Muon
Composition :
Statistics :
Generation :
Family :
Interaction forces :
Symbol :
Antiparticle :
Mass :
Decays into :
Electric charge :
Color charge
Spin :
Weak isospin :
Weak hypercharge :
Elementary particle
Fermionic
Second
Lepton
weak,
electromagnetic force
gravity
μ
Antimuon ( μ or μ⁺ )
105.6583755(23) MeV/c²
electron (e⁻)
electron antineutrino ( νₑ )
in 2,2 μsec [2]
-1 e
none
¹/₂
LH : - ¹/₂, RH : 0
LH : -1, RH : -2
_
MUON
Definition and general characteristics
The muon is denoted by the symbol "μ" and has a negative electric charge of -1e, where "e" is the elementary charge. The muon is similar to the electron in terms of its charge and spin properties, but it is much more massive. The electron has a mass of approximately 9.11 x 10^-31 kilograms, while the muon is about 207 times more massive, with a mass of approximately 1.88 x 10^-28 kilograms.
Key properties of muons include:
Charge: The muon carries a negative electric charge, which is equal in magnitude but opposite in sign to the charge of an electron.
Spin: Muons, like electrons, have spin-1/2, which is a fundamental property associated with quantum particles.
Lifetime: Muons are unstable particles with a relatively short lifetime. A free muon has a mean lifetime of approximately 2.2 microseconds (μs) in its rest frame. This limited lifetime is a result of the muon's tendency to decay into other particles through weak interactions.
Decay: Muons primarily decay via the weak force, transforming into an electron, an electron antineutrino, and a muon neutrino. The decay process is represented as follows:
Production: Muons can be produced in various high-energy processes, such as cosmic ray interactions with Earth's atmosphere or particle collisions in accelerators.
Penetration through matter: Due to their relatively high mass and charge, muons are less strongly affected by electromagnetic forces than electrons. They can penetrate matter more deeply and are often used in particle physics experiments to study the properties of materials.
Muon Neutrino Oscillations: Like other neutrinos, muon neutrinos can undergo flavor oscillations, changing from one type of neutrino to another (e.g., muon neutrino to electron neutrino) as they travel through space.
The discovery of the muon in cosmic ray experiments in the early 20th century played a crucial role in expanding our understanding of particle physics and the elementary particles that make up the universe. Muons have since been extensively studied in laboratory experiments, contributing to our understanding of the Standard Model of particle physics and providing insights into the fundamental forces and particles that govern the behavior of matter in the universe.
Muon Magnetism: We know that the muon possesses magnetism due to its electric charge and spin, resulting in a magnetic moment known as g-2. Previous experiments, including the Muon g-2 experiment at Fermilab, have measured this magnetism with high precision. These measurements initially showed a deviation from theoretical predictions, suggesting the possibility of new physics. However, recent results and computer simulations have reconciled this deviation, suggesting that our current theoretical framework may be more accurate than previously thought. Nonetheless, some experimental discrepancies remain, emphasizing the need for further research and investigation into the muon's magnetism.
The Muon g-2 experiment at Fermilab has confirmed earlier measurements of the muon's magnetism, resolving a long-standing discrepancy with theoretical predictions. Initially suggesting new physics, recent results have aligned with theoretical calculations due to improved precision and computer simulations. However, a separate experiment in Russia introduces a new discrepancy that needs further investigation. The Fermilab experiment plans to reduce uncertainty further with additional runs. Overall, while intriguing, the muon's behavior may not require major revisions to the existing theoretical framework. [1]
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Figure 235 - The Muon g-2 experiment at the Fermi National Accelerator Laboratory near Chicago, Illinois [2]
Combinations with muons
Particles that contain at least one muon as a constituent are typically mesons and hadrons. Mesons are composite particles made up of one quark and one antiquark bound together by the strong force, while hadrons include mesons as well as baryons, which are composed of three quarks. The muon itself is a lepton and is not composed of smaller constituents like quarks.
Here are some examples of particles that contain at least one muon:
Muon (μ): The muon itself is a lepton and does not have quark constituents. It is a fundamental particle, much like the electron but with greater mass.
Muonic Atoms: Atoms that incorporate muons instead of electrons in their orbits are known as muonic atoms. These atoms behave differently from normal atoms due to the muon's mass and shorter lifetime. However, muonic atoms are not stable in the long term, as muons eventually decay.
Muonium is a short-lived, exotic atom-like particle that consists of a muon and an antimuon. Unlike ordinary atoms, where electrons orbit a nucleus, muonium is formed by a positive muon (μ) orbiting an antimuon (). Both the muon and antimuon are elementary particles, belonging to the lepton family.
The symbol for muonium is often denoted as M, and its formation can be represented as:
The muon and antimuon in muonium are attracted to each other by electromagnetic forces, similar to the way electrons are attracted to protons in regular atoms. However, muonium is short-lived because muons themselves have a finite lifetime. The mean lifetime of a free muon is approximately 2.2 microseconds (μs).
Muonium has been studied in experiments to gain insights into fundamental physics, particularly in the realm of quantum electrodynamics (QED) and the interactions of charged particles. It serves as a unique system for testing theoretical predictions and understanding the behavior of lepton-antilepton pairs in bound states.
Due to its short lifetime, muonium is not present in significant quantities in nature and is typically produced in laboratory settings, often in particle physics experiments or using high-energy accelerators.
