An Introduction to Higher-Spin Fields to Higherto Higher ... · An Introduction to Higher-Spin Fields An Introduction to Higherto Higher-Spin FieldsSpin Fields Augusto SAGNOTTI

An Introductionto Higher-Spin Fields

An IntroductionAn Introductionto Higherto Higher--Spin FieldsSpin Fields

Augusto SAGNOTTIAugusto SAGNOTTIScuola Normale Superiore, PisaScuola Normale Superiore, Pisa

Eotvos Superstring Workshop, Budapest, Sept. 2007Eotvos Superstring Workshop, Budapest, Sept. 2007

Some reviews:

N. Bouatta, G. Compere, A.S., hep-th/0609068 D. Francia and A.S., hep-th/0601199

A.S., Sezgin, Sundell, hep-th/0501156 X. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hep-th/0503128A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 2008 (?)

Some reviews:Some reviews:

N. Bouatta, G. Compere, A.S., hepN. Bouatta, G. Compere, A.S., hep--th/0609068 th/0609068 D. Francia and A.S., hepD. Francia and A.S., hep--th/0601199 th/0601199

A.S., Sezgin, Sundell, hepA.S., Sezgin, Sundell, hep--th/0501156 th/0501156 X. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hepX. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hep--th/0503128th/0503128A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 200A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 2008 (?) 8 (?)

Budapest, Sept. 2007Budapest, Sept. 2007 22

Some Motivations for HSSome Motivations for HSSome Motivations for HS

Key role in String Theory:Key role in String Theory:(Non) Planar duality of tree amplitudes(Non) Planar duality of tree amplitudes

Modular invariance and soft U.V.Modular invariance and soft U.V.OpenOpen--closed dualityclosed duality……………………………………

Key (old) problem inKey (old) problem in (classical)(classical) Field Theory:Field Theory:Only s=0,1/2,1,3/2,2Only s=0,1/2,1,3/2,2


For instance …For instance For instance ……

(Non(Non--) planar duality) planar duality rests on infinitely many poles rests on infinitely many poles [Actual t (or s) dependence implies a growing sequence of spins[Actual t (or s) dependence implies a growing sequence of spins] ] Similarly for Similarly for modular invariancemodular invariance::


What is “Spin” here ?What is What is ““SpinSpin”” here ?here ?

D=4 :D=4 :Up to dualities, all cases exhausted by fully symmetric (spinor)Up to dualities, all cases exhausted by fully symmetric (spinor)tensors:tensors:

D > 4 :D > 4 :Arbitrary Young tableaux : Arbitrary Young tableaux : ““spinspin”” somehow the number of columns. somehow the number of columns. Less developed, clumsy in general, many general lessons can be dLess developed, clumsy in general, many general lessons can be drawn rawn from the previous special set of fields.from the previous special set of fields.

(See X. Bekaert and N. Boulanger, hp(See X. Bekaert and N. Boulanger, hp--th/0606198 and seminar of A. Campoleoni)th/0606198 and seminar of A. Campoleoni)


Summary, ISummary, ISummary, I

Lecture I:Lecture I:Free HigherFree Higher--Spin fieldsSpin fields

FierzFierz--Pauli condition: the s=2 case in detailPauli condition: the s=2 case in detailSinghSingh--Hagen and Fronsdal formulations: Hagen and Fronsdal formulations: trace constraintstrace constraintsKaluzaKaluza--Klein massesKlein massesFangFang--Fronsdal formulation for Fermi fieldsFronsdal formulation for Fermi fields

Lecture II:Lecture II:Free HigherFree Higher--Spin fieldsSpin fields

Constrained vs unconstrained formulationsConstrained vs unconstrained formulationsNonNon--local bosonic formulation and Higherlocal bosonic formulation and Higher--Spin GeometrySpin GeometryNonNon--local fermionic constructionlocal fermionic construction


Summary, IISummary, IISummary, IILecture III:Lecture III:

Relation with String TheoryRelation with String TheoryBRST formulation for the bosonic stringBRST formulation for the bosonic stringLowLow--tension limit and tripletstension limit and triplets

Compensator equations for Higher SpinsCompensator equations for Higher SpinsCompensator equations and minimal LagrangiansCompensator equations and minimal LagrangiansAn alternative offAn alternative off--shell formulationshell formulation

Lecture IV:Lecture IV:External currentsExternal currents

Lessons for the nonLessons for the non--local formulationlocal formulationVDVZ discontinuityVDVZ discontinuity

The Vasiliev construction (and the compensator)The Vasiliev construction (and the compensator)Problems with higherProblems with higher--spin interactionsspin interactionsThe Vasiliev construction and the compensatorThe Vasiliev construction and the compensator


Lecture ILecture ILecture I

Free HigherFree Higher--Spin fieldsSpin fields

FierzFierz--Pauli condition: the s=2 case in detailPauli condition: the s=2 case in detailSinghSingh--Hagen and Fronsdal formulations, Hagen and Fronsdal formulations, trace constraintstrace constraintsKaluzaKaluza--Klein massesKlein massesFangFang--Fronsdal formulation for Fermi fieldsFronsdal formulation for Fermi fields


Fierz-Pauli conditions: BoseFierzFierz--Pauli conditions: BosePauli conditions: Bose

