# Particle Physics

In an attempt to explain the $\beta$ decay and to understand internal structure of the neutron a new branch of physics was born, the particle physics. The only way to explore the structure of sub-atomic particles is to strike them with other particles in order to knock out their constituent parts. The simple logic says: The more powerful the impact, the smaller parts can be knocked out.
At the beginning the only source of energetic particles to strike other particles were the cosmic rays. Earth is constantly bombarded by all sort of particles coming from the outer space. Atmosphere protects us from most of them, but many still reach the ground.

## Antiparticles

In 1932, studying the cosmic rays with a bubble chamber, Carl Anderson made a photograph of two symmetrical tracks of charged particles. The measurements of the track curvatures showed that one track belonged to an electron and the other was made by a particle having the same mass and equal but positive charge. These particles were created when a cosmic $\gamma$ quantum of a high energy collided with a nucleus.
The discovered particle was called positron and denoted as $e^{+}$ to distinguish it from the electron, which sometimes is denoted as $e^{-}$. It was the first antiparticle discovered. Later, it was found that every particle has its mirror reflection, the antiparticle. To denote an antiparticle, it is used bar over a particle symbol. For example, ${\bar {p}}$ is the anti-proton, which has the same mass as an ordinary proton but a negative charge.
When a particle collides with its mirror reflection, they annihilate, i.e. they burn out completely. In this collision, all their mass is transformed into electromagnetic energy in the form of $\gamma$ quanta. For example, if an electron collides with a positron, the following reaction may take place
${\begin{matrix}e^{-}+e^{+}\,\longrightarrow \,\gamma +\gamma \ ,\end{matrix}}$
where two photons are needed to conserve the total momentum of the system.
In principle, stable antimatter can exist. For example, the pair of ${\bar {p}}$ and $e^{+}$ can form an atom of anti-hydrogen with exactly the same energy states as the ordinary hydrogen. Experimentally, atoms of anti-helium were obtained. The problem with them is that, surrounded by ordinary matter, they cannot live long. Colliding with ordinary atoms, they annihilate very fast.
There are speculations that our universe should be symmetric with respect to particles and antiparticles. Indeed, why should preference be given to matter and not to anti-matter? This implies that somewhere very far, there must be equal amount of anti-matter, i.e. anti-universe. Can you imagine what happens if they meet?

## Muon, mesons, and the others

In yet another cosmic-ray experiment a particle having the same properties as the electron but $\sim$207 times heavier, was discovered in 1935. It was given the name muon and the symbol $\mu$. For a long time it remained unnecessary particle in the picture of the world. Only the modern theories harmonically included the muon as a constituent part of matter (see Elementary particles: Quarks and leptons).
The same inexhaustible cosmic rays revealed the $\pi$ and $K$ mesons in 1947. The $\pi$ mesons (or simply pions) were theoretically predicted twelve years before by Yukawa, as the mediators of the strong forces between nucleons. The $K$ mesons, however, were unexpected. Furthermore, they showed very strange behaviour. They were easily created only in pairs. The probability of the inverse process (i.e. their decay) was $10^{13}$ times lower than the probability of their creation.
It was suggested that these particles possess a new type of charge, the strangeness, which is conserving in the strong interactions. When a pair of such particles is created, one of them has strangeness $+1$ and the other $-1$, so the total strangeness remains zero. When decaying, they act individually and therefore the strangeness is not conserving. According to the suggestion, this is only possible through the weak interactions that are much weaker than the strong interactions (see Sec. Elementary particles: Forces of nature) and thus the decay probability is much lower.
The golden age of particle physics began in 1950-s with the advent of particle accelerators, the machines that produced beams of electrons or protons with high kinetic energy. Having such beams available, experimentalists can plan the experiment and repeat it, while with the cosmic rays they were at the mercy of chance. When the accelerators became the main tool of exploration, the particle physics acquired its second name, the high energy physics.
During the last half a century, experimentalists discovered so many new particles (few of them are listed in Table 15.5) that it became obvious that they cannot all be elementary. When colliding with each other, they produce some other particles. Mutual transformations of the particles is their main property.
Table: Few representatives of different particle families. :
| family | particle | symbol | mass (MeV) | mean lifetime (s) |
| photon | photon | $\gamma$ | 0 | stable |
| | electron | $e^{-}$, $e^{+}$ | 0.511 | stable |
| | muon | $\mu ^{-}$, $\mu ^{+}$ | 105.7 | $2.2\times 10^{-6}$ |
| leptons | tau | $\tau ^{-}$, $\tau ^{+}$ | 1777 | $10^{-13}$ |
| | electron neutrino | $\nu _{e}$ | $\sim 0$ | stable |
| | muon neutrino | $\nu _{\mu }$ | $\sim 0$ | stable |
| | tau neutrino | $\nu _{\tau }$ | $\sim 0$ | stable |
| | pion | $\pi ^{+}$, $\pi ^{-}$ | 139.6 | $2.6\times 10^{-8}$ |
| | pion | $\pi ^{0}$ | 135.0 | $0.8\times 10^{-16}$ |
| | kaon | $K^{+}$, $K^{-}$ | 493.7 | $1.2\times 10^{-8}$ |
| | kaon | $K_{S}^{0}$ | 497.7 | $0.9\times 10^{-10}$ |
| hadrons | kaon | $K_{L}^{0}$ | 497.7 | $5.2\times 10^{-8}$ |
| | eta meson | $\eta ^{0}$ | 548.8 | $10^{-18}$ |
| | proton | $p$ | 938.3 | stable |
| | neutron | $n$ | 939.6 | 900 |
| | lambda | $\Lambda ^{0}$ | 1116 | $2.6\times 10^{-10}$ |
| | sigma | $\Sigma ^{+}$ | 1189 | $0.8\times 10^{-10}$ |
| | sigma | $\Sigma ^{0}$ | 1192 | $6\times 10^{-20}$ |
| | sigma | $\Sigma ^{-}$ | 1197 | $1.5\times 10^{-10}$ |
| | omega | $\Omega ^{-}$ | 1672 | $0.8\times 10^{-10}$ |
Physicists faced the problem of particle classification similar to the problems of classification of animals, plants, and chemical elements. The first approach was very simple. The particles were divided in four groups according to their mass: leptons (light particles, like electron), mesons (intermediate mass, like pion), baryons (heavy particles, like proton or neutron), and hyperons (very heavy particles).
Then it was realized that it would be more logical to divide the particles in three families according to their ability to interact via weak, electromagnetic, and strong forces (in addition to that, all particles experience gravitational attraction towards each other). Except for the gravitational interaction, the photon ($\gamma$ quantum) participates only in electromagnetic interactions, the leptons take part in both weak and electromagnetic interactions, and hadrons are able to interact via all forces of nature (see Elementary particles: Forces_of_nature).
In addition to conservation of the strangeness, several other conservation laws were discovered. For example, number of leptons is conserving. This is why in the reaction (15.6) we have an electron (lepton number $+1$) and anti-neutrino (lepton number $-1$) in the final state. Similarly, the number of baryons is conserving in all reactions.
The quest for the constituent parts of the neutron has led us to something unexpected. We found that there are several hundreds of different particles that can be knocked out of the neutron but none of them are its parts. Actually, the neutron itself can be knocked out of some of them! What a mess! Further efforts of experimentalists could not find an order, which was finally discovered by theoreticians who introduced the notion of quarks.