3
$\begingroup$

In 2D CFT there is a bijection between states and operators. In one direction it is easy: if $\phi(z)$ is a primary field then $$|\phi \rangle:=\lim_{z \to 0} \phi(z)|0\rangle$$ is a highest weight state. Now, suppose that a highest weight vector $|\phi\rangle$ is given, how one can compute the corresponding operator $\phi(z)$? In other words, how to compute action of this operator on any state?

The Hilbert space is obtained by acting raising operators $L_n$, $n<0$ on highest weight states and we looking for a primary operator that satisfy $$ [L_n,\phi(z)]=z^n(z\partial +(n+1)h) \phi(z), $$ thus, it is enough to determine action on all highest weight states.

We also know action of such operator on the vacuum $$ \phi(z) |0\rangle = e^{z L_{-1}} |\phi\rangle, $$ and therefore we know the action of $\phi(z)$ on all descendants of the vacuum.

How $\phi(z)$ acts on other highest weight states in the Hilbert space?

$\endgroup$

1 Answer 1

3
$\begingroup$

Suppose $\left|\psi\right\rangle =\psi\left(0\right)\left|0\right\rangle $ is another primary state, created by the primary operator $\psi\left(z\right)$. Now you want to calculate

\begin{align*} \phi\left(z\right)\left|\psi\right\rangle & =\phi\left(z\right)\psi\left(0\right)\left|0\right\rangle \\ & =\sum_{p}C_{\phi\psi p}O_{p}\left(0\right)\left|0\right\rangle \end{align*}

where in the second line I expand $\phi\left(z\right)\psi\left(0\right)$ at $0$, which is called the operator product expansion. $O_{p}\left(0\right)$ represents the contribution to the OPE from a primary operator and its decedents. $C_{\phi\psi p}$ are the OPE coefficients.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.