Muon

Muon ( μ - )

The muon ( muon in English ) is an elementary particle similar to the electron, however, an approximately 200 times higher mass ( 105.6 MeV/c2 instead of 0.511 MeV/c2 ) has. As the electron is negatively charged with an elementary charge and has a half-integer spin. Both are subject to the electro-weak but not the strong interaction. The formula symbol of the muon is.

According to the standard model the electron and muon are related particles. Both belong to the class of leptons, only that the electron belongs to the first of the three families, the muon to the second. The corresponding particles of the third family, the τ - lepton, has also been demonstrated. The antiparticle of the muon, the positive muon or antimuon is how the positron simply positively charged.

The muons were discovered in 1936 by Carl D. Anderson and Seth Neddermeyer in the study of cosmic rays. They can be artificially generated by high-energy particle accelerators.

Until about the 1960s, the muon was called My- meson; " Meson " meant at that time simply " particle with a mass between the electron and proton ."

Cosmic radiation

Muons are one of the main components of the secondary cosmic radiation, they are created by reactions of the actual cosmic rays (mainly protons coming from outer space ) with atomic nuclei and molecules of the atmosphere. In the investigation of cosmic ray muons were first discovered in 1936. Most muons are produced in the outer atmosphere: At an altitude of about 10 km already 90 % of all produced in the entire atmosphere muons are created. Initially created pions and kaons to a lesser extent. The decay of these very short-lived particles by weak interactions arise, among others, muons and muon neutrinos. At sea level, positive and negative muons in the ratio / ≈ 1.27 are measured.

Time dilation

Because of the time dilation of moving particles, the muons secondary cosmic radiation can reach the earth's surface in spite of its short half-life; without this relativistic effect would be only about 600 m range. Was measured for the first time experimentally the number of muons that arrive at different heights in 1941 by Bruno Rossi and D. B. Hall. The measurement was limited to those muons through a special filter assembly which move with 99.94 % of the speed of light. The comparison of the measured numbers has made it possible to determine the half -life of these rapidly moving muons. This is 13 microseconds approximately nine times longer than the half-life of muons at rest, which is 1.5 microseconds ( this half-life results from the above mentioned life by multiplying by 0.693 ..., the natural logarithm of 2 ). Thus, the fast-moving muons decay slower than their stationary counterparts. A similar experiment was performed with increased precision of fresh and Smith ( 1963).

The flux density of cosmic muons at sea level is about 100 per square meter per second. The ratio μ / μ - is about 1.2.

Decay

The free muon decays according to the Feynman diagram shown on the right in a muon neutrino, an anti- electron neutrino and an electron:

The antimuon decomposes accordingly, except that in each case the antiparticles of the aforementioned particles are formed.

In addition, the decay nor gamma rays ( photons) are generated with a probability of ( 1.4 ± 0.4 ) %

Or an electron -positron pair with a probability of ( 3.4 ± 0.4 ) · 10-5:

According to the standard model the decay of the muon is mediated by a W boson ( see also boson ).

The experimentally determined mean lifetime of the positive muon is 2.196 980 3 microseconds ± 2.2 ps. The negative muon in matter has an additional decay channel by trapping in an atomic nucleus, corresponding to the K- capture of an electron; there is a proton to a neutron and the muon to a muon neutrino. Therefore, the experimentally determinable mean lifetime of the negative muon is shorter in matter. In a vacuum, so without this additional decay channel, the measured lifetimes of positive and negative muon to 1 ‰ match exactly.

In the standard model forbidden decay channels

Certain neutrinoless decay modes of the muon are indeed kinematically possible, but in the standard model (ie without neutrino oscillations ) is prohibited and not previously observed. This is expressed by the laws of conservation of lepton flavors ( conservation of Leptonenfamilienzahlen in each interaction vertex ), from which it also follows that the muon is not an excited state of the electron. Examples of such decays, which would change the lepton -flavor are

As well as

The observation of such a decay would be a clear indication of new physics beyond the Standard Model ( New Physics ). The upper limit for the branching ratios of these decays has been in numerous experiments over the last 50 years constantly improved. The current value for the decay was determined in 2011 in the MEG experiment and is < 2.4 x 10-12.

