It's not surprising that I'm not up to date in this exotic field, but I didn't expect to be totally blindsided by something. How long has this been going on? I assume this is the reason that no free quarks exist in nature. Still, it's a darn curious thing.
Since the universe was 10-5 seconds old, when the hadrons formed. ;^)
The theory was worked out in the 1960's. The key features of QCD are confinement and asymptotic freedom. Confinement means that the quarks are confined to hadrons, as you mentioned. Asymptotic freedom means that as the quarks get closer and closer together, the strong-force interaction between them asymptotically approaches zero, i.e. they behave as free particles as far as the strong force is concerned. (They still interact electromagnetically, etc.)
Both of these features are exhibited by a force that is directly proportional to distance. Asymptotic freedom is obvious--the force goes to zero at zero distance--but confinement is less obvious. That arises from the fact that the quarks have finite masses. If the force is proportional to distance, you can only pull a quark and an antiquark so far apart before you put in enough energy to pry a new quark-antiquark pair out of the vacuum. The color charges cancel those of the quarks you're pulling apart, and they "hadronize": the new quark pairs up with the old antiquark, the new antiquark pairs up with the quark, and you have two "colorless" mesons instead of two quarks.
The usual pedagogical analogy is to try to obtain monopoles by pulling a bar magnet apart. It just won't work.
No they just decided to change the rules recently. Pay better attention. Protons, for the moment, do not "decay".
Color ForceA property of quarks labeled color is an essential part of the quark model. The force between quarks is called the color force. Since quarks make up the baryons, and the strong interaction takes place between baryons, you could say that the color force is the source of the strong interaction, or that the strong interaction is like a residual color force which extends beyond the proton or neutron to bind them together in a nucleus. Inside a baryon, however, the color force has some extraordinary properties not seen in the strong interaction. The color force does not drop off with distance and is responsible for the confinement of quarks. The color force involves the exhange of gluons and is so strong that the quark-antiquark pair production energy is reached before quarks can be separated. Another property of the color force is that it appears to exert little force at short distances so that the quarks are like free particles within the confining boundary of the color force and only experience the strong confining force when they begin to get too far apart. |