3d supersymmetric gauge theories and Hilbert series

0.37 MB·

by  Stefano Cremonesi

7 3d supersymmetric gauge theories and Hilbert series
1
0
Stefano Cremonesi
2

n
a Abstract. TheHilbertseriesisageneratingfunctionthatenumeratesgauge
J invariant chiral operators of a supersymmetric field theory with four super-

3 charges and an R-symmetry. In this article I review how counting dressed
’t Hooft monopole operators leads to a formulafor the Hilbertseries of a 3d

] N ≥ 2 gauge theory, which captures precious information about the chiral
h ringandthemodulispaceofsupersymmetricvacuaofthetheory. (Conference
t paper basedonatalkgivenatString-Math2016,Coll`egedeFrance,Paris.)
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p
e
h
[
1. Introduction

1
v There is a long and illustrious tradition of fruitful interplay between super-
1 symmetric quantum field theory and geometry [51, 55, 64, 55]. The main bridge
4 betweenthetwotopicsistheconceptofthemodulispaceofsupersymmetricvacua,
6
the set of zero energy configurationsof the field theory, which in the context of su-
0
persymmetric field theories with at least four supercharges is a complex algebraic
0
. variety equipped with a K¨ahler metric.
1
Moduli spaces of vacua of quantum field theories with four supercharges in
0
7 four spacetime dimensions have been studied in great detail since the 1990’s, and
1 their algebro-geometric structure is well understood. Less understood are their
: counterparts in three dimensions, due to new scalar fields which are obtained by
v
i dualizing vectors in three dimensions. Interesting results on the moduli spaces of
X
vacuaofthree-dimensionaltheorieswithfoursuperchargeswereobtainedbymeans
r of semiclassical analysis [29, 1], but a precise understanding of the underlying
a
algebraic geometry was lacking, except for a few simple theories [11].
In this article I will review recent developments that allow one to make exact
statementsonthealgebraicgeometryofthemodulispacesofsupersymmetricvacua
ofthree-dimensionalgaugetheorieswithfourormoresupercharges[26,24,23,22,
27]. The key idea is to countthe gaugeinvariantchiraloperatorsthatparametrize
the moduli space of supersymmetric vacua, using a generating function called the
Hilbert series. In the context of three-dimensional supersymmetric field theories,
the gauge invariant chiral operators are dressed ’t Hooft monopole operators: I
will describe their properties and how to count them, leading to a formula for
the Hilbert series. A peculiarity of ’t Hooft monopole operators, that hindered
the understanding of the algebraic geometry of moduli spaces of vacua of three-
dimensional supersymmetric gauge theories, is that they obey relations that arise
ContributionsubmittedtotheProceedingsofString-Math2016.
1
2 STEFANOCREMONESI
in the quantum theory. However, by applying plethystic techniques to the Hilbert
series that counts dressed monopole operators, one can deduce information on the
chargesofgeneratorsandrelationsofthechiralringofthefieldtheory,whichisthe
coordinate ring of its moduli space of vacua. The formalism is therefore capable
of producing predictions on the quantum relations between monopole operators,
without making any assumptions on them.
The article is organizedas follows. In section 2 I recall the concepts of moduli
spaceofvacuaandofchiralringinthefamiliargroundoffour-dimensionaltheories
with four supercharges (4d N = 1 theories). In section 3 I introduce the Hilbert
series in that context and give a few useful examples for what follows. In section 4
I introduce three-dimensional theories with four supercharges (3d N =2 theories)
andcontrastthemwith4dN =1theories. Insection5Idiscusssupersymmetric’t
Hooft monopole operators and some of their properties. In section 6 I present the
main formula (6.3) for the Hilbert series, that counts dressed monopole operators.
In section 7 I apply these ideas to Coulomb branches of 3d N = 4 theories, and
in section 8 to moduli spaces of vacua of 3d N = 2 Yang-Mills and Chern-Simons
theories. I conclude with a few remarks and open questions in section 9.
