The muon is a tiny particle, but it has enormous potential to revolutionize our understanding of the subatomic world and reveal an unknown type of fundamental physics.
This possibility seems increasingly likely, according to early results from an international collaboration – hosted by the US Department of Energy’s National Fermi Accelerator Laboratory – that involved key contributions from a Cornell team led by Lawrence Gibbons. , professor of physics at the College of Arts and Sciences.
The collaboration, which brought together 200 scientists from 35 institutions in seven countries, aimed to confirm the findings of a 1998 experiment that surprised physicists by indicating that the muon magnetic field deviates significantly from the Standard Model, which is used to explain the laws that govern fundamental particles.
The digitization modules undergo tests in the laboratory of Lawrence Gibbons, professor of physics, before being shipped to the Fermi National Accelerator Laboratory. Twenty-eight cases of these modules were installed around the muon g-2 ring.
“The question was: what is going on? Was the experiment wrong? Or is the theory incomplete?” Gibbons said. “And if the theory is incomplete, then confirming what is happening becomes the first earthly evidence of an entirely new type of particle or fundamental force that we do not know about. It would be the first experience on Earth that is somehow here. equivalent of the discovery of dark matter in space. “
On April 7, the team confirmed that the original results were correct, which means there must be more physics surrounding the muon than previously known.
Muons are like electrons but are over 200 times more massive. Both are essentially tiny magnets with their own magnetic fields. Muons are much more unstable, however, and decay in a few millionths of a second. They are also notoriously difficult to observe at the quantum mechanical level because the vacuum in which they exist is not a large empty cavity, but rather a bubbling, foaming dynamic environment.
“It’s your cappuccino froth version of the vacuum cleaner, where there are virtual particles that flicker and disappear all the time,” Gibbons said. “And that turns out to affect the strength of a muon’s magnetic field.”
To understand why, researchers at Brookhaven National Laboratory set out 20 years ago to measure the absolute strength of the muon’s magnetic field. They did this by shooting a muon beam through a 14-meter-diameter magnetic ring at almost the speed of light while a series of detectors captured data. Scientists found a major deviation in the muon’s magnetic field: it was more than 3.5 standard deviations from the Standard Model predicted by theoretical physicists.
A plan was eventually made to repeat the Brookhaven experiment with greater precision. In 2013, the Brookhaven magnetic ring was transported to the Fermilab plant in Batavia, Ill., Where it was paired with an even more powerful particle accelerator capable of producing more than 20 times the amount of muons. In 2018, the first of several series of experiments was launched.
This experiment on muon g-2 – “g” refers to the value of the force of the magnet caused by its intrinsic spin, slightly greater than two – was successful thanks to a system of detectors developed thanks to a partnership between Cornell and the University of Washington.
The Washington University group built a set of 24 calorimeters from lead fluoride crystals and silicon photomultipliers that measure blue light, known as Cherenkov radiation, which occurs when positrons in the muon decay strike the crystals. By measuring the time and amount of light for each of about 8 billion positrons, researchers can identify the muon’s precession rate, which is the frequency of its rotational oscillation. The rate is directly related to the value of g-2.
The Cornell team built the digitizers that could look at the electronic signal coming out of the detectors and create a digitized version of the waveform that could be analyzed offline. The researchers were supported in the effort by the Elementary Particle Physics Laboratory (LEPP), and their digitizers incorporated $ 200,000 worth of specialized analog-to-digital converter chips donated by Texas Instruments.
Gibbons’ group also built one of two reconstruction packages that helped their collaborators analyze and analyze the data collected, and they were helped to get the most accurate measurements by David Rubin, Emeritus Professor of Physics Boyce. D. McDaniel (A&S), who helped correct the propagation of muon pulses in the stored beam and the small vertical movement as the beam moves around the magnetic ring. Two other Cornell professors, Toichiro “Tom” Kinoshita, professor emeritus of physics, and G. Peter Lepage, professor of physics Goldwin Smith, both in A&S, contributed to the prediction of the standard model of g-2, to which the project compared its results.
As a final touch, Gibbons chose to make the front of the Cornell Digitizer red.
With so much subatomic information to analyze, six different groups worked to separately confirm the muon precession frequency. Gibbons helped design blinding software that would ensure groups performed their calculations independently.
Then the time has come to compare the results.
“I have to say that was scary. You walk into the room, and there are all these dots scattered all over the place from all the offsets, and you have to decide, OK, are we going to compare the results now? do they agree? ”Gibbons said. “We were trying to measure something at 500 parts per billion. The range we had was plus or minus 25 parts per million on the frequencies we are trying to measure. There was a huge sigh of relief when we found everything to be perfectly alright. “
And when all the international collaborators got together online for the final unblocking of the magnetic field measurement and checked it against the original Brookhaven result?
“Oh man. It was like hats flying through the air,” Gibbons said. “It was a combination of elation and relief.”
The results of this first experimental trial represent only 6% of the data that the researchers hope to collect in the long term. Additional analysis has already started on a second and third pass, which will generate three to four times more data. It will take 10 years for the entire analysis to be completed.
“We landed just above that result which could really indicate that something totally new is happening. We really want to push the uncertainty, the precision, to make the strongest possible statement that we can experience.” said Gibbons, who started working. on the project in 2011. “We might be on something really deep, something that we don’t understand. And we still have to figure out what it is.”