# The Haag-Lopuszanski-Sohnius theorem

1. Jul 9, 2006

### Perturbation

Hey, guys. I recently bought Weinberg's QFT Vol. III on Supersymmetry and I'm a bit stuck with part of the proof he gives for the Haag-Lopuszanski-Sohnius theorem in chapter 25.2. He starts off by giving the usual way of classifying representations of the Homo' Lorentz group by a pair of integers (A, B) according to

$$\mathbf{A}=\tfrac{1}{2}\left(\mathbf{J}+i\mathbf{K}\right)$$
$$\mathbf{B}=\tfrac{1}{2}\left(\mathbf{J}-i\mathbf{K}\right)$$

Where J and K are the generators of rotations and boost respectively. This I'm familiar with. Then he introduces a set of (2A+1)(2B+1) fermionic operators $Q_{ab}^{AB}$ (with a=-A...A and b=-B...B) that furnish an (A, B) representation of the Homo' Lorentz group, ok. But what I don't get is the commutation relations he gives for these operators with A and B as above

$$[\mathbf{A}, Q_{ab}^{AB}]=-\sum_{a'}\mathbf{J}^{(A)}_{aa'}Q_{a'b}^{AB}$$.
$$[\mathbf{B}, Q_{ab}^{AB}]=-\sum_{b'}\mathbf{J}^{(B)}_{bb'}Q_{ab'}^{AB}$$

Where $\mathbf{J}^{(j)}$ is the spin j three-vector matrix. The commutation relations make sense intuitively: the commutator of A and Q should be a sum of Q's that belong to the A rep', likewise with the commutator with B. But I don't quite get the introduction of J, does anyone have a proof they can give or link me to? I follow the rest of the proof of the theorem, but these commutation relations are quite important to establish a starting point of the theorem, namely the relation between the Hermitian adjoint of an (A, B) operator and a (B, A) operator. I'd skip over it and just accept it but it's bothering me and I'm not usually the sort to assume important results.

I was thinking I could write $Q_{ab}^{AB}$ as a tensor product of A and B spinor operators and work it through like that, and given that A and B satisfy the usual commutation relations of angular momentum it makes sense that J should pop out at the end, but I'm not sure. Perhaps I'm not looking at it right and the Q's are just adjusted so that they obey said relations...if so I wasted five minutes writing this. All Weinberg says in relation to them is "Moreover the Q's satisfy the following commutation relations [the ones referenced above]", or something like that.

Any help would be appreciated, this damn thing is stopping me from progressing through the topic, something I've been interested in for a while, but haven't had the money to buy a book on.

Cheers, folk

Last edited: Jul 9, 2006
2. Jul 10, 2006

### Perturbation

Nobody...?

3. Jul 10, 2006

Staff Emeritus
Not I, said selfy-welfy.

4. Jul 10, 2006

### Perturbation

Bummer

Perhaps in the "Beyond the Standard Model" forum?

5. Jul 10, 2006