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.
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