Thirty years ago, the discovery of a long-sought particle was announced at the U.S. Department of Energy’s Fermi National Accelerator Laboratory by researchers with the Collider Detector at Fermilab experiment and the DZero experiment. This announcement marked the culmination of a worldwide hunt for the last quark predicted by the Standard Model.
CDF and DZero were both international scientific collaborations, each with about 450 researchers at the time of the top quark discovery. They included institutions from Brazil, Canada, Colombia, France, India, Italy, Japan, Korea, Mexico, Poland, Russia, Taiwan and the United States.
To mark the thirtieth anniversary of the top quark discovery at Fermilab, we spoke with two researchers who contributed to this breakthrough: Douglas Glenzinski, who worked on CDF, and Pushpalatha Bhat, who worked on DZero.
What is the top quark?
Bhat: “Once upon a time we thought atoms were the most elementary building blocks of matter. Then we discovered the nucleus, and its constituents: protons and neutrons. As we looked deeper and deeper into these particles, we found that they are made up of tinier particles called up and down quarks. Then we found out that there are other types of quarks, the strange quark and the charm quark. Fermilab then discovered the bottom quark in 1977. Once the fifth quark was discovered it was widely expected that there would be a sixth quark, the top quark, since quarks seem to come in pairs.”
Glenzinski: “The top quark is by far the heaviest of the quarks. It’s about the same mass as a gold atom, except this is a single quark. It was a little surprising that it was so much more massive than the other quarks.”
What are CDF and DZero?
Glenzinski: “CDF, or the Collider Detector at Fermilab, was one of the two international collaborations that operated at the Tevatron. CDF used a variety of technologies to measure the position, momentum and energy of particles created when protons and anti-protons from the Tevatron collided. The detector is about the size of a three-story house and looks like something out of a science fiction story.”
Bhat: “DZero was one of the two major collider experiments at the Tevatron. Its name comes from the interaction point in the Tevatron at which it’s located. These detectors are huge and have very complex detector subsystems, using different technologies and detection techniques.”
What were you working on in the CDF and DZero collaborations when the top quark was discovered?
Glenzinski: “When I joined CDF in 1992 as a graduate student, it was a very mature collaboration. The experiment had already been taking data for a long time and was in the middle of some upgrades. I was part of a team working on putting together a silicon microstrip vertex detector for the upgrade. CDF was the first to use this type of detector in a hadron collider. And the idea was that this upgrade would help reconstruct and study bottom quarks, which turned out to play an important role in the top quark discovery.”
Bhat: “As soon as I came to Fermilab in 1989, I joined DZero. At that time, we were putting the detector together, installing it and commissioning it. I took charge of the test beamlines and helped calibrate our calorimeters. Then we started taking data in late 1992. But finding this rare signal in data that had huge background events was like finding a needle in a haystack. I was in charge of the multivariate analysis group that used neural networks and other advanced techniques in top searches to discriminate against that background.”
Was there a specific moment when the collaborations knew they had the top quark?
Bhat: “On DZero, we were running data analysis from what was called the ‘express line’, with special events that had been filtered out from the whole data set and processed almost immediately. One event that came through the filter was striking. We calculated the probabilities that this was a top quark event using a multivariate method. And it was very highly likely. So that was the thing that made us go ‘Oh my God. We may be seeing the beginnings of top quark events!’ But we needed a significant number of top quark candidate events and careful estimates of background to be able to claim a discovery. With continued intense periods of data-taking and sophisticated analysis efforts by many people working relentlessly, we were able to do that.”
Glenzinski: “In CDF’s case, there had already been strong hints, and the collaboration published a paper in 1994 called ‘Evidence for top quark production,’ which was very suggestive but not statistically definitive. So, when data-taking resumed there was a lot of pressure to analyze the data as fast as we could. Many people worked to improve and update the analyses. And, as I remember it, there was a collaboration meeting where the updated results were unveiled. When it came time for questions, someone stood up and just said, ‘This is it, we did it. This is the top quark.’ The final result combined multiple analyses to get a more complete picture, and there was careful work to double-check everything with many people contributing, but, as I remember it, it was pretty clear to everyone what had happened.”
Looking back over the past 30 years, what has being part of this discovery meant?
