Thursday, March 10, 2011

Who Ordered That? - Part 4

The Discovery of Quarks and Leptons

By the mid 1960’s the hadron family had grown to over a hundred members. These hadrons all fitted within the framework of this new quark model. So the experimentalists were tasked with finding these newly predicted quarks. As noted earlier these quarks had a fractional charge, so identifying these new particles should be easy. The early searches for the elusive quark failed. It was suggested that it may not be possible to find them as no accelerator currently operating was powerful enough to generate collision energies needed to observe quarks. It was also possible that there was no such thing as quarks but these were just abstract mathematical tools to help give a pattern to the particles we observed.

In 1968 at the Stanford Linear Accelerator, SLAC, direct evidence was observed for their existence. This was again observed at CERN in early 1970. This discovery owed a lot to the Rutherford experiment back in 1911 which used the idea of scattering to probe the inner structure of the atom.

Standford Linear Accelerator

The scale of the atom is generally 10-10m and the nucleus about 10-15m. The electron is far smaller again at 10-18m, why is the proton almost a thousand times bigger? It was assumed prior to the quark model that the proton was this apparent size due to the force carrying particle the pion.

Now with the advent of the quark model, the size that the proton had could be attributed to the way the quarks moved about, similar to how the electron orbiting the nucleus gives an atom its ‘size’. Due to its size and the energies the electron was the perfect particle to probe the inner structure of the neutron and the proton. With quarks having fractional charge it could be predicted how electrons would scatter as they passed by these quarks. If the proton had charge evenly spread throughout its structure the electrons would show a diffuse pattern from which no evidence could be gleaned. On the other hand, if the proton was indeed made up of three point like particles, there should be a very discernable pattern emerging from the electron scattering.

It was observed that indeed the proton and neutron were made up of three quarks as predicted. It also showed another particle was present within the innards of the proton and neutron. This particle it seemed might be the mediator of the force which binds quarks together and was called the gluon. From the initial results it seemed that the quarks were free to move about within the confines of the proton and that they were weakly bound together. So why then could we not observe a free unbound quark? No matter how energetic the collisions no unbound quark was forthcoming.

By the early 1970’s the quark model was refined and a more complete picture emerged. From this new model a fourth quark was theorized, called charm. At this time no hadrons were known of that required this quark. Work continued to produce ever more hadrons and finally in 1974 two teams of experimenters produced a new particle. One team was led by Samuel Ting (Brookhaven) and the other by Burton Richter (SLAC). Both groups have given this particle a different name so the particle was known by its joint name J/Ψ. This new particle required the new charm quark to explain its properties. Both Ting and Richter received the Nobel prize for this ground breaking work.

By now we had four quarks (up, down, strange and charm) and four leptons (electron, muon, neutrino and muon neutrino), were these two distinct groups linked in some manner? In 1975 a fifth lepton was discovered called the tau. Due to the similar nature of these two groups it was suggested that there may be a fifth quark. Since all the previous lepton had a paired neutrino was there also a sixth quark? In 1977 a group at Fermi Lab led by Leon Lederman discovered a massive meson which could only be made by a new quark which they dubbed the bottom quark. The discovery of the sixth lepton and quark would take nearly twenty more years of work. These two new particles, the top quark and tau neutrino, were so massive no accelerators were available until the late 1990’s that could provide collision energies high enough to produce these particles.


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