# nLab beta-gamma system

### Context

#### Quantum field theory

functorial quantum field theory

complex geometry

# Contents

## Idea

What is called the $\beta$-$\gamma$ system is a 2-dimensional quantum field theory defined on Riemann surfaces $X$ whose fields are pairs consisting of a $(0,0)$-form and a $(1,0)$-form and whose equations of motion demand these fields to be holomorphic differential forms.

The name results from the traditional symbols for these fields, which are

$(\gamma,\beta) \in \Omega^{0,\bullet}(X) \oplus \Omega^{1, \bullet}(X) \,.$

## Definition

We state the definition of the $\beta$-$\gamma$-system as a free field theory (see there) in BV-BRST formalism, following (Gwilliam, section 6.1).

We first give the standard variant of the theory, the

Then we consider the

### Abelian massless theory

Let $X$ be a Riemann surface.

kinematics

• the field bundle $E \to X$ is

$E \coloneqq \wedge^{0,\bullet}\Gamma(T X) \oplus \wedge^{1,\bullet} \Gamma(T X)$
• hence the (abelian) sheaf of local sections is

$\mathcal{E} = \Omega_X^{0,\bullet} \oplus \Omega_X^{1, \bullet} \,,$

we write

$\mathcal{E}_c \hookrightarrow \Gamma_X(E)$

for the sections of compact support

• the local pairing

$\langle -,-\rangle_{loc} \colon E \otimes E \to Dens_X$

with values in the density bundle is given by wedge product followed by projection on the $(1,1)$-forms

$\langle \gamma_1 + \beta_1, \gamma_2, \beta_2\rangle_{loc} \coloneqq (\gamma_1 \wedge \beta_2 + \gamma_2 \wedge \beta_1)_{|(1,1)}$
• hence the global pairing

$\langle -,-\rangle \colon \mathcal{E}_c \otimes \mathcal{E}_c \to \mathbb{C}$

is given by

$\langle \gamma_1 + \beta_1, \gamma_2, \beta_2\rangle_{loc} \coloneqq \int_{X}\left(\gamma_1 \wedge \beta_2 + \gamma_2 \wedge \beta_1\right)$

dynamics

• $Q \colon \mathcal{E} \to \mathcal{E}$

is the Dolbeault differential $\bar \partial$

• hence the elliptic complex of fields is

$(\mathcal{E}, Q) = (\Omega_X^{0,\bullet}\oplus \Omega_X^{1,\bullet}, \bar \partial)$

is the Dolbeault complex;

• and hence the action functional

$S \colon \mathcal{E}_c \to \mathcal{C}$

is

\begin{aligned} (\gamma + \beta) & \mapsto \frac{1}{2}\int_X \langle \gamma+ \beta, \; \bar \partial (\gamma + \beta)\rangle \\ & = \int_X \beta \wedge \bar \partial \gamma \end{aligned}

(…)

## Properties

### Euler-Lagrange equations of motion

The equations of motion are

$\bar \partial \gamma = 0 \;\;, \;\; \bar \partial\beta = 0 \,.$

## Relation with $\sigma$-models

Consider a sigma-model $X\hookrightarrow \mathbb{R}^{1,1}$ to the target space $\mathbb{R}^{1,1}$

$S[q,\gamma] = \int_X \partial q \wedge \bar{\partial}\gamma,$

which has an abelian right-moving Kac-Moody symmetry $q\mapsto q+\lambda$ with $\partial\lambda=0$. We can can consider a theory where this symmetry is promoted to a gauge symmetry, i.e.

$S_{\mathrm{gauged}}[q,\gamma] = \int_X \partial_\beta q \wedge \bar{\partial}\gamma,$

where $\partial_\beta q := \partial q + \beta$ where $\beta\in\Omega^{1,\bullet}(X)$ is the connection. If we choose the gauge with $q=0$, we obtain the $\beta$-$\gamma$ system with action

$S_{\beta\gamma}[\beta,\gamma] = \int_X \beta \wedge \bar{\partial}\gamma.$

Thus, a $\beta$-$\gamma$ system can be interpreted as a chiral (or Kac-Moody) quotient along a null killing vector of a sigma-model with target space $\mathbb{R}^{1,1}$ (LinRoc20).

• pure spinor formalism?

## References

Discussion in the context of BV-quantization and factorization algebras is in chapter 6 of

• Owen Gwilliam, Factorization algebras and free field theories PhD thesis (pdf)

A construction of chiral differential operators via quantization of $\beta\gamma$ system in BV formalism with an intermediate step using factorization algebras: