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Encyclopedia > Muon
Composition: Muon The Moon's cosmic ray shadow, as seen in secondary muons detected 700m below ground, at the Soudan 2 detector. Elementary particle Fermion Lepton Second Gravity, Electromagnetic, Weak Antimuon Carl D. Anderson, 1936 105.658369(9) MeV/c2 −1 e None ½

For historical reasons, muons are sometimes referred to as mu mesons, even though they are not classified as mesons by modern particle physicists (see History). Muons have a mass of 105.6 MeV/c2, which is 206.7 times the electron mass. Since their interactions are very similar to those of the electron, a muon can be thought of as a much heavier version of the electron. Due to their greater mass, muons do not emit as much bremsstrahlung radiation; consequently, they are much more penetrating than electrons. Mesons of spin 1 form a nonet In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. It is the speed of all electromagnetic radiation in a vacuum, not just visible light. ... (helpÂ· info), (from the German bremsen, to brake and Strahlung, radiation, thus, braking radiation), is electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. ...

As with the case of the other charged leptons, there is a muon-neutrino which has the same flavor as the muon. Muon-neutrinos are denoted by νμ. The neutrino is an elementary particle. ... ...

## Contents

Since the production of muons requires an available COM frame energy of over 105 MeV, neither ordinary radioactive decay events, or nuclear fission and fusion events (such as happen in nuclear reactors and nuclear weapons), are energetic enough to produce muons. Only nuclear fission produces single-nuclear-event energies in this range, but due to conservation constraints, this is not available to produce since particles such as the muon. The center of mass frame (also called the center of momentum frame, CM frame, or COM frame) is defined as being the particular inertial frame in which the center of mass of a system of interest, is at rest (has zero velocity). ... Radioactive decay is the set of various processes by which unstable atomic nuclei emit subatomic particles (radiation). ... Nuclear power station at Leibstadt, Switzerland. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the epicenter. ...

On earth, all naturally occurring muons are apparently created by cosmic rays, which consist mostly of protons, many arriving from deep space at very high energy. When a cosmic ray proton impacts atomic nuclei of air atoms in the upper atmosphere, pions are created. These decay within a relatively short distance (meters) into muons (the pion's favorite decay product), and neutrinos. The muons from these high energy cosmic rays, generally continuing essentially in the same direction as the original proton, do so at very high velocities. Despite their lifetime, which without relativistic effects would allow a half-survival distance of only about 0.66 km at most, the time dilation effect of special relativity allows cosmic ray secondary muons to survive the flight to the earth's surface. Indeed, since muons are unusually penetrative of ordinary matter, like neutrinos, they are also detectable deep underground and underwater, where they form a major part of the natural background ionizing radiation. Like cosmic rays, as noted, this secondary muon radiation is also directional. See the illustration above of the moon's cosmic ray shadow, detected when 700 m of soil and rock filters secondary radiation, but allows enough muons to form a crude image of the moon, in a directional detector. Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: &#960;0, &#960;+ and &#960;&#8722;. Pions are the lightest mesons. ... The neutrino is an elementary particle. ... Time dilation is the phenomenon whereby an observer finds that anothers clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest...

The same nuclear reaction described above (i.e., hadron-hadron impacts to produce pion beams, which then quickly decay to muon beams over short distances) is used by particle physicists to produce muon beams, such as the beam used for the muon g-2 gyromagnetic ratio experiment (see link below). In naturally-produced muons, the very high-energy protons to begin the process are thought to originate from acceleration by electromagnetic fields over long distances between stars or galaxies, in a manner somewhat analogous to the mechanism of proton acceleration used in laboratory particle accelerators. In physics, the gyromagnetic ratio or LandÃ© g-factor is a dimensionless unit which expresses the ratio of the magnetic dipole moment to the angular momentum of an elementary particle or atomic nucleus. ... A particle accelerator uses electric fields to propel charged particles to great energies. ...

## Muon decays

The most common decay of the muon involves a W boson

Since muons are the lightest charged particle except for the electron, they must decay to an electron and other particles with a net charge of zero. Nearly all of the time, they decay into an electron, an electron-antineutrino, and a muon-neutrino. Antimuons decay to a positron, an electron-neutrino, and a muon-antineutrino: Image File history File links Muon-Electron-Decay. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ...

$mu^-to e^-barnu_enu_mu,~~~mu^+to e^+nu_ebarnu_mu$.

Rarely, a photon or electron-positron pair is also present in the decay products. The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, microwaves, radio waves, and visible light are all forms of light. ...

## Muonic atoms

The muon was the first elementary particle discovered that does not appear in ordinary atoms. Negative muons can, however, form muonic atoms by replacing an electron in ordinary atoms. Muonic atoms are much smaller than typical atoms because the larger mass of the muon gives it a smaller ground-state wavefunction than the electron. In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. ... Atomic redirects here. ... An exotic atom is the analogue of a normal atom in which one or more of the negatively charged electrons found in an ordinary atom are replaced by other negative particles, such as a muon or a pion, or a positively charged proton found in the nucleus of an ordinary... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ...

## 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). Carl David Anderson (3 September 1905 &#8211; 11 January 1991) was a U.S. experimental physicist. ... 1936 (MCMXXXVI) was a leap year starting on Wednesday (link will take you to calendar). ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Current (I) flowing through a wire produces a magnetic field () around the wire. ... // For alternative meanings see proton (disambiguation). ...

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 μ (mu) used to approximate the sound of the Latin letter m).

However, it was soon found that the mu meson significantly differed from other mesons; for example, its decay products included a neutrino and an antineutrino, rather than just one or the other, as was observed in other mesons. Other mesons were eventually understood to be hadrons, that is, particles made of quarks, and thus subject to the residual strong force. In the quark model, each meson is composed of two quarks. Mu mesons, however, were found to be fundamental particles (leptons) like electrons, with no quark structure. Thus mu mesons were not mesons at all (in the new sense of the term), and so the term mu meson was abandoned, and replaced with the modern term muon. Neutrinos are elementary particles. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the &#931;c++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... This article is about the force sometimes called the strong nuclear force. For the weak nuclear force or weak interaction, see that article. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ...

A muonium particle is an exotic atom made up of a positive muon and an electron, and is given the symbol Mu or Î¼+eâ€“. During the muons 2 microsecond lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu). ... Muon spin spectroscopy is an experimental technique based on the implantation of spin polarized muons in matter and on the detection of the influence of the atomic, molecular or crystalline surroundings on their spin motion. ... Muon-catalyzed fusion is a process allowing nuclear fusion to take place at room temperature. ... This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them. ...

Results from FactBites:

 muon. The Columbia Encyclopedia, Sixth Edition. 2001-05 (355 words) However, the muon’s behavior did not conform to that of Yukawa’s meson theory (which actually describes the pion, discovered more than 10 years later), and the muon is now classed as a lepton rather than a meson. Muons are produced by the weak decay of pions into a muon and a muon antineutrino. Muons can be substituted for electrons in orbit around the nucleus of an atom; the resulting atom is long-lived enough to exhibit behavior that further supports the close resemblance between the muon and the electron.
 Leptons (635 words) The muon is a lepton which decays to form an electron or positron. The muon is produced in the upper atmosphere by the decay of pions produced by cosmic rays: Muons make up more than half of the cosmic radiation at sea level, the remainder being mostly electrons, positrons and photons from cascade events.(Richtmyer) The average sea level muon flux is about 1 muon per square centimeter per minute.
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