1. Aug 2, 2013

### AlbertEi

Hi,

I'm getting a bit confused about the adjoint representation. I learnt about Lie algrebras using the book by Howard Georgi (i.e. it is very "physics-like" and we did not distinguish between the abstract approach to group theory and the matrix approach to group theory). He defines the adjoint as follows:

[T_a]_{bc} = i f_{abc} T_c

where $T_i$ are the generators and $f_abc$ are the structure constants. This definition clearly means that matrix in the adjoint representation must have the same amount of dimensions as there are generators. However, recently I have reading some papers where they talk about the adjoint representation of for instance the Higgs field $\phi$ as follows:

\phi=\phi^a T^a

with the following transformation properties:

\phi \mapsto g \phi g^{-1}

where the generators do not necessarily have to be of the same dimensions as the number of generators. I understand that this basically just means that $\phi$ always takes values in the Lie algebra, but I think it is really odd that two very different properties of the Lie algebra have been given the same name. My confusion even grows further when they discuss roots/weight. More specifically, are the eigenvalues of the latter generators called roots or weights?

Last edited: Aug 2, 2013