# nLab orbit method

### Context

#### Representation theory

representation theory

geometric representation theory

# Contents

## Idea

The Orbit method (or Kirillov’s method, or method of coadjoint orbits) is a method in geometric representation theory concerned with identifying unitary representations of Lie groups with the canonical $G$-actions on spaces of sections of certain line bundles over coadjoint orbits of the Lie group. In terms of quantum physics this realizes $G$-representations as action of global gauge groups on quantum states under geometric quantization.

More in detail, the dual ${𝔤}^{*}$ of a (say finite-dimensional real) Lie algebra has a canonical structure of a Poisson manifold (with the Poisson structure due to Alexandre Kirillov and Jean-Marie Souriau), namely for any $a\in {𝔤}^{*}$,

$\left\{f,g\right\}\left(a\right)≔⟨\left[d{f}_{a},d{g}_{a}\right],a⟩\phantom{\rule{thinmathspace}{0ex}}.$\{ f, g\}(a) \coloneqq \langle [d f_a, d g_a],a\rangle \,.

This Poisson manifold foliates into symplectic leaves which are the coadjoint orbits. The line bundles in question are the prequantum line bundles of these symplectic manifolds.

Hence, in the language of quantum physics, the orbit methods identifies unitary representations of Lie groups $G$ with the $G$-action on spaces of states of the geometric quantization of a classical mechanical system with a global $G$-symmetry.

Many important classes of unitary representations are obtained by that method.

Notably in the case of compact Lie groups, co-adjoint orbits are flag manifolds and the Borel-Weil theorem says that under certain further conditions the expected unitary representations are obtained.

The case of non-compact Lie groups is much less well understood, see for instance (Graham-Vogan, Vogan 99).

## Definitions and constructions

We list and discuss the basic notions, definitions and constructions in the context of the orbit method. A useful review is also in (Beasley, section 4).

### The group and its Lie algebra

Throughout, let $G$ be a semisimple compact Lie group. For some considerations below we furthermore assume it to be simply connected.

Write $𝔤$ for its Lie algebra. Its canonical (up to scale) binary invariant polynomial we write

$⟨-,-⟩:𝔤\otimes 𝔤\to ℝ\phantom{\rule{thinmathspace}{0ex}}.$\langle -,-\rangle \colon \mathfrak{g} \otimes \mathfrak{g} \to \mathbb{R} \,.

Since this is non-degenerate, we may equivalently think of this as an isomorphism

$𝔤\simeq {𝔤}^{*}$\mathfrak{g} \simeq \mathfrak{g}^*

that identifies the vector space underlying the Lie algebra with its dual vector space ${𝔤}^{*}$.

### The coadjoint orbit and the coset space/ flag manifold

We discuss the coadjoint orbits of $G$ and their relation to the coset space/flag manifolds of $G$.

Write

1. $T↪G$ inclusion of the maximal torus of $G$.

2. $𝔱↪𝔤$ the corresponding Cartan subalgebra

In all of the following we consider an element $⟨\lambda ,-⟩\in {𝔤}^{*}$.

###### Definition

For $⟨\lambda ,-⟩\in {𝔤}^{*}$ write

${𝒪}_{\lambda }↪{𝔤}^{*}$\mathcal{O}_\lambda \hookrightarrow \mathfrak{g}^*

${𝒪}_{\lambda }=\left\{{\mathrm{Ad}}_{g}^{*}\left(⟨\lambda ,-⟩\right)\in {𝔤}^{*}\mid g\in G\right\}\phantom{\rule{thinmathspace}{0ex}}.$\mathcal{O}_{\lambda} = \{ Ad_g^*(\langle\lambda,-\rangle) \in \mathfrak{g}^* | g \in G \} \,.

Write ${G}_{\lambda }↪G$ for the stabilizer subgroup of $⟨\lambda ,-⟩$ under the coadjoint action.

###### Proposition

There is an equivalence

$G/{G}_{\lambda }\stackrel{\simeq }{\to }{𝒪}_{\lambda }$G/G_\lambda \stackrel{\simeq}{\to} \mathcal{O}_\lambda

given by

$g{G}_{\lambda }↦{\mathrm{Ad}}_{g}^{*}⟨\lambda ,-⟩\phantom{\rule{thinmathspace}{0ex}}.$g G_\lambda \mapsto Ad_g^* \langle\lambda,-\rangle \,.
###### Definition

An element $⟨\lambda ,-⟩\in {𝔤}^{*}$ is regular if its coadjoint action stabilizer subgroup coincides with the maximal torus: ${G}_{\lambda }\simeq T$.

###### Example

For generic values of $\lambda$ it is regular. The element in ${𝔤}^{*}$ farthest from regularity is $\lambda =0$ for which ${G}_{\lambda }=G$ instead.

### The symplectic form

We describe a canonical symplectic form on the coadjoint orbit/coset ${𝒪}_{\lambda }\simeq G/{G}_{\lambda }$.

Write $\theta \in {\Omega }^{1}\left(G,𝔤\right)$ for the Maurer-Cartan form on $G$.

###### Definition

Write

${\Theta }_{\lambda }≔⟨\lambda ,\theta ⟩\in {\Omega }^{1}\left(G\right)$\Theta_\lambda \coloneqq \langle \lambda, \theta \rangle \in \Omega^1(G)

for the 1-form obtained by pairing the value of the Maurer-Cartan form at each point with the gixed element $\lambda \in {𝔤}^{*}$.

Write

${\nu }_{\lambda }≔{d}_{\mathrm{dR}}{\Theta }_{\lambda }$\nu_\lambda \coloneqq d_{dR} \Theta_\lambda

for its de Rham differential.

