The 12-Dimensional Real Vector Subspaces

Together the set of mapsC⊗su(2),C⊗su(3),F₃,F₃^∗,F₁, andF₁^∗span a 51C-dimensional subspace of the 64C-dimensional space M(C⊗O). Therefore there exists a 13 dimensional

5Note that this argument for linear independence relies on the full-rank nature of anyM ∈C⊗su(2) when restricted to act on 2 or 2^∗. As we will see in subsection 4.4.4, this is precisely what demands our additional U(1) transformations must act on the same decomposition as SU(3).

4.4 Construction 49

space linearly independent maps not encapsulated by the irreducible representation spaces thus far described. The only subspace of M(C⊗O) which has not been fully spanned is the space of maps 3⊕3^∗ →2⊕2^∗. The rest of the space being fully spanned by the maps F₃, F₃^∗, F₁, and F₁^∗.

The maps in the subspace of M(C⊗O) which map 3⊕3^∗ to 2⊕2^∗ break down into four distinct ideals corresponding to the spaces of maps:

φ : 3→2, φ^∗ : 3^∗ →2^∗, ψ : 3^∗ →2, ψ^∗ : 3→2^∗. (4.9) As these maps belong to different ideals thanF₁,F₁^∗,F₃, andF₃^∗, linear independence between these subspaces is trivial. Further, any map inφ⊕φ^∗⊕ψ⊕ψ^∗⊕C⊗(su(2)⊕su(3)) is linearly independent from F1⊕ F1^∗⊕ F3⊕ F3^∗ if and only if the projection of this map on the space 3⊕3^∗ →2⊕2^∗ is non-zero.

Any generator, i.e. basis element, γ of our Lie algebras satisfies γ^∗ = −γ. So clearly our complexified Lie group elements can be written as a combination of basis elements {κ_i} which satisfy κ^∗_i = ±κ_i. Since the operation of projecting on the space of maps 3⊕3^∗ →2⊕2^∗ is invariant under complex conjugation, this implies that after projecting the generators κi on this space we still have basis elements which are invariant, up to an overall sign, under complex conjugation. Thus to ensure that any map inφ⊕φ^∗⊕ψ⊕ψ^∗ is linearly independent from our complexified Lie algebras, we need only ensure our linear combination is not expressible in terms of basis elements which are invariant, up to an overall sign, under complex conjugation.

Let the space φ be spanned by basis elements φ_i as a 12-dimensional real vector space.

Then we can span φ^∗ by the basis elements φ^∗_i. Similarly we span ψ and ψ^∗ by the 12 basis elements ψ_i and ψ_i^∗ respectively. That is, we treat our spaces of complex dimension 6 as spaces of real dimension 12. In addition to requiring linear independence with the projection of our complexified Lie algebras, we also require that our spaces are irreducible representations of the Lie groups. Correspondingly, we must find which sets of maps, out of all the maps in 3⊕3^∗ → 2⊕2^∗, span irreducible representation spaces, to be denoted by the letter H and referred to as H-spaces, of SU(2)×SU(3) while still being linearly independent. This greatly restricts our choice of relevant spaces, H, as all the subspaces in (4.9) are all irreducible representation spaces of SU(2)×SU(3). Thus for any relevant subspace H, we must have that the projection of H on any of the subspaces in (4.9) is either the zero element or the entire space.

The argument for linear independence of ourH-spaces and the complexified Lie algebra centres on these H-spaces not containing any elements A that satisfy A^∗ =±A. As such we can analyse subspaces of φ⊕φ^∗ and ψ⊕ψ^∗ independently. Thus we will only present the following analysis for the subspace φ⊕φ^∗, with the arguments for ψ ⊕ψ^∗ following analogously. Clearly, we cannot use the entirety of φ⊕φ^∗, as it is no difficult task to find basis elements in this space which are their own complex conjugate. Indeed, we can at most span a 12-dimensional real subspace of the 24-dimensional real subspaceφ⊕φ^∗. This follows immediately from the observation that choosing 13 or more basis elements in the 24-dimensional real space requires choosing some φ ∈ φ and its corresponding φ^∗ ∈ φ^∗, such that φ+φ^∗ is a basis element which is its own complex conjugate.

Additionally, since the projection of H on φ is either {0}or φ, and similarly for φ^∗, H must be spanned by basis elements

{φ_i+f(φ_i)}¹²_i=1, (4.10)

where f : φ → φ^∗ is either the zero map (i.e. maps all elements to the zero element) or some bijection. This bijection must be such that for φ → U φV, with U ∈ SU(3) and V ∈ SU(2), we have f(φ) → f(U φV) ≡ U^∗f(φ)V^∗. As the 3 and 3^∗ representations of SU(3) are distinct representations they cannot be transformed into each other via a linear map, i.e. matrix multiplication. From this we know that f must contain the operation of complex conjugation. Then any other operation in f must commute with the group elements U and V, which implies multiplication by an overall scalar.

Thus we can write our relevant subspaces as

H^α := Span_R{φ_i+αφ^∗_i} (4.11)

for some α∈C. Note that we are considering this space as a real vector space, as opposed to our previous analysis which has always focused on complex vector spaces. This is because for some complex coefficient γ ∈Cin front of one of our basis elements,

γ(φ_i+αφ_i) = (γφ_i) +α(γ^∗φ_i)^∗ (4.12) does not yield a vector of the form φ+αφ^∗ unless γ ∈R, and so is an element of H^α ⇐⇒

γ ∈ R. This feature arises because f is an anti-linear map, and so commutes only with multiplication by real numbers. Ergo, the spaceH^α is only a vector space under real scalar multiplication.

