دوستان سلام

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Leptons

Leptons are fundamental particles that have no strong interactions. The six known types of leptons are shown in the table below. There are also six anti-lepton types, one for each lepton.

The electron has a mass of 0.000511 GeV/c². The electron is the least massive charged particle of any type. It is absolutely stable because conservation of energy and electric charge together forbid any decay.

The antiparticle of the electron is called a positron. It has exactly the same mass as the electron, but the opposite sign (+1) for its electric charge. Positrons are also stable particles. However, positrons can annihilate when they meet an electron. Both the electron and the positron vanish, and their energy goes into photons and, possibly, more massive particles. Conversely, photons with sufficient energy (E > 2x (mass of electron) x c²) can produce an electron and a positron -- this is called pair production.

The muon has a mass of 0.106 GeV/c². The negatively charged muon (mu-minus) is just like an electron, except it is more massive. Muons are unstable -- they decay to produce a virtual W-boson and the matching neutrino type. The W-boson then decays to produce an electron and an electron-type anti-neutrino.

The antiparticle of a mu-minus is a mu-plus. Particle physicists use the name muon for either mu-plus or a mu-minus a muon. The mu-plus decays to produce an anti-muon type neutrino and a W-plus boson, which then decays to a positron and an electron-type neutrino.

Muons are produced in particle physics experiments. They also are produced by cosmic rays. Because they are much more massive than electrons, muons readily pass through the electric fields inside matter with very little deflection. So, muons do not radiate and slow down as electrons do. However, they can cause ionization and this makes them readily detectable in matter, for example, with a Geiger counter.

The tau-minus is a electron-like particle with a mass of 1.784 GeV/c².  Its antiparticle, the tau-plus, has the same mass but a positive electric charge. These particles were discovered at SLAC in experiments at SPEAR . The 1995 Nobel Prize was awarded for this discovery.

This third type of charged lepton is also unstable. The tau-minus decays to produce its matching neutrino and a virtual W-minus boson. The W-minus has enough energy that there are several possible ways for it to decay, such as:

1. An electron and an electron-type antineutrino.
2. A um-minus and an muon- type antineutrino.
3. A down quark and an up-type antiquark.
4. An s quark and an up-type antiquark.

The quark and antiquark do not emerge individually. One or more mesons emerge from the decay that contain the initial quark and antiquark, and possible additional quark-antiquark pairs produced from the energy in the strong force field between them.

For tau-plus, a similar set of decays occurs -- just replace every particle by its antiparticle (and vice-versa, every antiparticle by the matching particle.) Thus, for example, tau-plus can decay to give a tau type anti-neutrino and a positron and an electron-type neutrino.

There are three types of neutrinos, one associated with each type of charged lepton. All are particles that are somewhat like electrons: they have half a quantum unit of spin angular momentum, and do not participate in strong interactions.

However, neutrinos differ from electrons in that they have zero electric charge and, as far as we know today, zero mass. (See, however, evidence for neutrino mass discovered in Japan.) Experimentally, all we can do is set an upper limit on their masses -- they are smaller than some value. Larger masses would have had observable effects in some experiment. The limits are:

• less than 0.00000002 GeV/c² for electron type neutrinos (or antineutrinos).
• less than 0.0003 GeV/c²for muon type neutrinos (or antineutrinos).
• less than 0.04 GeV/c²for tau type neutrinos (or antineutrinos).

The only known difference between the three neutrino types is which type of the charged lepton they are associated with during production or decay processes.

Since neutrinos have no electric charge, they participate only in weak interaction or gravitational processes. For this reason, they are very difficult to detect. We observe them only by the effects they have on other particles with which they interact.

For example, a high-energy electron-type neutrino can convert to an electron by exchanging a W-boson with a neutron (which becomes a proton when it absorbs the W boson). This rarely happens. With an intense source of neutrinos and a large detector containing many neutrons, one can observe events with no visible initiating particles that can only be explained as neutrino-initiated processes. What is seen in the detector is the recoiling electron and proton after the process occurs.   (Experimental work demonstrating this process resulted in Frederick Reines sharing the 1995 Nobel Prize with Martin Perl.)

Even harder to see is the process where the neutrino is deflected by exchanging a Z-boson with a proton or neutron. The proton or neutron gains energy from this exchange, so one searches for events where a recoiling proton or neutron is seen with no associated electron and no visible initiating particle.

In high-energy particle experiments, we often use energy and momentum conservation to infer that production of one or more neutrinos occurred. If the detector detects everything but neutrinos, then an event where the total final energy detected (or the total final momentum) does not match the initial energy (or momentum) in the incoming particles, then neutrinos must have been produced.  The neutrinos carried off the missing energy (and momentum).