Making an Impact

by Pamela Zerbinos

In the late 1980s, Luciano Ristori had an idea.

Researchers had just gotten their hands on a new technology that allowed them to design and build their own silicon microchips. The technology, called VLSI for Very Large Scale Integration, had previously been available only to large companies like Intel and Motorola, but gradually it leaked into the research sector and allowed scientists and engineers to design and build integrated circuits for very specific purposes.

Ristori, a CDF collaborator from the Italian Institute of Nuclear Physics (INFN) laboratory at Pisa, convinced himself that chips could be built for the specific purpose of matching up patterns created by particles flying through the layers of a silicon detector — which did not yet exist. The silicon detector is now the innermost layer in a series that makes up a large detector like CDF or DZero, and is therefore the first to “see” the products of a particle collision.

The particles in question are the protons and antiprotons that Fermilab’s Tevatron smashes together at very high energies to watch what comes out. One kind of particle that sometimes comes out is the B meson, made of a bottom quark and a lighter anti-quark. The B meson travels about one millimeter (that’s a long way in particle physics) and then decays into other particles called pions and kaons. As these particles pass through the detector, they leave tracks that scientists can use to figure out exactly what kind of particle it was.

Decays, collisions and other events happen at the Tevatron at a rate of millions per second, and even Fermilab doesn’t have enough disk space to save them all. So they have to filter out the important events and throw everything else away. This is accomplished by a set of electronic devices collectively known as “the trigger” that measure all kinds of factors — speed, energy, charge, etc. The trigger has to make extremely fast decisions about which events to keep and which ones to throw away.

“This is important,” Ristori said,“because there is no going back. Once you reject an event, it is lost forever.” CDF is currently keeping an average of 50 events per second,“so we’d better make sure they’re the right 50.”

And that’s where his idea came in. He thought it would be possible to use VLSI to design circuits that would match new particle tracks with previously identified geometric patterns to find out how long the particle had traveled before decaying — all in about 20 microseconds. This information could then be used in the trigger to keep valuable B meson events from being thrown out.

“In the early days,” said CDF cospokesman Nigel Lockyer, “nobody believed it would work. It was too ambitious, and we didn’t understand whether the beam was stable. We didn’t know whether you could find these tracks fast enough. We hadn’t even put silicon detectors in the experiment at that point. None of the technology was available to do this.”

But Ristori went ahead with it anyway, and the project, called the Silicon Vertex Tracker (SVT), was proposed to CDF in 1991.

“I was convinced that it was a good thing to do, that it would be important, and I was convinced that we could do it,” he said.“There was no reason why we couldn ’t do it. I did the first step, which was to convince people that it was not totally crazy.”Once he had convinced the first two or three people at the University of Pisa, others quickly followed and now the collaboration includes the University of Chicago, the University of Geneva and the INFN labs at Rome and Trieste.

“I think the real virtue of the people who started working on this was to believe it could be done,” said Ristori.“It took some courage — it was a new technology and it was very easy to make mistakes. There were many, many ways we could have failed.”

Over the more than 10 years he’s been working on the SVT, he’s had help from more than 20 people, several of whom worked full-time on the device for several years. Ristori estimates the total amount of work put into the project to be around 50 man-years.

“There were a lot of good people who worked for many years,” Ristori said.“It took a lot of ideas. We had to solve many problems. It was much harder than I anticipated, but we had enough people who were good enough to solve all these problems and go through to the end.”

Their work was funded in large part by INFN, the U.S. National Science Foundation and the Department of Energy. Ristori thinks the project cost around $1 million, but the exchange rate has fluctuated so much in the past 12 years that it’s hard to keep track. More importantly,“they always gave me the money I asked for.”

All the hard work has paid off; the SVT has been up and running since Run II began in March 2001. It’s the only device like it in the world, and it gives CDF important — and sometimes unexpected — capabilities.

“It’s given us the ability to do all kinds of things we never thought we could do,” said Lockyer.“We now have the world’s largest sample of charm [quarks]. We never planned on doing charm physics, but it turns out that the charm lifetime is very similar to the B lifetime, and all of a sudden we’re inundated with all this charm. It’s very exciting for us.”

The 53-year-old Ristori was born in Prato, just outside Florence. He has always been interested in mathematics and physics, and attended the University of Pisa, where he “had to make this choice between physics and mathematics. I chose physics, and I do not regret it. ”He received his Laurea in Fiscia (Italy was not offering Ph.D.s at the time) from Pisa in 1971 and soon joined the NA1 experiment at CERN, the European Particle Physics Laboratory, as an INFN-Pisa collaborator. He stayed for several years, and then in 1980 he visited Fermilab and decided to join CDF.

“I liked the place, I liked the physics, I liked what they were planning to do at CDF and at the Tevatron,” he said.“CERN was a very nice place, but I like the style here, the way people are working and interacting. I also like the lifestyle here, this place, this part of the country.”

When he first joined CDF, he lived mostly in Pisa and would commute back and forth several times a year, staying a few weeks or a few months. Now he and his wife live full-time in the Fermilab village, and he visits Pisa several times a year.

“It’s important to keep the relationship with your home institution, even if most of the work is here at the moment,” he said.

His work here still involves the SVT,“but there aren’t any major problems to solve. It needs to be optimized, and there is fine tuning to do, but soon we should be able to just turn it on and have it work.” Once that happens, Ristori hopes to turn his attention to some of the data produced by the device.

“I’d like to play a role at CDF in terms of looking at the important physics results we can get with SVT,” he said.“I think it’s the natural outcome of your work — you’ve been working so many years to build something, and then it works, and you want to see the results in terms of physics. I’d like to play a role in that, have more time to look at the outcome. That’s the real motivation for going through all this trouble. It’s fun, building something and using the detectors. But in the end all of this would not be worth it if it’s not useful for physics.”