Muon

Moon's shadow in muons

Hard-component-muon-868x1024

Muon Decay

Muon is an elementary particle in the subatomic particle family, which also includes particles such as the electron, proton, and neutron. Muons are denoted by the Greek letter mu (μ). They are similar to electrons, with a negative electric charge and a spin of 1/2, but are about 207 times more massive. Despite their greater mass, muons are still considered leptons, a class of particles that do not participate in the strong nuclear force.

Discovery

The muon was discovered in 1936 by Carl D. Anderson and Seth Neddermeyer at the California Institute of Technology during their study of cosmic rays. Initially, it was mistaken for the particle predicted by Yukawa Hideki to mediate the strong nuclear force, now known to be the pion. The unexpected mass of the muon and its failure to interact with nuclei puzzled scientists, leading Isidor Isaac Rabi to famously remark, "Who ordered that?"

Properties

Muons have a mass of approximately 105.7 MeV/c^2 and a mean lifetime of 2.2 microseconds, making them unstable but long-lived by subatomic standards. They decay into an electron, an antineutrino, and a neutrino, a process mediated by the weak nuclear force. This decay can be represented by the equation:

\[\mu^- \rightarrow e^- + \bar{\nu}_e + \nu_\mu\]

Despite their short lifespans, muons are highly penetrative and can travel through significant amounts of matter, which makes them useful in various scientific and practical applications, such as muon tomography.

Applications

Muon tomography is a technique that uses muons to generate images of the interior of large objects, such as pyramids or volcanoes, similar to how X-rays are used in medical imaging. This method exploits the natural flux of muons produced by cosmic rays interacting with the Earth's atmosphere.

Cosmic Rays

Muons produced in the upper atmosphere by cosmic rays provide a natural source for studying muon properties and for applications like muon tomography. The flux of atmospheric muons at the Earth's surface is a significant component of the natural background radiation.

Experimental Research

In particle physics, muons are used in experiments to probe the Standard Model and search for new physics beyond it. The anomalous magnetic moment of the muon, for example, is a key test of the Standard Model. Recent experiments, such as those conducted at the Fermi National Accelerator Laboratory, have measured this property with high precision, finding results that hint at possible deviations from the Standard Model predictions.

See Also

• Lepton
• Electron
• Neutrino
• Standard Model
• Particle physics

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