by Virginia Greco. Published: 18 December 2017

John Harris, a professor of physics at Yale University and the founding spokesperson of the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC), is now Chair of the ALICE Collaboration Board (CB). We talked with him about his role in the collaboration, his scientific interests and his vision for the future of heavy-ion physics.

Professor Harris, what pushed you towards heavy-ion physics and what are the steps of your career that you consider as the most important?

I think that good education and some positive influences from people you meet in your life path are very important. They certainly were for me. My father used to work as a hobby on cars and TVs, I used to help and I got interested in that. Then, in high school, I had an excellent physics teacher who was a mentor for me. I also enjoyed very much my college advanced laboratory courses, including electronics. This motivated me and gave me the confidence to work on detectors and hardware as well, even though physics was – and still is – my primary interest.

The influence of people has been a determinant in pushing me towards relativistic heavy-ion physics as well. I think that it started when I was an undergraduate and attended a summer course on quantum mechanics given by Nobel Prize laureate Hans Bethe. In 1971 he had written a more-than-100-page-long review article on what he considered hot topics in nuclear physics theory at the time. I read that article as well as other papers that were published in the 1970’s on Lee-Wick matter, on pion condensation and other exotic states at high density. All this attracted my attention and interest.

In 1977 I attended a 7-week-long summer school on nuclear theory at Les Houches (France), where Herman Feshbach, a professor at MIT, gave a fascinating talk on relativistic heavy ion physics, something that I had never heard of before as a field itself.

Hence, right after completing my Ph.D., I moved from research in low energy nuclear physics to high energy heavy ion physics. I was given the opportunity to go to LBL at the University of California (UC) in Berkeley and work with Reinhard Stock at the Bevalac - where ions were pre-accelerated by the SuperHILAC linear accelerator and then injected into the Bevatron synchrotron. Stock, who is a strong, clever and passionate physicist, then became my life-long mentor. At Berkeley I made the first important steps of my career: I went from post-doc to divisional fellow and I worked in the NA35 and NA49 experiments at the CERN Super ProtonSynchrotron (SPS). Then, I was given the responsibility to develop a concept for an experiment at the Relativistic Heavy-Ion Collider (RHIC). Together with some colleagues, we designed and constructed the STAR experiment, of which I was spokesperson for 11 years.

Twenty-one years ago, I was invited to join the Faculty of Yale University, where I was able to put together from scratch my own research group. I am glad for that, I really like having undergraduate and graduate students, as well as PhD scientists, working with me.  

How and when did you join the ALICE experiment?

In Spring 2002 at an ITP Workshop in Santa Barbara (California), Jurgen Schukraft and Peter Braun-Munzinger approached me and proposed to me that my group - and the US –join ALICE. Then in 2006 after many discussions with other groups and funding agencies in the US, we applied to enter the ALICE collaboration and proposed to work on building the calorimeter. I was enthusiastic about taking part in this experiment, which I felt was really top-notch.

Since last January you have been the Chair of the ALICE Collaboration Board (CB). Were you happy being elected for this role?

Yes, very much so. During the years spent in ALICE, and in particular being deputy of the previous Chair, I developed my idea of how a collaboration should be. Not that one person alone can make big changes, but certainly the collaboration board chair can influence things.

When I was spokesperson of STAR I struggled with the collaboration council, which I think was not very active or productive in ensuring that everybody worked effectively and at their best. The spokesperson’s team is responsible for the day-to-day activity, but there are longer term issues that the collaboration board has to take care of, in particular engaging every member - down to the individual student - ensuring that everyone feels welcome and a collarative environment that encourages and values his or her contribution.

This year the collaboration board has continued with the work of updating the constitution that had been started during the previous mandate. Written more than 20 years ago and already modified once, it absolutely needed some updates. We have changed the publication policy and addressed some diversity issues, both key to creating a collaborative and inclusive climate in the experiment.

Is it difficult to cover this role being physically distant from where the experiment sits?

Actually, it is. Frankly I hadn’t realised how difficult it is until this year once I started. I am trying to spend about a week a month at CERN, which is already a lot of travelling - and on top of that there are conferences. I find it difficult to do more because I am teaching most of the year, during both semesters. I have already arranged to have a bit less of teaching duties, but on the other hand I really like giving classes and engaging in University courses.

