Active Reading Solution: Building Hadrons

  1. A baryon is a combination of a red, a blue, and a green quark. Those colors do not impose any restriction on flavor: remember that every flavor of quark (up, charm, etc) can come in all three colors. But we do have to use exactly three quarks to make a baryon. Each quark will have a charge of (2/3)e or -(1/3)e. (They cannot have the opposites of those charges, because then they would be antiquarks, and they would make an antibaryon.) Since we want the total charge to be -1, we want all three quarks to have charge -(1/3)e. The lightest way to achieve that is to choose all three of our quarks from the left-most (lightest) family.

    So the answer is: three down quarks, one of each color.

  2. A meson is a combination of a quark (charge (2/3)e or -(1/3)e) and an antiquark (charge -(2/3)e or (1/3)e) with complementary colors. Since we once again want the lighest possible combination, we once again go to the left-most family and combine a down (charge -(1/3)e) with an antiup (charge -(2/3)e). The colors must match: for instance, if the former is blue, the latter must be antiblue.