Lev Borisovich Okun

Lev Borisovich Okun died on 23 November 2015 in Moscow, Russia. He commanded greatest respect among scientific community. He was elected to the USSR (now Russian) Academy of Sciences in 1966 (full member since 1990) and received numerous national and international scientific prizes and awards. He was the first Soviet physicist elected to the Scientific Policy Committee of CERN.

Okun was born on 7 July 1929 in Sukhinichi, Kaluga region, USSR. He attended the Moscow Engineering Physics Institute with Kogan and Migdal as his advisors and in 1954 became a post- graduate student at the Institute for Theoretical and Experimental Physics in Moscow that remained his scientific home for the rest of his life. There he received his Ph.D. in 1956 under the supervision of Isaak Pomeranchuk, and in 1961 he received his Doctor of Science degree. At ITEP he organized and headed for more than 30 years a laboratory of elementary particle theory. ITEP was the Center of the “School of Okun” with his colleagues, students and students of his students. Many visitors from USSR and the World visited ITEP to discuss physics with Lev. He always sought to achieve clarity and maximum possible simplicity in understanding physics phenomena and concepts. His deep intuition and vision of most fundamental ideas influenced the development of particle physics for over half a century.

One of Okun’s favorite subjects was the study of weak interactions. From his early works he contributed several fundamental results to this development, such as the conclusion that violation of P-parity in beta-decay also means the violation of C-parity (1957, together with Ioffe and Rudik) , as well as an evaluation of the difference between the masses of neutral K-mesons (with Pontecorvo, 1957). His remarkable book “Weak Interaction of Elementary Particles”, published in 1963, became a textbook and a desktop reference material for several generations of students and academics. This book, that appeared before the Quark Model was based on one of the first successful composite models of hadrons (this term was introduced into scientific language by Okun), the so-called Sakata-Okun model, that he was developing since 1958. In this model, all known particles were constructed of three proto-particles predecessors of quarks. He had predicted the existence of η and η′ mesons, and formulated the selection rule ∆Q= ∆S for semi-leptonic decays of strange particles.

In the field of strong interactions the famous Okun-Pomeranchuk theorem on the equality of cross sections for scattering of the particles from the same isomultiplet at asymptotically high energies was proved in 1956. In the 70-s he and co-authors developed a new method of QCD sum rules, that became known in the literature as the “ITEP Sum Rules”.

He has made seminal contributions to the new field of research at the intersection of particle physics, cosmology and astrophysics. A method for calculating relic abundance of elementary particles during the expansion of the Universe was developed in his 1965 paper with Zel’dovich and Pikel’ner. They performed a calculation of the abundance of free quarks. Non-observation of free quarks was one of the arguments for quark confinement. Now the approach that emerged from this paper became a standard tool in the studies of the origin of dark matter in the Universe.

In 1964, in the paper written together with Pomeranchuk and Kobzarev, the idea of “mirror world” came into existence. “Mirror matter” is still a possible candidate for dark matter.

Vacuum domain walls investigated by him in 1974 were the first macroscopic object of QFT that could determine the evolution of the Universe. In the same year Okun together with Voloshin and Kobzarev published a pioneering paper on the decay of the false vacuum -- a subject that unexpectedly became of a relevance to the physical vacuum in our Universe after the discovery of Higgs boson with mass 125 GeV.

Lev was contemplating a lot and in-depth about the limits of applicability of the fundamental concepts of physics, such as the Pauli principle, CPT invariance, conservation of electric charge, the absence or existence of “other photons”, electrical neutrality of atoms, masslessness of photon. In each of these questions he sought to quantify the limits of applicability of such principles and had analyzed practical ways to improve those limits.

Scientific discussions with Okun played an invaluable role for his colleagues and students. His relentless desire to achieve clarity of understanding of the subject was often harassing, but always beneficial at the end. Many are grateful to him for his ability to ask the “right” questions. A famous example is his comment during the talk of A.M. Polyakov in 1975, when he pointed out that solution of the Yang-Mills equations found by speaker was nothing else but a magnetic monopole.

He was absolutely devoted to physics and embodied the quest for truth that is at the heart of physics. He had an extraordinary pedagogical ability and his course of particle physics that he taught for many years was legendary among the students.

Okun was a preeminent physicist, an extraordinary person of outstanding integrity, a friend of great warmth and a source of deep knowledge and wisdom. He will be remembered by his family, friends and colleagues. He is survived by his wife Erika, daughters Olga and Inna and son Boris, and their families.