# Braiding matrix from CFT first principles

Various CFT models are known to produce representations of braid groups.

A famous example is the $$SU(2)$$ WZW model at level $$k$$, for which the braiding matrix for the case of two fundamental irreps on $$S^2$$ is

$$B = \exp \left( \frac{4 \pi i}{k + 2} T^a \otimes T^a \right).$$

I only know this formula because WZW models are holographically dual to the 3-dimensional gauge theory with Chern-Simons action, which is known to produce knot polynomials related to quantum groups (the Jones polynomial). So I guessed the expression above by simply writing the $$R$$-matrix of $$U_q(\mathfrak{su_2})$$ for two fundamental irreps.

However, I would very much like to understand how the action of $$B$$ on conformal blocks of the WZW model can be explicitly calculated (at least for the simplest possible case of two fundamental irreps on $$S^2$$). The calculation should only use the first principles of CFT, not clever connections to Chern-Simons and quantum groups (as I already know how to obtain the answer from them).

I also admit to not fully understand the conformal blocks construction, so please add any information that you think is relevant.

As a follow-up question, which CFT models produce representations of the braid group? Is there a specific criterion?

## 2 Answers

This question is really about the conformal blocks of two-dimensional CFT, not specific to WZW models. An $$n$$-point conformal block on the sphere is a locally holomorphic function of $$n$$ points $$(z_1,\dots,z_n)$$, which is singular when some points coincide. Elements of the braid group act on conformal blocks by moving the $$n$$ points so that they never collide, and that they end up in a permutation of $$(z_1,\dots,z_n)$$.

The braiding matrix describes how conformal blocks behave under a transposition, for example the exchange of $$z_1$$ with $$z_2$$. The matrix can be diagonalized using a basis of blocks that behave as $$\mathcal{F}_{\Delta}(z_1, z_2,\dots)\underset{z_1\to z_2}{=} (z_1-z_2)^{\Delta-\Delta_1-\Delta_2}(1 + O(z_1-z_2))$$ where $$\Delta_1,\Delta_2$$ are the conformal dimensions of the primary fields at $$z_1,z_2$$. Under braiding, such a block indeed just picks the factor $$B_\Delta = e^{\pi i(\Delta-\Delta_1-\Delta_2)}$$ To relate this to your WZW example, let us write $$T^a_1,T^a_2$$ the Lie algebra elements acting on the fields $$1$$ and $$2$$. By the Sugawara construction you have $$\Delta_i = \frac{T^a_i T^a_i}{2(k+2)}$$. Moreover, for our particular bases of blocks, we also have $$\Delta = \frac{(T^a_1+T^a_2)(T^a_1+T^a_2)}{2(k+2)}$$ thus $$\Delta-\Delta_1-\Delta_2 = \frac{T^a_1T^a_2}{k+2}$$. This explains your braiding matrix except for the factor $$4$$, which I do not see what to do with at the moment.

For more on conformal blocks in 2d CFT, see Wikipedia https://en.wikipedia.org/wiki/Virasoro_conformal_block . For all the details, see Moore-Seiberg, "Classical and Quantum Conformal Field Theory".

• Are all the transpositions mutually diagonal in this basis? I thought that there are nonabelian representations which appear too, when the OPE doesn't just contain a single primary. – Ryan Thorngren Dec 27 '18 at 20:02
• No, the transpositions are not mutually diagonal. You need different bases for diagonalizing different transpositions. – Sylvain Ribault Dec 28 '18 at 19:53
• How to derive the expression for $\Delta$? – Prof. Legolasov May 5 at 16:31

The representation of the $$n$$-strand braid group arises from integrating the $$n$$-variable Knizhnik-Zamolodchikov (KZ) equations, which are Ward identities for the movement of the insertion points for the $$n$$-point correlation function. Wikipedia has a nice description, and there is a short book by T. Kohno which explains the connection between 2D CFT and 3D TQFT as a clever encoding of the KZ equations.

All rational CFTs will produce braid group representations. Beyond that I don't know.

• Would you mind writing a self contained answer please? – Prof. Legolasov Dec 25 '18 at 19:17
• @SolenodonParadoxus The explicit integration of the KZ equations is an active area of mathematical research and far too complicated to explain in a stackexchange answer. I suggest you check out the references I gave. – Ryan Thorngren Dec 25 '18 at 19:45
• I’m sorry Ryan, but this isn’t at all what I was looking for. This isn’t a resource recommendation question... I don’t buy that it is impossible to compose a short summary of the derivation of $B$. Here’s how I imagine it going: introducing KZ equation, explaining how to decompose into conformal blocks and how they relate to integrability... That all is doable. Look at my contributions — I always try to explain the relevant physics and math in simple terms, and many of my answers are upvoted for this reason. I understand your frustration, but the whole point of SE is to answer questions. – Prof. Legolasov Dec 25 '18 at 22:02
• @SolenodonParadoxus Sorry to disappoint you. I wish you luck in finding a simple derivation of the monodromy representation. If you find one I'd be interested in having a look at it. – Ryan Thorngren Dec 25 '18 at 23:20
• No worries. That’s the plan. You’ll be the first one to know when I have enough state to answer this question. Thanks for trying to help, much appreciated. – Prof. Legolasov Dec 26 '18 at 3:16