Being physically at CERN is very important for developing relationships and trust, you need to be able to discuss things and be open. Also, people need to get to know you. I think this is the case between Federico Antinori and me: we work together and I don’t have any qualm or problem approaching him. This is essential in my opinion, since the CB has an oversight on the needs of the collaboration, can determine directions, thus it must be able to raise the attention of the spokesperson’s team on some issues.

As I said, I am trying to be at CERN as much as I can and I do realize that maybe it would be more effective if I were more present. However, it is important to have an outside perspective. I think that along the 10 years spent in ALICE before becoming the CB chair I developed relationships sufficiently to know people well and interact with them effectively.

Do you have sometime a member of the collaboration coming directly to you to discuss about an issue?

Yes, that happens. It normally starts with an email and when I am at CERN I try to meet with the person and talk. If they are not at CERN, I try to arrange a phone call. Phone calls do not work as well as talking in person, especially if the topic is somewhat sensitive, but they are useful anyway.

What is your main field of interest?

My personal interests have been always quite broad in physics in general. With respect to detectors and instrumentation, I have worked primarily on tracking, time projection chambers and silicon devices.

Physics-wise, in my group at Yale we realised that it was advantageous to carry out research on hard probes, high-pt probes and heavy flavour jets at the LHC more than at RHIC - as in STAR jets were barely visible - and to utilize those to understand the properties of the QGP. So we work on that.

Ever since I joined ALICE, I have also been particularly interested in tying together some investigation on hard probes (heavy quarks, jets, flavour dependences) and soft observables.

Of course, I don’t analyse data myself anymore, but I am very fortunate to have talented people working with me and throughout ALICE in general.

How many people are in your group now?

My group comprises two faculty members, leading respectively the STAR-effort and the ALICE-effort: Helen Caines – who has been recently elected co-spokesperson of STAR - and myself. Between the two, in total we have three postdocs, two detector physicists, six graduate students and three undergraduates. Of these, two postdocs and four graduate students are in my ALICE subgroup, while the two detector physicists are working both on the ALICE TPC upgrade and on some general detector research and development. We are lucky to be well supported and we feel like we can carry out a lot of interesting research activities.

What is your vision for the future of ALICE and heavy-ion physics in general?

I think that heavy-ion physics is in a very interesting phase, but there is a lot to be done. We need to understand better the properties of the QGP and the initial state. I think that some innovative thinking and cross-pollination with other fields, such as condensed matter for the dynamics and hadronization time scales, high-energy physics for observables like jet sub-structure, would be very fruitful. In my opinion, in Run 3 of LHC and in future projects - if there will be a Future Circular Collider (FCC) for example - we really have to make measurements looking at heavy-quarks, jets and flavour dependencies, and correlating those with soft observables.

We are under scrutiny of the entire field of physics, it is our responsibility to provide the scientific evidence of the properties of the plasma to really convince the rest of the field of its existence. The bottom line for me is that there is both a need and a lot of room for new ideas and innovation on how to do all this, both in experiment and theory.

Looking beyond Run 3, a future CERN collider could also address topics like topological defects and even entanglement, which go well beyond our field of heavy-ion physics.

Do you think that with the data taken before the next long shut-down ALICE will be able to make some important measurements? Or will we have to wait for Run3?

I think that Run 2, especially the data-taking of 2018, will be central for giving us a good idea of how the hard probes and heavy flavour correlate with the soft observables. Another priority at the moment is trying to understand better high-multiplicity pp and p-Pb collisions and how that relates to what we see in Pb-Pb. This is really essential and I think may point back to the need for a better understanding and measurements of the initial state.

So, yes, I think that we will have interesting results in Run 2 and then Run 3 will bring additional statistics for correlating observables, and should allow us to make more definitive statements.

“What are your interests outside your job? Do you find time for hobbies?”

I feel like I have too many outside interests to find time to satisfy them all. But I make time (and travel) for kitesurfing (California, Hawaii, Panama), backpacking (Rocky Mountains) and some kayaking (recently Alaska and Puget Sound). More frequent local interests include hiking, swimming and staying healthy. I long to write, but I find little time for that at the moment.