Re: Hi, I've got a question about isospin and quark generations:

Charles,

Nice question. The short answer (as I see it) is that there is nothing particularly special about quantities like isospin or hypercharge. One could just as well classify all of the hadrons in terms of their quark content, spin, and angular momentum properties.

As you may already suspect, most of the textbooks on particle physics are written by professors who were active in the field before quarks were accepted as established theory. Before all of the different baryons and mesons were understood to be different combinations of quarks, physicists in the 1960s like Murray Gell-Mann were making sense of the particle "zoo" by organizing particles with the same spin according to their isospin and hypercharge. New particles could be predicted and their properties estimated simply by observing the symmetries of each particle group. Gell-Mann's so-called "Eightfold Way" was only one of two leading theories of the time. Other physicists subscribed to "S-Matrix Theory", which related hadrons to each other by having them lie on straight lines (called "trajectories") in graphs of their mass versus their spin. Observable mesons (and baryons) appeared at points where these trajectories crossed lines of integer (or half integer) spin.

Both the Eightfold way and S-matrix theories were viewed as "orthodox" physics before they were superseded in the early 1970s by the quark model. Hence, much of the terminology used at the time still survives.

While in some sense "obsolete", the older terms like isospin and hypercharge still have their uses. Isospin, for example, is perfectly valid for understanding nucleon-nucleon interactions, since the u and d quarks have nearly identical masses anyway. Furthermore, with all the literature still used that have the older terminology, I suspect that these terms will not go away any time soon.

Still, I find courses and textbooks which introduce particle physics to new students in an "historical" fashion to be unnecessarily confusing. I like the textbook Introduction to Elementary Particles by David Griffiths, which begins with an historical introduction in the first chapter and then gives a modern presentation of particle physics based on the quark model, symmetries, and Feynman calculus. I also recommend the popular book The Hunting of the Quark by Michael Riordan. It is a well written book on the history of particle physics, written by someone who took part in the quark revolution. It can be quite an eye opener for people who believe that science progresses in orderly, logical stages, or that physics terminology is carefully formulated. :-)

I hope this helps. Please feel free to email me at silver@physto.se if I can be of further assistance.

Cheers,

Sam Silverstein