Guido Altarelli 1941–2015

Guido Altarelli passed away on 30 September in his 74th year. A great theoretical physicist, Guido spent most of his scientific carrier in Rome and at CERN. He received the Julius Wess Prize in 2011, the Sakurai Prize in 2012 and the EPS Prize in 2015, but these very celebrated awards only partially reflect the richness of his contributions: his deep and long-lasting influence is ubiquitous in the field of high-energy physics.

Guido was born on 12 July 1941. He studied physics at the University of Rome, and in 1963 he graduated under the supervision of Raul Gatto (who was in Florence) with a thesis in collaboration with Franco Buccella on single-photon emission in e+e collisions. In 1964, he joined the very lively and large group of young researchers (known as "gattini", which means "young cats" in Italian) that were working together under the careful and inspiring supervision of Gatto ("cat", in Italian). In Florence, where he remained until 1968, Guido worked with Gatto and the other gattini (among them Franco Buccella, Luciano Maiani and Giuliano Preparata), primarily on topics that were fashionable at the time (e.g. SU(6) symmetries, Regge poles). After two years in New York, he returned to Rome as professor, initially at La Sapienza until 1992 and later at the University of Roma Tre. During this period, he was a senior staff physicist in the CERN Theory Division (1987–2006) and theory division leader from 2000 to 2004.

After his return to Rome, his scientific interests veered towards the parton model and QCD. Many of these works were written with Nicola Cabibbo, Luciano Maiani, Guido Martinelli, Roberto Petronzio and myself. Among the most important ones are the papers on octet enhancement of non-leptonic weak interactions in asymptotically free gauge theories – a crucial and seminal step towards understanding the interplay between QCD strong interactions and weak interactions – as well as his paper on the discovery of large QCD correction to the naïve parton model prediction in μ+μ production, and the paper where we derived the so-called Altarelli–Parisi equation. This last paper stemmed from one of his ideas, which was to make previously obtained results on scale violation clearer and more exploitable. It was written while both of us were in Paris, and Guido liked to remark that it is the most cited French paper.

New interests

In the 1980s he became more interested in deriving predictions for future CERN experiments, e.g. the production of jets, heavy-vector mesons, and other exotic objects like Higgs and supersymmetric particles. In the same period, he continued his deep analysis of the consequences of QCD on weak-interactions theory, e.g. computing, for the first time, the two-loop contributions. He also wrote seminal papers on the decay of heavy quarks. During this period, he became deeply interested in polarised proton structure function, where he discovered, together with Graham Ross, the crucial interplay among the gluon anomaly and polarisation effects.

Later on, after he went to CERN, Guido worked on the construction of a model independent analysis of electroweak data (with Riccardo Barbieri as a precious collaborator), on the Higgs mesons (theoretical predictions on the mass and the cross-section at colliders), and on many other problems. In the new millennium, Guido went on to work on a new subject without neglecting any of the old ones: he became fascinated by the elegance of tri-bimaximal neutrino mixing. Many of his papers (mostly written with Ferruccio Feruglio) are dedicated to the search for the possible origins of this baffling symmetry.

As often happens, his scientific success was inseparable from his human qualities, and was not only due to his technical capabilities. Perhaps his most characteristic features were his great kindness and intellectual honesty, coupled with a rather ironic view of himself and of life in general. His great inquisitiveness, the enjoyment he derived from learning new things and putting together the pieces of a puzzle, allowed him to make summaries of topical subjects. These were crucial, not only because they allowed us to take stock of the current state of a field of research, but also because they indicated new directions to take. He liked clear, precise formulations that could be understood by all.

He was not a reclusive or selfish scientist, only interested in the personal prestige that could be gained from his research. Guido was also a researcher who worked with others within a large community – that of CERN, and the high-energy particle-physics community in general. Many of Guido’s works, from the most famous to the lesser known, were conceived in a spirit not only of research, but of service to the community to which he belonged. It is difficult to imagine what the status of the field would be without his contributions.

We all miss him very much, not only as an invaluable scientist, but also as a dear friend.

• Giorgio Parisi.

Harry (Zvi) Lipkin 1921–2015

Harry (Zvi) Lipkin passed away on 15 September. He was one of the leading theoretical physicists of Israel, one of the founders of the Physics Department of the Weizmann Institute, and a major contributor to a broad spectrum of topics. Unlike most theoretical physicists, his originality and creativity continued for many decades; he was the prolific author of excellent new ideas at an age when others retire and most have forgotten their last important contribution.

Lipkin was born in New York City in 1921 and grew up in Rochester, New York. His life was very rich: he graduated in engineering; contributed to the crucial WWII anti U-boat microwave radar project at MIT; undertook an experimental-physics PhD thesis at Princeton; immigrated to Israel with his wife Malka to start a pioneering life in an agricultural kibbutz on the Lebanese border; was sent to France to study nuclear reactors; joined an early R&D unit of the Israeli army; co-founded and moved into the newly created Department of Nuclear Physics at the Weizmann Institute; became a theoretical nuclear physicist… and we have only reached 1955 in his history. For the remaining 60 years of his life, he also contributed to theoretical condensed-matter physics, particularly the Mössbauer effect; basic problems in quantum mechanics; and, especially, particle physics, with an emphasis on symmetries, quark-model analysis, applications of group theory and a wide variety of other topics. His book Lie Groups for Pedestrians introduced many generations of physicists to the subject. He received several major prizes, including the Wigner Medal, the Emet Prize and the Rothschild Prize. He spent long periods of research in the US, especially at Argonne National Lab and, for decades, was a frequently invited speaker at just about every major physics department and conference.

But his original contributions to physics research were only one aspect of his incredible career. He always felt that one should never take oneself too seriously, even as a scientist. Together with virologist Alexander Kohn, he founded the Journal of Irreproducible Results, in which no allegedly serious scientific topic remained immune to parodies, jokes and ridicule. Lipkin was also passionate about the teaching of reading in elementary schools, a subject about which he held strong, well-informed views, often arguing his case in widely distributed written contributions. He did the same on his own interpretations regarding events in the Middle East, and was essentially a prolific blogger, decades before the word "blog" was coined. In the 1980s, Lipkin corresponded with the exiled Andrei Sakharov, and was instrumental in keeping Sakharov’s fate in the focus of public opinion.

His research, as well as his attitude to everything else, was enriched by a unique ability to provide simple descriptions and explanations, often using analogies to better understood topics. His physics work always stood on several basic, solid legs: maximal contact with experiments, both already performed and newly proposed; a rare intuition for complex quantum-mechanical paradoxes and dilemmas, a feature that most great physicists understand but have no intuition for; and an ability to see through a myriad of irrelevant details, straight to the heart of the matter.

Lipkin was an excellent scientist, great mind and a wonderful tour guide through many labyrinths. We are proud to have been his friends and collaborators.

• Haim Harari, Weizmann Institute and Marek Karliner, Tel Aviv University.