2. Moduli space of supersymmetric vacua and chiral ring
Letusfirstrecallsomewell-knownfactsaboutfour-dimensionalgaugetheories
with four Poincar´e supercharges (4d N = 1 theories). Most of the structure of 4d
N =1 supersymmetric theories carries over to 3d N =2 supersymmetric theories,
which will be our main focus in the following.
The 4dN =1 supersymmetryalgebraconsistsofthe followinggenerators: the
Lorentz generators M =−M (µ,ν = 0,...,3) for rotations and boosts, which
µν νµ
generatethe LorentzgroupSO(1,3); the momentum P , an SO(1,3) vector which
µ
generates translations in the Minkowski spacetime R1,3; the complex supercharges
Q and Qα˙ = (Q )† (α = 1,2 and α˙ = 1,2), left-handed and right-handed Weyl
α α
spinors transforming in the two-dimensional representations [1;0] and [0;1] of the
double cover of the Lorentz group Spin(1,3) = SL(2,C) ∼= SL(2,R)×SL(2,R),
which generate translations along the Grassmann odd directions of superspace;
and possibly R, a Lorentz scalar which generates a U(1) symmetry that acts
R
nontriviallyonthe supercharges. The commutationrelations arethose requiredby
the Lorentz properties of the generators recalled above, together with
{Q ,Q }=2(σµ) P
α α˙ αα˙ µ
(2.1)
[R,Q ]=−Q , [R,Q ]=Q ,
α α α˙ α˙
where(σµ) =(−1,σ ) andσ arethe Paulimatricesthatsatisfy σ σ =δ +
αα˙ i αα˙ i h j hj
iǫ σ . In addition there might be a globalnon-R symmetry algebra,often called
hjk k
flavoursymmetry, generatedby scalar chargeswhich commute with the generators
of the supersymmetry algebra.
Fields in a 4d N = 1 supersymmetric field theory fit in irreducible repre-
sentations of the 4d N = 1 superalgebra, which in turn constrains the form of
interactions. Altogether, a 4d N = 1 supersymmetric Lagrangian gauge theory is
specified by the following data:
(1) Gauge group: a compact semisimple Lie group G, to which one asso-
ciates real vector multiplets Va, with a=1,...,rk(G);
3D SUPERSYMMETRIC GAUGE THEORIES AND HILBERT SERIES 3
(2) Matter content: a representationR of G, to which one associatescom-
plex chiral multiplets Xi, with i=1,...,dim(R);
(3) Superpotential: a G-invariant holomorphic polynomial W(X) in the
matter fields.
The scalar fields in the chiral multiplets, which we also denote as Xi with a
common abuse of notation, interact through a potential
g2
(2.2) V = |F |2+ (Da)2 .
i
2
i a
X X
Here F (X) = ∂W are the F-terms, which equal the derivatives of the super-
i ∂Xi
potential, Da(X,X†) = X†(Ta)i Xj are the D-terms, which equal the mo-
i i j
ment maps of the action of the gauge group G on the matter representation R,
and g is the Yang-Mills cPoupling constant.1 For U(1) gauge factors, the moment
map can be shifted by a constant, the Fayet-Iliopoulos (FI) parameter ξ, so that
DU(1) = q |Xi|2−ξ, with q the U(1) charge of Xi.
i i i
One can associate to a supersymmetric gauge theory specified by these data
P
an object of great physical and mathematical interest: its moduli space of super-
symmetric vacua M [64]. Physically, M controls the low energy behaviour of the
quantum field theory. Many of the impressive results on the dynamics of super-
symmetric field theories obtained in the 1990’s were indeed rooted in the analysis
oftheir moduli spaces of vacua [55]. Mathematically, M provides a naturalbridge
between supersymmetric field theories and (differential and algebraic) geometry.
Physicallybasedresultsonmodulispacesofvacuaofsupersymmetricfieldtheories
can thus lead to interesting mathematical predictions.
Concretely, the moduli space of supersymmetric vacua M is defined as
the set of constant field configurations that minimize the potential (2.2), modulo
gauge equivalence:
M={(X,X†)| F (X)=0 ∀i, Da(X,X†)=0 ∀a}/G=F//G
i
(2.3)
∼={(X)| ∂W(X)=0}/GC =F/GC .