Next we must identify the range of values we may chose for α∈Cwhile still retaining linear independence from the projection of the complexified Lie algebras. To do this we need only exclude those values forα for which there exists some A∈H^α for which we also haveA^∗ ∈H^α. Without loss of generality, we write A=φ1+αφ^∗₁for someφ1 ∈φ. We may then span H^α by the basis elements {A, φ_i +αφ^∗_i}¹²_i=2. H^α has a non-trivial intersection with the projection of the complexified Lie groups if and only if there exists some set of real parameters {di} such that

A^∗ =φ^∗₁+α^∗φ₁ =X

i

d_i(φ_i+αφ^∗_i)∈H₁^α. (4.13) Now since all the basis elements φ_i are linearly independent this implies a non-trivial intersection with the projection of the complexified Lie algebras if and only if

1 =d₁α and α^∗ =d₁. (4.14)

This implies clearly that we may only have a non-trivial intersection if α = ±1, as d₁ ∈ R demands α ∈ R. Employing the same arguments straightforwardly to the space

4.4 Construction 51

ψ⊕ψ^∗, we find the pair of 12-dimensional real vector spaces H₁^α:= Span_Rn

h∈M(V);h =φ₁+φ₂kφ₁ : 3→2,φ₂ : 3^∗ →2^∗; s.t. φ₂ =αφ^∗₁o

, (4.15) H₂^β := Span_Rn

h∈M(V); h=ψ₁+ψ₂kψ₁ : 3^∗ →2,ψ₂ : 3→2^∗; s.t. ψ₂ =βψ₁^∗o

, (4.16) for some α, β ∈ C, where α, β 6=±1 is required for linear independence. Note that while the spacesH₁^α andH₂^β describe spaces of complex matrices, they each have the structure of a 12-dimensional real vector space. Thus together, H :=H₁^α⊕H₂^β span a 24 dimensional real vector subspace of M(8,C).

Let us also take a brief moment to comment on the limits of α, β ∈ C. Clearly as

|α|,|β| →0 we have thatH₁^αand H₂^β go toφ andψ respectively. In these limits we recover a fundamental 6-dimensional complex vector space representations of SU(2)×SU(3), i.e.

the tensor product of a doublet of SU(2) and triplet of SU(3). On the other hand, in (4.15) and (4.16) we could, for any |α|,|β| > 0, have used the relations φ1 = α⁻¹φ^∗₂ and ψ₁ = α⁻¹ψ₂^∗. So in this case we see that the limits |α|,|β| → ∞ implies H₁^α and H₂^β go to φ^∗ and ψ^∗ respectively. In this limit we recover the complex conjugate representation spaces to the limit |α|,|β| → 0. This relationship of our limits is a general feature of the interesting behaviour of these H-spaces, namely under complex conjugationH₁^α ↔H₁^(α−1) and H₂^β ↔ H₂^(β−1). This differs from our complexified Lie algebras, which are invariant under complex conjugation, and from our fundamental representations Fn, which all have corresponding complex conjugate spaces F_n^∗ ≡ F_n^∗. In subsection 4.5.3 we further discuss this behaviour under complex conjugation as well as other interesting features about these representation spaces.

We comment here on the identification of these H-spaces as the spaces which are not expressible in terms of complex conjugate invariant basis elements. Naturally this may seem like a sufficient, but not necessary, condition for linear independence with our complexified Lie algebras. In reality this condition is both sufficient and necessary. When projected onto the space 3⊕3^∗ →2⊕2^∗, our complexified Lie algebrasC⊗(su(2)⊕su(3)) span an 11-dimensional complex subspace. On the other hand, the set of all elements γ^∗ = −γ together span a 12-dimensional complex subspace. Thus at first glance the condition that no elements h in our H-spaces are of the form h^∗ = ±h may seem too restrictive. However, as we are only interested in irreducible representations it is clear that our H-spaces must be of real dimension divisible by four.⁶ As such we may minimally have irreducible representation spaces of complex dimension 6 or real dimension 12. This implies that having any one element h^∗ =±h in our H-spaces demands having some 12-dimensional real subspace of elements transforming ash^∗ =±h. Clearly this is not possible without overlapping with elements in C⊗(su(2)⊕su(3)), and thus we must only consider maps which are not expressible as h^∗ =±h.

6The space of maps 3→2 is a six dimensional complex map but a 12 dimensional real map. The even nature of complex spaces then manifests itself as the divisibility of four when considered in terms of real dimensions.

Additionally, we comment on the vector space structure of our H-spaces. While these H−spaces are real vector spaces under scalar multiplication, they do transform as a triplet of SU(3) and a doublet of SU(2), as is evident purely from dimensional counting. However due to the different transformations of φ_i and φ^∗_i the complex structure cannot simply be represented by multiplication via i, the complex unit imaginary. Instead, the complex linear structure describing the complex nature of the fundamental transformations of the space H₁^α becomes multiplication by i on φ and by −i on φ^∗. This is evident from the appearance of the anti-linear bijection f.

In total we now have only a one dimensional complex subspace left to span the entirety of M(8,C). This remaining subspace is found in the following subsection.