muon

muon
Name	Muon
Composition	Elementary particle
Statistics	Fermi–Dirac statistics
Group	Lepton
Generation	Second
Interaction	Gravitation, Electromagnetism, Weak interaction
Particle	μ−
Antiparticle	μ+
Discovered	Carl D. Anderson and Seth Neddermeyer (1936)
Mass	105.6583755
Decay time	2.1969811
Electric charge	±1 ''e''
Spin	1, 2

muon. A muon is an elementary particle with negative electric charge and a spin of 1/2, classified as a second-generation lepton. It is similar to the electron but is approximately 207 times more massive, with a mean lifetime of 2.2 microseconds before decaying via the weak interaction. These particles are common constituents of cosmic ray showers and are produced in high-energy collisions in particle accelerators like the Large Hadron Collider.

Discovery and history

The existence of the muon was first suspected from observations of cosmic ray interactions in cloud chambers during the early 1930s. In 1936, the team of Carl D. Anderson and Seth Neddermeyer at the California Institute of Technology conclusively identified a new particle with a mass intermediate between the electron and the proton, which they initially termed a "mesotron." This discovery puzzled theorists like Niels Bohr and Werner Heisenberg, as the particle did not fit the then-current models of nuclear forces. The confusion was famously summarized by Isidor Isaac Rabi, who quipped, "Who ordered that?" when the particle, later renamed the muon, was found not to be the predicted pion that mediates the strong interaction.

Properties

The muon is a fermion obeying Fermi–Dirac statistics and is fundamentally unstable. It has a rest mass of about 105.66 MeV/c2, which is between that of the electron and the tau (particle), the other charged leptons. With a mean lifetime of 2.2 microseconds, it decays primarily into an electron, a muon neutrino, and an electron antineutrino. Like the electron, it experiences interactions described by quantum electrodynamics and the electroweak theory, but it does not participate in the strong interaction. The magnetic moment of the muon has been measured with extreme precision in experiments like the Muon g-2 experiment at Brookhaven National Laboratory and Fermilab, where a persistent anomaly compared to predictions of the Standard Model has generated significant interest.

Sources and production

Muons are produced copiously in nature through the decay of pions generated when high-energy cosmic ray protons collide with nuclei in the Earth's atmosphere. In artificial settings, they are created in particle accelerators such as the Large Hadron Collider at CERN and the J-PARC facility in Japan. These facilities typically produce pions from proton collisions with fixed targets, with the pions then decaying into muons. Dedicated muon sources, like the Muon Campus at Fermilab, use specialized beamlines to generate intense, pure beams for precision experiments.

Interactions and decay

The muon interacts via the gravitational, electromagnetic, and weak interaction forces. It can form bound states, such as muonium (an atom consisting of an antimuon and an electron), and can replace an electron in atoms to form "muonic atoms," where its greater mass leads to a tighter orbit. The dominant decay mode, governed by the weak interaction, is into an electron, a muon neutrino, and an electron antineutrino. This process involves the virtual exchange of a W boson. Rare decay channels, searched for in experiments at the LHCb experiment and Belle II experiment, are sensitive probes for physics beyond the Standard Model.

Applications

Due to their penetrating nature, muons are used in muon tomography to image dense structures, with applications ranging from scanning the interior of pyramids of Giza to monitoring volcanoes like Mount Vesuvius. In materials science, techniques like muon spin spectroscopy (μSR), pioneered at facilities like the ISIS Neutron and Muon Source and the Paul Scherrer Institute, probe magnetic properties and diffusion in superconductors and semiconductors. Historically, the study of muon decay provided critical tests for the V-A theory of the weak interaction and the concept of parity violation.

Cosmic-ray muons

Cosmic-ray muons are a major component of secondary radiation at Earth's surface, originating from the decay of pions and kaons produced in the upper atmosphere. Their flux and energy spectrum have been measured by experiments like the Pierre Auger Observatory and are influenced by factors such as solar activity and atmospheric conditions. These muons are used as a natural probe for muon tomography in geophysics and archaeology, and their lifetime dilation, a direct consequence of special relativity from Albert Einstein, provides a classic demonstration of time dilation for relativistic particles.

Category:Elementary particles Category:Leptons