Wednesday, March 9, 2011

Who Ordered That? - Part 3

The Eight Fold Way and the Quark Model


Theoretical physicists were now playing catch up as the experiments of the 1950's had expanded the range of sub atomic particles to ever increasing heights. The first task was to identify shared properties which might hint to some underlying pattern.

This problem is something very similar to what chemists faced in the late 1800's. At this time there was a wide array of elements which at first glance did not appear to share any common ground. Many attempts were made to group and classify these elements into some pattern. It wasn't until Russian chemist Dmitri Mendeleev (1834-1907) grouped them by similar chemical properties and increasing atomic mass, designing the now familiar table we know as the Periodic Table. Could there be a similar table for the particle zoo?

Dmitri Mendeleev
To get any hint of this we must first try to understand what properties you can attribute to sub atomic particles. It would appear that there were only six leptons (the electron and muon are members of this family) and six corresponding anti particles, which behaved like point particles, while the hadrons (the proton and neutron are members of this family) had a far greater number of family members and a definite extension in space. Leptons obey a conservation of lepton number while in the hadron family, only the baryons obeyed a conservation law. There was another new property which was attributed to the newly discovered kaons called strangeness. This new property was due to these new particles only forming in pairs and so their associated strange value must be zero. With so many new properties and conservation laws it was easy for scientists to think that there was no way to make sense to this particle zoo. It would take one of the most talented physicists of the 20th century to solve this disturbing problem.





All these tables taken from an excellent CPEP poster.

In 1961 Murray Gell-Mann (b.1929) discovered there was an underlying pattern to the hadrons. Using a branch of mathematics called group theory, he was able to group the hadrons into families of particles. The proton and neutron became part of a family called the baryon octet while the pion became a member of a different octet called the meson octet. Gell-Mann dubbed this new view the eightfold way in homage to a Buddhist teaching belief. This model was very successful and in 1962 it enabled Gell-Mann to predict a new particle (much in the same vein as Mendeleev did years earlier) with certain properties and mass. This new particle, omega minus (Ω-), was found by a team of researchers at Brookhaven in 1963. Now armed with this powerful new theory could they now unravel what the underlying relationship between this menagerie of particles.

The question now was why did this eightfold way work? The hadrons could be grouped in patterns of 1,8,10 and 27, so was there some common ground they shared which gives rise to this pattern? Gell-Mann and George Zweig (b. 1937) would solve this puzzle. Both of them developed a theory independently that there must be some new fundamental particle. These new particles, which they believed there were three of, made up all the observed hadrons. Combining these three new particles in triplets could give rise to all the hadrons. This new particle was named the quark. These new quarks were named up, down and strange. These new particles were similar to the leptons with spin ½ and acted like point particles. The baryon family of the hadrons must be made up of a triplet of quarks similar to the proton and neutron. The meson family on the other hand are made up of just two quarks, a quark and anti-quark pair. The reason for this grouping is due to the rules of conservation developed in mathematical symmetry by Gell-Mann earlier which we refer to as SU(3). These new quarks were required to have a fractional charge for this new system to function. Now armed with this theory the experimentalists set out to find these new particles.


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