My favourite particle: the muon

Mark Lancaster describes the discovery of his particular favourite, and how it could help crack one of the big puzzles of particle physics.

There is nothing new to be discovered in physics. All that remains is more and more precise measurement

He would be proved horribly wrong. The discovery of the nucleus and then its constituents, the proton and neutron, revolutionised our view of what the world was made of. Our understanding of the world changed from the classical to the quantum and up to 1933 quantum mechanics went from success to success in describing experimental observations. This culminated in the Dirac equation, which predicted the existence of anti-matter, confirmed shortly afterwards by the discovery of the anti-electron (the positron). However, the physicists’ smugness was short-lived. Behind the scenes, all was not well. Quantum Mechanics was struggling to provide an explanation for particles that were raining down on earth from the cosmos at a rate of 10,000 per minute per m2. A veritable who’s-who of physics luminaries were trying to understand the nature of these “cosmic-ray” particles. Since at that time the only known particles were electrons, protons, neutrons, photons and (yet to be directly detected) neutrinos. It was assumed that these cosmic-ray particles arriving at the earth were electrons.

The problem with this (wrong) assumption was that the “electrons” raining down on the earth seemed to come in two varieties –1. those which were easily absorbed by blocks of lead and which created a secondary shower of electrons, positrons and photons when they interacted with the lead and 2. those that penetrated the lead blocks with aplomb.

Read the whole of Mark’s interesting article here My favourite particle: the muon | Science | guardian.co.uk.

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