Pinning down the ampere with a supersensitive particle detector

From light bulbs to cell phones, all electronic devices in everyday life rely on the flow of electrons to function. Just as scientists use meters to describe the length of an object or seconds to measure the passage of time, they use amperes, or amps, to quantify electric current — the rate at which electric charge moves through a circuit.

In everyday life, you can safely use a hair dryer or toaster without knowing exactly how many electrons are flowing through it every second. But researchers on the frontiers of physics must have a precise definition of the ampere to detect when experiments unexpectedly deviate from theoretical predictions.

“As technology progresses, lots of measurements that we could not do before become available, and then you can have extremely high-precision measurements,” said Fermilab scientist Javier Tiffenberg. “So you want to have a definition of the unit that is much more precise than whatever you are trying to measure.”

For decades, scientists have struggled to achieve the necessary precision for the ampere. But now, a device called the skipper CCD, developed by Tiffenberg and his collaborators at Fermilab and the Lawrence Berkeley National Laboratory Microsystems Lab, could spark an advance in measurement science.

Counting electrons, one by one

Two current-carrying wires exert a force on each other that depends on the distance between the wires as well as the value of the current. Until recently, 1 amp was defined to be the current that would cause two infinitely long wires placed parallel to each other a meter apart to experience a force of exactly 0.2 millionths of a newton per meter of length.

But that definition troubled the scientific community — an experiment requiring infinitely long wires is impossible to perform. Other base units also had unsatisfactory definitions: For example, the kilogram was defined to be the mass of a particular metal cylinder in a vault near Paris. So in 2019, the General Conference on Weights and Measures adopted new definitions for four of the seven base units of the International System of Units, or SI, including the kilogram and the ampere.

“Now the idea is to link all the units to fundamental constants of the universe,” Tiffenberg said. “In the case of the ampere, the link is done through the charge of the electron.”

Yet one problem remains: The charge of a single electron is minuscule. Under the new definition, the current generated by a single electron passing a given point each second is exactly 1.602176634×10^-19 amps, or less than 2 tenths of a billionth of a billionth of an amp. Many experts say that an instrument to calibrate the definition of the ampere must generate a current of at least 1 microamp, or 1 millionth of an amp, while counting individual electrons — trillions of them every second. No such device exists yet.

Enter Fermilab’s skipper charge-coupled device, which builds on improvements made in the 1990s to standard CCDs. Pixels connected in a grid store the electrons produced when light hits them. Then the electrons are shuttled to a detector that measures the charge contained in each pixel.

Widely used in digital cameras and scientific instruments, standard CCDs can measure the charge in each pixel only once before losing the information. Skipper CCDs, on the other hand, can measure each pixel repeatedly at a rate of 100 times per millisecond. This allows skipper CCDs, unlike standard ones, to count individual electrons.

“Because these measurements are independent, just by taking many, many samples and averaging them, you are able to reduce the uncertainty on how much charge was sitting in the pixel,” explained Tiffenberg, who won the 2021 New Horizons in Physics Prize and the 2020 URA Early Career Award for his work on skipper CCDs. “In principle, you can reduce this to a number that is arbitrarily small. We have done this to uncertainty levels of 0.06 electrons.”

Tiffenberg and his collaborators began the skipper CCD project with the goal of detecting dark matter, the mysterious substance that makes up about 85 percent of the matter in the universe. Some theories predict that collisions with lightweight dark matter particles would cause individual electrons to recoil, which a skipper CCD could detect with extreme precision.

Now that the ampere is defined in terms of single electrons, researchers at Fermilab are working to scale up skipper CCD technology to reach the current needed for a successful calibration of the definition.

“I’m not saying this is going to be easy, but there’s no theoretical limitation,” said Guillermo Fernandez Moroni, a postdoc at Fermilab working on skipper CCDs.

Building a larger current source

In the 2019 redefinition of the SI units, the General Conference on Weights and Measures provided three candidate methods to calibrate the ampere. The most promising hinges on single-electron transistors, which, like skipper CCDs, can count individual electrons. But the current produced by today’s SETs falls far short of the threshold for a precise calibration.

The first generation of skipper CCDs can already produce a larger current than SETs. Tiffenberg and Moroni expect that future refinements will allow them to build skipper CCDs that generate a current as large as 1 billionth of an amp while still counting individual electrons.

To reach the 1 microamp threshold from there, researchers would need to link together a thousand skipper CCDs. This, too, seems feasible to Tiffenberg. His team’s prototype dark matter detector contains around a hundred skipper CCDs. While SETs must be chilled to a few thousandths of a degree above absolute zero, skipper CCDs can operate at minus 133 degrees Celsius — a balmy temperature by comparison. As a result, scaling up the latter is more practical.

In the meantime, Fermilab researchers are exploring a host of other uses for skipper CCDs.

“We have been adding a lot of people to this effort, and now our days are full of meetings. Every day is a different subject around the skipper,” said Moroni, who received the 2019 URA Tollestrup Award for his skipper CCD research. “Monday and Wednesday are dark matter, Wednesday and Friday are neutrinos, Tuesday is quantum, Thursday is astronomy and satellites. It is very exciting.”

Tiffenberg agrees that skipper CCDs hold great promise for measurement science and physics research more broadly.

“The applications seem to just pop out everywhere, so it’s a lot of fun,” he said.

Fermilab research on particle detector technology is supported by the DOE Office of Science.

