a particle first discovered by Anderson and Neddermeyer at Caltech in 1936; originally thought to be Yukawa's meson, but later shown to be a "heavy electron", now called muon. Cowan et al. (1956) See the nuclear physics section. An important experiment (based on a theoretical analysis by Lee and Yang) that proved that parity conservation was disobeyed by the weak force, later confirmed by another group in the same year. This won Lee and Yang the Nobel Prize in Physics for 1957. Sakharov, A. D. (1967). See cosmology section. Standard undergraduate particle physics textbook. Quantum mechanics For relevant publications before 1900, see atomic and molecular physics section. Introduced Planck's law of black body radiation in an attempt to interpolate between the Rayleigh–Jeans law (which worked at long wavelengths) and Wien's law (which worked at short wavelengths). He found that the above function fit the data for all wavelengths remarkably well. This paper is considered to be the beginning of quantum theory and discovery of photon. English translations: "On a Heuristic Point of View about the Creation and Conversion of Light". Translated by Dirk ter Haar "On a Heuristic Point of View about the Creation and Conversion of Light. Translated by Wikisource Introduced the concept of light quanta (called photons today) to explain the photoelectric effect. Cited for Nobel Physics Prize (1921). Part of the Annus Mirabilis papers. Bohr model papers: Introduced the Bohr model of the (hydrogen) atom, which later formed the foundation for the more sophisticated atomic shell model of larger atoms. An experiment on the electrical conductivity of gases that supported the conclusions of the Bohr model. Arnold Sommerfeld (1919). Atombau und Spektrallinien. Friedrich Vieweg und Sohn, Braunschweig' . Arnold Sommerfeld, translated from the third German edition by Henry L. Brose Atomic Structure and Spectral Lines (Methuen, 1923) Added a relativisitic correction to Bohr's model achieved in 1916, by Sommerfeld. Together with Planck (1901), Einstein (1905) and Bohr model (1913) considered stanchion of old quantum theory. This important experiment on a beam of particles through a magnetic field described the experimental observation that their deflection takes only certain quantized values was important in leading to the concept of a new quantum number, spin. de Broglie, Louis (1924). Recherches sur la théorie des quanta (in French) (Researches on the theory of quanta), Thesis, Paris. Ann. de Physique (10) 3, 22 (1925) Introduced formally the concept of the de Broglie