by Virginia Greco. Published: 06 December 2016

William Zajc, who has been working at RHIC since its early times, talks about the discovery of the defining property of quark-gluon plasma.

Columbia University’s physicist William Zajc, a former spokesperson of RHIC’s PHENIX collaboration, took the stage at the 30 Years of Heavy Ions celebration at CERN to report on the history of RHIC and the most important results achieved by its experiments. In the early stages of his career, Zajc worked at CERN’s Intersecting Storage Rings (ISR), a machine that operated from 1971 to 1984 and became the first collider to study both proton-proton and proton-antiproton collisions. The ISR was also used to study collisions between α particles, thus it has been the world’s first light-ion collider.

The Relativistic Heavy-Ion Collider, RHIC, has been in turn the first ‘purpose-built’ heavy-ion collider. The four experiments installed around its rings are STAR, PHENIX, PHOBOS, and BRAHMS. Colliding gold ions, physicists at RHIC not only provided the definitive proof of the existence of QGP, but also demonstrated that this new state of matter behaves as an almost perfect liquid. In particular, the STAR experiment observed the phenomenon of the ‘strong elliptic flow’ and the PHENIX experiment the ‘jet quenching’, as Zajc reminded in his talk. In characterizing and measuring the properties of this perfect liquid, RHIC physicists are developing hydrodynamic theories whose interest goes beyond nuclear and heavy-ion physics. After talking about RHIC’s results, Zajc briefly discussed new heavy-ion physics at LHC and its perspectives.

At the end of the conference, we asked him for some comments.

Professor Zajc, how did your adventure with heavy-ion physics begin?

I started working at the ISR and certainly this was for me a formative experience. It was the first time we had a collider environment, where we were studying not proton-proton but nuclei collisions. In addition, we were using a detector that was a preview of the one that would be built later at RHIC. It was very exciting and we made many interesting measurements. We didn’t see any particular signal that could be interpreted as QGP, nevertheless the experience was very valuable and gave us expertise that could be reused at RHIC. In fact, the PHENIX experiment, in which I participated, had some resemblance to the ISR R807 experiment, in particular the configuration of the magnet. R807 had been conceived and built by Bill Willis and Chris Fabjan, who were both CERN scientists at that time.

You said that discovering that the QGP is characterized by a ‘near-perfect liquidity’ signed a paradigm shift. Can you explain why?

It was a paradigm shift because we had to change our notion of what the essential property of quark gluon plasma was. Our experimental activity had been inspired by the search for asymptotically free quarks and gluons and most people imagined QGP as something in which quarks and gluons could easily move. Then the data forced us to reconsider this notion. In fact, we observed that this new state of matter was not some sort of gaseous plasma, but behaved much like a liquid. And actually it is the most perfect liquid, in the sense that it has the least internal friction compared to any liquid that has ever been studied in the laboratory. So we were surprised to discover that near-perfect liquidity was indeed the defining property of QGP.


 What’s the scientific programme of RHIC for the next future?

In the next 2-3 years, the STAR experiment will undertake what is known as the “Beam Energy Scan II”, mapping out the phase diagram of thermal quantum chromodynamics in regions with high densities of protons and neutrons. In particular, they will be searching for signatures of the predicted critical end point, which marks the end of a sharp distinction between an assembly of ordinary particles and the QGP. In the next decade a new experiment, sPHENIX, will begin operations at RHIC. This experiment, which very recently received a critical approval milestone from the Office of Nuclear Physics in the U.S. Department of Energy, will measure jets and heavy flavor production at RHIC. These sPHENIX measurements will provide direct comparisons to those from the LHC, which will allow us to understand the precise mechanism that produces the perfect liquid behavior of the QGP.