###### Proposition

The 2-form ${\nu }_{\lambda }$ from def. 3

1. satisfies

${\nu }_{\lambda }=\frac{1}{2}⟨\lambda ,\left[\theta \wedge \theta \right]⟩\phantom{\rule{thinmathspace}{0ex}}.$\nu_\lambda = \frac{1}{2}\langle \lambda, [\theta\wedge \theta]\rangle \,.
2. it descends to a closed $G$-invariant 2-form on the coset space, to be denoted by the same symbol

${\nu }_{\lambda }\in {\Omega }_{\mathrm{cl}}^{2}\left(G/{G}_{\lambda }{\right)}^{G}\phantom{\rule{thinmathspace}{0ex}}.$\nu_\lambda \in \Omega^2_{cl}(G/G_\lambda)^G \,.
3. this is non-degenerate and hence defines a symplectic form on $G/{G}_{\lambda }$.

### The prequantum bundle

We discuss the geometric prequantization of the symplectic manifold given by the coadjoint orbit ${𝒪}_{\lambda }$ equipped with its symplectic form ${\nu }_{\lambda }$ of def. 2.

Assume now that $G$ is simply connected.

###### Proposition

The weight lattice ${\Gamma }_{\mathrm{wt}}\subset {𝔱}^{*}\simeq 𝔱$ of the Lie group $G$ is isomorphic to the group of group characters

${\Gamma }_{\mathrm{wt}}\stackrel{\simeq }{\to }{\mathrm{Hom}}_{\mathrm{LieGrp}}\left(G,U\left(1\right)\right)$\Gamma_{wt} \stackrel{\simeq}{\to} Hom_{LieGrp}(G,U(1))

where the identification takes $⟨\alpha ,-⟩\in {𝔱}^{*}$ to ${\rho }_{\alpha }:T\to U\left(1\right)$ given on $t=\mathrm{exp}\left(\xi \right)$ for $\xi \in 𝔱$ by

${\rho }_{\alpha }:\mathrm{exp}\left(\xi \right)↦\mathrm{exp}\left(i⟨\alpha ,\xi ⟩\right)\phantom{\rule{thinmathspace}{0ex}}.$\rho_\alpha \colon \exp(\xi) \mapsto \exp(i \langle \alpha, \xi\rangle) \,.
###### Proposition

The symplectic form ${\nu }_{\lambda }\in {\Omega }_{\mathrm{cl}}^{2}\left(G/T\right)$ of prop. 2 is integral precisely if $⟨\lambda ,-⟩$ is in the weight lattice.

### The Hamiltonian $G$-action / coadjoint moment map

The group $G$ canonically acts on the coset space $G/{G}_{\lambda }$ (by multiplication from the left). We discuss a lift of this action to a Hamiltonian action with respect to the symplectic manifold structure $\left(G/T,{\nu }_{\lambda }\right)$ of prop. 2, equivalently a momentum map exhibiting this Hamiltonian action.

### Wilson loops and 1d Chern-Simons $\sigma$-models with target the coadjoint orbit

Above (…) we discussed how an irreducible unitary representation of $G$ is encoded by the prequantization of a coadjoint orbit $\left({𝒪}_{\lambda },{\nu }_{\lambda }\right)$. Here we discuss how to express Wilson loops/holonomy of $G$-principal connections in this representation as the path integral of a topological particle charged under this background field, whose action functional is that of a 1-dimensional Chern-Simons theory.

Let $A{\mid }_{{S}^{1}}\in {\Omega }^{1}\left({S}^{1},𝔤\right)$ be a Lie algebra valued 1-form on the circle, equivalently a $G$-principal connection on the circle.

For

$\rho :G\to \mathrm{Aut}\left(V\right)$\rho \colon G \to Aut(V)

a representation of $G$, write

${W}_{{S}^{1}}^{R}\left(A\right)≔{\mathrm{hol}}_{{S}^{1}}^{R}\left(A\right)≔{\mathrm{Tr}}_{R}\left({\mathrm{tra}}_{{S}^{1}}\left(A\right)\right)$W_{S^1}^R(A) \coloneqq hol^R_{S^1}(A) \coloneqq Tr_R( tra_{S^1}(A) )

for the holonomy of $A$ around the circle in this representation, which is the trace of its parallel transport around the circle (for any basepoint). If one thinks of $A$ as a background gauge field then this is alse called a Wilson loop.

###### Definition

Let the action functional

$\mathrm{exp}\left(i{\mathrm{CS}}_{\lambda }\left(-{\right)}^{A}\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\left[{S}^{1},G/T\right]\to U\left(1\right)$\exp(i CS_\lambda(-)^A) \;\colon\; [S^1, G/T] \to U(1)

be given by sending $gT:{S}^{1}\to G/T$ represented by $g:{S}^{1}\to G$ to

$\mathrm{exp}\left(i{\int }_{{S}^{1}}⟨\lambda ,{A}^{g}⟩\right)\phantom{\rule{thinmathspace}{0ex}},$\exp(i \int_{S^1} \langle \lambda, A^g\rangle ) \,,

where

${A}^{g}≔{\mathrm{Ad}}_{g}\left(A\right)+{g}^{*}\theta$A^g \coloneqq Ad_g(A) + g^* \theta

is the gauge transformation of $A$ under $g$.

