# Spin in elementary particles

1. Dec 16, 2012

### TheAnalogKid2

Looking at the Standard model, I noticed that all the quarks and leptons have a spin of 1/2 and all of the gauge bosons have a spin of 1. Can someone give me a general definition of what "spin" is and the difference between a particle that has a spin of 1/2 and a particle that has a spin of 1?

2. Dec 17, 2012

### andrien

3. Dec 17, 2012

### dextercioby

Since the Standard Model comes from putting QED + EW together, the particles involved (rather their fields) must have spin 1 tops, as spins 3/2 and 2 are coming from SuperGravity theories.

The elementary leptons are fermions and must have spin 1/2.
The gauge particles are bosons with spin 1.

4. Dec 18, 2012

### h1010134

Wanted to ask the same thing. The question is what exactly IS this mysterious thing called spin? Answers above only state what are the spin values of different particles, not what this "spin" is, or WHY these particles have such spin values.

5. Jan 6, 2013

### DrewD

Spin is an intrinsic property of fundamental particles. It arises from relativistic QM but I am not an expert in that area, so I won't comment on how. Long before that was shown, spin was used in non-relativistic QM. It is called spin because it acts like spin angular momentum of a large body. If you don't think too hard about it, you could probably convince yourself that it is the particle spinning. In an atom, the spin and orbital angular momentum add to make a total angular momentum.

For a while, it seemed reasonable that electrons were really rotating, but E&M would not allow this (you can probably find a nice explanation of why via Google). Spin is now considered a fundamental property of fundamental particles.

I assume that you want more than that. Specific questions might help, but you should probably just get a decent QM book and spend a lot of time with it. I doubt you will be able to get much deeper from a layman's perspective.

6. Jan 6, 2013

### cosmic dust

"What is spin?" is a question of the species "What mass, charge, or color is?" and the answer is the same: "A fundamental property of elementary particles". I think that this property had been observed for the fist time in the famous "Stern–Gerlach experiment" or, according to wikipedia, "in the context of the emission spectrum of alkali metals".

The first attempt to theorize spin was done by W. Pauli, although, except than a two-valued degree of freedom, he had no idea what was it exactly. Some physicists thought that the fundamental particles were actually spinning, so they had some internal angular momentum, but this idea was abandoned at once, because the velocity required to have the observed value of angular momentum exceeds the speed of light.

The correct theoretical treatment of spin arises from the combination of QM and special relativity. Specifically, when you calculate the angular momentum of a particle in the rest frame of that particle, you find that it can have a non-zero value. That means that that the particle carries some angular momentum that is not due to its motion, i.e. spin is an internal amount of angular momentum.

Spin is quantized, that is it can have only certain values, and these values can be integer or half-integer multiples of $\hbar$. According to QFT and specifically because of the “causality principle”, particles of half-integer spin have to be “fermions” and those of integer spin have to be “bosons”, that is, a large number of identical particles have to obey “Fermi-Dirac” or “Bose-Einstein” statistics, respectively.

Those are some general information about spin and I hope will help you clear up the concept of spin.

Last edited: Jan 6, 2013
7. Jan 6, 2013

### DiracPool

Nice summary Dust.