4d N =1 supersymmetry implies that the moduli space of vacua M is a (possibly
singular) K¨ahler manifold. The first line of (2.3) expresses M as a symplectic (in
factK¨ahler)quotientofF bythegaugegroupG,whereasthesecondlineexpresses
M equivalently as a holomorphic (GIT) quotient by the complexified gauge group
GC. Here
(2.4) F ={(X)| ∂W(X)=0}
is the space of solutions of the F-term equations, often called F-flat moduli space
or master space [35]. Algebraically, it is a complex affine variety defined by the
vanishing of the F-term relations ∂W(X)=0.
In the following I will adopt the holomorphic viewpoint in the second line of
(2.3) and view the moduli space of vacua M as a complex algebraic variety. M is
typically an affine variety. Of particular interest are superconformal field theories,
whose moduli spaces of vacua are cones. The C∗ action whose radial part dilates
the cone is the complexification of the U(1) symmetry of the field theory.
R
1IntheD2 termIhaveusedabasisoftheLiealgebrathatdiagonalizes theKillingform. If
thegaugegroupGissemisimplethereisoneYang-Millscouplingconstant persimplefactor.
4 STEFANOCREMONESI
Closely related to the moduli space of supersymmetric vacua are the concepts
of chiral operators and chiral ring [63]. Local gauge invariant chiral operators
O (x) form a subset of observables in a 4d N =1 field theory which are protected
i
from quantum corrections. These are 1-BPS operators that are annihilated by all
2
the supercharges of positive R-charge:
(2.5) Q O (x)=0 ∀ α˙ =1,2 .
α˙ i
A crucial property of chiral operators is that their spacetime derivatives are
Q-exact and therefore vanish in expectation values, provided supersymmetry is
unbroken. It follows that a product of chiral operators is free of short distance
divergences and that its expectation value factorizes into the product of spacetime
constant one-point functions: hO (x )...O (x )i=hO i...hO i.
i1 1 in n i1 in
Chiral operators form a commutative ring, the chiral ring R, with product
(2.6) O O =c kO +Q (...)α˙ ,
i j ij k α˙
where the only spacetime dependence is in the Q-exact term, and repeated indices
are summed over. Since we are physically interested in taking expectation values,
we will work at the level of Q-cohomology and omit Q-exact terms in (2.6) from
now on. The chiral ring is then specified once a basis of chiral operators {O } and
i
the structure constants c k are provided.
ij
TheexpectationvalueshO iofgaugeinvariantchiraloperators,orequivalently
i
Q-cohomologyclasses,areholomorphicfunctions onthe moduli spaceofvacua M.
It is generally expected, though not proven to the best of my knowledge, that the
correspondence between expectation values of chiral operators and holomorphic
functions on M is one-to-one, once relations are taken into account. With this
physically motivated assumption, the chiral ring R of the supersymmetric field
theory is identified with the coordinate ring of its moduli space of vacua M. We
would then like to characterize the chiral ring as a quotient ring
(2.7) R=C[O ,...,O ]/I ,
1 n
determine the generators O ,...,O of the polynomial ring and the defining rela-
1 n
tions of the ideal I.
In a 4d N = 1 gauge theory, the chiral operators are G-invariant polynomials
in the matter fields X.2 If there is no gauge symmetry, the chiral ring is just the
jacobian ring of the superpotential W. In a gauge theory, however, the quotient
by the gauge group in (2.3) makes it often hard to explicitly determine generators
and relations of the chiral ring, and therefore the defining equations of the moduli
space M as an algebraic variety.
3. The Hilbert series
Sincedetermininggeneratorsandrelationsofthechiralringofasupersymmet-
ricgaugetheoryis ingeneraladifficult task,ithelpsto exploitasmuchaspossible
the symmetries of the theory. A very useful tool in this respect is the Hilbert se-
ries [8],3 a generating function that counts scalar gaugeinvariantchiraloperators,
2Weneglecthereglueballoperators [17],sincetheydonotplayaroleinthreedimensions.