SpinSpin--s boson of mass m :s boson of mass m :

Correct degrees of freedom:Correct degrees of freedom:

Combine with trace: Only traceless spatial componentsCombine with trace: Combine with trace: OnlyOnly traceless spatial componentstraceless spatial components

(Fierz, Pauli, 1939)


Fierz-Pauli conditions: FermiFierzFierz--Pauli conditions: FermiPauli conditions: Fermi

SpinSpin--s fermion of mass m:s fermion of mass m:

Correct degrees of freedom, with:Correct degrees of freedom, with:

Combine with γ-trace: only γ-traceless spatial componentsCombine with Combine with γγ--trace: trace: only only γγ--tracelesstraceless spatial componentsspatial components

(Fierz, Pauli, 1939)


Massive case: low spinsMassive case: low spinsMassive case: low spinsSpin 1:Spin 1:

Gives Proca equation:Gives Proca equation:

For s=2 try (For s=2 try (ΦΦ traceless):traceless):

Does not give the correct Fierz-Pauli conditionsDoes not give the correct FierzDoes not give the correct Fierz--Pauli conditionsPauli conditions

(Singh, Hagen, 1974)


Massive case: spin 2Massive case: spin 2Massive case: spin 2

Can still take the divergence:Can still take the divergence:

Take Take αα=2 (eliminate ) =2 (eliminate ) If we also impose: If we also impose:

Add a Lagrange multiplier (with its kinetic and mass terms):Add a Lagrange multiplier (with its kinetic and mass terms):


Massive case: spin 2Massive case: spin 2Massive case: spin 2

2x2 homogeneous2x2 homogeneous system:system:

Determinant is algebraic if : Determinant is algebraic if :


Massless case: spin 2Massless case: spin 2Massless case: spin 2Not surprisingly:Not surprisingly: gauge symmetry as Mgauge symmetry as M 00

The equations become:The equations become:

Gauge invariant under: Gauge invariant under:

(Fronsdal, 1978)


Massless case: spin 2Massless case: spin 2Massless case: spin 2

In terms of a traceful spin 2:In terms of a traceful spin 2:

Fronsdal eq: Fronsdal eq:

Fronsdal action: Fronsdal action:

This gives:This gives:

(Einstein, actually, in this case !)(Einstein, actually, in this case !)(Einstein, actually, in this case !)


Kaluza-Klein massKaluzaKaluza--Klein massKlein mass

Spin 1: Spin 1:

Upon D+1 Upon D+1 D D reduction :reduction :

Stueckelberg symmetry:Stueckelberg symmetry:

Masses can be (re)generated by KMasses can be (re)generated by K--K reductions (complex notation):K reductions (complex notation):

For spin 2: For spin 2:


Fronsdal equation: spin 3Fronsdal equation: spin 3Fronsdal equation: spin 3

Not a Lagrangian equationNot a Lagrangian equation

Bianchi identity:Bianchi identity:

The Lagrangian actually yields:The Lagrangian actually yields:

(Fronsdal, 1978)


Implicit NotationImplicit NotationImplicit NotationFor all spins, one can eliminate all indicesFor all spins, one can eliminate all indicesNeed only some unfamiliar combinatoric rules Need only some unfamiliar combinatoric rules

(Francia, AS, 2002)


Fronsdal equations: spin sFronsdal equations: spin sFronsdal equations: spin s

33 ''2

ϕ− ∂

We have seen that gauge invariance requires:We have seen that gauge invariance requires:

We can also derive the We can also derive the Bianchi identityBianchi identity: :

(Fronsdal, 1978)


Fronsdal equations: spin sFronsdal equations: spin sFronsdal equations: spin s

Fronsdal construction:Fronsdal construction:

Constraints:Constraints:

Lagrangians: Lagrangians:


Massless case: spin 3/2Massless case: spin 3/2Massless case: spin 3/2RaritaRarita--Schwinger equation: familiar from supergravitySchwinger equation: familiar from supergravity

gauge invariant under:gauge invariant under:


Massless case: spin 5/2Massless case: spin 5/2Massless case: spin 5/2

Try to repeat for spinTry to repeat for spin--5/2:5/2:

NOT gauge invariant : NOT gauge invariant :

BUT: can again constrain the gauge parameter!BUT:BUT: can again constrain the gauge parameter!can again constrain the gauge parameter!

(Fradkin,1941; Fang,Fronsdal, 1978)


Massless case: spin s=n+1/2Massless case: spin s=n+1/2Massless case: spin s=n+1/2

Can extend to all Can extend to all ½½--integer spins:integer spins:


Additional constraint:Additional constraint:


Bianchi identity for spin s+1/2Bianchi identity for spin s+1/2Bianchi identity for spin s+1/2

Now derive the Bianchi identity:Now derive the Bianchi identity:


Kaluza-Klein mass: BoseKaluzaKaluza--Klein mass: BoseKlein mass: BoseCan extend the KCan extend the K--K construction to spinK construction to spin--s cases case(Stueckelberg gauge symmetries)(Stueckelberg gauge symmetries)

[ (s[ (s--3)3)-- parameter missing due to trace condition]parameter missing due to trace condition][(s[(s--4)4)--field missing due to double trace condition]field missing due to double trace condition]

