View Full Version : Isospin and Hypercharge
04-21-2002, 12:37 PM
In Quantum Chromodynamics...the quantum/relativistic theory
which describe the Strong Nuclear Force...has two factors which
puzzle me: Isospin and Hypercharge.
Now spin, AFAIK, is analogous to an object rotating,
while charge is like an electrical positive or negative.
How do spin and charge relate to isospin and hypercharge?
GOOGLE only gave me a bunch of sites giving me incomprehensible equasions.
Until someone comes along to give a better answer maybe this will help.
isospin Particle Physics. a quantum number representing a fictitious spin vector, conserved in all nuclear reactions, in which charge independence of hadronic forces treats neutrons and protons as two states of one particle without electromagnetic interaction. Also, ISOBARIC SPIN, ISOTOPIC SPIN.
hypercharge Particle Physics. a quantum number (Y) that is calculated by a particle's strangeness. Hypercharge is analogous to electric charge, except that hypercharge is not conserved when the particle decays.
04-21-2002, 02:10 PM
Isospin reflects the fact that QCD says that the force between quarks doesn't depend on what those quarks are - it's the same whether they're up quarks, down quarks whatever. Thus, from the point of view of the strong force, a proton (uud) and a neutron (udd) are pretty much the same. It's therefore sometimes useful to think of them as merely two states of the same thing.
At this stage, you can introduce an analogy with spin. In QM, a spin 1/2 particle, when observed, will be in one of two states, spin up or spin down. Similarly, the isospin of a nucleon is 1/2 and the proton corresponds to "isospin up" and the neutron to "isospin down". You can also extend the same idea to other sets of particles. Pions come in three types, with charges of +1, 0 and -1. Pions therefore have isospin 1, since spin-1 particles can have three states. All the maths involved is then identical to that for spin; nasty enough, but familiar to particle physicists. (Actually, there are pretty deep reasons why the maths here have to be the same, to do with symmetry groups.) If you want, you can think of an abstract isospin space in which a proton can become a neutron simply by being rotated.
The real usefulness of isospin is that it's a conserved quantity in strong interactions and so is a very powerful tool for deciding which reactions are possible and the like. As you can probably guess from the fact that protons and neutrons are different, isospin isn't conserved in electromagnetic interactions. Of course, historically isospin was introduced (by Heisenberg in 1932) long before it could be explained in terms of QCD; it was a handy approximation in understanding nuclear physics.
Hypercharge is just Y = B + S (well, it gets a little more complicated when you introduce charm etc.), where B is the baryon number and S is the strangeness. There's nothing terribly profound: the textbook I checked this definition in just now (Halzen and Martin) even states "This choice has no physical significance; it simply centers the multiplets on the origin."
I believe the significance of hypercharge is that it is conserved in EM and strong interactions, but not in weak interactions.
04-21-2002, 05:07 PM
rsa is perfectly correct. The thing about the hypercharge is that it's effectively equivalent to strangeness. That's rather more intuitive, since it basically counts the number of strange quarks in a particle. One of the consequences of QCD not caring about the flavour of quarks is that it doesn't change those flavours. Thus in a strong interaction the number of strange quarks is conserved (with antiquarks counting as negative). Hence hypercharge is as well. Similarly for EM interactions, but not for weak ones.
Because hypercharge is equivalent to this more intuitive quantity, particle physicists tend not to think in terms of it in practice. It's defined in all the textbooks and certainly useful in classifying particles, but when it comes to actual reactions one's more likely simply to visualise which quarks are going where to decide whether the process is possible rather than totting up hypercharges.
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