3Seealso[71]foranearlyincarnationofthisconcept.
3D SUPERSYMMETRIC GAUGE THEORIES AND HILBERT SERIES 5
graded by their charges under a Cartan subalgebra of the global symmetry:
(3.1) H(t,x)=TrH tR xQbbi .
i
(cid:18) b (cid:19)
Ybi
b b
Here H = {O |Q O = 0, M O = 0} denotes the vector space of gauge invari-
i α˙ i µν i
ant scalar chiral operators, that parametrize the moduli space of supersymmetric
vacua.4 It can be decomposed into common eigenspaces of the U(1) generator R
R
andthe generatorsQ of the Cartansubalgebraofthe flavoursymmetry. For a su-
i
perconformalfieldtheborytheR-chargescanbetakenpositive,thus(3.1)isaTaylor
series in t (and a Laburent series in x ), and the eigenspaces are finite dimensional.
i
The Hilbert series is a character on tbhe vector space of scalar chiral operators: the
coefficientsoftheseriesarethedimensionsofthecommoneigenspacesoftheglobal
b
symmetry.
In light of the correspondence between the space H of scalar chiral operators
of the supersymmetric field theory in (3.1) and the space H0(M) of holomorphic
functionsonitsmodulispaceofvacuaM,theHilbertseries(3.1)canbeinterpreted
geometricallyasacharacteroftheactionoftheglobalsymmetrygrouponH0(M).
Forasuperconformalfieldtheory,whosemodulispaceofvacuaisacone,theHilbert
series equals the equivariant index of the Dolbeault operator on M
(3.2) H(t,x)=Tr tR xQbbi H0(M) = (−1)p Tr tR xQbbi Hp(M) ,
i i
(cid:18) Ybi b (cid:12)(cid:12) (cid:19) Xp (cid:18) Ybi b (cid:12)(cid:12) (cid:19)
dubbedindebx-characterin[b66],(cid:12)becausehigherDolbeaultcohomolbogy(cid:12)groupsHp(M)
vanishforp>1. In(3.1)-(3.2)wehavedistinguishedtheR-charge,whichgenerates
a C∗ action that rescalesthe holomorphic top form ofM, fromthe flavourcharges
Q , which generate a torus action that leave the holomorphic top form invariant.
i
b Useful information on the moduli space of vacua M can be extracted from
tbhe Hilbert series [8]. For instance, the complex dimension d of M is the order
of the pole at t = 1 of the unrefined Hilbert series H(t,1), and the coefficient
of (1−t)−d is proportional to the volume of the (d−1)-dimensional base of M.
Most importantly, the charges of the generators and relations can be extracted
using plethystic techniques, if higher syzygies can be disentangled.5 Once this is
achieved, the problem of presenting the moduli space M as an algebraic variety
(or equivalently the chiral ring as a quotient ring (2.7)) is reduced to determining
a finite number of coefficients, that specify which linear combinations of the chiral
operators having the appropriate charges appear as generators or relations.
After this general discussion, let us see how the Hilbert series is computed in
practice for a sample of 4d N = 1 supersymmetric quantum field theories. We
start by considering theories with no gauge group, so that the moduli space (2.3)
coincides with the F-flat space (2.4). For the theory of a free chiral multiplet X of
R-charge R[X] = r, the moduli space is the complex plane, the chiral ring is the
4Analternativecountofprotectedoperatorsisprovidedbythe“superconformal”index[73],
which also counts fermionic and short non-chiral operators, and depends on the superpotential
only through the R-charges of matter fields. Since our interest is in the moduli space of vacua
andthechiralring,wefocusontheHilbertseriesratherthanthesuperconformalindex.
5This is often possible with some physical input, such as an independent determination of
thedimensionofthemodulispace,andthehelpofcomputer algebrasuchasMacaulay2 [42].
6 STEFANOCREMONESI
polynomial ring in one complex variable C[X] and the Hilbert series is simply the
geometric series counting powers of X:6
1
(3.3) H =1+τ +τ2+···= =PE[τ] , τ =tr .
