muon decay

muon decay is the spontaneous radioactive decay process of the unstable muon, a fundamental lepton in the Standard Model of particle physics. It is a prime example of a weak interaction governed by the electroweak theory, proceeding via the exchange of a virtual W boson. The decay's precise characteristics have been crucial for testing fundamental symmetries and probing physics beyond the Standard Model.

Overview

The muon, discovered in cosmic rays by Carl David Anderson, is essentially a heavier, unstable cousin of the electron. With a mean lifetime of approximately 2.2 microseconds, it decays via the weak force, which violates parity symmetry maximally. This process was historically vital in establishing the V−A theory developed by Richard Feynman, Murray Gell-Mann, Robert Marshak, and George Sudarshan. Studies of this decay at facilities like CERN and Fermilab have provided stringent tests of lepton universality.

Decay process and products

The dominant decay mode for a negative muon (μ⁻) produces an electron, an electron antineutrino (ν̄_e), and a muon neutrino (ν_μ). The corresponding Feynman diagram depicts a muon transforming into its neutrino by emitting a virtual W boson, which subsequently decays into the electron and its antineutrino. The energy spectrum of the emitted electron is continuous, a hallmark of three-body decays, with a maximum energy determined by the mass difference between the muon and the electron. The Michel parameters, named after Louis Michel, describe the electron's energy and angular distribution, encoding details of the weak interaction's structure.

Theoretical description

The theoretical framework is provided by the electroweak theory within the Standard Model. The interaction is described by a current–current interaction Lagrangian of the V−A form, coupling left-handed lepton currents. The matrix element for the process involves the Fermi constant, which sets the strength of the weak interaction at low energies. Higher-order quantum electrodynamics corrections, calculated using techniques like the S-matrix, are necessary for precise predictions of the decay rate and spectra. The theoretical lifetime calculation agrees spectacularly with experiment, confirming the consistency of the Standard Model.

Experimental measurements

The muon lifetime has been measured with extraordinary precision using techniques like the MuLan experiment at the Paul Scherrer Institute. These experiments often store muons in magnetic storage rings or use advanced scintillation counter arrays to detect decay events. Measurements of the Michel parameters and the electron's polarization have been performed at laboratories including Brookhaven National Laboratory and KEK. These results have constrained possible deviations from the V−A structure and placed limits on the mass of the W boson before its direct discovery at the Super Proton Synchrotron.

Role in particle physics

This decay serves as a fundamental laboratory for testing the Standard Model and searching for new physics. It provides one of the most precise determinations of the Fermi constant, a key parameter in electroweak theory. Discrepancies in measurements related to lepton flavor universality could hint at phenomena beyond the Standard Model, such as interactions mediated by a hypothetical Z' boson. Furthermore, the process is analogous to beta decay in nuclear physics but involves purely leptonic currents, offering a cleaner system for study. Its investigation remains a active area of research at major facilities like the Muon g-2 experiment at Fermilab.

Category:Particle physics Category:Radioactivity Category:Weak interaction