Family patterns history 6 ext a
Murray Gell-Mann and Yuval Ne’eman wanted to simplify the picture of particles that was emerging in the 1950s. There were simply too many hadrons. They put these hadrons into family tables (like Mendeleev’s periodic table) to look for patterns which they could then explain using more fundamental particles.
Splitting up the family
The family tree of baryons with a missing member with rollover highlights
Figure 1. The baryon decuplet
First of all they split the hadrons into three groups according to their spin. Those with spin zero are known as mesons 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.
Periodicity again
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. Again, Gell-Mann predicted the existence and properties of the omega-minus (W-) particle which was later discovered.
More fundamental
The quark composition of a neutron The quark composition of a proton
A neutron is made from two down quarks and an up quark A proton is made from two ups and a down
The success of their family arrangements encouraged physicists to look for underlying reasons for the patterns. 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.
The quark composition of some other hadrons
Some other hadrons: a delata minus is made from three down quarks and a sigma minus is two downs and a strange. There are hundreds of hadrons made from different combinations of the six quarks.
There are six quarks in total. However, only two of them are needed to make protons and neutrons. We call these two quarks up and down. A proton is made from two up quarks and a down quark and a neutron from two downs and an up. There are four other types, or flavours, of quark: top, bottom, strange and charm.
Strange days
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. However, using experiments similar to those of Ernest Rutherford, it was possible to delve inside protons and neutrons and find evidence of these quarks.
Probing the quarks
The story so far
Particle accelerators produced hundreds of new hadrons
Physicists wanted a simpler picture
Gell-Mann and Ne'eman put the hadrons into families and made tables to show the pattersn in their properties
The quark model explains the families of hadrons using just six fundamental particles
Question H8

Most hadrons are not found in ordinary matter. So why are they important to particle physicists?

The story so far
  • Protons and neutrons are members of the family of hadrons
  • Protons and neutrons are the only hadrons found in normal matter
  • Particle acclerators were creating lots of new particles in the 1960s