1−τ
If X is subject to a superpotential W(X)=XN+1, the Hilbert series becomes
1−τN
(3.4) H =1+τ +τ2+···+τN−1 = =PE[τ −τN] , τ =tN2+1 .
1−τ
ThechiralringisC[X]/hXNiandthemodulispaceMconsistsofapointofmulti-
plicity N. The generatorX andthe relationXN =0arerespectivelyassociatedto
the positive term+τ andthe negative term−τN in the argumentofthe plethystic
exponential. In general, the plethystic logarithm of the Hilbert series terminates
fortheorieswhosemodulispacesarecompleteintersections,thedimensionofwhich
is the number of generators minus the number of relations.
ThecaseofagenericpolynomialsuperpotentialW(X)=XN+1+ N c XN−i
i=1 i
of degree N + 1 can be treated similarly: even though the U(1) symmetry is
R
P
explicitly broken by the subleading terms in the superpotential, it can be restored
by assigning R-chargesto the parameters c , as is common practice in the analysis
i
of supersymmetric field theories [74]. The parameters c are not dynamical and
i
are not counted by the Hilbert series, which is insensitive to them and remains
(3.4), but they may (and do) appear in the relations. The Hilbert series only
constrains the charges of the relations, which in this case must be of the form
XN + α c XN−i = 0, but does not fix the coefficients, which in this case we
i i i
know to be α =(N −i)/(N +1). Of course there is no need to invoke the Hilbert
i
P
series to study the chiral ring of such a simple theory, but this example makes
it clear which information can be extracted from the Hilbert series (i.e. charges
of operators, generators and relations) and which cannot (i.e. the precise form
of the relations, unless they are entirely fixed by symmetry). Even when the are
coefficients in the relations that cannot be determined by symmetry alone, the
Hilbert series is a very useful tool for deducing the most general form of the chiral
ring relations that is consistent with symmetry.
Another simple but more interesting example of moduli space is provided by
the XYZ model, a theory of three chiral multiplets X, Y and Z with the trilinear
superpotential W = XYZ. From the F-term relations ∂W = 0 we deduce that
the chiralringis C[X,Y,Z]/hYZ,ZX,XYi. The modulispace M consistsofthree
1-dimensional components (in physical jargon “branches”) parametrized by X, Y
6PE isthe plethystic exponential, thegenerating function of symmetricpowers. For amul-
tivariatefunctionf(x1,...,xn)thatvanishesattheorigin,
∞
PE[f(x1,...,xn)]=exp(cid:18) p1f(xp1,...,xpn)(cid:19).
pX=1
This implies that PE[ iai jxbjij]= i(1− jxbjij)−ai. The inverse of the plethystic expo-
nentialistheplethystiPclogaQrithmPL. FQoramuQltivariatefunctiong thatequals1attheorigin,
∞
µ(k)
PL[g(x1,...,xn)]= k logg(xk1,...,xkn),
kX=1
whereµ(k)istheM¨obiusfunction.
3D SUPERSYMMETRIC GAUGE THEORIES AND HILBERT SERIES 7
and Z respectively, meeting at a point. The Hilbert series reads
1 1 1
H = + + −2= (τ =t2/3 , xyz =1)
1−τx 1−τy 1−τz
(3.5) 1−τ2(yz+xz+xy)+2τ3xyz
= .
(1−τx)(1−τy)(1−τz)
PL[H]=(x+y+z)τ −(yz+xz+xy)τ2+2τ3−(x+y+z)τ4+... .
The firstline shows that the moduli space M consists of three copies ofC meeting
atapoint; thesecondlineshowsthe threegeneratorsinthedenominator,andthat
the moduli space is not a complete intersection because the numerator does not
factorize. The plethystic logarithmin the last line allows us to extractinformation
aboutthering: thegeneratorsX,Y,Z ofthepolynomialringappearatorderτ,the
generatorsofthe idealof relationsby whichwe quotient ∂ W,∂ W,∂ W at order
X Y Z
τ2, and then we see higher order syzygies: X∂ W −Y∂ W and X∂ W −Z∂ W
X Y X Z
at order τ3, ∂ W∂ W −Z2∂ W and cyclic permutations at order τ4, and so on.