Gauge fixing the Stueckelberg symmetries one is left with:Gauge fixing the Stueckelberg symmetries one is left with:

In terms of traceless tensors Singh-Hagen fieldsIn terms of traceless tensors In terms of traceless tensors SinghSingh--Hagen fieldsHagen fields


Kaluza-Klein mass: FermiKaluzaKaluza--Klein mass: FermiKlein mass: FermiCan again extend the KCan again extend the K--K construction to the spinK construction to the spin--(n+1/2) case : (n+1/2) case : (Stueckelberg gauge symmetries) (Stueckelberg gauge symmetries)

[ (s[ (s--2)2)-- parameter missing due to parameter missing due to γγ--trace condition] trace condition] [(s[(s--3)3)--field missing due to triple field missing due to triple γγ--trace condition] trace condition]

Gauge fixing the Stueckelberg symmetries one is left with:Gauge fixing the Stueckelberg symmetries one is left with:

In terms of γ-traceless tensors Singh-Hagen fieldsIn terms of In terms of γγ--traceless tensors traceless tensors SinghSingh--Hagen fieldsHagen fields


Lecture IILecture IILecture II

Free HigherFree Higher--Spin fieldsSpin fields

Constrained vs unconstrained formulationsConstrained vs unconstrained formulationsNonNon--local bosonic formulation and Higherlocal bosonic formulation and Higher--Spin GeometrySpin GeometryNonNon--local fermionic constructionlocal fermionic construction


Fronsdal equations, revisitedFronsdal equations, revisitedFronsdal equations, revisited(Fronsdal, 1978)

1 1 2 1 1

... 1 1 2 1 2 31

... ... ...

... ... ...

...

( . ...) ( ' ...) 0s s s s

s s ssF

μ μ

μ μ μ μ μ μ μ μ

μ μ μ μ μ μ μ μ μ

δϕ

ϕ ϕ ϕ−

= ∂ Λ + + ∂ Λ

≡ − ∂ ∂ + + ∂ ∂ + =

Originally from massive SinghOriginally from massive Singh--Hagen equationsHagen equations

(Singh and Hagen, 1974)

Unusual constraints:Unusual constraints: ' 0, '' 0ϕΛ = =

Gauge invariance for massless symmetric tensors:Gauge invariance for massless symmetric tensors:


Bianchi identitiesBianchi identitiesBianchi identities

Why the unusual constraints:Why the unusual constraints:1.1. Gauge variation of F Gauge variation of F

1 1 2 3 4... ...3 ( ' ... )s s

Fμ μ μ μ μ μ μδ = ∂ ∂ ∂ Λ +

2.2. Gauge invariance of the LagrangianGauge invariance of the Lagrangian•• As in the spinAs in the spin--2 case, F not integrable2 case, F not integrable

•• Bianchi identity:Bianchi identity:

( ) ( )2 3 42 2 3 5... ... ...1 ' ... 3 '' ...2 2s s s

F F μ μ μμ μ μ μμ μ μ ϕ∂⋅ − − ∂ ∂ ∂ +∂ + =


The Fronsdal actionsThe Fronsdal actionsThe Fronsdal actions

NaiveNaive equations:equations: 1...0 0

sF Rμ μ μν= ↔ =

1 1 2 3... ...1 10 ( ' ...)2 2s s

R R F Fμν μν μ μ μ μ μ μη η− = ↔ − +

Do not follow directly from a LagrangianDo not follow directly from a Lagrangian

But:But: when combined with traces they dowhen combined with traces they do


Constrained gauge invarianceConstrained gauge invarianceConstrained gauge invariance

( )1 1 1 2 3... ... ...1 ' ...2s s s

L F Fμ μ μ μ μ μ μ μδ δϕ η⎡ ⎤= − +⎢ ⎥⎣ ⎦

If in the variation of L one insertsIf in the variation of L one inserts 1 1 2... ... ...s sμ μ μ μ μδϕ = ∂ Λ +

( ) ( )2 2 2 3 2 3 4... ... ... ...

'Bianchi identity: "

1 1' ... ' ..2 2s s s s

L s F F Fμ μ μ μ μ μ μ μ μ μ μ

ϕ

δ η

Λ

⎡ ⎤⎢ ⎥⎢ ⎥= − Λ ∂ ⋅ − ∂ + − ∂ ⋅ +⎢ ⎥⎢ ⎥⎣ ⎦1444442444443 144424443

Are the constraints really necessary?Are the constraints really necessary?Are the constraints really necessary? (Francia, AS, 2002)


The spin-3 caseThe spinThe spin--3 case3 case

23 ' ' 3 'F Fμ ν ρμνρ μ ν ρ μ ν ρδ δ

∂ ∂ ∂⎛ ⎞= ∂ ∂ ∂ Λ → ∂ ⋅ = ∂ ∂ ∂ Λ⎜ ⎟

⎝ ⎠

A fully gauge invariant (nonA fully gauge invariant (non--local) equation:local) equation:

0Fμνρ =

Reduces to local Fronsdal form upon partial gauge fixingReduces to local Fronsdal form upon partial gauge fixing

2 0'FF μμν

νρ

ρ∂ ∂ ∂− ∂ ⋅ =


Spin 3: other non-local eqsSpin 3: other nonSpin 3: other non--local eqslocal eqs

Other equivalent forms:Other equivalent forms:

( )1 ' 0' '3

F F FF μ ν ρ ν ρ μ ρν μ νμ ρ − ∂ ∂ + ∂ ∂ + ∂ ∂ =

Lesson:Lesson: full gauge invariance with nonfull gauge invariance with non--local termslocal terms

( )1 03

FF F Fμ νρ νμ ρν μρ μ ρ ν− ∂ ∂ ⋅ + ∂ ∂ ⋅ + ∂ ∂ ⋅ =


Spin 3: non-local actionSpin 3: nonSpin 3: non--local actionlocal action

32 ' '1 ' ...