Magnetic anomaly of the muon

Muons are particularly well suited to study fundamental forces in physics at the highest level of precision. As leptons they are to be considered according to current knowledge as a point. Thus, their properties can be very precisely calculated in the framework of quantum electrodynamics. The influence of other forces than the electromagnetic force is small, but by virtual particles surrounding the muon observable. This leads to a variation of the magnetic properties of the muon.

A precision measurement of this magnetic anomaly was carried out at Brookhaven National Laboratory by a worldwide collaboration around the year 2000. Should there be other than the currently known particles of particle physics, and have this not overly large masses, then they would have to make itself felt in the magnetic anomaly of the muon. Because the experiment could not find a large deviation, so the standard model of particle physics has been confirmed in an impressive way. The magnetic anomaly of the muon is called g -2 value and a value at which all particle theories have to be measured.

Muonic atoms

Muons (but not antimuons ) can be bound to the atomic nuclei due to their charge as electrons. However, the corresponding Bohr radius of the " Myonbahn " around the nucleus in the ratio of the masses of the electron and muon is smaller. The result is that the muons are much more strongly bound than the electrons. Usually going muons shortly after capture in a 1s state. In heavy nuclei due to the small orbital radii, much of the muon probability is in the nucleus. There it can then come to the inverse beta decay, in which absorbs the muon and a proton is converted into a neutron. Here, in addition, a neutrino, and possibly one or more gamma quanta produced. The newly formed nucleus is radioactive in many cases. Going through these in sequence a normal beta decay, the original nucleus is restored.

A bound muon has a much shorter service life, for example, about 0.163 microseconds in copper, which is also used for muon -spin analysis due to the additional reaction probability.

Since the bound muon shields a part of the nuclear charge, to shift the energy levels of the bound electrons. Add a muonic atom, however, a muon and two electrons could well (ie both) are in a 1s state. The ban (Pauli principle) that no two fermions in the same system can reside in the same condition does not apply to different types of particles such as the electron and the muon.

The bound muon is the only additional decay pathway - in addition to all decay channels of the free muon - the Kerneinfang open. Kerneinfang is the dominant process for heavy nuclei. After further decay possibilities is currently being sought, such as the so-called muon -electron conversion. This would be a clear sign of so-called new physics, because this process is not possible in the standard model of particle physics.

Antimuons can with their positive charge, however, like protons or positrons itself to capture an electron. This creates an exotic atom muonium is called.

Muon -catalyzed fusion

If a muon captured (D2 or DT ) in a deuterium or a deuterium -tritium molecule, the result is a positive muonic molecular ion, because the relatively large binding energy of the muon releases the two electrons of the molecule. In this muonic molecular ion, the two nuclei are each about 200 times closer than in an electronic molecule. This allows the tunnel effect, the fusion of the two nuclei. The very large released by the fusion energy (at D D about 3 MeV for D T 14 MeV) also sets the muon free again, and it can vary depending on the environmental conditions during its lifetime, many more ( order of 100 ) catalyze individual mergers.

To produce this myonisch catalyzed fusion useful energy, you should be able to gain more energy from the approximately 100 individual mergers than is necessary for the generation of the muon. The efficiencies achieved to date the particle accelerator facilities with which muons can be produced, are not sufficient.

The muon -catalyzed fusion is also known under the name of cold fusion. It was originally proposed by Andrei Sakharov.

Applications

With the help of muons, it is possible to illuminate larger objects. These muons of cosmic radiation are used and measured their absorption. So the Khafre Pyramid of Luis Walter Alvarez was X-rayed in the 1960s. More recently, the Iwodake volcano was illuminated by the density distribution of the volcano could be determined.

Trivia

The muon was initially mistaken for a 1935 postulated by Hideki Yukawa exchange particles of nuclear power, which is known as Pion today. This oversight was due to the similar rest mass of the two particles ago; it took almost ten years to the confusion was cleared up.