Monday, March 7, 2011

Who Ordered That? - Part 2

The Particle Zoo



By the 1930's we had developed a model of what was believed to be the fundamental building blocks of matter; electron, proton, and neutron. This belief was turned on its head by several ground breaking theories.

In 1928 a young British physicist, Paul Dirac (1902-1984), published a paper which predicted the possibility of there being an anti-electron. This theory had several holes which he could not explain. A revised paper 1929 tried to tackle the confusing negative energy result but to avail. However, Dirac persisted and in 1931 he published a paper which gave rise to a particle known as the positron (e+), a particle which would be discovered a year later by Carl David Anderson (1905-1991). This was just the beginning for the discovery of many strange new particles.

At this time the formulation of theories for both the strong and weak nuclear forces had thrown up the possibility of new particles. Fermi had shown with the weak nuclear force that a neutron could decay into a proton and an electron. There was a snag, the energies of the reaction did not add up nor did the spin. Did he just discover some physical effect which didn't obey the conservation of energy and conservation of angular momentum? Wolfgang Pauli (1900-1958) suggested that there might be some new particle responsible for the missing energy and spin. Based on this Fermi came up with the neutrino (n). This seemly mass-less and charge-less particle could have a range of energies so would be responsible for the various energies of beta decay that we see. We know now that what he actually discovered was the antineutrino.

Yukawa, with the strong nuclear force theory, had postulated that this force was mediated by the exchanging of a particle between the neutrons and protons. He named this new particle the pion. The pion would come in several different forms; position pion (W+), negative pion (W-) and neutral pion (Z0). This bewildering array of new particles gave rise to the term particle zoo.

Summary of elementary particles

Armed with all of these new theories, physicists raced to find these elusive particles. A new age of experimentation had begun. There were two distinct paths to the discovery of these new particles; particle accelerators and cosmic rays.

It was known at this time that the upper atmosphere of the earth was bombarded by high energy stream of particles (referred to as cosmic rays). These cosmic rays would collide with particles and shower the Earth in a large assortment of exotic particles.  Using balloons and working on mountain tops provided the best means from observing these strange particles. With the aid of cloud chambers, the tracks made by these new particles were analysed and from this their properties deduced. It was then in 1936 when research teams were looking for Yukawa's new particle that another particle was observed which did not match any theory. This was a great surprise to the field of particle physics, was there even more strange particles lying in wait? What they had discovered was the muon. This would later been seen to be a new elementary particle.

This image is a pion (right track) which decays into a muon (the upper left track anti clockwise rotation) which then decays into an electron (top track that spirals towards the top).
Yukawa's pion would eventually be found by a mountaintop observatory in 1948 in the French Pyrenees. In 1950 there was a shock in store for experimenters when a strange track was observed in a cloud chamber. It was a v shaped track which appeared to come from nowhere. By 1955 they had identified 3 similar particles which they named kaons. The particle zoo was growing beyond expectations, adding to the confusion that was spreading throughout particle physics.

As the field of particle physics became more prevalent, the search for more powerful particle accelerators expanded. For the first true accelerators we must first return to the early 1930s at Cambridge University. In 1932 John Cockcroft (1897-1967) and Ernest Walton (1903-1995) developed the linear accelerator. It was used to study the transmutation of atomic nuclei. From this early success more advanced models were pioneered and by the 1960's several different types of accelerators in institutes around the world were beginning to replicate results seen from cosmic ray observations.

Walton in the observation hut of the linear accelerator at Cambridge

By the 1960's the particle zoo had grown to almost nightmarish proportions. What had begun as an exploration as to the force than binds the nucleus had lead down a path of what seemed like a never ending discovery of new particles. The beautiful simplicity of the early atomic physics was now gone and replaced by this unwieldy monster. To solve this we now shift focus from the experimental to theoretical. 

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