4 Hadrons
As well as the leptons and quarks, there is another quite different group in the mix - hadrons! Perhaps the most familiar and even intuitive class of particles. A hadron is a particle which is composed of two of more quarks, a composite particle of which protons and neutrons are examples of. The quarks are held together by the ‘strong force’ (yes, that is what it is called, for hopefully obvious reasons!), much in the same way that the electromagnetic force holds molecules together. This helps explain the name- the word hadron comes from the Greek hadros, meaning ‘strong’ or ‘robust’. This is a good point to briefly introduce the fundamental forces found in nature. In everyday parlance, the word fundamental is often used with reckless abandon, but it has a very particular, important meaning here. These forces are named as such because they cannot be explained due to the action of another force. For example, friction is NOT a fundamental force as it can be explained as occurring as a result of the interaction of the electromagnetic forces in one atom with those in another. Table 3 shows a summary of their key properties and they are listed in order of strength with the strongest at the top. Notice how weak gravity really is! And it’s just as well, it would be awfully difficult to lift your foot up if it wasn’t!
| Force | Experienced by | Range (m) (distance between 2 objects before force becomes negligible) | Strength (relative to EM force) |
|---|---|---|---|
| Strong | Quarks | 10-15 | 100 |
| Electromagnetic | Charged particles | ∞ | 1 |
| Weak | Any particle | 10-17 | 10-5 |
| Gravitation | Any particle with a mass | ∞ | 10-38 |
Although the only hadrons existing in the everyday world are protons and neutrons, many more types of hadron can be created in high-energy particle collisions. Such reactions are common in the upper atmosphere where high-energy protons from outer space (known as cosmic rays) collide with nuclei of nitrogen and oxygen, smashing them apart and creating new hadrons. Since the 1960s, such reactions have been closely studied in laboratories such as CERN (the European Organization for Nuclear Research), where high-energy beams of particles are smashed together.
Although many dozens of different types of hadron may be created in this way, all of the new ones are unstable and rapidly decay into other, long-lived particles, such as leptons, protons and neutrons. Fortunately, it’s not necessary to dwell on (let alone remember) the names and properties of all the types of hadron, because there is a straightforward description for building them from particles that are believed to be fundamental, namely from quarks and antiquarks.