###### Proposition

The Wilson loop of $A$ over ${S}^{1}$ in the unitarry irreducible representation $R$ is proportional to the path integral of the 1-dimensional sigma-model with

1. target space the coadjoint orbit ${𝒪}_{\lambda }\simeq G/T$ for $⟨\lambda ,-⟩$ the weight corresponding to $R$ under the Borel-Weil-Bott theorem

2. action functional the functional of def. 4:

${W}_{{S}^{1}}^{R}\left(A\right)\propto {\int }_{\left[{S}^{1},{𝒪}_{\lambda }\right]}D\left(gT\right)\mathrm{exp}\left(i{\int }_{{S}^{1}}⟨\lambda ,{A}^{g}⟩\right)\phantom{\rule{thinmathspace}{0ex}}.$W_{S^1}^R(A) \propto \int_{[S^1, \mathcal{O}_\lambda]} D(g T) \exp(i \int_{S^1} \langle \lambda, A^g\rangle) \,.

See for instance (Beasley, (4.55)).

###### Remark

Notice that since ${𝒪}_{\lambda }$ is a manifold of finite dimension, the path integral for a point particle with this target space can be and has been defined rigorously, see at path integral.

## Formulation in higher geometry

We discuss here a natural equivalent reformulation of the above ingredients of the orbit method in terms of the higher geometry of smooth ∞-groupoids, and specifically in terms of the extended prequantum field theory of Chern-Simons theory with Wilson line defects (FSS).

### Survey

We discuss how for $\lambda \in 𝔤$ a regular element, there is a canonical diagram of smooth moduli stacks of the form

$\begin{array}{ccccccc}{𝒪}_{\lambda }& \stackrel{\simeq }{\to }& G/T& \stackrel{\theta }{\to }& {\Omega }^{1}\left(-,𝔤\right)//T& \stackrel{⟨\lambda ,-⟩}{\to }& BU\left(1{\right)}_{\mathrm{conn}}\\ & & ↓& {⇙}_{\simeq }& {↓}^{J}\\ & & *& \stackrel{}{\to }& B{G}_{\mathrm{conn}}& \stackrel{c}{\to }& {B}^{3}U\left(1{\right)}_{\mathrm{conn}}\end{array}\phantom{\rule{thinmathspace}{0ex}},$\array{ \mathcal{O}_\lambda &\stackrel{\simeq}{\to}& G/T &\stackrel{\mathbf{\theta}}{\to}& \Omega^1(-,\mathfrak{g})//T &\stackrel{\langle \lambda, - \rangle}{\to}& \mathbf{B} U(1)_{conn} \\ && \downarrow &\swArrow_{\simeq}& \downarrow^{\mathrlap{\mathbf{J}}} \\ && * &\stackrel{}{\to}& \mathbf{B}G_{conn} &\stackrel{\mathbf{c}}{\to}& \mathbf{B}^3 U(1)_{conn} } \,,

where

1. $J$ is the canonical 2-monomorphism;

2. the left square is a homotopy pullback square, hence $\theta$ is the homotopy fiber of $J$;

3. the bottom map is the extended Lagrangian for $G$-Chern-Simons theory, equivalently the universal Chern-Simons circle 3-bundle with connection;

4. the top map denoted $⟨\lambda ,-⟩$ is an extended Lagrangian for a 1-dimensional Chern-Simons theory;

5. the total top composite modulates a prequantum circle bundle which is a prequantization of the canonical symplectic manifold structure on the coadjoint orbit ${\Omega }_{\lambda }\simeq G/T$.

### Definitions and constructions

Write $H=$ Smooth∞Grpd for the cohesive (∞,1)-topos of smooth $\infty$-groupoids.

For the following, let $⟨\lambda ,-⟩\in {𝔤}^{*}$ be a regular element, def. 2, so that the stabilizer subgroup is identified with a maximal torus: ${G}_{\lambda }\simeq T$.

As usual, write

$B{G}_{\mathrm{conn}}\simeq {\Omega }^{1}\left(-,𝔤\right)//G\in H$\mathbf{B}G_{conn} \simeq \Omega^1(-,\mathfrak{g})//G \in \mathbf{H}

for the moduli stack of $G$-principal connections.

###### Definition

Write

$J≔\left(\phantom{\rule{thickmathspace}{0ex}}{\Omega }^{1}\left(-,𝔤\right)//T\phantom{\rule{thickmathspace}{0ex}}\to \phantom{\rule{thickmathspace}{0ex}}{\Omega }^{1}\left(-,𝔤\right)//G\simeq B{G}_{\mathrm{conn}}\phantom{\rule{thickmathspace}{0ex}}\right)\in {H}^{\left({\Delta }^{1}\right)}$\mathbf{J} \coloneqq \left( \; \Omega^1(-,\mathfrak{g})//T \;\to\; \Omega^1(-,\mathfrak{g})//G \simeq \mathbf{B}G_{conn} \; \right) \in \mathbf{H}^{(\Delta^1)}

for the canonical map, as indicated.

###### Remark

The map $J$ is the differential refinement of the delooping $BT\to BG$ of the defining inclusion. By the general discussion at coset space we have a homotopy fiber sequence

$\begin{array}{cccc}{𝒪}_{\lambda }\simeq G/T& \to & BT& \simeq *//T\\ & & ↓\\ & & BG& \simeq *//G\end{array}\phantom{\rule{thinmathspace}{0ex}}.$\array{ \mathcal{O}_\lambda \simeq G/T &\to& \mathbf{B}T & \simeq *//T \\ && \downarrow \\ && \mathbf{B}G & \simeq *//G } \,.

By the discussion at ∞-action this exhibits the canonical action $\rho$ of $G$ on its coset space: it is the universal rho-associated bundle.