X Y Z
The plethystic logarithm is a series which does not terminate: this is the general
structure for theories with non-complete intersection moduli spaces.
Nextweconsidergaugetheories. GaugingasubgroupGoftheflavoursymme-
tryleadsintheholomorphicdescriptionofthemodulispacein(2.3)tothequotient
M = F/GC of the F-flat moduli space by the complexified gauge group. At the
levelof the Hilbert series, the projection to gauge singlets is achievedby averaging
the Hilbert series H of the ungauged theory (whose moduli space is the F-flat
F
moduli space F) over the gauge group
(3.6) H(t,x)= dµ (x)H (t,x,x)
G F
I
using the Haar measure
b b
r
dx
(3.7) dµ (x)= j (1−xα) .
G
2πix
I (cid:18)jY=1I j(cid:19)αY∈∆+
The integral is over the maximal torus of G, r = rk(G) is the rank of the gauge
group, ∆ is the set of positive roots of its Lie algebra,and I have used the short-
+
hand xα = r xαi. Gauge fugacities are denoted by x and ungauged flavour
i=1 i
fugacities by x.
Q
A simple class of examples, that will be useful in the following, is provided by
theorieswithbgaugegroupGanda chiralmultiplet Φ inthe adjointrepresentation.
(This is also the vector multiplet sector of 4d N =2 theories with gauge group G.
The branchof the moduli space of vacua where the vector multiplet scalarΦ takes
expectation values is called the Coulomb branch.) Setting τ = tR[Φ], the Hilbert
series reads
r
1
(3.8) H(τ)= dµ (x)PE[τχG(x)]= ,
G ad 1−τdi(G)
I i=1
Y
where χG(x) is the character of the adjoint representation of the gauge group.
ad
The result expresses the well-known fact that the ring of invariants of the adjoint
representationis freely generatedby Casimir invariantsu of degreesd (G). Hence
i i
R= C[g]G =C[φ ,...,φ ]/W =C[u ,...,u ]. E.g. for G=SU(N) the Casimir
1 r G 1 r
invariants are d (SU(N))=2,3,...,N.
i
8 STEFANOCREMONESI
Q Qi
i
2 e 1 2
e
v x u
Figure 1. Quiver diagram for SQED with two flavours.
B
B<0 B=0 B>0
Figure 2. Line bundles on the conifold and resolutions.
Another example is a U(1) gauge theory with two matter fields Qi of charge
1 and two matter fields Q of charge −1, also known as SQED with two flavours.
i
See figure 1 for the quiverediagram of this theory. It turns out to be interesting to
compute the Hilbert seriees in the presence of a background electric charge −B for
theU(1)gaugesymmetry,eventhoughthismayseemartificialfromtheperspective
of four-dimensional gauge theory. (We will see that B has a more 4natural inter-
pretation in three dimensions.) This modified Hilbert series, often called baryonic
Hilbertseries[36],countspolynomialsinthematterfieldsQandQoftotalelectric
charge B to compensate the background electric charge −B, and is computed by
the formula [36] e
dx 1 1 1
H (τ,u,v)≡g (τ,u,v;B)= x−BPE τx u+ +τ v+
−B 1
2πix u x v
(3.9) I (cid:20) (cid:16) (cid:17) (cid:16) (cid:17)(cid:21)
∞ [n+B;n] τ2n+B B ≥0
= n=0 u,v .
(P∞n=0[n;n−B]u,vτ2n−B B ≤0
Hereuandv arefugacitPiesfortheSU(2) ×SU(2) flavoursymmetriesthatrotate
u v
Qi and Q respectively, and [n;m] denotes the character of the representation
i u,v
[n;m] of SeU(2) ×SU(2) .
u v
Theebackground electric charge or “baryonic charge” B in (3.9) is a discrete
analogueof the Fayet-Iliopoulosparameter ξ introduced after (2.2), which leads to
a resolution of the conical moduli space of vacua in the symplectic reduction in
the first line of (2.3). The theory that we are discussing is nothing but the gauged
linear sigma model for the conifold, albeit viewed as a four-dimensional theory:
its moduli space of vacua M is the conifold if ξ = 0, and the resolved conifold if
ξ 6= 0, with the resolutions at ξ > 0 and ξ < 0 being related by a flop transition.