Figure 236 - A muonium atom
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Creation of muons
Muons can be created through various processes in nature and in laboratory settings, typically involving high-energy interactions. Here are some of the main processes by which muons can be produced:
Cosmic Ray Interactions:
Muons are frequently created in the Earth's atmosphere through interactions with high-energy cosmic rays. Cosmic rays, which consist of energetic particles from outer space, collide with particles in the atmosphere, leading to the production of muons.
Particle Decays in Cosmic Rays:
Muons are produced as secondary particles in the decay of other particles generated in cosmic ray interactions. For example, pions (π+ or π−) produced in cosmic ray collisions can decay into muons and muon neutrinos.
Particle Collisions in Accelerators:
High-energy particle accelerators, such as those found in research institutions like CERN, can produce muons by colliding particles at very high speeds. The energy released in these collisions can be sufficient to create muon-antimuon pairs.
Decays of Charmed Particles:
Certain particles containing charm quarks, such as D mesons (D+, D−, etc.), can decay into muons. Charmed particles are often produced in high-energy experiments or through the study of cosmic ray interactions.
Decays of Beauty (B) Mesons:
B mesons, which contain a bottom (or beauty) quark, can also decay into muons. These decays are studied in experiments to investigate the properties of heavy quarks.
Decays of Tau Leptons:
Tau leptons, which are heavier cousins of muons, can decay into (anti)muons. This occurs in certain particle physics experiments where tau leptons are produced.
Muon-Neutrino Interactions:
Muon neutrinos (νμ) can interact with matter, producing muons in the process. This is observed in neutrino experiments, where neutrino beams are directed at detectors to study neutrino properties.
These processes contribute to the production of muons in different environments, from the natural cosmic ray interactions in the Earth's atmosphere to controlled experiments in particle physics laboratories. Studying muon production and interactions is crucial for advancing our understanding of fundamental particles and the underlying forces in the universe.
Figure 237 - D-meson decay creating a muon
Figure 238 - B-meson decay creating muon
Figure 239 - Tau decay into muon
Figure 240 - muon-neutrino interaction
Muon decay
The muon (μ) is an unstable elementary particle, and it undergoes decay processes primarily through the weak force. The dominant decay mode of a free muon is into an electron (e), an electron antineutrino (νe), and a muon neutrino (νμ). The decay process is represented as follows:
It's important to note that the decay process of the muon is characterized by its relatively short mean lifetime, which is approximately 2.2 microseconds (2.2×10−6 seconds) in its rest frame. This limited lifetime is a result of the weak force governing the decay.
Figure 241 - Muon decay
Muon annihilation
Muon annihilation refers to the process where a muon (μ and an antimuon () come into close proximity and annihilate each other, producing other particles in the process. Annihilation occurs when a particle and its corresponding antiparticle meet and convert their rest mass energy into other particles, typically photons (gamma-ray photons). The process is governed by the principles of quantum field theory.
The reaction for muon annihilation can be represented as follows:
The annihilation process adheres to the principles of conservation of energy and momentum. The total energy before annihilation (rest mass energy plus kinetic energy) is equal to the total energy after annihilation.
Figure 242 - Muon-Muon annihilation
Fusion of muons
Fusion involving muons, often referred to as muon-catalyzed fusion, is a theoretical process that proposes a novel way to induce nuclear fusion reactions at lower temperatures than those required for traditional fusion reactions. The concept involves using muons to overcome the electrostatic repulsion between positively charged atomic nuclei, allowing them to come close enough for nuclear fusion to occur. It's important to note that while muon-catalyzed fusion has been studied theoretically and experimentally, it has not yet demonstrated practical viability for energy production.
Figure 243 - Muon-catalyzed fusion
Recent Developments
US and Europe should team up on muon collider [3]
The Particle Physics Project Prioritization Panel (P5) in the United States has proposed a groundbreaking vision for the future of high-energy particle physics, urging the development of a muon collider. This recommendation comes as a response to the need for prioritizing new facility construction. Unlike traditional colliders, a muon collider would utilize muons, heavier particles than electrons, and could potentially offer ten times more energy with lower costs.
However, realizing a muon collider presents significant challenges, including the instability and rapid decay of muons. Despite the hurdles, the P5 panel calls for a US-led initiative to advance muon collider technology, reminiscent of the Apollo Moon program. This initiative aims to restore US leadership in high-energy physics, which has waned since the cancellation of the Superconducting Super Collider in 1993.
While CERN, Europe's particle-physics laboratory, also considers muon-collider research, its preferred project is the Future Circular Collider (FCC), involving a massive accelerator tunnel and a high-energy proton collider. The FCC's hefty price tag and uncertain scientific justification raise questions about its feasibility and necessity.
Furthermore, some physicists argue for alternative approaches, such as a linear electron–antielectron collider, which could be cheaper and more precise for studying the Higgs boson. Despite the geopolitical implications and the substantial investment required, the case for such projects should focus on global collaboration and scientific exploration.
The proposal for a muon collider represents an opportunity for physicists worldwide to collaborate and determine its feasibility and cost-effectiveness. If successful, it could offer an exciting and potentially more affordable means of exploring nature's fundamental particles and forces.
Literature
References
[1] Nature Vol. 620 No 7974 p. 473 "Dreams ofnew physics fade with latest muon magnetism result"
[2] The Muon g – 2 experiment at the Fermi National Accelerator Laboratory near Chicago, Illinois, has made the best measure of the muon’s magnetic moment. Credit: Science History Images/Alamy
[3] Nature 625, 423-424 (2024) doi: https://doi.org/10.1038/d41586-024-00105-9