The following proposition says what happens to this statement under differential refinement

###### Proposition

The homotopy fiber of $J$ in def. 5 is

$\theta :G/T\stackrel{}{\to }{\Omega }^{1}\left(-,𝔤\right)//T$\mathbf{\theta} \colon G/T \stackrel{}{\to} \Omega^1(-,\mathfrak{g})//T

given over a test manifold $U\in$ CartSp by the map

${\theta }_{U}:{C}^{\infty }\left(U,G/T\right)\to {\Omega }^{1}\left(U,𝔤\right)$\mathbf{\theta}_U \colon C^\infty(U,G/T) \to \Omega^1(U,\mathfrak{g})

which sends $g↦{g}^{*}\theta$, where $\theta$ is the Maurer-Cartan form on $G$.

###### Proof

We compute the homotopy pullback of $J$ along the point inclusion by the factorization lemma as discussed at homotopy pullback – Constructions.

This says that with $J$ presented canonically as a map of presheaves of groupoids via the above definitions, its homotopy fiber is presented by the presheaf of groupids $\mathrm{hofib}\left(J\right)$ which is the limit cone in

$\begin{array}{ccccc}\mathrm{hofib}\left(J\right)& \to & & \to & {\Omega }^{1}\left(-,𝔤\right)\\ ↓& & ↓& & ↓\\ & & \left(B{G}_{\mathrm{conn}}{\right)}^{I}& \to & B{G}_{\mathrm{conn}}\\ ↓& & ↓\\ *& \stackrel{}{\to }& B{G}_{\mathrm{conn}}\end{array}\phantom{\rule{thinmathspace}{0ex}}.$\array{ hofib(\mathbf{J}) &\to& &\to& \Omega^1(-, \mathfrak{g}) \\ \downarrow && \downarrow && \downarrow \\ && (\mathbf{B}G_{conn})^I &\to& \mathbf{B}G_{conn} \\ \downarrow && \downarrow \\ * &\stackrel{}{\to}& \mathbf{B}G_{conn} } \,.

Unwinding the definitions shows that $\mathrm{hofib}\left(J\right)$ has

1. objects over a $U\in$ CartSp are equivalently morphisms $0\stackrel{g}{\to }{g}^{*}\theta$ in ${\Omega }^{1}\left(U,𝔤\right)//{C}^{\infty }\left(U,G\right)$, hence equivalently elements $g\in {C}^{\infty }\left(U,G\right)$;

2. morphisms are over $U$ commuting triangles

$\begin{array}{ccccc}{g}_{1}^{*}\theta & & \stackrel{t}{\to }& & {g}_{2}^{*}\theta \\ & {}_{{g}_{1}}↖& & {↗}_{{g}_{2}}\\ & & 0\end{array}$\array{ g_1^* \theta &&\stackrel{t}{\to}&& g_2^* \theta \\ & {}_{\mathllap{g_1}}\nwarrow && \nearrow_{\mathrlap{g_2}} \\ && 0 }

in ${\Omega }^{1}\left(U,𝔤\right)//{C}^{\infty }\left(U,G\right)$ with $t\in {C}^{\infty }\left(U,T\right)$, hence equivalently morphisms

${g}_{1}\stackrel{t}{\to }{g}_{2}$g_1 \stackrel{t}{\to} g_2

in ${C}^{\infty }\left(U,G\right)//{C}^{\infty }\left(U,T\right)$.

3. The canonical map $\mathrm{hofib}\left(J\right)\to {\Omega }^{1}\left(-,𝔤\right)//T$ picks the top horizontal part of these commuting triangles hence equivalently sends $g$ to ${g}^{*}\theta$.

###### Remark

There is yet one more fiber sequence of similar structure. If we let $LG≔\left[{S}^{1},G\right]$ denote the free loop group, then there is a fiber sequence

$\begin{array}{ccc}G/T& \to & LG/T\\ & & ↓\\ & & LG/G& \simeq \Omega G\end{array}\phantom{\rule{thinmathspace}{0ex}}.$\array{ G/T &\to& L G / T \\ && \downarrow \\ && L G / G & \simeq \Omega G } \,.

The geometric quantization of $LG/T$ yields the positive energy representations? of the loop group $L{G}_{𝒞}$. See at loop group – Properties – Representations for more on this.

###### Proposition

If $⟨\lambda ,-⟩\in {\Gamma }_{\mathrm{wt}}↪{𝔤}^{*}$ is in the weight lattice, then there is a morphism of moduli stacks

$⟨\lambda ,-⟩\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}{\Omega }^{1}\left(-,𝔤\right)//T\to BU\left(1{\right)}_{\mathrm{conn}}$\langle \lambda, - \rangle \;\colon\; \Omega^1(-,\mathfrak{g})//T \to \mathbf{B}U(1)_{conn}

in $H$ given over a test manifold $U\in$ CartSp by the functor

$⟨\lambda ,-{⟩}_{U}\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}{\Omega }^{1}\left(U,𝔤\right)//{C}^{\infty }\left(U,G\right)\to {\Omega }^{1}\left(U\right)//{C}^{\infty }\left(U,U\left(1\right)\right)$\langle \lambda, - \rangle_U \;:\; \Omega^1(U,\mathfrak{g})//C^\infty(U,G) \to \Omega^1(U)//C^\infty(U,U(1))

which is given on objects by

$A↦⟨\lambda ,A⟩$A \mapsto \langle \lambda, A\rangle

and which maps morphisms labeled by $\mathrm{exp}\left(\xi \right)\in T$, $\xi \in {C}^{\infty }\left(-,𝔱\right)$ as

$\mathrm{exp}\left(\xi \right)↦\mathrm{exp}\left(i⟨\lambda ,\xi ⟩\right)\phantom{\rule{thinmathspace}{0ex}}.$\exp(\xi) \mapsto \exp( i \langle \lambda, \xi \rangle ) \,.
###### Proof

That this construction defines a map $*//T\to *//U\left(1\right)$ is the statement of prop. 3. It remains to check that the differential 1-forms gauge-transform accordingly.