2F FG F Fμνρ μνρ μν ρ

μ ν ρ η⎡ ⎤⎛ ⎞≡ − +⎜ ⎟⎢ ⎥⎝ ⎠

∂ ∂ ∂ ∂− ∂ ⋅

⎦∂

⎣⋅ −

One can define an EinsteinOne can define an Einstein--like tensor:like tensor:

Now:Now: 0Gνρ∂ ⋅ ≡

Gauge invariance of L with NO constraintsGauge invariance of L with NO constraintsGauge invariance of L with NO constraints


Spin 3: non-local actionSpin 3: nonSpin 3: non--local actionlocal actionOne can simply associate to the previous nonOne can simply associate to the previous non--locallocalequation the nonequation the non--local actionlocal action

( ) ( ) ( ) ( )2

2

2 221 3 1 3' '2 2 2 2

3 ' 1 13 '

L μ αβγ βγ μ α

α α ϕ

ϕ ϕ ϕ

ϕ

ϕ

ϕϕ ϕ ϕ+

= − ∂ + ∂ ⋅ − ∂

∂ ⋅∂ ⋅∂ ⋅ ∂ ⋅ − ∂ ⋅

⋅ + ∂

+ ∂ ⋅∂ ⋅ ∂ ⋅∂ ⋅ ∂ ⋅∂ ⋅∂ ⋅

αβγ α βγ β γα γ αβδϕ = ∂ Λ + ∂ Λ + ∂ Λ

fullyfully invariant underinvariant under


Kinetic operators for integer spinKinetic operators for integer spinKinetic operators for integer spin

IndexIndex--free notation:free notation:

(1) 2 ' 0

( )!. .! !

p q p q

F

p qe gp q

ϕ ϕ ϕ

+

≡ − ∂ ∂ ⋅ + ∂ =

⎛ ⎞+∂ ∂ ≡ ∂⎜ ⎟

⎝ ⎠Now define:Now define:

2( 1) ( ) ( ) ( )1 1'

( 1)(2 1) 1n n n nF F F F

n n n+ ∂ ∂

= + − ∂ ⋅+ + +

2 1( ) [ ]

1(2 1)n

n nnF nδ

+

−

∂= + ΛThen:Then:



2(1)1 11 ( ) 0

( 1)(2 1) 1k

Fk k kξ ξ ξ

⎡ ⎤∂ ∂+ ∂ ⋅ ∂ − ∂ ⋅ ∂ Φ =⎢ ⎥+ + +⎣ ⎦

∏

•• is the generic kinetic operator for higher spinsis the generic kinetic operator for higher spins•• when combined with traces can be reduced towhen combined with traces can be reduced to

Defining:Defining:

3 ( 3 ')F H Hδ= ∂ = Λ

1

1 ...1( , ) ...!

s

sx

sμμ

μ μ

ξ

ξ ξ ξ ϕ

ξ

Φ =

∂∂ =

∂



•• Are gauge invariant for n > [(sAre gauge invariant for n > [(s--1)/2]1)/2]•• Satisfy the Bianchi identitiesSatisfy the Bianchi identities

2 1[ 1]

1( ) ( ) 11

21 '2

n nn

nnn

F Fn

ϕ+

+−∂ ⋅ − ∂ =

∂⎛ ⎞− +⎜ ⎟⎝ ⎠

•• For n> [(sFor n> [(s--1)/2] allow Einstein1)/2] allow Einstein--like operatorslike operators

The FThe F(n)(n)::

1( ) ( )[ ]

0

( 1) ( )!2 !

pnn p n p

pp

n pG Fn

η−

=

− −= ∑


Geometric equationsGeometric equationsGeometric equationsChristoffel connection:Christoffel connection:

hμν μ ν ν μδ ε ε= ∂ + ∂

Generalizes to ALL symmetric tensorsGeneralizes to ALL symmetric tensors(De Wit and Freedman, 1980)

1 2 1 1 1

1 2 1 2 1 1

; ... ; ...

