by Virginia Greco. Published: 10 November 2017

On the occasion of his 80th birthday, we talked with Guy Paić, a brilliant physicist and a long-time member of the ALICE collaboration, about his career and his passions.

Guy Paić During the celebration of his 80th birthday at the Universidad Autónoma de Mexico.

Recently turned 80, Guy Paić is one of the founders of the ALICE experiment and has been in heavy-ion physics since its infancy. To celebrate his birthday, his colleagues at the Universidad Autónoma de Mexico have organised a symposium in his honour, held on the 30th of October. The poster of the event features Paić in a fighting ring and with a typical ‘luchalibre’ mask on his face. He never practised free fight, but the image is a metaphorical representation of his personality. ‘This is how they see me, as a fighter’, Paić explains. 'I have always fought, because I am not the kind of person who likes to go with the flow, I prefer to go counter-flow. It is my nature.'

The son of a medical doctor turned physicist (his mother) and a chemist turned physicist (his father), he was born in France, where his parents settled during the Second World War. After the end of the conflict, they moved to Zagreb, in Croatia, where Paić studied first Electrotechnical Engineering and then Physics, thus following the tradition of the family. 'When I was young I was fascinated by physics,’ he recalls, ’but my father had told me that I should go for a profession that could help me feed myself and my future family. It was an age when children used to follow the guidance of parents. So, I enrolled for electrotechnical studies and, when I completed them, I started the courses of physics as well, with the support of my family."

This double training actually marked his career, since he has always worked at the same time in hardware and in analysis. In ALICE, for example, he was co-spokesperson of a R&D project for the production of Caesium-Iodide (CsI) photocathodes (RD26) and was also involved in the development of the High Momentum Particle Identification Detector (HMPID). On the other hand, he covered the role of physics coordinator for a long time and he was in charge of the ALICE physics performance report.

Paić entered heavy-ion physics in 1986 thanks to Reinhard Stock, who had invited the physics group of the University of Zagreb to join the NA35 experiment at the SPS. It was a fortunate occasion, since Paić - who at that time was working in low energy physics - was looking for a new challenge and because it opened for him the way to a brilliant career in heavy-ion physics. ‘I learned a lot from Reinhard Stock. Even though he is a bit younger than me, I consider him as my mentor.

Following NA35, Paić joined the NA44 and the ALICE collaborations. Indeed, he is one of the early 50 members of ALICE. ‘At that time,’ he comments, 'we all knew each other, we were all friends. Now the collaboration counts more than 1500 members, there are many groups in it and everything is very structured. Of course, managing such a big experiment is completely different than a small one, and it is not at all easy.

Since 2003, Paić is Professor at the Universidad Autónoma de Mexico, in Mexico City. ‘It was a turn in my life that I hadn’t planned at all, it was an occasion that just arrived and I took it. Paolo Giubellino informed me that our Mexican collaborators were looking for candidates for a full professorship and to establish a laboratory at their University. He told me that he considered me as the right person, given my experience and skills.’ Paić accepted the proposal and went to Mexico, a country that he didn’t know at all, and there he built up a research group and a laboratory: ‘I was given full support, but no instrumentation or students. I had to start from scratch, as my father did at the University of Zagreb’.

Anytime I look back and think of the fact that I was offered this position at UNAM when I was already 65, I feel lucky and very much in debt of my colleagues,’ he confesses. ‘They have always supported me and I try to be worth the money they invested - and keep investing - on me.'

The poster of the Symposium organized at the Universidad Autónoma de Mexico in honor of Guy Paić on occasion of his 80th birthday.

Currently, he is mainly focusing on the study of high-multiplicity events in pp collisions, since he firmly believes that in such collisions lie much information complementary to the one that can be extracted from Pb-Pb collisions. This research considers pp collisions not only as a benchmark for Pb-Pb analysis, but as an interesting system per se. Indeed, in 2000 Paić and six of his ALICE colleagues wrote the first note on the possible importance of pp collisions for the field (ALICE2000-28).

If it is true that this year ALICE is pursuing a full pp programme - no lead runs are scheduled until the end of 2018 - and that Paić has always had internal support, it is also worth mentioning that his opinions are often contradictory to the established ones. ‘I think there is the need for somebody who proposes something different, who tries other ways,’ he explains. 'What is the point of having thousands of people working on the same thing?'

The group he formed at the Instituto de Ciencias Nucleares at UNAM is well recognised in ALICE: Antonio Ortiz is convener of a Physics analysis group member of numerous paper committees   and we have a number of students, three of whom presented talks on behalf of ALICE in international conferences this year. He believes that this represents the best impact he has had in Mexico for the benefit of the science there and for ALICE.

Even though he could enjoy a nice and relaxing retirement, Paić prefers to continue his research activities. “Physics has always been a passion for me, I don’t take it as a job. I still have ideas so I keep thinking and doing research. The day I have no ideas anymore, I will leave.’

He is convinced that within five years ALICE will come up with important results and interesting breakthroughs. ‘Our field of research is very open and ALICE’s potential for discovery, in my opinion, is large,’ Paić comments. 'The Higgs boson was a sort of known-unknown, in the sense that we knew it was there, even though we didn’t know where exactly. On the contrary, with ALICE we look into something that is still the “unknown unknown”. I think we will understand the interactions of these systems of particles, be it through pp high-multiplicity or heavy-ion collisions.’

While he probably reached the most important achievements of his career while working in ALICE - and he has recently become one of the few researchers in the world having a publication impact factor (h-index) higher than 100 (108 at the date) - he has produced interesting results in other fields as well. In particular, before focusing on high-energy physics, he had worked on low energy physics and had also become an expert in dosimetry and radiation protection. This expertise allowed him to participate in many missions in Africa for the International Atomic Energy Agency (IAEA) between 1973 and 1999. He spent long periods in Morocco, Madagascar, Senegal, Sudan, Ghana and the Ivory Coast.

Since during his PhD he had worked on a neutron generator - ‘my father had built it in Zagreb,’ he recounts - and the IAEA was following the installation of similar machines in various places around the world, he was proposed to help commissioning and putting them into operation. He also performed some physics studies with them.

Paić’s high expertise in this field is testified by the fact that he wrote a book on dosimetry, called “Ionizing Radiation: Protection and Dosimetry”, which was published in 1988 by CRC Press. ‘The interest for dosimetry is another thing I have inherited,’ he tells. 'In this case, it was from my mother, who worked in radiation protection when we moved to Zagreb. She was a physician by training, but she ended up working in physics as well.’

Not only does he keep his mind at work, but his body as well, since he usually plays tennis two or three times a week and practices bicycling regularly: ‘I used to come to work always by bike. Now I have an electric one since I live in Gex and the way back is quite hard because I have to climb a bit.’ Considering that Gex is about 15 km from the Meyrin site of CERN, it is quite impressive.