For this the key point is that since $T\simeq {G}_{\lambda }$ stabilizes $⟨\lambda ,-⟩$ under the coadjoint action, the gauge transformation law for points $A:U\to B{G}_{\mathrm{conn}}$, which for $g\in {C}^{\infty }\left(U,G\right)$ is

$A↦{\mathrm{Ad}}_{g}A+{g}^{*}\theta \phantom{\rule{thinmathspace}{0ex}},$A \mapsto Ad_g A + g^* \theta \,,

maps for $g=\mathrm{exp}\left(\xi \right)\in {C}^{\infty }\left(U,T\right)↪{C}^{\infty }\left(U,G\right)$ to the gauge transformation law in $BU\left(1{\right)}_{\mathrm{conn}}$:

$\begin{array}{rl}⟨\lambda ,A⟩& ↦⟨\lambda ,{\mathrm{Ad}}_{g}A⟩+⟨\lambda ,{g}^{*}\theta ⟩\\ & =⟨\lambda ,A⟩+d⟨\lambda ,\xi ⟩\end{array}$\begin{aligned} \langle \lambda, A \rangle & \mapsto \langle \lambda, Ad_g A\rangle + \langle \lambda, g^* \theta\rangle \\ & = \langle \lambda, A \rangle + d \langle\lambda, \xi \rangle \end{aligned}
###### Remark

The composite of the canonical maps of prop. 6 and prop. 7 modulates a canonical circle bundle with connection on the coset space/coadjoint orbit:

$⟨\lambda ,\theta ⟩:G/T\stackrel{\theta }{\to }{\Omega }^{1}\left(-,𝔤\right)//T\stackrel{⟨\lambda ,-⟩}{\to }BU\left(1{\right)}_{\mathrm{conn}}\phantom{\rule{thinmathspace}{0ex}}.$\langle \lambda, \mathbf{\theta}\rangle \colon G/T \stackrel{\mathbf{\theta}}{\to} \Omega^1(-,\mathfrak{g})//T \stackrel{\langle \lambda, - \rangle}{\to} \mathbf{B}U(1)_{conn} \,.
###### Proposition

The curvature 2-form of the circle bundle $⟨\lambda ,\theta ⟩$ from remark 4 is the symplectic form of prop. 2. Therefore $⟨\lambda ,\theta ⟩$ is a prequantization of the coadjoint orbit $\left({𝒪}_{\lambda }\simeq G/T,{\nu }_{\lambda }\right)$.

###### Proof

The curvature 2-form is modulated by the composite

$\omega :G/T\stackrel{\theta }{\to }{\Omega }^{1}\left(-,𝔤\right)//T\stackrel{⟨\lambda ,-⟩}{\to }BU\left(1{\right)}_{\mathrm{conn}}\stackrel{{F}_{\left(-\right)}}{\to }{\Omega }_{\mathrm{cl}}^{2}\phantom{\rule{thinmathspace}{0ex}}.$\omega \colon G/T \stackrel{\mathbf{\theta}}{\to} \Omega^1(-,\mathfrak{g})//T \stackrel{\langle \lambda, - \rangle}{\to} \mathbf{B}U(1)_{conn} \stackrel{F_{(-)}}{\to} \Omega^2_{cl} \,.

Unwinding the above definitions and propositions, one finds that this is given over a test manifold $U\in$ CartSp by the map

${\omega }_{U}:{C}^{\infty }\left(G/T\right)\to {\Omega }_{\mathrm{cl}}^{2}\left(U\right)$\omega_U \colon C^\infty(G/T) \to \Omega^2_{cl}(U)

which sends

$\left[g\right]↦d⟨\lambda ,{g}^{*}\theta ⟩\phantom{\rule{thinmathspace}{0ex}}.$[g] \mapsto d \langle \lambda, g^* \theta \rangle \,.

### Nonabelian charged particle trajectories – Wilson loops

Let $\Sigma$ be an oriented closed smooth manifold of dimension 3 and let

$C\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}{S}^{1}↪\Sigma$C \;\colon\; S^1 \hookrightarrow \Sigma

be a submanifold inclusion of the circle: a knot in $\Sigma$.

Let $R$ be an irreducible unitary representation of $G$ and let $⟨\lambda ,-⟩$ be a weight corresponding to it by the Borel-Weil-Bott theorem.

Regarding the inclusion $C$ as an object in the arrow category ${H}^{{\Delta }^{1}}$, say that a gauge field configuration for $G$-Chern-Simons theory on $\Sigma$ with Wilson loop $C$ and labeled by the representation $R$ is a map

$\varphi \phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}C\to J$\phi \;\colon\; C \to \mathbf{J}

in the arrow category ${H}^{\left({\Delta }^{1}\right)}$ of the ambient cohesive (∞,1)-topos. Such a map is equivalently by a square

$\begin{array}{ccc}{S}^{1}& \stackrel{\left(A{\mid }_{{S}^{1}}{\right)}^{g}}{\to }& {\Omega }^{1}\left(-,𝔤\right)//T\\ {↓}^{C}& {⇙}_{g}& {↓}^{J}\\ \Sigma & \stackrel{A}{\to }& B{G}_{\mathrm{conn}}\end{array}$\array{ S^1 &\stackrel{(A|_{S^1})^g}{\to}& \Omega^1(-,\mathfrak{g})//T \\ \downarrow^{\mathrlap{C}} &\swArrow_{g}& \downarrow^{\mathrlap{\mathbf{J}}} \\ \Sigma &\stackrel{A}{\to}& \mathbf{B}G_{conn} }

in $H$. In components this is

• a $G$-principal connection $A$ on $\Sigma$;

• a $G$-valued function $g$ on ${S}^{1}$

which fixes the field on the circle defect to be $\left(A{\mid }_{{S}^{1}}{\right)}^{g}$, as indicated.

