![]() Therefore, a quantum number is introduced for each family: the lepton family number. It is also observed that the net number of leptons and antileptons within each generation is conserved in each interaction. TABLE 1 Characteristics and Quantum Numbers Associated with Leptons Prior to the interaction L = (+1) + (-1) = 0, and after the interaction L = (+1) + (-1) = 0. As an example of L conservation, consider the case where an electron and positron annihilate and create a muon and an antimuon ( e + e - → μ +μ -). Leptons have L = +1 and antileptons have L = -1, whereas quarks have L = 0. This is known as lepton conservation, which has an associated quantum number of lepton L. It has been experimentally observed that the net difference in the number of leptons compared to anti-leptons before and after an interaction is unchanged. Through the weak interaction, the more massive charged leptons may decay into their less massive counterparts. All fermions participate in the weak interaction and carry weak charge. Leptons do not participate in the strong interaction, so it is said that they carry zero strong (color) charge. By historical convention, the electrically charged leptons are assigned one unit of negative, rather than positive, electric charge. Each generation has two particles and two antiparticles, where the antiparticles have the same mass as the particle but opposite quantum numbers.Įach force has an associated charge. There is an electron, muon, and tau lepton family. ![]() The set of leptons can be arranged into three generations, as shown in Table 1. The defining feature of a lepton is that it does not participate in the strong interaction, allowing it to exist for substantial periods of time as an independent particle. Leptons were discovered much earlier than the other set of elementary fermions, the quarks, because they appear individually in nature rather than as composite particles. There is an electron neutrino, a muon neutrino, and a tau neutrino. ![]() For each of these three charged leptons, there is an uncharged partner particle, a neutrino. Two other family members that carry electric charge are the muon and the tau. One of the members of this family, the electron, was the first elementary particle to be discovered. This name is appropriate because leptons are a set of particles with no measurable dimensions, and hence they are elementary. The name lepton derives from the Greek word leptos, meaning thin or light.
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