The existence of antiparticles was predicted by Paul Dirac in 1928, when he theorized that energy can exist in two different states—positive and negative—but are identical to each other in both size and mass. He was proven right. In the 1930s, physicists discovered that particles have their opposites.
Every particle has an equivalent anti-particle that carries the same mass but possesses the opposite electric charge. A particle refers to protons, neutrons, electrons, and neutrinos, while an anti-particle refers to anti-protons, anti-neutrons, positrons (the opposite of electrons), and anti-neutrinos and, as a result, the anti-nucleus. All these particles make up the anti-atom (Fig. 2.8).
Figure 2.8. The Existence of Opposites
The first to be discovered was in 1932 when American Physicist Carl Anderson noticed a twin particle called positron, which is the positive state of electron. In 1934, Fermi broached the idea of a neutrino, Italian for “little neutron,” that has no electric charge, no mass, no tendency to interact with matter, but capable of knocking out protons out of nuclei.
He was also proven correct. In 1956, a team of American physicists led by Frederick Reines and Clyde Cowan, using a fission reactor, detected antineutrinos in their laboratory, namely, antiprotons, antineutrons, and positrons. Together with their opposites, these particles began to be known as the first generation of leptons (Fig. 2.9).
Figure 2.9. First Generation of Leptons
Of the three antiparticles, the neutrino is the one deserving of more attention because it is the least understood. It was predicted as a hypothetical particle by Wolfgang Pauli in 1931 based on his assumption that energy and momentum are not conserved in certain radioactive decays.
The term “neutrino” is an Italian term to mean “little neutral one” and coined by Enrico Fermi. But it was only in 1959 that this particle was discovered. Thereafter, three other types or flavors of neutrinos were discovered. Each of these types is related to a charged particle.
Neutrinos, like their counterpart “neutrons,” do not carry electric charge, which might even be zero. They are subject to the weak nuclear force, which, as we already know, only affects ranges that are shorter than electromagnetism. They are known to have three types or flavors, each of which is related to a charged particle (Fig. 2.10).
Figure 2.10. Neutrinos
These are: the “electron neutrino (ve),” discovered in 1968; “the muon neutrino (vu), discovered in 1962;” and the “tau neutrino (vt),” discovered in 1978. The electron neutrino is associated with the electron, which is positively charged.
It’s getting more technical and a bit boggling but let’s persevere keeping an eye always on the illustrations. We have to understand this tiny world because this is also what is happening in the bigger world, as we will see shortly.
The muon and tau neutrinos are heavier and bigger than the electrons. The muon has a mass of 200 times greater than the electron, while the tau is 3,500 times greater than that of the electron (https://www.scientificamerican.com/article/what-is-a-neutrino/).
Anderson discovered two other twin particles belonging to the second generation of the lepton family, namely: muon (later classified as meson) and antimuon. Muon is the same particle that Japanese physicist Hideki Yukawa also discovered and later confirmed by British Physicist Patrick Blackett (Percy Seymour, 1992:58).
Leptons and quarks, then, also have their twin particles (Fig. 2.11). Leptons are known as “electron-like” particles and they consist of six different types, namely, the electron, mau, tau, electron neutrino, mau neutrino, tau neutrino, while quarks also consist of six types, which are the up, down, charm, strange, top, and bottom.
Figure 2.11. Quarks and Leptons
Leptons interact only via the electromagnetic and weak forces. Unlike the leptons, quarks bind together through the strong interaction, the force that is responsible for the protons and neutrons to stay together as one nucleus.
In addition, physicists also discovered another subatomic particle that is similar to the electron but 207 times heavier. This is the muon. The muon was first discovered in 1936 by physicists Carl D. Anderson and Seth Neddermeyer as an integral component of cosmic-ray particle showers. It is assigned as a member of the lepton group of subatomic particles, under which family the electron also belongs. It also never reacts with nuclei or other particles through the strong interaction.
Like the electron, it is negatively charged and has its twin, which is positively charged antiparticle. A muon is said to be unstable, having a lifetime of only 2.2 microseconds before it decays by the weak force into an electron and neutrinos.
Back to the primary ingredients of atoms....
There is another way of categorizing atomic particles. All the known matter in the universe is made of atoms and atoms are made up of electrons, protons, and neutrons, which are considered “primary particles.” All the other particles that were discovered after them were labelled as “secondary particles”. They include the fermions and bosons.