# representation theory

1. ### A Gamma - traceless

I read this question https://physics.stackexchange.com/questions/95970/under-what-conditions-is-a-vector-spinor-gamma-trace-free . Also I read Sexl and Urbantke book about groups. But I dont understand why spinors is irreducible if these are gamma-tracelees. Also I read many papers about higher...
2. ### A (A,A) representation of Lorentz group-why is it tensor?

Why representation of Lorentz group of shape (A,A) corespond to totally symmetric traceless tensor of rank 2A? For example (5,5)=9+7+5+3+1 (where + is dirrect sum), but 1+5+3+9+7<>(5,5) implies that (5,5) isn't symmetric ? See Weinberg QFT Book Vol.1 page 231.

9. ### I Tensor representation of the Lorentz Group

I've been trying to understand representations of the Lorentz group. So as far as I understand, when an object is in an (m,n) representation, then it has two indices (let's say the object is $\phi^{ij}$), where one index $i$ transforms as $\exp(i(\theta_k-i\beta_k)A_k)$ and the other index...
10. ### I Minimum requisite to generalize Proca action

Hello guys, In 90% of the papers I've read about diferent ways to achieve generalizations of the Proca action I've found there's a common condition that has to be satisfied, i.e: The number of degrees of freedom allowed to be propagated by the theory has to be three at most (two if the fields...
11. ### I $A_\mu^a=0$ in global gauge symmetries ?

Hi, this question is related to global and local SU(n) gauge theories. First of all, some notation: $A$ will be the gauge field of the theory (i.e: the 'vector potential' in the case of electromagnetic interactions) also known as 'connection form'. In components: $A_\mu$ can be expanded in...
12. ### A Diagonalizing Hermitian matrices with adjoint representation

Suppose I have a hermitian $N \times N$ matrix $M$. Let $U \in SU(N)$ be the matrix that diagonalizes $M$: $M = U\Lambda U^\dagger$, where $\Lambda$ is the matrix of eigenvalues of $M$. This transformation can be considered as the adjoint action $Ad$ of $SU(N)$ over its...
13. ### A Tensor symmetries and the symmetric groups

In one General Relativity paper, the author states the following (we can assume tensor in question are tensors in a vector space $V$, i.e., they are elements of some tensor power of $V$) To discuss general properties of tensor symmetries, we shall use the representation theory of the...
14. ### A Who wrote "Ch 6 Groups & Representations in QM"?

Who really wrote the best introductory account of representation theory in QM that I've seen so far ? [Likely mis-attribution discussed here below; prefixed "Advanced" to reach lecturers who are more likely to know the answer to this question.] It's available via...
15. ### Massive spin-s representations of the Poincare group

Context The following is from the book "Ideas and methods in supersymmetry and supergravity" by I.L. Buchbinder and S.M Kuzenko, pg 56-60. It is about realizing the irreducible massive representations of the Poincare group as spin tensor fields which transform under certain representations of...
16. ### I Bootstraping a space from its tensor square

By space, I mean a vector space which could be a representation of a group or even have some expanded algebraic structure. So I am not sure if this question goes here or in the Algebra subforum. Consider the tensor square r\otimes r of an irreducible group representation r with itself, and...
17. ### A How parity exchanges right handed and left handed spinors

Reading through David Tong lecture notes on QFT. On pages 94, he shows the action of parity on spinors. See below link: [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf In (4.75) he confirms that parity exchanges right handed and left handed spinors. Or for an arbitrary...
18. ### A Notation/Site for Representations of an Algebra

I'm currently reading the paper "Higher Spin extension of cosmological spacetimes in 3d: asymptotically flat behaviour with chemical potentials in thermodynamics" I'm looking at equation (3) on page 4. I know that symmetrization brackets work like this A_(a b) = (A_ab + A_ba)/2. However I have...
19. ### I Representation of a vector

Hello. If I represent a vector space using matrices, for example if a 3x1 vector, V, is represented by 3x3 matrix, A, and if this vector was the eigenvector of another matrix, M, with eigenvalue v, if I apply M to the matrix representation of this vector, does this holds: MA=vA? Also, if I...
20. ### I SU(2) representations

Hello! I just started reading about SU(2) (the book is Lie Algebras in Particle Physics by Howard Georgi) and I am confused about something - I attached a screenshot of those parts. So, for what I understood by now, the SU(2) are 2x2 matrices whose generators are Pauli matrices and they act on a...
21. ### I About Lie group product ($U(1)\times U(1)$ ex.)

I recently got confused about Lie group products. Say, I have a group U(1)\times U(1)'. Is this group reducible into two U(1)'s, i.e. possible to resepent with a matrix \rho(U(1)\times U(1)')=\rho_{1}(U(1))\oplus\rho_{1}(U(1)')=e^{i\theta_{1}}\oplus e^{i\theta_{2}}=\begin{pmatrix}e^{i\theta_{1}}...
22. ### Lorentz transformations and vector fields

Hi Everyone. There is an equation which I have known for a long time but quite never used really. Now I have doubts I really understand it. Consider the unitary operator implementing a Lorentz transformation. Many books show the following equation for vector fields: U(\Lambda)^{-1}A^\mu...
23. ### Representations of spinors

I'm currently reading a book on relativistic field theory and I'm trying to understand spinors. After the author introduces the four parts of the Lorentz group he talks about spinors and group representations: "....With this concept we see that the 2x2 unimodular matrices A discussed in the...
24. ### Introduction to Young-tableaux and weight diagrams?

I am looking for a short pedagogical introduction to Young-tableaux and weight diagrams and the relationship between them, which contains many detailled and worked out examples of how these methods are applied in physics, such as in the context of the standard model and beyond for example. I am...
25. ### 2j+1 d representation for Poincaré group

I want to learn how to write down a particle state in some inertial coordinate frame starting from the state $| j m \rangle$, in which the particle is in a rest frame. I know how to rotate this state in the rest frame, but how does one write down a Lorentz boost for it? Note that I am not...
26. ### Why are invariant tensors also Clebsch-Gordan coefficients?

On one hand, in reading Georgi's book in group theory, I comprehend the invariant tensor as a special "tensor", which is unchanged under the action of any generators. On the other hand, CG decomposition is to decompose the product of two irreps into different irreps. Now it is claimed that...