# From irreducible representations of the Lorentz algebra to irreducible representations of the Lorentz group

My lecture notes state that we need to classify all finite-dimensional irreducible representations of the proper, orthochronous Lorentz group in order to formulate a QFT for particles with non-zero spin.

This is done by characterising the Lorentz algebra by the eigenvalues $a (a + 1)$ and $b (b + 1)$ of the square of the operators $$\vec{A} = \frac{1}{2} (\vec{J} + i \vec{K}) \\ \vec{B} = \frac{1}{2} (\vec{J} - i \vec{K}) ,$$ where $\vec{J}$ is the generator of rotation and $\vec{K}$ the generator of boosts.

The corresponding representation of the Lorentz group is then obtained by taking the exponential map of particular operators like $\frac{\vec{\sigma}}{2}, 0$ for $a = \frac{1}{2}, b = 0$.

Can $\vec{A}^2$ an $\vec{B}^2$ be understood as the Casimirs of the Lie algebra or do they have something in common with the concept (I am missing some understanding here)?

How can I guarantee that taking the exponential map of an irreducible representation of the Lie algebra gives me an irreducible representation in the corresponding Lie group?

• I've asked a related question on Mathematics -math.stackexchange.com/q/2316362 - perhaps it is of some help. – user1620696 Sep 14 '17 at 13:44
• Comment to the question (v2): Are you talking about the restricted Lorentz group/algebra, or its complexification? – Qmechanic Sep 14 '17 at 13:48
• To be honest, I can't tell. We have not dived that deep into group theory in our theoretical particle physics course. I came across this complexification thing, when searching here and on Wikipedia, but wasn't able to wrap my head around it. Please feel free to explain how those things relate! – rgba Sep 14 '17 at 13:55
• WP not helpful? – Cosmas Zachos Sep 14 '17 at 14:21
• Two good references for projective representations are Weinberg's The Quantum Theory of Fields, Vol I, chapter 2, and Brian C. Hall's Lie groups, Lie algebras, and Representations: An Elementary Entroduction. (The first one is not an easy read, but it covers infinite-dimensional representations.) There is also a detailed version of the Wikipedia article on the Lorentz group representations at Wikiversity. It follows Hall's construction. – YohanN7 Sep 15 '17 at 7:18

• (i) representations of the double cover $$Spin^+(1,3,\mathbb{R})\cong SL(2,\mathbb{C})$$ of the restricted Lorentz group $$SO^+(1,3;\mathbb{R})$$,

• (ii) representations of the corresponding Lie algebra $$so(1,3;\mathbb{R})$$,

• (iii) projective representations of the restricted Lorentz group $$SO^+(1,3;\mathbb{R})$$,

are all labelled by two non-negative half-integers $$(a,b)~\in~\frac{1}{2}\mathbb{N}_0 \times\frac{1}{2}\mathbb{N}_0.$$

1. If $$a+b~\in~\mathbb{N}_0$$ is an integer, it is also a group representation of the restricted Lorentz group $$SO^+(1,3;\mathbb{R})$$ itself.
2. $$A_i$$ and $$B_i$$, $$i\in\{1,2,3\},$$ are the $$3+3=6$$ generators of the complexified Lie algebra $$so(1,3;\mathbb{C})~\cong~sl(2,\mathbb{C})_A\oplus sl(2,\mathbb{C})_B,$$ with quadratic Casimirs $$\vec{A}^2$$ and $$\vec{B}^2$$.
3. The exponential map $$\exp: so(1,3;\mathbb{R})\to SO^+(1,3;\mathbb{R})$$ for the restricted Lorentz group is surjective, cf. e.g. this Phys.SE post.