Science

Have Fermilab Scientists Broken Modern Physics?

The past half century has been relatively uneventful for scientist’s understanding of the subatomic world. Theories developed in the 1960s and early 1970s have been combined into what is now called the standard model of particle physics. While there are a few unexplained phenomena (for example dark matter and dark energy), scientists have tested predictions of the standard model against measurements and the theory has passed with flying colors. Well, except for a few loose ends, including a decade-old disagreement between data and theory pertaining to the magnetic properties of a subatomic particle called the muon. Scientists have waited for two decades to see if this discrepancy is real. And today, the wait is over. A new measurement has been announced that goes a long way towards telling us if the venerable theory will need revising.

Muons are ephemeral subatomic particles, much like the more familiar electron. Like their electron brethren, muons have electric charge and spin. They also decay in about a millionth of a second, which makes them challenging to study.

Objects that are both electrically charged and spin are also magnets, and muons are no exception. Physicists call the magnetic strength of a magnet made in this way the “magnetic moment” of a particle. One can predict the magnetic moment of both electrons and muons using the conventional quantum mechanics of the 1930s. However, when the first measurement of the magnetic moment of the electron was accomplished in 1948, it was 0.1% too high. The cause of this tiny discrepancy was traced to some truly odd quantum behavior. At the very smallest size scales, space is not quiescent. Instead, it’s a writhing mess, with pairs of particles and antimatter particles appearing and disappearing in the blink of an eye.

We can’t see this frenetic sea of objects appearing and disappearing, but if you accept that it is true and calculate its effect on the magnetic moment of both muon and electron, it is in exact agreement with the tiny, 0.1%, excess, first reported back in 1948.

In the intervening 70 years, scientists have both predicted and measured the magnetic moment of the both the muon and electron to a staggering precision of twelve digits of accuracy. And measurement and prediction agree, digit for digit, for the first ten digits. But they disagree for the last two. Furthermore, the disagreement is larger than can be explained by the uncertainty on either the prediction or measurement. It appears that the two disagree.

If data and theory disagree, one (or both) is wrong. It’s possible that the measurement was inaccurate in some way. It’s also possible that the calculation has an error, or the calculation doesn’t include all relevant effects. If that last option is true – overlooked effects – it means that the standard model of particle physics is incomplete. There is at least something new and unexpected.

For the past two decades, the best measurement of the magnetic moment of the muon is one made by the Muon g-2 experiment at Brookhaven National Laboratory, on Long Island, New York. (The experiment is pronounced “muon gee minus two.”) The “g-2” is historical and refers specifically only to the 0.1% excess over the prediction of standard quantum mechanics. Standard quantum mechanics predicts that the magnetic moment of the electron or muon is “g.”

The discrepancy between theory and measurement was pretty large. If you divided the difference by the combined experimental and theoretical uncertainty, the result was 3.7. Scientists call that ratio “sigma,” and use sigma to rate how important a measurement is. If a sigma is under 3, scientists say it is not interesting. If sigma is between 3 and 5, scientists start to get interested and call that state of affairs to be “evidence of a discovery.” If sigma is above 5, scientists are confident that the discrepancy is real and meaningful. For sigmas above 5, scientists usually title their papers as “Observation of…” Five sigma is a big deal.

So, the Muon g-2 experiment at Brookhaven reported a 3.7 sigma result, which is a big deal, but not big enough to be super excited. Another measurement was needed.

However, the accelerator facility at Brookhaven had done all it could do. A more powerful source of muons was needed. Enter Fermilab, America’s flagship particle physics laboratory, located just west of Chicago. Fermilab could make more muons than Brookhaven could.

So, researchers bundled up the g-2 apparatus and sent it to Fermilab. Because the g-2 apparatus is shaped like a plate, but 50’ across and 6’ thick, it couldn’t easily be shipped on roads. So, the equipment was put on a barge that went down the east coast of the U.S., up the Mississippi and some of its tributaries, until it was at a debarkation point near Fermilab in northeast Illinois. Then the equipment was put on a flatbed truck and driven in the dead of night to Fermilab. It took two nights, but on July 26, 2013, the g-2 experiment was located at Fermilab.

Scientists then set to work, building the buildings, accelerator, and infrastructure necessary to perform an improved measurement.  In the spring of 2018, the scientists began taking data. Each year, the experiment operates for many months, collecting data. Each year is called a “run” and the Fermilab Muon g-2 experiment is expected to make five runs, including a few in the future.

The measurement is incredibly precise. They are measuring something with twelve digits of accuracy. That is like measuring the distance around the Earth to a precision a little smaller than the thickness of a sheet of computer printer paper.

This recent measurement using the g-2 equipment at Fermilab confirmed the earlier measurement at Brookhaven. When the data from the two laboratories are combined, the discrepancy between data and theory is now 4.2 sigma, tantalizingly close to the desired “Observation of” standard, but not quite there.

On the other hand, the measurement reported today is based on a single run. Given improvements to the accelerator and facilities, researchers expect to record sixteen times more data than has been reported so far. If the measurement involving all of the data is consistent with the measurement reported today, and the precision of the measurement improves as expected, it is very likely that the g-2 experiment will definitively prove that the standard model is not a complete theory. That conclusion is premature, but it is looking likely.

So, what does this mean? The most robust conclusion one can draw is that if future measurements tell the same story, the standard model needs modification. It appears that there is something going on in the subatomic realm that is giving the muon a different magnetic moment than the standard model predicts.

What could that new physics be? Well, it is unlikely that the standard model will need to be completely discarded. It simply works too well on other measurements that aren’t quite as precise. What is more likely is that there exists an unknown class of subatomic particles that have not yet been discovered. One possibility is that an extension of the standard model, called supersymmetry, is true. If supersymmetry is real, it predicts twice as many subatomic particles as the standard model. In a pure supersymmetric theory, these new particles would have the same mass as the known ones, but this is ruled out by many measurements. However, there could be a modified version of supersymmetry, which makes the undiscovered cousin particles heavier than the known ones. If true, it would modify the prediction of the magnetic moment of the muon in just the right way to make data and theory agree.

But supersymmetry is just one possible explanation. The simple fact is that there could be many different kinds of subatomic particles that haven’t been discovered. Perhaps some new theory that explains dark matter might be relevant. Or something entirely unimagined by anyone at this point. We just don’t know.

But not knowing isn’t bad. It just means that there are new things to learn, problems to solve. Theoretical physicists are already thinking through what might be the implications of the new measurement and what sorts of theories might explain it. The important thing is to accept that a venerable and long-accepted theory is incomplete, and that we need to rethink things. That’s how science is done.

But I’m getting ahead of myself. The researchers need to analyze the other runs and verify that the more precise results validate today’s measurement. But things are definitely beginning to look interesting.

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