Muon

In the Standard Model of particle physics, a muon (from Greek letter mu used to represent it) is a semistable fundamental particle with negative electric charge and a spin of 1/2. Together with the electron, the tauon and the neutrinos, it is classified as part of the lepton family of fermions. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon.

History

Muons were discovered by Carl D. Anderson in 1936 while he studied cosmic radiation. He had noticed particles that curved in a manner distinct from that of electrons and other known particles when passed through a magnetic field. In particular, these new particles curved to a smaller degree than electrons, but more sharply than protons. It was assumed that their electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that these particles were of intermediate mass (lying somewhere between that of an electron and that of a proton).

Related Topics:
Carl D. Anderson - 1936 - Cosmic radiation - Magnetic field - Proton

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For this reason, Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for "intermediate". Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. Faced with the need to differentiate between different types of mesons, the mesotron was renamed the mu meson (with the Greek letter mu used to approximate the sound of the English letter m).

Related Topics:
Greek - English

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However, it was soon found that the mu meson significantly differed from other mesons (e.g. its breakdown products included a neutrino and an antineutrino, rather than one or the other as was observed in other mesons). Thus mu mesons were not mesons at all, and so the term mu meson was abandoned and replaced with the modern term muon.

Related Topics:
Neutrino - Antineutrino

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In the mid 1970s, experimental physicists working at the European Center for Nuclear Research fired neutrinos at a proton target. According to what was then known about the weak interaction, they expected the collision to turn the neutrino into a muon, and the proton into debris. They were surprised to discover that two muons, one negatively and one positively charged, result from such collision.

Related Topics:
1970s - European Center for Nuclear Research - Weak interaction - Neutrino - Proton

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This generated a good deal of theoretical discussion, until a consensus emerged on how that positive muon comes about. The neutrino/proton collision produces not only proton debris and a negative muon, but a charm quark, and the quark soon decays into a strange quark, a muon neutrino, and a positive muon.

Related Topics:
Charm quark - Strange quark

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