by Virginia Greco. Published: 21 January 2019

Barbara V. Jacak has been recently awarded the American Physical Society’s Tom W. Bonner Prize in Nuclear Physics for her contribution to the study of the Quark-Gluon Plasma (QGP) and its characteristics. We talked with her about her research interests and future plans.

When Barbara Jacak was still an undergraduate student of Chemistry at the University of California, Berkeley, she participated in a research project on superheavy elements. Working on it, she came to realize that she was more interested in the nuclei themselves than in the chemistry. Thus, when she went to graduate school at the Michigan State University, she joined a programme of chemical physics, which allowed her to follow her newly discovered love for physics, even though she was officially in the Chemistry Department. 

After receiving her PhD, in 1994 she obtained an Oppenheimer Fellowship at Los Alamos National Laboratory and later, in 1997, she became a staff member. There, she took part in the heavy ion programme of the CERN’s Super Proton Synchrotron, first as a member of the HELIOS Collaboration and then of the NA44 experiment. After 12 years at the Los Alamos lab, she was offered the opportunity to join a University and become a professor. Intrigued by this perspective, she moved to the Stoney Brook University, where she started to work on the PHENIX experiment – to the proposal and design of which she had already contributed when in Los Alamos. “It was the perfect time, since some years later the RHIC Collider started delivering collisions,”Jacak comments, “so I could work on the experiment and sleep on my own bed every night.” 

From 2007 to 2012 she served as Spokesperson of the PHENIX experiment, an experience that she really enjoyed.“It was an exciting period,” she recalls, “in which we obtained important results, such as the first observation of a big suppression of charm quarks.  We also started a discussion on what to do after PHENIX would conclude its data taking and the idea of sPHENIX came out”.

At the beginning of 2015 she was appointed Director of Nuclear Science at the Lawrence Berkeley Laboratory (LBNL) and professor of physics at UC, Berkeley. Hence, she moved back to the San Francisco Bay.“I was very excited about this opportunity for both professional and personal reasons,” Jacak explains. “Becoming division director was appealing; it would mean learning how a laboratory works and having the chance to really interact with scientists doing research on all kinds of nuclear physics, not just Quark-Gluon Plasma.”

From the personal side,” she continues “I was happy to come back here, since it is where I grew up and my sister lives 20 minutes away. Also, I can sit at my desk in my office and look through the window at San Francisco…

Upon arrival at Berkeley, she had to choose the collaboration to join, which was not straightforward. Colleagues at LBNL, in fact, were very active in the STAR experiment at RHIC, while she had been Spokesperson of the “competitor” experiment, PHENIX. She doubted about what to do and finally she decided to join the ALICE Collaboration. She gradually formed her group, which now includes two postdocs and four PhD students.

Her passion and dedication to heavy ion physics has been recently recognized by the American Physical Society, which has awarded her the Tom W. Bonner Prize in Nuclear Physics, “for her leadership in the discovery and characterization of the quark-gluon plasma, in particular for her contributions to the PHENIX experiment and its explorations of jets as probes."

Indeed, Jacak is very fascinated by the properties of the QGP, which she defines “the weirdest and most interesting stuff that one can study”, since, despite the huge temperature, it is still strongly-coupled enough that it acts like a liquid. Interested in understanding how the energy is transported by the plasma, she has been focusing on the study of the jet fragmentation function.

At RHIC, we had neither an optimized detector nor enough events to perform this study by reconstructing the jets and their substructure,” she explains “so we used gamma-hadron correlations: the photon tags the momentum of the scattered parton – mostly quarks – and, by looking at the hadrons from the fragmentation of that parton, one can directly reconstruct the fragmentation function both in ptand in angle. This way we found that there is extra soft radiation at large angles.”

She continued working on this topic in the different environment of the ALICE experiment. Here, the higher temperature could affect differently the transport dynamics. “My group has been looking at gamma-hadron and gamma- jet correlations in pp and p-Pb collisions in ALICE and we have presented some preliminary results at the latest Hard Probes Conference; our goal now is to apply our methods to Pb-Pb collisions. The study is still ongoing, we will certainly profit from the coming new data.

Jacak is also participating in the ALICE upgrade activities. Her group at UC Berkeley and her colleagues at LBNL are working together on building the inner layers of the Inner Tracking System (ITS), by assembling staves sent to them from various places around the world.

While in the next two years she will be quite busy studying jet fragmentation and the interaction of partons with hot, dense matter in ALICE, Jacak is already making plans for future activities. Interesting opportunities would come along if the discussed electron-ion collider is actually built in the US. That, in fact, would allow the study of cold dense gluonic matter. “My bet is that it is strongly coupled and has surprising properties, as the high temperature one,” Jacak declares. “I would really like to study it next. By the time the electron-ion collider is built I will probably be formally retired, but I will certainly continue to do physics.” 

Together with her colleagues at other universities in California, she made a proposal to create a University of California Collaboration around the electron-ion collider, which has been just accepted. The goal of this California Consortium is to actively participate in this development and to build a piece of the detector.

We do not understand very well the strongly coupled matter yet,” she concludes “so there is still a lot of research to do on it, both at high and low temperature.