Document Type
Course
Journal/Book Title/Conference
Physics 3710 – Intermediate Modern Physics, Spring 2018
Publication Date
1-8-2018
First Page
1
Last Page
5
Abstract
The particle zoo
Prior to the 1930s the fundamental structure of matter was believed to be extremely simple: there were electrons (each with mass about 0.5 MeV), e− , photons (no mass), γ , and protons (mass about 938 MeV), p+ . Starting in 1932 the world began to get a lot more complicated. First came Dirac’s positron ( e+ , with same mass as the electron), postulated in 1928 but mostly ignored until Anderson’s accidental discovery (see SM 1). Soon after, the neutron ( n ) was identified (mass about 940 MeV). In beta decay, the neutron transforms into a proton and an electron. The energy of the electron in beta decay has a maximum cutoff and is otherwise “never” observed to be the same–as it would be if there were only two products. It is almost as if energy is not conserved in beta decay. In 1930, Wolfgang Pauli proposed that a third, unseen, particle was also emitted and that the three products conserved energy and momentum by sharing them in a variety of unpredictable ways. The new particle would have to have spin-1/2 (because the neutron, proton, and electron all have spin-1/2 and not even the crazy rules of addition in quantum mechanics allow 1/2 +1/2 = 1/2 ) and be electrically neutral (because the neutron is neutral and the proton plus electron is also neutral). Eventually, Pauli’s particle–the neutrino, ν –was directly detected in the 1950s. This set of particles was all that was needed to make sense of nuclei and their properties.
Recommended Citation
Peak, David, "Structure of matter, 3" (2018). Structure of Matter. Paper 3.
https://digitalcommons.usu.edu/intermediate_modernphysics_matter/3