; ... ; ...

s s

s sR R

μ ν ν μ μ ν ν

μ μ ν ν μ μ ν ν

−Γ ⇒ Γ

⇒

μ μνρ ν ρδ εΓ = ∂ ∂


ConnectionsConnectionsConnections

( )11

2 12 11

s ss s

ssss

δ ϕ−−

−⎛ ⎞−⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠−⎝ ⎠

∂ = ∂ Λ12314243

( )

0

1 ( 1)1

kmm m k k

k mmk

ϕ−

=

−Γ = ∂ ∇

+ ⎛ ⎞⎜ ⎟⎝ ⎠

∑In general:In general:

, :∂ ∇ Derivatives w.r.t. two sets of sym. indicesDerivatives w.r.t. two sets of sym. indices


ConnectionsConnectionsConnections•• In Einstein gravity:In Einstein gravity: metric (vielbein) postulatemetric (vielbein) postulate

0g g g gα αρ μν ρ μν ρμ αν ρν μα∇ ≡ ∂ −Γ −Γ =

•• For spin 3For spin 3 (linearized):(linearized):

; ; ;σ τ στμνρ νρ μ ρμ ν μν ρστ στφ∂ ∂ = Γ + Γ + Γ

•• Linearizing:Linearizing: g hμν μν μνη→ +

; ;hρ ρμν ν μ νρμ∂ = Γ + Γ


Geometric equationsGeometric equationsGeometric equations

1.1. Odd spins (s=2n+1):Odd spins (s=2n+1):

2.2. Even spins (s=2n):Even spins (s=2n):

0F μνμ∂ = 1[ ] ; ...1 0sn v v

n R μμ∂ =

0Rμν = 1[ ] ; ...1

1 0sn v vn R− =


FermionsFermionsFermions

(S i≡ ∂ψ ψ−∂ 2) 0 2S iδ ε= → = − ∂

(Fang and Fronsdal, 1978)


1 1'2 2

S S∂⋅ − ∂ − ∂ S 2i ψ= ∂ '

Constraints:Constraints: ψ ' 0, ε= 0=


FermionsFermionsFermions

Notice: 12

12s

S⎛ ⎞+⎜ ⎟⎝ ⎠

∂− ∂ S

12

s

i⎛ ⎞+⎜ ⎟⎝ ⎠

∂=

( )sF

Example: spin 3/2 (Rarita-Schwinger)

12

S μμ

∂− ∂ S i ∂

= vν

μν μη ψ⎡ ⎤−∂ ∂⎣ ⎦

0μνρν ργ ψ∂ = S iμ ≡ ∂ μ μψ ψ− ∂( ) 0=

(see Francia(see Francia’’s seminar)s seminar)


FermionsFermionsFermionsOne can again iterate:One can again iterate:

The relation to bosons generalizes to:The relation to bosons generalizes to:

Bianchi identities:Bianchi identities:

Gauge transformations: Gauge transformations:


Lecture IIILecture IIILecture III

Relation with String TheoryRelation with String Theory

BRST formulation for the bosonic stringBRST formulation for the bosonic stringLowLow--tension limit and tripletstension limit and triplets

Compensator equations for Higher SpinsCompensator equations for Higher SpinsCompensator equations and minimal LagrangiansCompensator equations and minimal LagrangiansAn alternative offAn alternative off--shell formulationshell formulation


Bosonic string: BRSTBosonic string: BRSTBosonic string: BRSTThe starting point is the Virasoro algebra: The starting point is the Virasoro algebra:

In the tensionless limit, one is left with:In the tensionless limit, one is left with:

Virasoro contracts (no c. charge):Virasoro contracts (no c. charge):


Low-tension limitLowLow--tension limittension limitSimilar simplifications for BRST charge:Similar simplifications for BRST charge:

Making zeroMaking zero--modes manifest: modes manifest:


String Field equationString Field equationString Field equationHigherHigher--spin massive modes:spin massive modes:

•• massless for 1/massless for 1/αα’’ 00•• Free dynamics can be encoded in:Free dynamics can be encoded in:

20 ( 0)Q Q

Q

ψ

δ ψ

= =

= Λ

(Kato and Ogawa, 1982)(Kato and Ogawa, 1982)(Witten, 1985)(Witten, 1985)(Neveu, West et al, 1985)(Neveu, West et al, 1985)

NONO trace constraints on j or Ltrace constraints on j or L


Symmetric tripletsSymmetric tripletsSymmetric triplets(A. Bengtsson, 1986)(A. Bengtsson, 1986)(Henneaux, Teitelboim, 1987)(Henneaux, Teitelboim, 1987)(Pashnev, Tsulaia, 1998)(Pashnev, Tsulaia, 1998)(Francia, AS, 2002)(Francia, AS, 2002)Emerge from Emerge from

The triplets are:The triplets are:


Symmetric tripletsSymmetric tripletsSymmetric triplets

Can also eliminate C:Can also eliminate C:

Gauge theories ofGauge theories of

Physical state conditions:Physical state conditions:Propagate spins s,sPropagate spins s,s--2, 2, ……, 0 or 1, 0 or 1


(A)dS symmetric triplets(A)dS symmetric triplets(A)dS symmetric tripletsCan build directly, deforming flatCan build directly, deforming flat--space triplets, or via BRSTspace triplets, or via BRST

Directly:Directly: insist on relation between C and othersinsist on relation between C and othersBRST: BRST: gauge nongauge non--linear constraint algebralinear constraint algebra

Basic commutator: Basic commutator:


(A)dS symmetric triplets(A)dS symmetric triplets(A)dS symmetric tripletsCan also deform directly the equations without C:Can also deform directly the equations without C:

It is convenient to define It is convenient to define

Bianchi identity and first equation then become:Bianchi identity and first equation then become:


(A)dS symmetric triplets(A)dS symmetric triplets(A)dS symmetric tripletsThe deformed equations can be derived from The deformed equations can be derived from

