Quarks are the building blocks of hadrons, such as protons and neutrons. The six types of quarks are up (u), down (d), strange (s), top (or truth, t), charm (c), and bottom (or beauty, b). The first three are known as "light quarks", while the latter three are "heavy quarks." (See the chart for more information.) There are six quantities called quantum numbers that describe every elementary particle, and each type of quark has its own set of quantum numbers. Here are the quantum numbers of the light particles:

 Particle
 Isotopic Isotopic
   Spin     Spin    Hypercharge     Strangeness    Baryon Number     Charge
    I        I3          Y               S               B              Q
u  1/2      +1/2        +1/3             0              1/3            2/3
d  1/2      -1/2        +1/3             0              1/3           -1/3
s   0         0         -2/3            -1              1/3           -1/3

Strangeness is the name given to the fifth quantum number. It was postulated (discovered) in 1953, by M. Gell-Mann, T. Nakano and K. Nishijima, each working independently. The next year it was clearly demonstrated experimentally. It is a property of subatomic particles, and only applies to those known as hadrons, which include protons, neutrons, pions, kaons, and lambda, omega, and rho particles, among others. The symbol for strangeness is S.

The strangeness of a particle is the sum of the strangeness of its component quarks. Of the six flavors of quarks, only the strange quark has a nonzero strangeness. The strangeness of nucleons is zero, because they only contain up and down quarks and no strange (also called sideways) quarks. For more information see the chart The Standard Model of Fundamental Particles and Interactions.

We can find the strangeness of a particle by using the law of conservation of strangeness. For example, in a reaction where a negatively-charged pion interacts with a proton, a neutral kaon and a neutral lambda particle are formed. Since the strange numbers of the pion and proton are both zero and the kaon has a strangeness of +1, we know that the lambda particle's strangeness is -1.

"It is found that strangeness is conserved in all processes mediated by the strong and electromagnetic interactions."(Beiser p.534) An example is the high-energy proton-proton collision (protons have S=0), which creates a kaon(S=1), a lambda hyperon (S=-1),a proton (S=0) and a positively-charged pion (S=0). "On the other hand, strangeness can change in an event governed by the weak interaction."(Beiser p.534) However, the violation of this strangeness can only be +/- 1. Therefore, a particle such as a negatively-charged hyperon (S=-2) can not decay directly into another particle with a strangeness that is different by 2 or more, such as a neutron (S=0). Such a decay must proceed via two steps: The negatively-charged hyperon (S=-2) decays into a lambda hyperon (S=-1) and a negatively-charged pion (S=0). The lambda hyperon then decays into a neutron (S=0) and a neutrl pion (S=0).

The hypercharge (Y) is defined as the sum of baryon number and strangeness. Because both of these quantities are conserved under the influence of the stong and the electromagnetic forces, the hypercharge, therefore, should also be conserved.

Four of the five quantum numbers listed above can be related by the equation: Q=I3 + B/2 + S/2.

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