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

Editor’s note: Fermilab joins the global celebration of Dark Matter Day. Hear from Fermilab scientists during a special webinar on Saturday, Oct. 31, at 1 p.m. CT. Take a virtual tour of the lab’s dark matter experiments and detectors, and learn how Fermilab is helping answer questions about the mysterious stuff that makes up 25% of our universe. Register now to attend the free event.

Below is an adaptation of a press release published by the Interactions collaboration.

 

There’s far more to our universe than meets the eye. Everything we can see, everything we know exists, makes up just five percent of the matter and energy in the universe. So, what about the other 95%? Astronomers and astrophysicists believe that approximately 25% of the missing mass and energy in the universe is made up of dark matter. This ubiquitous substance is everywhere, yet, so far, remains a mystery.

Dark Matter Day, an international event, aims to shed some light on that mystery. From Oct. 26-31, a series of Dark Matter Day events will highlight the global search for dark matter, which, together with dark energy, makes up about 95% of the mass and energy in our universe. Dark Matter Day spreads the word about the many fascinating ways scientists search for dark matter, and the importance of devoting scientific resources to unraveling this cosmic riddle.

Dark Matter Day is going virtual this year, making every event accessible to a worldwide audience. To explore the many opportunities to participate, visit the Dark Matter Day website.

Although scientists have yet to detect dark matter, indirect evidence tells us it exists — in the gravitational effects of galaxies and the way light bends around unseen objects in space. Understanding the nature of dark matter will help us better understand the universe in which we live. But scientists are not sure yet what this mysterious substance is composed of or whether the answer, when it comes, will require a complete rewrite of our understanding of physics.

A host of innovative experiments are searching for the source of dark matter using different types of tools, such as detectors built over a mile underground, powerful particle beams and telescopes based both on Earth and in space. For more on the global hunt for dark matter, visit the Interactions collaboration’s Dark Matter Hub.

Sponsored by the Interactions Collaboration, an international community of particle physics communication specialists, Dark Matter Day celebrates the work being done in laboratories and institutions around the world, and shares what we do know about this cosmic puzzle with audiences worldwide.

To find resources or to register your event, go to the Dark Matter Day website.

Fermilab research on dark matter is supported by the DOE Office of Science.

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

 

Fermilab’s Robert Bernstein had been conducting neutrino research for 25 years when he began searching for a new project.

“One of my friends came up to me and said, ‘You should be Mu2e spokesperson.’ And I said, ‘What’s Mu2e?’ I went off and did some homework and I said, ‘OK, it’s terrifically interesting, really hard, and I don’t know anything about it. Sure. Sign me up!” he said.

Although the muon was discovered in 1936, physicists still seek to understand why the particle exists, thus the need for the Mu2e experiment (pronounced “mew 2 E,” which stands for muon-to-electron conversion). It’s part of a larger question: Why do multiple generations of subatomic particles exist? The quark family is made of three generations of particles — up and down, strange and charm, bottom and top — each heavier than the next. Similarly, the lepton family has three generations — electrons, muons and taus — and again, the members have different masses.

“Why is that? Why are there these generations of particles. That’s a fundamental question,” Bernstein said.

Bernstein first became co-spokesperson with Boston University’s James Miller in 2007, the year Mu2e received formal approval. He continued in that role until 2014, when Fermilab scientist Doug Glenzinski succeeded him. In March, Bernstein returned to the co-spokesperson role after Glenzinski became Fermilab’s chief project officer.

“Bob and I were co-spokespersons at the beginning of Mu2e, and it’s great to work with him again,” Miller said. “He brings a deep knowledge of the experiment and a proven record of leadership.”

Bernstein returned the sentiments.

“He has a long and distinguished career in muon physics and precision experiments,” Bernstein said of Miller. “He’s been doing this even longer than I have.”

The Mu2e collaboration consists of nearly 250 scientists at 40 institutions. The MBA that Bernstein earned from the University of Chicago in 2006 has helped him grapple with the organizational challenges involved in working with such a large group.

“I majored in organizational behavior. In terms of thinking about how to get people to work together on a task or a big project or a big problem, I use the ideas I learned there every day,” he said.

Bernstein noted that the experiment is changing from construction to installation.

“The building, along with most components for the infrastructure, experiment and beam have been built, and over the next couple of years, we’ll turn Mu2e into a running experiment,” he said. “It’s an exciting time, and a great one for young people who will put the parts together. When someone is analyzing data, they’ll know they we’re the ones who installed the piece that gave them that data, and they’ll know all its quirks. That’s a lot of fun and incredibly satisfying.”

The collaboration has needed to invent technology to get to this point. No one, for example, has ever built anything like Mu2e’s solenoid system, which serves as the heart of the experiment. The tracker, the instrument that detects the electron, boasts novel technology that includes straw-like tubes whose walls are about half as thin as a typical human hair.

Mu2e meshes nicely with Fermilab’s NOvA neutrino experiment, which Bernstein also is involved in. NOvA’s goals include observing the oscillation of muon neutrinos into electron neutrinos. Mu2e similarly seeks evidence for the conversion of muons to electrons. Such a discovery could put physicists on the trail of new particles or new forces of nature. To get there, the collaboration continues to solve new problems as they arise.

“For me on a daily basis the most fun part of the experiment is working on really hard problems with really smart people,” Bernstein said. “That’s motivating for me.”

Muon research at Fermilab is supported by the Department of Energy Office of Science.

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.