Moreover, a gauge transformation between two such fields $\kappa :\varphi ⇒\varphi \prime$ is a $G$-gauge transformation of $A$ and a $T$-gauge transformation of $A{\mid }_{{S}^{1}}$ such that these intertwine the component maps $g$ and $g\prime$. If we keep the bulk gauge field $A$ fixed, then his means that two fields $\varphi$ and $\varphi \prime$ as above are gauge equivalent precisely if there is a function $t\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}{S}^{1}\to T$ such that $g=g\prime t$, hence gauge equivalence classes of fields for fixed bulk gauge field $A$ are parameterized by their components $\left[g\right]=\left[g\prime \right]\in \left[{S}^{1},G/T\right]$ with values in the coset space, hence in the coadjoint orbit.

For every such field configuration we can evaluate two action functionals:

1. that of 3d Chern-Simons theory, whose extended Lagrangian is $c:B{G}_{\mathrm{conn}}\to {B}^{3}U\left(1{\right)}_{\mathrm{conn}}$;

2. that of the 1-dimensional Chern-Simons theory discussed above whose extended Lagrangian is $⟨\lambda ,-⟩:{\Omega }^{1}\left(-,𝔤\right)//T\to BU\left(1{\right)}_{\mathrm{conn}}$, by prop. 7.

These are obtained by postcomposing the above square on the right by these extended Lagrangians

$\begin{array}{ccccc}{S}^{1}& \stackrel{\left(A{\mid }_{{S}^{1}}{\right)}^{g}}{\to }& {\Omega }^{1}\left(-,𝔤\right)//T& \stackrel{⟨\lambda ,-⟩}{\to }& BU\left(1{\right)}_{\mathrm{conn}}\\ {↓}^{C}& {⇙}_{g}& {↓}^{J}\\ \Sigma & \stackrel{A}{\to }& B{G}_{\mathrm{conn}}& \stackrel{c}{\to }& BU\left(1{\right)}_{\mathrm{conn}}\end{array}$\array{ S^1 &\stackrel{(A|_{S^1})^g}{\to}& \Omega^1(-,\mathfrak{g})//T &\stackrel{\langle \lambda, -\rangle}{\to}& \mathbf{B}U(1)_{conn} \\ \downarrow^{\mathrlap{C}} &\swArrow_{g}& \downarrow^{\mathrlap{\mathbf{J}}} \\ \Sigma &\stackrel{A}{\to}& \mathbf{B}G_{conn} &\stackrel{\mathbf{c}}{\to}& \mathbf{B}U(1)_{conn} }

and then preforming the fiber integration in ordinary differential cohomology over ${S}^{1}$ and over $\Sigma$, respectively.

For the bottom map this gives the ordinary action functional of Chern-Simons theory. For the top map inspection of the proof of prop. 7 shows that this gives the 1d Chern-Simons action whose partition function is the Wilson loop observable by prop. 5 above.

### 3d Chern-Simons theory with Wilson loops

We discuss how an extended Lagrangian for $G$-Chern-Simons theory with Wilson loop defects is naturally obtained from the above higher geometric formulation of the orbit method. In particular we discuss how the relation between Wilson loops and 1-dimensional Chern-Simons theory sigma-models with target space the coadjoint orbit, as discussed above is naturally obtained this way.

More formally, we have an extended Chern-Simons theory as follows.

The moduli stack of fields $\varphi :C\to J$ in ${H}^{\left({\Delta }^{1}\right)}$ as above is the homotopy pullback

$\begin{array}{ccc}\mathrm{Fields}\left({S}^{1}↪\Sigma \right)& \stackrel{}{\to }& \left[{S}^{1},{\Omega }^{1}\left(-,𝔤\right)//T\right]\\ ↓& {⇙}_{\simeq }& ↓\\ \left[\Sigma ,B{G}_{\mathrm{conn}}\right]& \to & \left[{S}^{1},B{G}_{\mathrm{conn}}\right]\end{array}$\array{ \mathbf{Fields}(S^1 \hookrightarrow \Sigma) &\stackrel{}{\to}& [S^1, \Omega^1(-,\mathfrak{g})//T] \\ \downarrow &\swArrow_\simeq& \downarrow \\ [\Sigma, \mathbf{B}G_{conn}] &\to& [S^1, \mathbf{B}G_{conn}] }

in $H$, where square brackets indicate the internal hom in $H$.