Inthe holomorphiclanguage,the Hilbert series(3.9) withinsertionofthe baryonic
charge B counts holomorphic sections of the line bundle O(BD), where D is the
toricdivisorassociatedto Q fields, and−D the toric divisorassociatedto Q fields.
See figure 2 for a summary. The baryonic Hilbert series counts operators of the
schematic form QB(QQ)n for B ≥ 0 and Q−B(QQ)n for B ≤ 0. For Be= 0 we
obtain the Hilbert series of the conifold
e e e
(3.10) H (τ,u,v)=PE[τ[1;1] −τ2] ,
0 u,v
3D SUPERSYMMETRIC GAUGE THEORIES AND HILBERT SERIES 9
correspondingtothe ringC[M1,M1,M2,M2]/hM1M2−M1M2igeneratedbythe
1 2 1 2 1 2 2 1
four mesons Mi = QiQ subject to a singlet relation det(M) = 0. We have thus
recoveredthe aeilgebraic deiescription of the conifold [18].
e
4. 3d N =2 gauge theories vs 4d N =1 gauge theories
Moduli spaces of supersymmetric vacua of four-dimensional supersymmetric
gaugetheories, whichhave been described so far and analysedwith the help of the
Hilbertseries,areofmathematicalandphysicalinterestandmanynontrivialresults
have been obtained [8, 34, 41, 45, 47]. However, the construction is limited on
bothfronts. Mathematically,eventhoughsupersymmetryonlyrequiresthemoduli
spaceofvacuaMof4dN =1theoriestobeaK¨ahlermanifold(orhyperK¨ahlerfor
the Higgsbranchof4dN =2theories),the latter is actuallya K¨ahlerquotient(or
hyperK¨ahler quotient for the Higgs branch of 4d N =2 theories) for a Lagrangian
supersymmetricgaugetheory. Onthephysicalside,thecomputationoftheHilbert
series is essentially classical and reduces to counting gauge invariant polynomials
in the matter fields appearing in the UV Lagrangian.7
Moduli space of vacua which are not (hyper)Ka¨hler quotients occur for non-
Lagrangian supersymmetric theories in four dimensions, and string theory or M-
theory constructions have been used to study some of their properties, along the
lines of [37]. We will pursue here an alternative way to overcome the limitations
explained above, by considering three-dimensional supersymmetric gauge theories
with at least four supercharges (3d N ≥2) instead of four-dimensional theories.
The 3d N = 2 supersymmetry algebra can be obtained by dimensional re-
duction of the 4d N = 1 supersymmetry algebra and is therefore very similar to
(2.1). The main modifications are that the supercharges are complex conjugate
3d Dirac spinors Q and Q transforming as doublets of SL(2,R), and that the
α β
anticommutator of two supercharges is
(4.1) {Q ,Q }=2(γµ) P +2iǫ Z .
α β αβ µ αβ
Here γµ = (−1,σ ,σ ), and Z is a real generator of the centre of the supersym-
1 3
metry algebra. The central charge Z originates from the momentum along the
reduced dimension. In the same vein, the vector and chiralmultiplets of 3d N =2
supersymmetry can be obtained by dimensional reduction of the vector and chiral
multiplets of 4d N = 1 supersymmetry. Supersymmetric actions in three dimen-
sions take the same form as in four dimensions, except for the possibility, peculiar
to odd spacetime dimensions, to add supersymmetric Chern-Simons (CS) terms
k k
(4.2) S = ab d3x d4θ Σ V = ab (A dA +σ D +superpartners) ,
CS a b a b a b
4π 4π
Z Z
whereΣ =ǫαβD D V isthefieldstrengthmultipletandA isthegaugeconnec-
a α β a a
tionforaU(1)gaugegroup. Therealscalarσ isthe lowestcomponentofthefield
a
strengthmultiplet Σ , and originatesfromthe componentA ofthe gaugeconnec-
a 4
tion in the dimensional reduction from four dimensions. (4.2) is a Chern-Simons
interaction between abelian gauge groups, but can be generalized to nonabelian
7Quantum corrected moduli spaces of vacua of several four-dimensional supersymmetric
gauge theories were intensively studied in the 1990’s [55], but the Hilbert series is to a large
extentinsensitivetothecomplexstructuredeformationsinducedbyquantum effects.
