Molecule: Electrically neutral particle made of one or more atoms held together by covalent (electronsharing) bonds. Muon: The second-generation charged lepton (see pp. 84–5). Neutrino: Uncharged lepton that interacts only via the weak force (though recent results show they do have a very small mass). There are three flavours of neutrinos, corresponding to the charged leptons: the electron neutrino νe, the muon neutrino νμ and the tau neutrino ντ. Neutron: Neutral, spin-half hadron consisting of one up quark and two down quarks. Nucleons: Protons and neutrons (i.e. those that make up the nucleus). Nucleus: The centre of the atom, containing its protons and neutrons. Omega (baryon): Composed of three strange quarks, the Ω- baryon was predicted by the Eightfold Way, which in turn helped with the development of the quark model. Parity: A symmetry of nature relating to the inversion of an odd number of spatial coordinates (see p. 101). Parton: Feynman’s name for sub-nucleonic particles, i.e. quarks and gluons. Photon: Spin-one exchange boson of the electromagnetic force. Pion (a.k.a. pi meson): Hadron consisting of two up or down quarks; depending on the exact quark content, pions can be positively-charged, negatively-charged or neutral. Positron: The antimatter equivalent of the electron (electric charge +1). Proton: Positively-charged, spin-half hadron consisting of two up quarks and one down quark. Quantization: The division of a quantity into discrete (i.e. non-continuous) values. Quantum chromodynamics (QCD): Quantum field theory describing the interactions of quarks and gluons (see pp. 118–21). Quantum electrodynamics (QED): Quantum field theory describing the interactions of charged matter and photons (see pp. 76–81). Quantum Field Theory (QFT): A mathematical model in which every point in space is associated with an infinite number of tiny springs, each representing a potential chunk of energy. All matter and forces may be thought of as ripples passing through this network of springs. Quark: Spin-half elementary particle that experiences all four fundamental forces (see pp. 107–17). Radioactivity: The processes by which atomic nuclei undergo some sort of decay, losing energy via the release of ionizing radiation. Renormalization (cf. QFT): Mathematical technique used to remove infinities from quantum field theories by absorbing them into measurable quantities (see p. 78). Special Relativity: Theory that works from two postulates: that the laws of physics are the same wherever you test them, and that the speed of light is a fixed constant of the universe. See Introducing Relativity for more information. Spin: Quantum-mechanical property of a particle relating to a sort of internal angular momentum that is almost entirely unlike spinning about its own axis. See Introducing Quantum Theory for a more thorough explanation! Standard Model: The quantum field theory that combines our knowledge of the strong, weak and electromagnetic forces that describe the interactions of the known fundamental particles (see p. 104 onwards). Strangeness: Before the quark model was formulated, this was the property assigned to particles containing strange quarks. Strong force: One of the four fundamental forces of nature. It is described by quantum chromodynamics. Only particles with colour charge experience the strong force. Supersymmetry: A hypothetical symmetry of nature that relates fermions and bosons. Tau (lepton): The third-generation charged lepton (see p. 122). Not to be confused with the Tau of the Tau-Theta puzzle (see below). Tau-Theta puzzle: The two decay modes of the positive kaon (also known as the K+) have differing final state parities. When both were observed in cosmic rays, it was thought that they must be due to the decay of two different particles, even though the mass and charge were the same. The puzzle was resolved when it was realized that parity could be violated, i.e. the parity of the final state did not have to be the same as the parity of the initial state (see pp. 100–03). Wave mechanics: Formulation of quantum mechanics where a particle’s properties (position, momentum, etc.) are described in terms of statistical distributions that resemble the mathematical description of waves. Weak force: One of the four fundamental forces of nature. It is mediated by the massive W and Z bosons. Both quarks and leptons experience the weak force. X-ray: High-energy electromagnetic radiation.