This past week was an exciting one for scientists. No, not only because of the breathtaking launch of SpaceX’s Falcon Heavy rocket, but also because a new, ultra-precise, scientific facility is beginning operations at Fermi National Accelerator Laboratory, just outside Chicago. The facility, called the g-2 experiment (pronounced “g minus 2”) could lead to the world’s scientific community rewriting the textbooks.
The universe around us is an extraordinary place and it is equally amazing that we small humans can understand so much about it. We can use powerful telescopes to peer billions of light-years away and see the signature of its creation. We can use giant atom smashers to determine the building blocks of the cosmos and literally recreate the conditions present at the Big Bang. Humanity has learned an awful lot about the story of our existence.
But we don’t know everything. There are mysteries that keep scientists awake at night. We don’t know the exact nature of dark matter and dark energy, which make up 95% of the matter and energy in the universe. We don’t know what caused the universe to begin. We don’t know a lot of things.
Scientists like myself don’t like being ignorant, so we are constantly making measurements that might lift the veil on at least some of our ignorance. A particularly interesting measurement was made back in 2001 at Brookhaven National Laboratory on Long Island. Using the same g-2 equipment, researchers measured very precisely a property of a subatomic particle called the muon and the measurement disagreed dramatically from what scientists had predicted.
Muons are subatomic particles, essentially pudgy cousins of electrons. Because they both spin and have electric charge, each muon is a little magnet. And if you put a magnet in a magnetic field, it will wobble like a top. It was the rate of wobble that was measured.
Wobbling muons sounds like an esoteric thing to study, but there are many reasons why this measurement is really fascinating. First, the wobble of the muon is affected by what is called the quantum foam. The quantum foam is a crazy-sounding idea that, at the subatomic level, particles are flickering into and out of existence in far faster than the blink of an eye. This frenzied dance is occurring at every point of space, including the one you’re sitting in right now. You can kind of picture what it is like by imagining bubbles in the foam of the head of a good pint of Guinness — hence the term “quantum foam.”
In the foam, all possible subatomic particles briefly appear and disappear. Thus, when this measurement and calculation disagree, it could be because the calculation only takes into account known particles and that the discrepancy is caused by unknown particles that were not taken into account.
It’s kind of like if a rich couple has a staff to clean their house. If the house is cleaner than can be accounted for by the staff they see during the day, perhaps there is a night shift that they didn’t know about. If this is the cause of this discrepancy, it will cause us to rewrite our most fundamental theories about the very building blocks of the universe.
Another very interesting facet of the measurement of wobbling muons is its precison. Scientists at Brookhaven made one of the most precise measurements in modern science, with a precision of 12 digits. To give a sense of just how good that is, it’s like measuring the distance from the Earth to the Sun to an accuracy of one millimeter. This is just an incredible achievement.
This precision has another fascinating aspect: Normally in particle physics experiments, the key to discoveries is to concentrate an incredible amount of energy in a small space. This is Einstein’s equation E = mc^2 in action.
The biggest particle accelerator in the world is the Large Hadron Collider, or LHC, located at the CERN laboratory, just outside Geneva, Switzerland. However, although the LHC has been operating since 2010 and has made brilliant measurements, its very high-energy beams have generated no evidence for undiscovered particles. Maybe the path forward will come not from high energy, but rather precision.
The Fermilab g-2 equipment is composed of a series of powerful magnets arranged in a ring about 50 feet (15 meters) across. In contrast, the LHC is nearly 600 times larger, with much higher energies. It’s in precision that the g-2 equipment is far superior and this might be a case where a scalpel is a better instrument than a sledgehammer.
The original g-2 measurement was made at Brookhaven, with a tantalizing discrepancy. The likelihood of a discovery of currently unknown physics — what researchers call the sigma — is defined to be the difference between measurement and prediction, divided by the combined uncertainty of both. A sigma of 3.0 is considered the threshold for claiming evidence of seeing something new, but it takes a sigma of 5.0 to claim a discovery. The original measurement was 3.5 — very interesting, but not conclusive.
Normally what would happen is that the experimenters would repeat and extend the experiment to get a more precise measurement, but the facility had technical issues that were not easy to overcome.
Luckily, Fermilab’s particle accelerator could create more muons to study. So in 2013, scientists packed up the 50-foot wide ring of magnets and shipped them to Fermilab by way of a barge that went down the eastern seaboard and up the Mississippi River. (Fun fact: there is a story, told among some residents near Brookhaven, that the laboratory housed a captured flying saucer that they were studying. Then, one day in the dead of night, a truck carrying a tarpaulin-wrapped, 50-foot-wide disk left the laboratory, surrounded by a heavy police escort, with lights a-flashing. Tell me that this didn’t add to the legend!)
Fermilab scientists refurbished the g-2 ring and added new equipment. After years of testing, they have begun operations. It is possible that data that they record this spring will push the discrepancy over that all-important threshold of 5.0 sigma.
So, will g-2 make a discovery of new sub-atomic particles? Nobody can say it will. To quote Yogi Berra, it’s hard to make predictions, especially about the future. However, there are many examples of “almost discoveries” with a 3 sigma discrepancy, which have disappeared when confronted with additional data. This might be the case with the existing g-2 result. But I am personally hopeful that this measurement might be the game-changer.
The g-2 has begun operations and researchers will collect data through the spring and analyze it during the fall. Realistically, it will be about a year before they make an announcement on the outcome of the measurement.
Basically, today’s g-2 experimenters are like the intrepid sailors of the Age of Exploration. Armed with an incomplete map from an earlier expedition, they will sail off into the unknown, in search of El Dorado. Now we’ll just have to wait to see if they return with ships full of gold.