Glenzinski: “It’s important to note that this was the result of collaboration-wide and lab-wide efforts over many years. It was extremely exhilarating to work in an international collaboration with a collection of people as motivated and bright as that. To have experienced that and had some impact on the top quark result just solidified my excitement for particle physics. I am grateful for having had that opportunity and for all those colleagues that mentored me, and the other graduate students involved.”
Bhat: “Being part of the top quark discovery (and then the Higgs boson) has been incredible. The top quark is very special. Because it’s so heavy, it’s the quark that couples very strongly to the Higgs boson which helps us understand the electroweak theory and symmetry breaking better. Together with the Higgs boson, the top quark also has implications for the stability of the universe.”
Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.
Fermi National Accelerator Laboratory is a leader in physics research, offering world-class facilities and expertise, along with a talented group of emerging researchers. Among them are Sara Sussman, Dylan Temples, and Christina Wang, who are conducting groundbreaking quantum research as Lederman fellows. Their work aims to confirm the existence of dark matter, potentially uncovering insights into new physics beyond the Standard Model.
Named after the late Nobel Prize-winning physicist and Fermilab Director Emeritus Leon Lederman, the Lederman Fellowship has helped to launch the careers of many distinguished scientists, including current Fermilab Chief Research Officer and Assistant Laboratory Director Bonnie Fleming. This prestigious program was designed for exceptional postdoctoral researchers who not only excel in scientific research but also share a passion for education and outreach — areas that were deeply valued by Lederman.
“The importance of effectively communicating Fermilab’s science to audiences of all ages and academic backgrounds is essential to motivating and inspiring others to be even more inquisitive about science in general,” said Natalie Johnson, head of the Office of Education and Public Engagement at Fermilab. “We hope that every educational engagement will propel individuals on a trajectory toward STEM careers and lifelong enthusiasm for science.”
Quantum technology and the search for dark matter are fascinating topics that can inspire young people to think about careers in physics and science in general. At the same time, it is crucial to be able to explain to funding agencies and the public what is being done with their money and why it is important. But doing so can be challenging — even for physicists.
“Lederman understood that education and outreach go together,” said physicist Joe Lykken, head of Fermilab’s Quantum Division. “If you want to be able to educate and inspire the next generation, you need to be able to explain what you do in an interesting way. That’s an important thing for scientists to learn and value. Getting our junior scientists excited and giving them the skills they need to reach out to other people will be super valuable, and it will be valuable to them in their careers as well.”
Fermilab’s most recent Lederman fellows are each studying ways to use quantum technology to search for dark matter — the mysterious substance scientists believe makes up most of the universe — along with other applications of quantum science.
Dylan Temples
Dylan Temples joined Fermilab as a Lederman fellow in 2021 and has just renewed his fellowship for another two years. At Fermilab, he continues research he began while working on similar projects at Northwestern University while earning his doctorate in physics.
Temples splits his time between projects for the Quantum Science Center, a Department of Energy National Quantum Initiative research center headquartered at Oak Ridge National Laboratory, and the Matter-wave Atomic Gradiometer Interferometric Sensor, or MAGIS-100.
For the Quantum Science Center, Temples is part of the Cosmic Quantum group. This interdisciplinary group is testing superconducting quantum bits for use as quantum sensors. Scientists can manipulate qubits to place them in delicate quantum states that are sensitive to changes in the local environment, such as variations in temperature or electric fields. A change induced through a particle impact, for instance, can disrupt the quantum state, leading to decoherence. A high decoherence rate could signal particle interaction, possibly making qubits useful for dark matter detectors.
In addition, Temples is working on special devices called phonon-mediated kinetic inductance detectors. These detectors will search for dark matter in previously unexplored areas. Specifically, he is seeking ways to reduce radiofrequency noise, which degrades the performance of these highly sensitive devices. Dark matter produces very faint signals that are easily hidden by these factors.
Temples’ second area of focus, MAGIS-100, features a different type of detector. This one aims to detect dark matter but in a very different mass range.
“Because we have never observed dark matter in a particle detector, the majority of the evidence we have for its existence is from the dynamics of astrophysical objects — galaxies, galaxy clusters and so on,” said Temples. “All that tells us is how much of it there is, not what the properties are or whether it’s a new fundamental particle. If so, there are about 50 orders of magnitude in mass in which a potential particle could live.”
While testing and optimizing quantum equipment to search for ephemeral particles from space, Temples also enthusiastically shares his work with students of all ages through the Fermilab Office of Education and Public Engagement.
