere.
Muons are particularly good for probing the Standard Model, the basic framework physicists use to describe how the universe works on the most fundamental level. While the Standard Model has held up to our experiments so far, we also know there are major aspects of physics it doesn’t explain.
We’re using a specific characteristic of the muon to explore these aspects. Many particles, including muons, have positive and negative poles like magnets. When these particles are in a magnetic field, their internal magnets try to line up with the magnetic field’s axis. At the same time, the particles are spinning, similar to a top. As a result, the muon spin’s axis can’t line up with the magnetic field. Instead, it wobbles around the magnetic field’s axis. Physicists use the letter “g” to represent a ratio that compares the muon’s magnetic strength and orientation to the speed of its spin. According to one important equation in the Standard Model, a bare muon by itself should have a “g” value of exactly two.
But experiments have shown us that it doesn’t. We know some of that difference comes from known particles that flicker in and out of existence. But they don’t explain the entire difference. The rest may come from particles that the Standard Model doesn’t yet describe, if there is a real difference between the laboratory observation and theory at all. Physicists found evidence of this difference in experiments at the Department of Energy’s (DOE) Brookhaven National Laboratory in the early 2000s. But the experiment wasn’t precise enough to know if there was a true difference or if it was from an experimental or theoretical error. Since then, we’ve kept doing more and more precise experiments to find out for sure.
“This Science Sounds Awesome”: 1996-1999
I came to muons via math. I had loved math as a kid and when I took physics in college, I realized that these applications are what math is for.
When I decided to pursue a graduate degree in physics, I wanted to go to the University of Illinois. I emailed the nuclear physics professors to ask if I could work with them over the summer. My future advisor sent me a three-page personal letter about how his team was running a muon experiment at the Department of Energy’s (DOE) Brookhaven National Laboratory. I thought, “This science sounds awesome. It’s like a dream come true for me.”
Physicists make muons by using a particle accelerator to speed up protons and hit them against a tiny target. That process produces pions, unstable particles that decay into muons. In the Brookhaven g-2 experiment, we smashed more than a trillion protons nearly instantaneously into a target made of nickel. The machine then funneled the muons into a storage ring with a specific magnetic field. As the beam moved around the field, the muon’s spins wobbled. Along the way, muons decayed into positrons. The positrons then spiraled into a calorimeter, a machine that measures the total energy of an incoming particle. We repeated this process many times per second for many months of running. How the positrons moved as they decayed told us about how the muon’s spin wobbled, which we used to calculate “g.”
When I joined the Brookhaven experiment, I worked on the team developing the calorimeters. When we tested them, we found we weren’t getting nearly the strength of signal that we expected. They were weak and puny. So we loaded four of them up in a U-Haul and I drove them back to Illinois, to work on at our laboratory at the University of Illinois. One of my greatest moments as a grad student was this “aha!” moment when I realized which component was broken. When we tore open the calorimeters, we found that a particular acrylic piece was damaged. You’re always supposed to handle them with gloves, so you don’t leave fingerprints on them. If you leave a fingerprint, it cracks the plastic and lets out light. That’s exactly what happened—somebody had touched it with their bare hands. We had to rebuild a bunch of them.
In addition to measuring the muons, we also had to measure the strength of the magnetic field around them in the Brookhaven accelerator ring. To do that, we used one set of probes in the storage ring that stayed in place while the muons were running around it and a second set on a trolley that moved around the ring when the muons weren’t there. Over the course of three years, while I was a graduate student, we took several thousand runs of data.
A Surprising Result and the Aftermath: 1999-2004
Our very first result was about six times more precise than the CERN experiment that had most recently measured the “g” value of the muon. It wasn’t the ultimate precision, but it was better than any prior experiment.
That interestingly revealed a three-sigma anomaly. Statistically speaking, a three-sigma result should only happen by mistake about three times out of a thousand times you run an experiment. We thought, “Wow, that is really interesting. This could really be a five-sigma sign of new physics.” Five-sigma is the requirement in physics to proclaim solid evidence of new physics. We had only analyzed a small part of the data, so we hadn’t expected to get significant results yet. Time Magazine even named us winners in their Winners and Losers list in 2001.
But as soon as you make an improvement in the experiment, it drives the theorists to create a competitive calculation. In the process, they realized the previous calculation had an error. Fixing it shifted the theoretical calculation towards the experimental observation, making the difference less significant. All of the hoopla about the three-sigma potential for new physics suddenly dropped down to more like two-sigma.