Alternatively: Alternatively: modify momenta in contracted Virasoromodify momenta in contracted Virasorogauge algebra gauge algebra non linearnon linear (but M,N diagonal on triplets)(but M,N diagonal on triplets)


Compensator EquationsCompensator EquationsCompensator Equations

In the triplet:In the triplet:

spinspin--(s(s--3) 3) compensatorcompensator::

The second becomes:The second becomes:

The first becomes:The first becomes:

Combining them:Combining them:

Finally (also from Bianchi):Finally (also from Bianchi):



Summarizing: Summarizing:

Describe a spin-s gauge field with:NO trace constraints on the gauge parameterNO trace constraints on the gauge fieldFirst can be reduced to minimal non-local form

BUT:NOT Lagrangian equations

Describe a spin-s gauge field with:NONO trace constraints on the gauge parameterNONO trace constraints on the gauge fieldFirst can be reduced to minimal non-local form

BUT:BUT:NOTNOT Lagrangian equations


(A)dS Compensator Eqs(A)dS Compensator Eqs(A)dS Compensator EqsFlatFlat--space compensator equations can be extended to (A)dS:space compensator equations can be extended to (A)dS:

Gauge invariant underGauge invariant under

The first can be turned into the second via (A)dS BianchiThe first can be turned into the second via (A)dS Bianchi


Compensator EquationsCompensator EquationsCompensator EquationsIt is possible to obtain a Lagrangian form of the compensatorIt is possible to obtain a Lagrangian form of the compensator

equations, using BRST techniquesequations, using BRST techniquesEssentially in Pashnev and Tsulaia (1997)Essentially in Pashnev and Tsulaia (1997)Formulation involves number of fields ~ sFormulation involves number of fields ~ sInteresting BRST subtletiesInteresting BRST subtletiesHere we discuss explicitly spin s=3Here we discuss explicitly spin s=3

Fields:Fields:



Gauge transformations:Gauge transformations:

Field equations:Field equations:

Gauge fixing:Gauge fixing:

Other fields: zero by field equations Other fields: zero by field equations


Fermionic Triplets Fermionic Triplets Fermionic Triplets (Francia and AS, 2002)(Francia and AS, 2002)

Counterparts of bosonic tripletsCounterparts of bosonic triplets

GSO:GSO: not in 10D susy stringsnot in 10D susy strings

Yes:Yes: mixed sym generalizationsmixed sym generalizations

Enter directly Enter directly typetype--0 models0 models

Propagate s=n+1/2 and all lower ½-integer spinsPropagate s=n+1/2 and allall lower ½-integer spins


Fermionic Compensators Fermionic Compensators Fermionic Compensators

Recall:Recall:

SpinSpin--(s(s--2) compensator:2) compensator:

Gauge transformations:Gauge transformations:

First compensator equation second via BianchiFirst compensator equation second via Bianchi


Fermionic Compensators Fermionic Compensators Fermionic Compensators We We couldcould extend the fermionic compensator eqs to (A)dS backgrounds extend the fermionic compensator eqs to (A)dS backgrounds [We [We could notcould not extend the fermionic triplets] extend the fermionic triplets]

First compensator equation First compensator equation second via (A)dS Bianchi identity:second via (A)dS Bianchi identity:


Off-Shell truncation of tripletsOffOff--Shell truncation of tripletsShell truncation of triplets( B( Buchbinduchbinder, er, KrykhtinKrykhtin, , ReshetnyakReshetnyak 2007 )2007 )

•• start from a start from a triplettriplet (s,s(s,s--2,2,……) )

•• add add two (gauge invariant) Lagrange two (gauge invariant) Lagrange multipliersmultipliers

•• Lagrangian : Lagrangian :

λ λ and and μμ : set to zero by the field equations: set to zero by the field equations

OffOff--shell reduction of triplets :shell reduction of triplets :


Minimal local LagrangiansMinimal local LagrangiansMinimal local Lagrangians

““MinimalMinimal”” local Lagrangians with local Lagrangians with unconstrained unconstrained gauge symmetry:gauge symmetry:

The Lagrangians are:The Lagrangians are:

(Francia, AS, 2005; Francia, Mourad and AS, 2007)

Can be nicely extended to Can be nicely extended to (A)dS backgrounds(A)dS backgrounds


Lecture IVLecture IVLecture IV

External currentsExternal currents

Lessons for the nonLessons for the non--local formulationlocal formulationVDVZ discontinuityVDVZ discontinuity

The Vasiliev construction (and the compensator)The Vasiliev construction (and the compensator)Problems with higherProblems with higher--spin interactionsspin interactionsThe Vasiliev constructions and freeThe Vasiliev constructions and free--differential algebrasdifferential algebrasStrong vs weak projection: recovery of the compensatorStrong vs weak projection: recovery of the compensator


Minimal local LagrangiansMinimal local LagrangiansMinimal local Lagrangians

““MinimalMinimal”” local Lagrangians with local Lagrangians with unconstrained unconstrained gauge symmetry:gauge symmetry:

The Lagrangians are:The Lagrangians are:

(Francia, AS, 2005; Francia, Mourad and AS, 2007)

Can be nicely extended to Can be nicely extended to (A)dS backgrounds(A)dS backgrounds


External currentsExternal currentsExternal currents

•• Residues of current exchanges reflect the Residues of current exchanges reflect the degrees of freedomdegrees of freedom

•• For s=1 : For s=1 :

•• For all s : For all s :


External currents : local caseExternal currents : local caseExternal currents : local case

K K ““doubly tracelessdoubly traceless”” using double trace constraintusing double trace constraintB: determines multiplier B: determines multiplier β β for double trace constraintfor double trace constraint

The exchange involves, correctly, a The exchange involves, correctly, a traceless conserved currenttraceless conserved current


External currents : non-local caseExternal currents : nonExternal currents : non--local caselocal caseHow about the nonHow about the non--local version of the theory? local version of the theory?