Once his fellowship concludes, Temples hopes to obtain a faculty position at a university or national laboratory. As a former software engineer prior to graduate school, Temples enjoyed his work but found that he wanted more academic freedom.
“I wasn’t flexing all the creative parts of my brain that you need to do science, and it wasn’t sating my curiosity,” he said.
Sara Sussman
Sara Sussman, a Lederman fellow since 2023, designs and fabricates multi-qubit devices and contributes to the open-source Quantum Instrumentation Control Kit, which was developed at Fermilab. QICK is a system used by more than 350 researchers worldwide that helps scientists to control and read out the information from superconducting qubits.
Sussman earned her doctorate in physics from Princeton University, where she focused on designing, fabricating and measuring superconducting qubits. During the second year of her doctoral research, she got the chance to work on QICK with Fermilab engineer Gustavo Cancelo. This earlier experience influenced her to come back to Fermilab as a Lederman fellow.
In addition to her work on QICK, Sussman enjoys working in the Cosmic Quantum group with Temples as part of what Sussman said is “an amazing group of scientists.” Using superconducting qubits to search for dark matter, she explained, is an ideal application for qubits.
“Currently, qubits are noisy and not error corrected, Sussman added. “They’re very sensitive. They can set some of the most stringent limits on searches for dark matter, so it’s great to use them.”
Working on these two projects is interesting because she is addressing very different aspects of qubits. For the Cosmic Quantum group, she designs and fabricates qubits, while for QICK, she helps enable scientists to control and measure the quantum states of qubits to read out information from them.
“Being a Lederman fellow is great because I get to spend time doing outreach,” said Sussman, who helps create educational materials and teaches new and established researchers to use QICK. Last summer, she and colleague Sho Uemura taught at the U.S. Quantum Information Summer School in Knoxville, Tennessee, and she regularly gives tutorials covering the latest QICK features. In doing so, she hopes newcomers can use qubit control to advance their research.
“It’s kind of like when someone buys a really nice camera and learns how to do photography,” said Sussman. “I help students learn how to do qubit control and how to see, read out and measure the qubits in ways that that are useful for their particular environment and lab setup.”
Christina Wang
Christina Wang, Fermilab’s newest Lederman fellow, began her fellowship in September 2024.
While earning her doctorate in physics at California Institute of Technology, Wang received a DOE Office of Science Graduate Student Research Award in 2022 to work at Fermilab, exploring dark matter at the Large Hadron Collider and use quantum-sensing technology for dark matter searches. Her advisor for that project, Cristián Peña, a former Lederman fellow, now oversees part of Wang’s research in Fermilab’s Quantum Division. Once Wang completed her doctoral degree, she decided to continue her career at Fermilab as a postdoctoral research associate.
“The Lederman fellowship allows me to have flexibility to choose what I want to do within the core program,” Wang said. “This flexibility will allow me to continue to work both on quantum sensing and on the CMS experiment at the Large Hadron Collider. I like that the Lederman Fellowship allows me to do both.”
On the quantum side, Wang works primarily on using superconducting nanowire single-photon detectors to search for the existence of axions, hypothetical particles that may comprise dark matter. SNSPDs are incredibly fast light sensors that operate at cryogenic temperatures and are made of superconducting wire to detect single photons. In theory, these photons are produced when an axion encounters a strong magnetic field.
According to Wang, they are still at the early stage of the experiment, conducting R&D for the detector.
“I’m hoping in the next few years, we’ll be able to build the experiment, integrate the SNSPD and optical reflector into a cryostat and have a meaningful physics result,” she said.
Wang has engaged in several outreach activities, including planning and hosting events related to the annual Dark Matter Days celebration and a science trivia event for the local public. Most recently, she helped organize and present for an American Physical Society conference hosted at Fermilab. Wang said she is looking forward to helping launch a future public quantum network between two Fermilab sites.
Next generation leaders
While it will take many years to develop full-scale quantum computers, sensors, and other quantum technology, Fermilab researchers are working towards becoming leaders in these areas.
“This technology is really going to mature 20, 30 years from now,” said Lykken.
“We’re going to need that next generation. They’re going to be doing the stuff that we’re dreaming about today.”
Lykken added, “It’s important strategically for our nation and for U.S. industry to be leaders in quantum, so having these young people get people’s attention is good.” “When these fellows become leaders in the field, it will come as a reflex for them to explain their scientific work and why it’s important to the future of the nation.”
Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.