Lev Borisovich Okun (Russian: Лев Борисович Окунь; 7 July 1929 – 23 November 2015) was a Soviet theoretical physicist.
His book Weak Interaction of Elementary Particles, published in 1963,[2] became a textbook and a desktop reference material for several generations of students and academics.
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.
A method for calculating relic abundance of elementary particles during the expansion of the Universe was developed in his 1965 paper with Yakov Zel'dovich and S. B. Pikel'ner.
Vacuum domain walls investigated by him in 1974 were the first macroscopic object of quantum field theory that could determine the evolution of the Universe.
In the same year, Okun together with Mikhail B. Voloshin and I. Y. 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 the Higgs boson with mass 125 GeV.