Apparently:Apparently: different choices for the field equation, EQUIVALENT without cudifferent choices for the field equation, EQUIVALENT without currentsrrents

S=3 :S=3 :

Bianchi identityBianchi identity: changes after every iteration: changes after every iteration


Naively: Naively:

Solution:Solution: modify the nonmodify the non--local Lagrangian equationlocal Lagrangian equation

External currents : non-local caseExternal currents : nonExternal currents : non--local caselocal case

For instance : For instance :

Incorrect current exchange ! Incorrect current exchange !


VD-V-Z Discontinuity for HSVDVD--VV--Z Discontinuity for HSZ Discontinuity for HS(van Dam, Veltman; Zakharov, 1970)(van Dam, Veltman; Zakharov, 1970)

For all s and D, m=0 : For all s and D, m=0 :

VDVZ VDVZ discontinuitydiscontinuity follows in general comparing D and (D+1) massless exchanges follows in general comparing D and (D+1) massless exchanges First present for s=2 via DFirst present for s=2 via D--dependence of dependence of ρρ(D,s) (D,s) For all s:For all s: can describe irreducibly a massive field acan describe irreducibly a massive field a’’ la Scherkla Scherk--Schwarz from (D+1) dimensions : Schwarz from (D+1) dimensions :

[ e.g. for s=2 : [ e.g. for s=2 : hhMNMN (h(hμνμν cos(my) , Acos(my) , Aμμ sin(my), sin(my), ϕϕ cos(my) )cos(my) ) ] ]

(A)dS extension, first discussed, for s=2, by Higuchi and Porrat(A)dS extension, first discussed, for s=2, by Higuchi and Porrati i Discontinuity Discontinuity smooth interpolation in (msmooth interpolation in (mLL) ) 22


HS InteractionsHS InteractionsHS Interactions

Problems with interacting higher spins :Problems with interacting higher spins :

Inconsistent equations (derivatives imply further conditions)Inconsistent equations (derivatives imply further conditions)Coupling with gravity leaves Coupling with gravity leaves ““nakednaked”” Weyl tensors Weyl tensors (Aragone, Deser,1979)

Coleman Coleman -- MandulaMandula

(Berends, Burgers, van Dam, 1982)(Bengtsson2, Brink, 1983)

(Fradkin and Vasiliev, 1980’s)

(Vasiliev, 1990, 2003)(Sezgin, Sundell, 2001)

Way out:Way out:

Infinitely many interacting fields Infinitely many interacting fields NonNon--vanishing vanishing ““cosmological constantcosmological constant”” ΛΛVasiliev equationsVasiliev equations:: paradigmatic exampleparadigmatic example


Aragone-Deser problemAragoneAragone--Deser problemDeser problem

Flat space Lagrangians: usually gauge invariant as a result ofFlat space Lagrangians: usually gauge invariant as a result ofcancellations between terms that differ by commutatorscancellations between terms that differ by commutators

Moving to curved backgrounds introduces in general nakedMoving to curved backgrounds introduces in general nakedRiemann tensorsRiemann tensors

Miracle of supergravity: such terms combine into Einstein Miracle of supergravity: such terms combine into Einstein tensors, which allows cancellationstensors, which allows cancellations

Independent higherIndependent higher--spin propagation possible in spin propagation possible in conformally flat conformally flat backgroundsbackgrounds

(A)dS: (A)dS: massmass--like termslike terms, but , but ““freefree””. . Central role in Vasiliev equationsCentral role in Vasiliev equations

(Aragone, Deser, 1979)(Aragone, Deser, 1979)


HS InteractionsHS InteractionsHS InteractionsVasilievVasiliev’’ s setting:s setting:

1.1. Extend Extend the frame formulation of gravity the frame formulation of gravity ::

For spinFor spin--ss: :

2.2. ∞∞ -- dim. HSdim. HS--algebra via algebra via oscillatorsoscillators (coordinates and momenta):(coordinates and momenta):


HS InteractionsHS InteractionsHS Interactions3.3. [Weyl ordered (symmetric) polynomials in (x,p) or [Weyl ordered (symmetric) polynomials in (x,p) or ∗∗-- products ] products ]

4.4. A oneA one--form Aform A in adjoint of HS algebra :in adjoint of HS algebra :(all (all ωω’’s : HS vielbeins and connections) s : HS vielbeins and connections)

