4/8/2023 0 Comments Elementary particles![]() ![]() Finally, we note that there is symmetry between particles and particles, and it is a convention which is which for example, we could call the positron the particle, and the electron the antiparticle. When brought together, particle–antiparticle pairs can annihilate each other, releasing their combined rest energy 2 mc 2 as photons or other forms of radiation. We will meet other quantum numbers in Sections II and III. Many quantum numbers differ in sign for particle and antiparticle, and electric charge is an example of this. Electric charge is just one example of a so-called quantum number (spin, introduced earlier, is another), which is a quantity that characterizes a particle, whether it is elementary or composite. In this case the superscript denoting the charge makes explicit the fact that the antiparticle has the opposite electric charge to that of its associated particle. Thus, for example, associated with the negatively charged electron e − is an antiparticle e , called the positron. However, for very common particles the bar is often omitted. For example, associated with every quark, q, is an antiquark, q ˜. If we write the particle as P, then the antiparticle is in general written with a bar over it, i.e., P ˜. Experimental evidence confirms this important theoretical prediction. ![]() This has the same mass as the corresponding particle, but the opposite electric charge. Hence any explanation of the phenomena of elementary particle physics must take account of the requirements of the theory of special relativity, in addition to those of quantum theory.Ĭonstructing a quantum theory of elementary particles which is consistent with special relativity leads to the conclusion that for each charged particle of nature, whether it is an elementary particle or a hadron, there must exist an associated particle, called an antiparticle. This in turn requires electron energies which are greater than 10 3 times the rest energy of the electron, implying electron velocities very close to the speed of light. ![]() For example, to explore the internal structure of the proton using electrons requires wavelengths which are much smaller than the radius of the proton, which is roughly 10 −15 m. By the de Broglie relation λ = h/ p, this implies that the momentum p of the probing particle, and hence its energy, must be large. The second reason is that to explore the structure of a particle requires a probe whose wavelength λ is at least as small as the structure to be explored. One reason for this is that if we wish to produce new particles in a collision between two other particles, then because of the well-known relativistic mass–energy relation E = mc 2, high energies are needed, at least as great as the rest masses of the particles produced. Thus, electron-lepton and tau-lepton numbers are also conserved.Brian Martin, Graham Shaw, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 I.B Relativity and AntiparticlesĮlementary particle physics is often called high-energy physics. = 0\) for the initial particle and all decay products. ![]()
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