But we still had two more years of data to collect. To protect against bias, we purposely designed it so no one knew what the real results were until the end.
When we revealed the results, people from all over the world flew in to visit the lab. These experiments take decades to build and analyze, so you don’t get to go to very many of these events. We did a little “Drumroll, please” and then had the postdoc managing the spreadsheet hit the button to show it on the projector. Lo and behold, you could see that there was still a three-sigma discrepancy!
Even though our results were 14 times more precise than the last CERN experiment, we still didn’t know if that difference was because of something wrong in the experiment, something wrong in the theory, or a sign of new physics. Three-sigma is a viable threshold for when things start to become interesting and prompt a follow-up. But it’s not enough to claim discovery.
Since I was a graduate student, we’ve been left this mystery of, “Did we crawl up on the first signs of new physics entering in the g-2 quantum fluctuations or were we just unlucky?” Unlucky in the sense that it’s unlikely but still possible to have three-sigma and not discover new physics. For 20 years, I and my colleagues that were part of the Brookhaven experiment, as well as many other physicists around the world, have been waiting and wondering whether we saw a crack in the Standard Model or not.
A New G-2: 2011-2016
After the Brookhaven experiment ended, we kept thinking about how we might do an upgraded experiment. A team looked at doing it at Brookhaven, but concluded that it would be too expensive and not particularly compatible with their future program.
Meanwhile, I was doing neutrino physics at DOE’s Fermilab. Around 2009, it became clear that Fermilab was going to be shutting down the Tevatron, its particle accelerator. That opened up a world of possibility. At the same time, the much bigger Mu2e muon experiment was gaining a lot of traction at the lab. Mu2e was the elephant in the room and we were the small mouse running around saying, “Hey, what about g-2? We could do g-2!” We recycled 100 million dollars’ worth of equipment to make the new Muon Campus at Fermilab that now hosts both the Muon g-2 and Mu2e experiments.
Besides the equipment at Fermilab, the Muon g-2 experiment also reused the original storage ring from Brookhaven. Getting the storage ring from Brookhaven in Long Island to Fermilab outside of Chicago was a monumental task.
Prepared for transit, the ring looked exactly like a spaceship – a big disk with a cockpit in front of it. Because of a rumor a few years earlier that a UFO had been shot down and brought to Brookhaven National Lab, moving the ring sparked a lot of local interest. This thing was massive, so we brought it in the middle of the night down the William Floyd Parkway that runs through Long Island. People were lined up in lawn chairs along this stretch of road to see it leave. As it crawled down the highway, I walked along and talked to people about the science we were doing. Moving it through the Chicago suburbs to Fermilab offered another chance for outreach. It stayed over one night in a Costco parking lot. Well over a thousand people came out to see it and hear about the science.
Once we had all of the parts in place, we could finally put the whole thing together. Overall, Fermilab’s Muon g-2 experiment has a similar set-up to Brookhaven. One of the biggest challenges was being able to produce enough muons to take 21 times more data. To improve it, we looked at what systematic issues turned out to be the most important at Brookhaven and tackled them head-on. We built a lot of really nice pieces of scientific equipment that way. The particle detectors are exquisitely beautiful.
As we were putting it together, I became the project manager. During the construction phase, being project manager is what you want to be. There’s an art to collecting input from 180 people, providing feedback, and distilling things down to decisions that are made in consensus as a large group. That was a role I really enjoyed.
The Charlie Brown of Experiments: 2017-Present
We started collecting data three years ago. The first time we turned it on, we couldn’t store the muons and were like, “We can’t see anything.” Injecting the muon beam into the storage ring is like threading a needle with a thread that’s moving at 99.9% the speed of light. When we finally got the beam to store correctly, it was extremely exciting.
Now, the experiment is operating the way we wanted it to operate. Every month it’s running, we’re taking a set of data larger than the entire Brookhaven sample. Our latest run started and the experiment ran beautifully. Then it got shut down due to coronavirus. We’re the Charlie Brown of particle physics experiments.
Compared to the Brookhaven experiment, this experiment is rolling the dice 20 different times and seeing how often we get a discrepant result from the Standard Model. If that discrepancy stands the test of time, it will be between a five- to seven-sigma discrepancy at the end. That’s going from the chances of this error randomly happening from three in a thousand to less than one in a million. It’s been amazing to work with so many great people that in the end have come together and built something tremendous.
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