5.5. A zeroA zero--form form ΦΦ in the in the ““twisted adjointtwisted adjoint””::

One writes the spinOne writes the spin--2 equation in the form: 2 equation in the form: ““ Riemann = Weyl Riemann = Weyl ””whose trace gives the familiar equation whose trace gives the familiar equation ““ Ricci=0Ricci=0””““ Weyl Weyl ”” (+ derivatives) :(+ derivatives) : fields with Youngfields with Young--tableau structure tableau structure

Scalar : ϕ


HS InteractionsHS InteractionsHS InteractionsTWO forms of Vasilev equations :TWO forms of Vasilev equations :

1.1. 44--dim spinors dim spinors (2(2--component formalism) component formalism)

2.2. DD--dim vectorsdim vectors

Let us try to see what these equations mean Let us try to see what these equations mean (focusing on the case with vector oscillators)(focusing on the case with vector oscillators)



Background independent (nonBackground independent (non--Lagrangian) !Lagrangian) !A:A: s fields for every (even) rank s s fields for every (even) rank s ((““adjointadjoint”” of HS algebra) of HS algebra)

(Generalized vielbeins and connections) (Generalized vielbeins and connections) [Chan[Chan--Paton extension to all (even and odd) ranks] Paton extension to all (even and odd) ranks]

ΦΦ:: ∞∞ fields for every rank s fields for every rank s ((““twisted adjointtwisted adjoint”” of HS algebra)of HS algebra)(Generalized Weyl and their covariant derivatives) (Generalized Weyl and their covariant derivatives)

ΦΦ∗∗κκ:: converts converts ““twisted adjointtwisted adjoint”” ΦΦ to adjointto adjointConsistent (almost by inspection) : Bianchi for F implies seconConsistent (almost by inspection) : Bianchi for F implies second eq ! d eq !



Gauge field A in (x,z) space:Gauge field A in (x,z) space:

Internal equations:Internal equations: power series in power series in ΦΦ by successive iterations by successive iterations



Lowest order in Lowest order in ΦΦ ““Riemann = WeylRiemann = Weyl”” + + ……

with

““Internal Internal ”” equations: equations:



k is a crucial ingredient :k is a crucial ingredient :

Linearized Linearized Φ Φ –– equation : equation :

Unfolding :Unfolding :

Uniform description of HS interactions Uniform description of HS interactions



Cartan Integrable System :Cartan Integrable System :

e.g. Cherne.g. Chern--Simons theory Simons theory manifestly consistent eqsmanifestly consistent eqsgauge covariancegauge covariancemanifest diff covariancemanifest diff covariancenonnon--LagrangianLagrangian

NEW INGREDIENT :NEW INGREDIENT : 00--form form ΦΦ

(Sullivan, 1977; D’Auria, Fre’, 1982)



Some missing ingredientsSome missing ingredients : : Y Y iAiA, Z , Z iAiA to build HS algebra extending SO(2,D) to build HS algebra extending SO(2,D) must select Sp(2,R) singletsmust select Sp(2,R) singletsK K i j i j : Sp(2,R) generators (bilinears in Y, Z) : Sp(2,R) generators (bilinears in Y, Z)

REMOVE TRACES to obtain dynamical equationsREMOVE TRACES to obtain dynamical equations

Weak projection :Weak projection : remove traces symmetrically from A and remove traces symmetrically from A and ΦΦ (Vasiliev, 2003)(Vasiliev, 2003)

Strong :Strong : leave traces in A leave traces in A (AS,Sezgin,Sundell, 2004)(AS,Sezgin,Sundell, 2004)


HS InteractionsHS InteractionsHS InteractionsFree flat limit (w. Strong projection) : Free flat limit (w. Strong projection) :

s = 2 : s = 2 :

s = 3 :s = 3 :a first equationa first equation, analogous of the vielbein postulate giving , analogous of the vielbein postulate giving ωω(e)(e)a second equationa second equation, defining a second, defining a second--order kinetic operator order kinetic operator

a third equationa third equation giving the constraint giving the constraint



(Dubois-Violette, Henneaux, 1999)

(AS, Sezgin, Sundell, 2004)


ConclusionsConclusionsConclusionsWhy Higher Spins?Why Higher Spins?

Field TheoryField TheoryString Theory (an instance with String Theory (an instance with ““spontaneous breakingspontaneous breaking””))

In these lectures:In these lectures:Free (unconstrained) HigherFree (unconstrained) Higher--Spin fieldsSpin fieldsRelation with VasilievRelation with Vasiliev’’ constructionconstruction

Some reviews:

N. Bouatta, G. Compere, A.S., hep-th/0609068 D. Francia and A.S., hep-th/0601199

A.S., Sezgin, Sundell, hep-th/0501156 X. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hep-th/0503128A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 2008 (?)

Some reviews:Some reviews:

N. Bouatta, G. Compere, A.S., hepN. Bouatta, G. Compere, A.S., hep--th/0609068 th/0609068 D. Francia and A.S., hepD. Francia and A.S., hep--th/0601199 th/0601199

A.S., Sezgin, Sundell, hepA.S., Sezgin, Sundell, hep--th/0501156 th/0501156 X. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hepX. Bekaert, S. Cnockaert, C. Iazeolla, M.A. Vasiliev, hep--th/0503128th/0503128A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 200A.S., P. Sundell, D. Sorokin, M.A. Vasiliev, Phys. Reports, 2008 (?) 8 (?)

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