Paul and I first met when I went to Cornell in 1980. He was in the last year, writing his Ph.D. dissertation. It seemed to me all he did was smoke hand-rolled cigarettes (not an oddity in those days), drink espresso (a bit of an oddity in those days), and work things out on the blackboard with Peter Lepage. I was stunned in the summer of 1981 when a photo of Paul and a baby (his first daughter) went up on the bulletin board. “Who has time for family while in grad school?” I thought.
We met several times during the latter half of my Ph.D., when Paul was a postdoc (first at Fermilab, then at the Institute for Advanced Study at Princeton); by then I was also working with Lepage. A short chapter of my dissertation is basically a transcript of a conversation Peter, Paul and I had one afternoon on how to calculate moments of parton distribution functions in lattice QCD.
Paul returned to Fermilab as an associate scientist. Fellow scientists Tom Nash and Estia Eichten had launched a project to build a computer for lattice QCD (called ACPMAPS): Tom was pushing the computing and Estia the physics opportunities. Paul gave talks at conferences about ACPMAPS, which got me thinking that Fermilab would be a good place for a career myself. Paul and Estia hired me for my second postdoc in 1988 (well, the Theoretical Physics Department hired me, but they clearly pushed for it); the subsequent year I became an associate scientist, staying here (despite another attractive offer) for the prospect of collaborating with Paul.
Paul is a very strong physicist. He is also cautious in his remarks so some people who don’t know him well overlook his depth. We made a good team developing lattice QCD from a promising tool into a precision science: I was more inclined to push the envelope; Paul made sure that everything was rock solid. He is also creative — he was the first to see that lattice QCD calculations pointed to smaller values of the quark mass than earlier thought. This caused some controversy, but he turned out to be right.
After ACPMAPS had had its run, Paul got us (our group of collaborators in one sense, and Fermilab in another) involved in a national effort for lattice-QCD infrastructure. Former Fermilab scientist Vicky White, who was spending a few years at DOE, was instrumental on the DOE side (as was Jeff Mandula). Paul became a charter member of what is now known as the USQCD Executive Committee. USQCD is the national group that organizes computing hardware and software for people doing lattice gauge theory calculations in the United States, a group of over 150 people.
When the original chair stepped down, the rest of this committee selected him to be the chair. In this capacity, which he served from 2009-18, he was instrumental in shaping the computing landscape for lattice QCD in the United States, not only in high-energy physics (that is, in programs funded by the DOE Office of HEP) but nuclear physics (funded by the DOE Office of Nuclear Physics). When a turn in his health forced him to step down, it’s fair to say it was a crisis, because so much relied on him.
Marcela Carena, head of our Fermilab Theoretical Physics Department, notes that “Paul and his collaborators paved the way to our current ability to extract precise predictions from the Standard Model using lattice gauge theory, predictions critical for experimental programs such as the LHC, Muon g-2 and DUNE. Through his fundamental insights and leadership of the USQCD lattice program, Paul ensured a central role for Fermilab in the advancement of this important science, which combines cutting edge computing with some of the deepest and most challenging features of quantum physics.”
Marcela also expressed her hopes that Fermilab will continue to profit from his wisdom for many years to come.
Paul has been an excellent mentor for postdocs. His influence on Aida El-Khadra (University of Illinois at Urbana-Champaign), Shoji Hashimoto (KEK), and Ruth Van de Water (Fermilab) was especially profound.
In the field of lattice QCD, Paul has been one of the most influential figures: His work on various methods helped point out new directions; in our common work, we turned lattice QCD into precision science; he managed computer infrastructure in such a way that lattice QCD blossomed all over the United States; and his attitudes influenced our postdocs, many of whom are now leaders in their own right.
And he is always a gentleman.
Paul retires on March 13. He says he plans to travel and to learn about things besides QCD during retirement. But he will continue to do work at the lab as scientist emeritus, adding to his already considerable contributions to particle physics.
Andreas Kronfeld is a scientist in Fermilab’s Theoretical Physics Department.