How to classify quarks?

  1. when want to illustrate messon and bayyons , we put u,d,s together;
    but when talk about family we put u,d together and c,s together.
    why? I am really confused!
     
  2. jcsd
  3. There are three families of quarks, (u,d), (s,c), and (t,b). Each of these 6 quarks come in three possible colors. These can be mixed in all possible ways to create mesons and baryons. So for example, uud is a proton, and udd is a neutron, but you could in principle mix any combination of 3 (for example usc, or ssb, or ucb, ...) to make a baryon, and any combination of 2 (u-ubar, u-dbar, u-sbar, c-bbar, ...) to make a meson. The only requirement for mesons and baryons is that they be colorless, which means the baryons need to have one quark of each color, and the mesons need to have one color and its corresponding anti-color. Since the u,d,s quarks are the lightest, they make the particle with the longest lifetimes, so they make the most common baryons and mesons, but other types are possible. If you go to this link you will see that there is a whole zoo of possible particles, including D mesons (which have a c quark), and B mesons (which have a b quark). The only caveat here is that the lifetime of the t quark is so short that it is never really observed inside baryons and mesons.
     
  4. Oh, and the other key thing to mention is that each family comprises one up-type and one down-type quark. u, c and t are up-type; d, s and b down-type. Up-type quarks have electric charges of +2/3, down-type ones -1/3.
     
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