11.1 Introduction to particle physics  (Page 4/22)

Soon it was discovered that for every particle in nature, there is a corresponding antiparticle    . An antiparticle has the same mass and lifetime as its associated particle, and the opposite sign of electric charge. These particles are produced in high-energy reactions. Examples of high-energy particles include the antimuon ( ${\mu }^{+}$ ), anti-up quark ( $\stackrel{\text{−}}{u}$ ), and anti-down quark $(\stackrel{\text{−}}{d}).$ (Note that antiparticles for quarks are designated with an over-bar.) Many mesons and baryons contain antiparticles. For example, the antiproton ( $\stackrel{\text{−}}{\text{p}}$ ) is $\stackrel{\text{−}}{u}\stackrel{\text{−}}{u}\stackrel{\text{−}}{d}$ and the positively charged pion $({\pi }^{+})$ is $u\stackrel{\text{−}}{d}$ . Some neutral particles, such as the photon and the ${\pi }^0$ meson, are their own antiparticles. Sample particles, antiparticles, and their properties are listed in [link] .

The same forces that hold ordinary matter together also hold antimatter together. Under the right conditions, it is possible to create antiatoms such as antihydrogen, antioxygen, and even antiwater. In antiatoms, positrons orbit a negatively charged nucleus of antiprotons and antineutrons. [link] compares atoms and antiatoms.

Antimatter cannot exist for long in nature because particles and antiparticles annihilate each other to produce high-energy radiation. A common example is electron-positron annihilation. This process proceeds by the reaction

The electron and positron vanish completely and two photons are produced in their place. (It turns out that the production of a single photon would violate conservation of energy and momentum.) This reaction can also proceed in the reverse direction: Two photons can annihilate each other to produce an electron and positron pair. Or, a single photon can produce an electron-positron pair in the field of a nucleus, a process called pair production. Reactions of this kind are measured routinely in modern particle detectors. The existence of antiparticles in nature is not science fiction.

Summary

• The four fundamental forces of nature are, in order of strength: strong nuclear, electromagnetic, weak nuclear, and gravitational. Quarks interact via the strong force, but leptons do not. Both quark and leptons interact via the electromagnetic, weak, and gravitational forces.
• Elementary particles are classified into fermions and boson. Fermions have half-integral spin and obey the exclusion principle. Bosons have integral spin and do not obey this principle. Bosons are the force carriers of particle interactions.
• Quarks and leptons belong to particle families composed of three members each. Members of a family share many properties (charge, spin, participation in forces) but not mass.
• All particles have antiparticles. Particles share the same properties as their antimatter particles, but carry opposite charge.

Conceptual questions

Problems