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4. Fundamental families page 17

Quarks
In the second half of the 20th century, scientists discovered hundreds of new particles. They found them in cosmic rays and through the use of accelerators that were producing higher energy collisions (see chapter 5). Most of these particles were hadrons. This is the family of particles that feel the strong nuclear force and includes protons and neutrons.

This was rather similar to the situation at the end of the 18th centruy when scientists started finding new elements. Where it had once seemed that there were a few different types of atom, suddenly there were nearly a hundred. Similarly, where it had once seemed that there were two types of hadron (the proton and neutron), there were now scores of them.

However, apart from protons and neutrons, all the hadrons are very unstable and decay within a hundredth of a second and do not form part of normal matter. Nevertheless, these particles were and are important to physicists to help them form a complete picture and find unifying theories.

There are two groups of hadrons – baryons and mesons; we now know that they themselves are made up of quarks.

The family tree of baryons with a missing member with rollover highlights
Picture 4.3 The baryon decuplet.
Simplifying the picture
Murray Gell-Mann and Yuval Ne’eman wanted to simplify the picture. They put the hadrons into family tables (like Mendeleev’s periodic table) to look for patterns which they could then explain using more fundamental particles.

First of all they split the hadrons into three groups according to their spin. Mesons are the ones with spin zero and those with spin 1/2 or 3/2 are known as baryons. Then they arranged the particles on axes according to their charge and a new property called strangeness. The axes are tilted at 60o to emphasise the patterns that arise.

The known mesons and spin 1/2 baryons formed closed hexagons when they were placed on these axes. The table of spin 3/2 baryons formed a triangle with a missing point. Gell-Mann predicted the existence and properties of the omega-minus (W-) particle which was later discovered.

Quarks – strange days
The success of their family arrangements encouraged physicists to look for underlying reasons for the patterns found in these tables. In the 1960s, Murray Gell-Mann and George Zweig suggested that all hadrons (including protons and neutrons) are made from smaller particles called quarks. They explained the different properties of all the hadrons in terms of different arrangements of quarks. This is similar to the way that we explain the properties of an element in terms of the arrangement of protons and electrons in its atoms.

Gell-Mann and Zweig were working in separate laboratories and came up with the same theory independently of each other. Zweig was concerned that people would ridicule the idea of particles with fractions of the standard unit of charge, so he did not publish his theory. In 1968, using experiments similar to those of Ernest Rutherford, physicists at the Stanford Linear Accelerator Centre (SLAC) were able to delve inside protons and find evidence of these quarks. In these experiments, called deep inelastic scattering, high energy electrons were fired at protons. They had enough energy to penetrate the protons and the results showed that they were bouncing off structure inside the protons.

A rollover change of a neutron A rollover change of a proton
Picture 4.4 A proton is made from two ups and a down. A neutron is made from two down quarks and an up quark
1st generation Quarks
Particles Antiparticles
up down anti-up anti-
down
m 1/3 1/3 1/3 1/3
q +2/3 -1/3 -2/3 1/3
F EM,
strong, weak, gravity
EM,
strong, weak, gravity
EM,
strong, weak, gravity
EM,
strong, weak, gravity
Table 8. The first generation of quarks showing mass (compared with proton), charge and the forces they feel.
A fundamental family

Protons and neutrons are made of two types of quark. There are six quarks in total. Each quark has a different flavour. Physicists usually refer to them in three generations of pairs:

  • up and down
  • strange and charm
  • top and bottom.

Each quark also has a corresponding antiquark. Only two flavours of quark are needed to make protons and neutrons: up and down. The up and down quarks are the only quarks found in normal matter and they are known as the first generation.

  • A proton is made from two up quarks and a down quark.
  • A neutron is made from two down quarks and an up quark.

Table 8 shows the properties of these quarks and how they combine to give the charges of protons and neutrons.

Quarks are currently believed to be fundamental. Quarks are unusual in that they have a fractional electric charge (unlike protons and electrons).

Picturing properties

We can never see these tiny particles because they are smaller than the wavelength of visible light but we can look into their properties. For example, we can work out their charge and measure their mass. Charge and mass are familiar properties because we can also measure the charge and mass of everyday objects.

However, sub-atomic particles have other properties that do not appear in everyday objects. One of these is the flavour of a quark. This is nothing like the flavour of something we eat (like ice cream) but it is a word that means something particular to physicists, who know when they use it exactly what they are talking about. (There are some much crazier names that you will come across.)

Quarks
Gen'n Particles Antiparticles
1 up down anti-
up
anti-
down
2 top bottom anti-
top
anti-
bottom
3 strange charm anti-
strange
anti-
charm
Table 9. The three generations of quarks.
Question 16
a) No hadrons, except protons and neutrons, form part of normal matter. So why are they useful to physicists?
b) How many generations of quarks are there?
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