Postcomposing the two projections with the two transgressions of the extended Lagrangians

$\mathrm{exp}\left(2\pi i{\int }_{\Sigma }\left[\Sigma ,c\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\left[\Sigma ,B{G}_{\mathrm{conn}}\right]\stackrel{\left[\Sigma ,c\right]}{\to }\left[\Sigma ,{B}^{3}U\left(1{\right)}_{\mathrm{conn}}\right]\stackrel{\mathrm{exp}\left(2\pi i{\int }_{\Sigma }\left(-\right)\right)}{\to }U\left(1\right)$\exp(2 \pi i \int_\Sigma[\Sigma, \mathbf{c}]) \;\colon\; [\Sigma, \mathbf{B}G_{conn}] \stackrel{[\Sigma, \mathbf{c}]}{\to} [\Sigma, \mathbf{B}^3 U(1)_{conn}] \stackrel{\exp(2 \pi i \int_\Sigma (-))}{\to} U(1)

and

$\mathrm{exp}\left(2\pi i{\int }_{\Sigma }\left[{S}^{1},⟨\lambda ,-⟩\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\left[{S}^{1},{\Omega }^{1}\left(-,𝔤\right)//T\right]\stackrel{\left[\Sigma ,⟨\lambda ,-⟩\right]}{\to }\left[{S}^{1},BU\left(1{\right)}_{\mathrm{conn}}\right]\stackrel{\mathrm{exp}\left(2\pi i{\int }_{{S}^{1}}\left(-\right)\right)}{\to }U\left(1\right)$\exp(2 \pi i \int_\Sigma[S^1, \langle \lambda, -\rangle]) \;\colon\; [S^1, \Omega^1(-,\mathfrak{g})//T] \stackrel{[\Sigma, \langle \lambda , -\rangle]}{\to} [S^1, \mathbf{B} U(1)_{conn}] \stackrel{\exp(2 \pi i \int_{S^1} (-))}{\to} U(1)

to yield

$\begin{array}{ccccc}\mathrm{Fields}\left({S}^{1}↪\Sigma \right)& \stackrel{}{\to }& \left[{S}^{1},{\Omega }^{1}\left(-,𝔤\right)//T\right]& \stackrel{\mathrm{exp}\left(2\pi i{\int }_{{S}^{1}}\left[{S}^{1},⟨\lambda ,-⟩\right]\right)}{\to }& U\left(1\right)\\ ↓& {⇙}_{\simeq }& ↓\\ \left[\Sigma ,B{G}_{\mathrm{conn}}\right]& \to & \left[{S}^{1},B{G}_{\mathrm{conn}}\right]\\ {↓}^{\mathrm{exp}\left(2\pi i{\int }_{{\Sigma }_{2}}\left[{\Sigma }_{3},c\right]\right)}\\ U\left(1\right)\end{array}$\array{ \mathbf{Fields}(S^1 \hookrightarrow \Sigma) &\stackrel{}{\to}& [S^1, \Omega^1(-,\mathfrak{g})//T] &\stackrel{\exp(2 \pi i \int_{S^1} [S^1, \langle \lambda, -\rangle] ) }{\to}& U(1) \\ \downarrow &\swArrow_\simeq& \downarrow \\ [\Sigma, \mathbf{B}G_{conn}] &\to& [S^1, \mathbf{B}G_{conn}] \\ \downarrow^{\mathrlap{\exp(2\pi i \int_{\Sigma_2} [\Sigma_3, \mathbf{c}])}} \\ U(1) }

and then forming the product yields the action functional

$\mathrm{exp}\left(2\pi i{\int }_{{S}^{1}}\left[{S}^{1},⟨-⟩\right]\right)\cdot \mathrm{exp}\left(2\pi i{\int }_{\Sigma }\left[\Sigma ,c\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\mathrm{Fields}\left({S}^{1}↪\Sigma \right)\to U\left(1\right)\phantom{\rule{thinmathspace}{0ex}}.$\exp(2 \pi i \int_{S^1}[S^1, \langle -\rangle]) \cdot \exp(2 \pi i \int_{\Sigma}[\Sigma, \mathbf{c}]) \;:\; \mathbf{Fields}(S^1 \hookrightarrow \Sigma) \to U(1) \,.

This is the action functional of 3d $G$-Chern-Simons theory on $\Sigma$ with Wilson loop $C$ in the representation determined by $\lambda$.

Similarly, in codimension 1 let ${\Sigma }_{2}$ now be a 2-dimensional closed manifold, thought of as a slice of $\Sigma$ above, and let ${\coprod }_{i}*\to {\Sigma }_{2}$ be the inclusion of points, thought of as the punctures of the Wilson line above through this slice. Then we have prequantum bundles given by transgression of the extended Lagrangians to codimension 1

$\mathrm{exp}\left(2\pi i{\int }_{{\Sigma }_{2}}\left[\Sigma ,c\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\left[{\Sigma }_{2},B{G}_{\mathrm{conn}}\right]\stackrel{\left[{\Sigma }_{2},c\right]}{\to }\left[{\Sigma }_{2},{B}^{3}U\left(1{\right)}_{\mathrm{conn}}\right]\stackrel{\mathrm{exp}\left(2\pi i{\int }_{{\Sigma }_{2}}\left(-\right)\right)}{\to }BU{\left(1\right)}_{\mathrm{conn}}$\exp\left(2 \pi i \int_{\Sigma_2}\left[\Sigma, \mathbf{c}\right]\right) \;\colon\; \left[\Sigma_2, \mathbf{B}G_{conn}\right] \stackrel{\left[\Sigma_2, \mathbf{c}\right]}{\to} \left[\Sigma_2, \mathbf{B}^3 U(1)_{conn}\right] \stackrel{\exp\left(2 \pi i \int_{\Sigma_2} \left(-\right)\right)}{\to} \mathbf{B}U\left(1\right)_{conn}

