I) Representation theory for the double cover $SL(2,\mathbb{C})$ of the restricted$^1$ Lorentz group $SO^+(1,3;\mathbb{R})$ is a fairly broad subject covered in many textbooks, see e.g. Ref. 1 for further information.
An irreducible representation$^2$
$$ (j_L,j_R)~=~j_L\otimes_{\mathbb{C}} j_R, \qquad
j_L, j_R~\in~ \frac{1}{2}\mathbb{N}_0,\tag{1}$$
is a tensor product of $V=V_L\otimes_{\mathbb{C}} V_R$ of two complex vector spaces $V_L$ and $V_R$, of complex dimension $2j_L+1$ and $2j_R+1$, respectively. The tensor product $V$ is again a complex vector space and has complex dimension $(2j_L+1)(2j_R+1)$. See also this Phys.SE post.
Examples:
- $(j_L,j_R)=(0,0)$. This is the trivial/singlet representation. Then the vector-space is $V\cong\mathbb{C}$. Note that the trivial representation $(0,0)$ is the multiplicative identity for the tensor product $\otimes_{\mathbb{C}}$, i.e.
$$\forall V:~~(0,0)\otimes_{\mathbb{C}}V~\cong~ V~\cong~ V\otimes_{\mathbb{C}}(0,0).\tag{2}$$
$(j_L,j_R)=(\frac{1}{2},0)$. This is known as the left-handed Weyl-spinor representation. Then the vector-space is $V\cong\mathbb{C}^2$. It is the fundamental/defining representation of $SL(2,\mathbb{C})$.
$(j_L,j_R)=(0,\frac{1}{2})$. This is known as the right-handed Weyl-spinor representation. It is the complex conjugate representation of the left-handed Weyl-spinor representation.
$(j_L,j_R)=(\frac{1}{2},\frac{1}{2})$. This is isomorphic to the vector representation of the Lorentz group.
The Dirac-spinor representation $(\frac{1}{2},0) \oplus (0,\frac{1}{2})$ is a direct sum of the left- and right-handed Weyl-spinor representation.
An irreducible representation (1) can be written with the help of the symmetric tensor product $\odot$ of the left-handed and right-handed Weyl-spinor representation
$$(j_L,j_R)~=~(\frac{1}{2},0)^{\odot 2j_L} \otimes (0,\frac{1}{2})^{\odot 2j_R}$$
$$~:=~\underbrace{\left\{(\frac{1}{2},0)\odot\ldots\odot(\frac{1}{2},0)\right\}}_{2j_L\text{ symmetrized factors}} \otimes \underbrace{\left\{(0,\frac{1}{2})\odot\ldots\odot(0,\frac{1}{2})\right\}}_{2j_R\text{ symmetrized factors}} .\tag{3} $$
Here $\otimes$ denotes the standard (un-symmetrized) tensor product.
II) Complexification. The restricted Lorentz group $SO^+(1,3;\mathbb{R})$ is obviously a subgroup of the complexified$^2$ Lorentz group $SO(1,3;\mathbb{C})$. One can show that the double cover of the complexified Lorentz group $SO(1,3;\mathbb{C})$ is isomorphic to the direct or Cartesian product group
$$ G~=~SL(2,\mathbb{C})_L\times SL(2,\mathbb{C})_R,\tag{4}$$
cf. e.g. Ref. 1 and this Phys.SE post.
In more detail, the irreducible representation (1) for $SL(2,\mathbb{C})$ lifts to an irreducible representation
$$ \rho~=~\rho_L\otimes \rho_R:G\to GL(V,\mathbb{C})\tag{5}$$
for the product Lie group (4) given as
$$ \rho(g_L,g_R)(\sum_iv^i_L\otimes v^i_R)~=~\sum_i\rho_L(g_L)v^i_L\otimes\rho_R(g_R)v^i_R ,\tag{6}$$
where both
$$ \rho_{L/R}:SL(2,\mathbb{C})\to GL(V_{L/R},\mathbb{C})\tag{7}$$
are irreducible representations of $SL(2,\mathbb{C})$ of complex dimensions $2j_{L/R}+1$.
References:
- I.L. Buchbinder and S.M. Kuzenko, Ideas and Methods of Supersymmetry and Supergravity - Or a Walk Through Superspace, 1998; Chapter 1.
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$^1$ Let us here for simplicity consider the restricted Lorentz group $SO^+(1,3;\mathbb{R})$ rather than the Lorentz group $O(1,3;\mathbb{R})$. To allow for spinor representations, we need to go to the double cover $SL(2,\mathbb{C})$.
$^2$ We can w.l.o.g. assume that a representation over a real vector space is complexified to a complex vector space.
$^3$ It turns out that relativistic physical theories often have pertinent complex analytic properties. See also this related Phys.SE.
