Physics Asked by Sigh_at_psi on August 26, 2021
As seen in Section 4 of Chapter 5 of Costello, K. "Renormalization and Effective Field Theory", or in section 5.2 $L_infty$-Algebras of Classical Field Theories and the Batalin-Vilkovisky Formalism, the BV form of the Chern-Simons action is
$$S=frac{1}{2}langle A,dArangle+frac{1}{6}langle A,[Awedge A]rangle+langle A^*,D_Acrangle+frac{1}{2}langle c^*,[c,c]rangle,tag{1}$$
with $cinOmega^0(M)otimesmathfrak{g}[1]$, $AinOmega^1(M)otimesmathfrak{g}$, $A^*inOmega^2(M)otimesmathfrak{g}[-1]$, and $c^*inOmega^3(M)otimesmathfrak{g}[-2]$. In here $mathfrak{g}$ is a Lie algebra equipped with an invariant non-degenerate pairing $langlecdot,cdotrangle$. However, in the first reference it is also claimed that this action can be put into the form
$$S=frac{1}{2}langle e,derangle+frac{1}{6}langle e,[ewedge e]rangletag{2}$$
for some field $e$. I don’t see how this is possible.
Let me explain my reasoning. Let us first assume $e=c+A+A^*+c^*$. Note that $langlealpha,betarangle=0$ if $alphainOmega^p(M)otimesmathfrak g$ and $betainOmega^q(M)otimesmathfrak g$ with $p+qneq 3$. We can use this to expand $langle e,d{e}rangle$. For example, the only term that can be coupled with the $A$ coming from the left $e$ is the $d{A}$ coming from $d{e}$. We conclude that
begin{equation}
frac{1}{2}langle e,d{e}rangle=frac{1}{2}langle c,d{A^*}rangle+frac{1}{2}langle A,d{A}rangle+frac{1}{2}langle A^*,d{c}rangle.tag{3}
end{equation}
Now, remembering that $A^*$ and $c$ are fermionic, we have
begin{equation}
begin{aligned}
langle c,d{A^*}rangle&=int c^ad{A^{*b}}langle T_a,T_brangle_{mathfrak g}=-int d{A^{*b}}c^alangle T_a,T_brangle_{mathfrak g}
&=-int d{(A^{*b}c^a)}langle T_a,T_brangle_{mathfrak g}+int A^{*b}d{c^a}langle T_a,T_brangle_{mathfrak g}.
end{aligned}tag{4}
end{equation}
Thus, up to total derivatives we have
begin{equation}
frac{1}{2}langle e,d{e}rangle=frac{1}{2}langle A,d{A}rangle+langle A^*,d{c}rangle.tag{5}
end{equation}
To expand the term $langle e,[ewedge e]rangle$, note that $[ewedge e]$ can only have even forms. Indeed, an odd form in the expansion of $[ewedge e]$ must come from the coupling $[alphawedgebeta]$ of an odd form $alpha$ and an even form $beta$ in $e$. Since they are different, the term $[betawedgealpha]$ also appears in the expansion of $e$. Now, all even forms in $e$ are fermionic while all odd forms in $e$ are bosonic. We conclude that $alpha$ is bosonic while $beta$ is fermionic. Therefore
begin{equation}
[alphawedgebeta]=alpha^awedgebeta^b[T_a,T_b]=beta^bwedgealpha^a[T_a,T_b]=-beta^bwedgealpha^a[T_b,T_a]=-[betawedgealpha].tag{6}
end{equation}
Therefore the terms $[alphawedgebeta]$ and $[betawedgealpha]$ cancel. By the same token, the rest of the surviving terms in the expansion of $[ewedge e]$ are symmetric $[alphawedgebeta]=[betawedgealpha]$. Given that we are in three dimensions, they have to either be 0-forms or 2-forms. We conclude that
begin{equation}
[ewedge e]=[cwedge c]+2[cwedge A^*]+[Awedge A].tag{7}
end{equation}
Of course, for $0$-forms we have $[cwedge c]=[c,c]$. The second term is then
begin{equation}
frac{1}{6}langle e,[ewedge e]rangle=frac{1}{6}langle A,[Awedge A]rangle+frac{1}{3}langle A,[cwedge A^*]rangle+frac{1}{6}langle c^*,[cwedge c]rangle.tag{8}
end{equation}
We see that we have failed to recover our original action because of some factors. One could try to resolve this by combining the fields in $e$ with different numerical factors. However, since the action of $A$ already has the correct factors, we cannot rescale $A$. Indeed, any rescaling of $A$ would produce a mismatch in the scales of the quadratic and cubic terms in $A$. On the other hand, the term $langle A^*,dcrangle$ has also the correct factor, so that we must scale $c$ and $A^*$ inversely. This means that we will never get the correct factor for the cubic term in $c$, $A$, and $A^*$.
In this answer we will focus on the cubic term, which seems to be OP's main question.
The trilinear form $$tequivlanglecdot,[cdot,cdot]rangle: mathfrak{g}times mathfrak{g}timesmathfrak{g}to mathbb{C}tag{A}$$ is totally antisymmetric, because the bilinear form $langlecdot,cdotrangle$ is invariant/associative.
Consider fields ${bf e}$ that are both Lie-algebra-valued, form-valued & supernumber-valued. Note that in OP's references the $n$-forms are (implicitly) interpreted as carrying Grassmann-degree $n$ (modulo 2). The total Grassmann-parity of the fields ${bf e}$ is assumed to be odd, so that such fields anti-commute (in the appropriate graded symmetric tensor algebra). The trilinear form $t$ therefore becomes totally symmetric wrt. such fields.
In BV-CS theory (before gauge-fixing), we consider a minimal field $$ {bf e} ~=~ c ~+~underbrace{A_{mu}mathrm{d}x^{mu}}_{=~{bf A}}~+~underbrace{A^{astmu}(star mathrm{d}x)_{mu}}_{=~{bf A}^{ast}} ~+~underbrace{c^{ast}Omega}_{=~{bf c}^{ast}} tag{B}$$ of above type, where $$(star mathrm{d}x)_{mu}~:=~frac{1}{2}epsilon_{munulambda}mathrm{d}x^{nu}wedge mathrm{d}x^{lambda}tag{C}$$ and where $$Omega~:=~frac{1}{6}epsilon_{munulambda}mathrm{d}x^{mu}wedgemathrm{d}x^{nu}wedge mathrm{d}x^{lambda} ~=~frac{1}{3}mathrm{d}x^{mu}wedge(star mathrm{d}x)_{mu}.tag{D}$$ (The wedges will be not be written explicitly from now on.)
The cubic action term is a multinomial expression $$begin{align} left. frac{1}{6} t({bf e},{bf e},{bf e})right|_{text{top-form}}~=~& frac{1}{6}t({bf A},{bf A},{bf A})+ t({bf A}^{ast},{bf A},c) +frac{1}{2}t({bf c}^{ast},c,c)cr ~=~&left( t(A_1,A_2,A_3)+ t(A^{astmu},A_{mu},c) +frac{1}{2}t(c^{ast},c,c)right) Omega.end{align}tag{E}$$ Note that the (reciprocal) coefficient of each term of eq. (E) is precisely its symmetry factor. Eq. (E) agrees with OP's eq. (1).
Correct answer by Qmechanic on August 26, 2021
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