Hadrons

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HADRONS


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(In this article, and in furthermore, u means up quark and d means down quark. Similarly, u with a bar on top means anti-up quark and d with a bar on top means anti-down quark.)

As well as the leptons and quarks, there is another quite different group in the mix - hadrons!

A hadron is a particle that is composed of two or more quarks, a composite particle of which protons and neutrons are examples. The quarks are held together by the ‘strong force’, 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’.

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 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.

Quarks and antiquarks only occur bound together inside hadrons; they have never been observed in isolation. While it may seem like the process of ‘building’ a hadron would be something very complicated, there are in fact 3 simple ‘recipes’ to remember.

A hadron can consist of either:
  • Three quarks (in which case it is called a baryon).
  • Three antiquarks (in which case it is called an antibaryon).
  • One quark and one antiquark (in which case it is called a meson).








Any combination of quarks and antiquarks that obeys one of the three recipes above is a valid hadron. The net electric charge of a hadron is simply the sum of the electric charges of the quarks or antiquarks of which it is composed. The net charge of a hadron is therefore always a whole number, even though the quarks themselves have a non-whole number electric charge.

As a specific example of the hadron-building recipe, the proton is a baryon, so it is composed of three quarks, and as mentioned above, it is composed of up and down quarks only.

The proton has a charge of +e. The only way that three up or down quarks can be combined to make this net charge is by combining two up quarks with a down quark. So the quark content of a proton is (uud), giving a net charge of +(2/3) e + (2/3) e - (1/3) e = + e.

The tally of six leptons and six quarks, each with their own antiparticles, may seem like a huge number of fundamental particles, but virtually everything in the Universe is made up of merely the first generation of each type of particle namely:
  • electrons
  • up quarks, and
  • down quarks,
with electron neutrinos being created in radioactive decays.

As for the other generations:
  • the second generation of leptons (muon and muon neutrino)
  • the second generation of quarks (charm and strange)
  • the third generation of leptons (tauon and tauon neutrino), and
  • the third generation of quarks (top and bottom)

all have exactly the same properties as their first-generation counterparts except that they are more massive.

Quite why nature decided to repeat this invention three times over is not currently understood!

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With Warm Wishes,
Lavanya

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