10 STEFANOCREMONESI
gauge groups G in a straightforward way using the Killing form as the symmetric
pairing.
Despitethesimilaritiesbetween4dN =1and3dN =2supersymmetricgauge
theories at the classical level, they behave very differently quantum-mechanically.
InwhatfollowsIwilldescribehowthequantumphysicsofthree-dimensionalN ≥2
supersymmetricgaugetheoriesleadsto modulispacesofsupersymmetric vacuaM
whichare(hyper)Ka¨hler,asrequiredbysupersymmetry,thoughnot(hyper)Ka¨hler
quotients. Unlike most of the previous results on moduli spaces of vacua of 3d
supersymmetricgaugetheorieswhichwerebasedonsemiclassicalanalysis(see[56,
28,29,1,32,76,54]forapartialsample),theaimherewillbetounderstandthese
modulispacesasalgebraicvarietiesandtodevelopgeneralmethodstocharacterize
the chiral rings of such theories. In particular, I will review explicit formulae for
the Hilbert series ofthe Coulombbranchofthe moduli spaceof vacuaof 3dN =4
gauge theories and for the Hilbert series of the moduli space of vacua of 3d N =2
gauge theories, which have been obtained in a series of recent works [26, 22, 27]
(see also [43]).
The count of holomorphic functions in the Hilbert series encodes information
about the moduli space of vacua and hints at a new construction of (hyper)Ka¨hler
moduli spaces which is alternative to the standard (hyper)Ka¨hler quotient. In the
context of 3d N =4 gauge theories, the Hilbert series formula of [26] has spurred
recent activity both on the physical [24, 25, 23, 30, 31, 14, 67, 15, 44, 50, 16,
19, 49]andonthemathematicalfront[69, 68, 13, 12, 70, 61],leadingeventually
toamathematicaldefinitionoftheCoulombbranchandtoseveralotherinteresting
developments (see also Nakajima’s and Bullimore’s talk at this conference).
The novelty compared to four-dimensional theories is that three-dimensional
supersymmetric gauge theories contain chiral ’t Hooft monopole operators, a new
class of gauge invariant chiral operators which are not polynomials in the matter
fields. Monopoleoperatorsaresubjecttorelationsthatarisequantum-mechanically
andcannotbeobtainedbydifferentiatingasuperpotential. Itisdifficulttodirectly
determinetheserelationsforgeneraltheories,althoughimpressiveresultshavebeen
obtained by direct computation for simple theories using conformal field theory
techniques[11]. Nevertheless,wewillseethatphysicalargumentsleadtoageneral
group-theoreticformulafortheHilbertseriesthatcompletelybypassesthisproblem.
Oncethe Hilbertseriesiscomputed, exactlyorinaTaylorexpansion,wecanlearn
about the quantum relations among monopole operators by means of plethystic
methods, as was sketched in the four-dimensional examples described above.
5. ’t Hooft monopole operators
Before we can explain how to compute the Hilbert series of 3d N ≥ 2 su-
persymmetric gauge theories, we need to understand some of the properties of
supersymmetric ’t Hooft monopole operators.
In recent years it has been increasingly appreciated that local operators in a
quantum field theory need not be expressible as polynomials in the microscopic
fields that are used to write down the Lagrangian [59]. One also needs to include
“disorder” (or defect) operators, which may be defined by prescribing appropriate
singular boundary conditions in the path integral. While the definition of local
disorder operators appears to put them on a different footing from standard local
“order” operators, this difference is an artifact of our choice of field variables in