Strange Particles
In particle physics, strange particles are those particles that, in addition to containing up and down quarks, contain a strange quark. Strange particles were originally so named because they exhibited greatly increased lifespans in comparison with other particles.
Strange particles are particles that are only produced in pairs in electromagnetic and strong interaction processes. They possess an extra quantum number called strangeness, and this quantum number is conserved in all strong and electromagnetic scattering and decay processes. Strangeness was later related to the fact that strange particles contain a new quark, known as the strange quark.
Strangeness (and the strange quantum number) are elementary particles that are conserved by the strong force. The quantum number reflects the difference between the baryon number and the hypercharge.
By the 1960s, the number of elementary particles was increasing rapidly. Among the strange particles discovered were K mesons, K+ , K- , K0 , and K-bar0 , with strangeness +1 or -1; the [b.lambda]0 , 0 , + , and - baryons, also with strangeness +1 or -1; the (Ξ)0 , (Ξ)- , Y0 , Y+ , and Y- baryons, with strangeness +2 or -2, and the (Omega) with strangeness -3.
As an example of strangeness conservation, consider the scattering reaction p + ()+ [b.lambda]0 + K0 . Protons and pions are common and have strangeness number 0. This collision occurs via the strong interaction, so strangeness is conserved. Therefore, by conservation of strangeness, the [b.lambda]0 and K0 must have opposite strangeness numbers. By convention, the K0 has strangeness number +1, requiring the [b.lambda]0 to have strangeness number -1.
Strange particles are free to decay via the weak interaction, but are not allowed to decay through the strong interaction. Because of this, strange particles have lifetimes of about 10-10 seconds compared with the 10-23 second lifetime of an unstable hadron with no strangeness.
Once it was understood that hadrons and mesons were made up of quarks, the eξstence of strange particles required more than just the common up and down quarks. To explain strangeness, the strange quark was postulated. The strange quark is nearly identical to the down quark, but it has a heavier mass. In the standard model, strange quarks can only decay via the weak interaction, but strange quark-antiquark pairs can be produced in strong and electromagnetic interactions and thus provide for the conservation of strangeness.
In contrast to the strange quark, the eξstence of other exotic quarks such as the charm and bottom quark were inferred by looking at resonance in processes. A resonance is a sharp peak in the probability distribution for scattering between two particles, and it occurs when there is the exact amount of energy needed to produce a particle-antiparticle pair.
The eξstence of the strange particles, as well as the eξstence of the muon, were the first clues that a three-family structure of particles eξsts. Physicists suspect there are three copies of each kind of particle, the only difference being that the masses are larger in the later families. For example, the down, strange, and bottom quarks all interact identically, but the bottom is heavier than the strange which is heavier than the down quark.
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