and

$\mathrm{exp}\left(2\pi i{\int }_{\coprod _{i}*}\left[\coprod _{i}*,⟨\lambda ,-⟩\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\left[\coprod _{i}*,{\Omega }^{1}\left(-,𝔤\right)//T\right]\stackrel{\left[\coprod _{i}*,⟨\lambda ,-⟩\right]}{\to }\left[\coprod _{i}*,BU\left(1{\right)}_{\mathrm{conn}}\right]\stackrel{\mathrm{exp}\left(2\pi i{\int }_{\coprod _{i}*}\left(-\right)\right)}{\to }BU\left(1{\right)}_{\mathrm{conn}}$\exp\left(2 \pi i \int_{\coprod_i {*}}\left[\coprod_i {*}, \left\langle \lambda, -\right\rangle\right]\right) \;\colon\; \left[\coprod_i {*}, \Omega^1\left(-,\mathfrak{g}\right)//T\right] \stackrel{[\coprod_i {*}, \langle \lambda , -\rangle]}{\to} \left[\coprod_i {*}, \mathbf{B} U(1)_{conn}\right] \stackrel{\exp\left(2 \pi i \int_{\coprod_i {*}} \left(-\right)\right)}{\to} \mathbf{B}U(1)_{conn}

and hence a total prequantum bundle

$\mathrm{exp}\left(2\pi i{\int }_{\coprod _{i}*}\left[\coprod _{i}*,⟨\beta ,-⟩\right]\right)\otimes \mathrm{exp}\left(2\pi i{\int }_{{\Sigma }_{2}}\left[{\Sigma }_{2},c\right]\right)\phantom{\rule{thickmathspace}{0ex}}:\phantom{\rule{thickmathspace}{0ex}}\mathrm{Fields}\left(\coprod _{i}*↪\Sigma \right)\to BU{\left(1\right)}_{\mathrm{conn}}\phantom{\rule{thinmathspace}{0ex}}.$\exp\left(2 \pi i \int_{\coprod_i {*}}\left[\coprod_i {*}, \langle \beta, -\rangle\right]\right) \otimes \exp\left(2 \pi i \int_{\Sigma_2}\left[\Sigma_2, \mathbf{c}\right]\right) \;:\; \mathbf{Fields}\left(\coprod_i {*} \hookrightarrow \Sigma\right) \to \mathbf{B}U\left(1\right)_{conn} \,.

One checks that this is indeed the correct prequantization as considered in (Witten 98, p. 22).

## References

Introductions and survey include

• Alexandre Kirillov, Lectures on the orbit method, Graduate Studies in Mathematics, 64, American Mathematical Society, (2004)

David Vogan, Review of: Lectures on the orbit method (pdf)

• David Vogan, Geometry and representations of reductive gorups (2007) (pdf)

• J. Maes, An introduction to the orbit method Master thesis (2011) (web)

• Craig Jackson, Symplectic manifolds, geometric quantization, and unitary representations of Lie groups (pdf)

• Reyer Sjamaar, Notes on the orbit method and quantization (1997) (pdf)

Original references include

• В. А. Гинзбург, Метод орбит в теории представлений комплексных групп Ли, Функц. анализ и его прил., 1981, том 15, в. 1, стр. 23–37, pdf; transl. V. A. Ginzburg, Method of orbits in the representation theory of complex Lie groups, Funct. Analysis and Its Appl. 1981, 15:1, 18–28, doi

• Bertram Kostant, Orbits and quantization theory, Proc. ICM Nice 1970, 395-406, djvu:597 K, pdf:1.1 M

• Bertram Kostant, Quantization and unitary representations. I. Prequantization, in: Lectures in Modern Analysis and Applications III, Lec. Notes in Math. 170, 87–208, MR294568; Russ. transl. by A. Kirillov: Uspehi Mat. Nauk 28 (1973), no. 1(169), 163–225, pdf

• Alexandre Kirillov, Унитарные представления нильпотентных групп Ли, , Uspehi. Mat. Nauk. 17 (1962), 57-110, Rus. pdf; transl. Unitary representations of nilpotent Lie groups, Russian Math. Surveys, 1962, 17:4, 53–104, doi, MR142001

• L. Auslander, Bertram Kostant, Quantization and representations of solvable Lie groups, Bull. Amer. Math. Soc. 73, 1967, 692–695, pdf; Polarization and unitary representations of solvable Lie groups, Invent. Math. 14 (1971), 255–354, MR293012, doi

• W. Graham, David Vogan, Geometric quantization for nilpotent coadjoint orbits, in Geometry and Representation Theory of real and p-adic groups. Birkhäuser, Boston-Basel-Berlin (1998)

• David Vogan, The method of coadjoint orbits for real reductive groups, in Representation Theory of Lie Groups. IAS/Park City Mathematics Series 8 (1999), 179–238

Discussion with an eye towards application in gauge theory and in particular for Wilson loop observables in Chern-Simons theory (hinted at on p. 22, 23 of Edward Witten’s QFT and the Jones polynomial) is in section 4 of

• Chris Beasley, Localization for Wilson Loops in Chern-Simons Theory, in J. Andersen, H. Boden, A. Hahn, and B. Himpel (eds.) Chern-Simons Gauge Theory: 20 Years After, , AMS/IP Studies in Adv. Math., Vol. 50, AMS, Providence, RI, 2011. (arXiv:0911.2687)

referring to

• S. Elitzur, Greg Moore, A. Schwimmer, and Nathan Seiberg, Remarks on the Canonical Quantization of the Chern-Simons-Witten Theory, Nucl. Phys. B 326 (1989) 108–134.

Generalizations to supergeometry are discussed in

• Gijs M. Tuynman, Geometric Quantization of Superorbits: a Case Study (arXiv:0901.1811)

A generalization to higher geometry and 2-group 2-representations is proposed in

The above discussion of the interpretation of th eorbit method in terms of higher moduli stacks for differential cohomology appears in