Theoretical physicist Lev Borisovich Okun died on 23 November 2015 in Moscow. For his work he had received numerous national and international scientific prizes and awards, including the 1988 Matteucci Medal from the Italian National Academy of Sciences and the 1996 Bruno Pontecorvo Prize from Russia’s Joint Institute for Nuclear Research. In 1981 he became the first Soviet physicist elected to CERN’s Scientific Policy Committee.
Okun was born on 7 July 1929 in Sukhinichi, USSR. He attended the Moscow Engineering Physics Institute, with Vladimir Kogan and Arkady Migdal as his advisers. In 1954 he became a postgraduate student at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow, which remained his scientific home for the rest of his life. He received his PhD on the topic of heavy mesons and hyperons in 1956 under the supervision of Isaak Pomeranchuk, and in 1961 he received his doctor of science degree.
At ITEP Okun organized and for more than 30 years headed a laboratory for elementary-particle theory. ITEP was the center of the “School of Okun” comprising his colleagues, his students, and students of his students. Many scientists from the USSR and around the world visited ITEP to discuss physics with him. He was an exceptional instructor, and the course on particle physics that he taught for many years was legendary. He always aimed for clarity and simplicity in understanding physics phenomena and concepts. His deep intuition for and approach to most fundamental ideas influenced the development of particle physics for more than half a century.
Okun contributed several fundamental results to the development of weak interactions, a favorite subject of his. In 1957, with Boris Ioffe and Alexei Rudik, he showed that violation of P (parity) symmetry in beta decay also means the violation of C (charge conjugation) symmetry. That same year he and Bruno Pontecorvo evaluated the difference between the masses of neutral K mesons.
In 1963 in the USSR, Okun published his influential book Slaboe vzaimodeystvie elementarnych chastiz (Fizmatgiz), which in 1965 was translated into English as Weak Interaction of Elementary Particles (Pergamon). Several generations of students and academics have used it as a textbook and desktop reference. Published before the quark model was introduced, the book was based on one of the first successful composite models of hadrons, which Okun and Shoichi Sakata had been developing since 1958. In the Sakata–Okun model, all known particles were constructed of three protoparticle predecessors of quarks. Okun introduced the term “hadron” into the scientific language in 1962 during a talk he gave in Geneva at the 11th International Conference on High Energy Physics. Okun had also proposed in 1958 the existence of η and η’ mesons.
His work on strong interactions included a 1956 paper in which he and Pomeranchuk proposed the equality of cross sections for scattering of particles from the same isomultiplet at asymptotically high energies, now known as the Okun–Pomeranchuk rule. In the 1970s Okun and colleagues developed a new approach based on quantum chromodynamics sum rules that became known in the literature as the ITEP sum rules.
In the 1960s Okun also made seminal contributions to the study of dark matter and what was then a new field of study that encompassed astrophysics, cosmology, and particle physics. He, Pomeranchuk, and Igor Kobzarev proposed in a 1964 paper the concept of a “mirror world” and its “mirror matter,” particles that have the sign of parity violation opposite that for our world’s particles and that interact only very weakly, possibly exclusively by gravity, with ordinary matter. The mirror matter continues to be a possible dark-matter candidate. In a 1965 article that contains what is now a standard tool for studying dark matter’s origins, Okun, Yakov Zel’dovich, and Solomon Pikel’ner devised a way to determine the relic abundance of elementary particles during the universe’s expansion. They calculated the abundance of free quarks and strengthened one argument for quark confinement, the non-observation of free quarks.
In 1974 Okun produced two groundbreaking papers. In one, he examined vacuum domain walls, the first macroscopic object of quantum field theory to help determine how the universe evolved. In the other, he, Kobzarev, and one of us (Voloshin) wrote about the quantum tunneling mechanism of metastable vacuum decay; the 2012 discovery of the 125 GeV Higgs boson makes their finding particularly relevant to the study of our physical vacuum.
Okun thought deeply about many fundamental concepts of physics, such as the Pauli exclusion principle, CPT invariance, conservation of electric charge, the absence or existence of “other photons,” electrical neutrality of atoms, and the masslessness of the photon. He sought to quantify the limits of applicability of each of the concepts and analyzed practical ways to improve those limits.
His colleagues and students found their scientific discussions with Okun to be invaluable and gained inspiration from his devotion to physics and his quest for truth. His relentless desire to clearly understand a subject was often intimidating, but it would undoubtedly prove beneficial in the end. Many are grateful to him for his ability to ask the right questions. A famous example is his comment during a talk by Alexander Polyakov in 1975, when Okun pointed out that Polyakov’s solution to the Yang–Mills equations was nothing but a magnetic monopole.
To his family, friends, and colleagues, the preeminent physicist will be warmly remembered for his extraordinary integrity, intelligence, wisdom, and kindness.
