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  Some questions about the paper, "AdS description of induced higher spin gauge theory"

+ 4 like - 0 dislike

I am referring to this paper.

I guess that in this paper one is trying to relate the massless spin $s$ gauge fields in $AdS_4$ to conformal spin $s$ theory on $S^3$.

  • So am I right that the operator $K$ that has been defined here in $2.8$ is something in the boundary? How does one derive the explicit expression for $K$ as given in $2.12$?

    Is it solely through this particular choice of $K$ in section $2.12$ that one is implementing in section $3$ the fact that the spin-$s$ theory on the boundary is conformal?

  • In section $3$ they seem to be solely focussed on symmetric traceless rank $s$ tensors (to represent spin-$s$ on the boundary $S^3$). But why is this enough? I would think that the spin-s fields to be considered are the fields on $S^3$ which lie in those representation of $SO(4)$ which when restricted to $S0(3)$ become its highest weight $s$ representation and these are not just symmetric and traceless but also have to be transverse and also satisfy some harmonic wave equation. What about these two conditions? (This was the definition of spin-$s$ as was discussed here.)

    But when considering spin-$s$ fields on the bulk $AdS$ in equation $5.1$ the condition of transversality and the wave-equation condition seem to be back!

    I basically don't understand equations $3.1$ and $3.6$. It would be great if someone could help explain these two.

  • Is there a value of $m^2$ (in equation 5.1) at which this spin-s field on $AdS$ will be conformally coupled? (...in this paper they are focussed at the massless case ($m^2 =0$) which I would think is not necessarily conformal..)

  • With reference to the discussion below equation 5.6,

    When the bulk spin-$s$ field is massless, there are two possible dimensions of the boundary spin-$s$ current, $J_{(s)}$ - at the UV fixed point it has dimensions, $\Delta_{-} = 2-s $ and at the IR fixed point it has dimensions, $\Delta_{+} = s+d-2$

    Here two things are not being very clear to me,

    (1) How does one see the claim that at the IR fixed point the value of $\Delta_{+}$ somehow implies that now $J_{(s)}$ is a conserved current and hence the spin-s field in the boundary is now a gauge field?

    (2) Is it also being claimed that at the UV fixed point the value of $\Delta_{-}$ is precisely the same as the dimension of a spin-$s$ gauge field? What theory is this? How do we understand this? I can't wrap my head around the fact that this $J_{s}$ which I thought of as the conserved current spin-s current till now happens to have the same dimension as a gauge field!?

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user user6818
asked Sep 13, 2013 in Theoretical Physics by user6818 (960 points) [ no revision ]
I will answer one by one. K is indeed defined in 2.8. as a two-point fuction of a spin-s operator - it knows nothing about bulk, purely boundary CFT object. All two point functions are fixed by conformal symmetry, so 2.12 is the unique expression in particular coordinates. As they say, they perturb a CFT with J^2 and then flow to a fixed-point, which meand they are again in CFT

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
I have to warn you that the questions you asked are complicated and suprising answers to some of them have been obtained quite recently. So I am not sure if the scope of the forum allows one to answer them comprehensively.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
@John: Wait, what are you saying? Do you mean to say that this is non-mainstream? It is fine (And good) to bring up modern mainstream research in questions.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user Dimensio1n0
I have in mind a question on the relation between gauge invariace and being conformall. The paper I refer to is just 5 years old. The rest of the questions have pretty standard answers.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John

1 Answer

+ 5 like - 0 dislike

On the last question, I am not sure how good you are at the representation theory, but the following fact is true: take so(d,2) (we need so(3,2) for this work), use the conformal base, i.e. Lorentz generators $L_{ab}$, translations $P_a$, conformal boosts $K_a$ and dilatation $D$, $a,b=1..d$. $P$ and $K$ behave as raising/lowering generators with respect to $D$, $[D,P]=+P$, $[D,K]=-K$. Take the vacuum to carry a spin-s representation of the Lorentz algebra and a weight $\Delta$ with respect to $D$, i.e. $|\Delta\rangle^{a_1...a_s}$. When $\Delta=d+s-2$, there is a singular vector, $P_m|\Delta\rangle^{ma_2...a_s}$. This is a standard representation theory: finding raising/lowering operators, defining vacuum, looking for singular vectors. Actually, singular vectors are exactly the conformally-invariant equations one can impose.

On the field language this means that $\partial_m J^{m a_2...a_s}=0$ is a conformally invariant equation iff the conformal dimension of $J$ is $\Delta=d+s-2$. Despite the fact that $J^{a_1...a_s}$ is a good conformal operator for any value of the conformal dimension, only for $d+s-2$ its divergence decouples. (Perhaps you have seen $L_{-2}+\alpha L_{-1}^2$ as a singular vector in the Virasoro algebra, now it is replaced with $P_m$ or $\partial_m$).

Now, having $J^{a_1..a_s}$ of weight $\Delta$ we can consider its contragradient representation or on the field language couple it via $\int \phi_{a_1..a_s}J^{a_1...a_s}$ to some other field $\phi$. That we need a conformally invariant coupling implies $\Delta_\phi=d-\Delta_J=s-2$. Not surprisingly something special must happen for $\Delta_J=d+s-2$.

$$\int (\phi_{a_1...a_s}+\partial_{a_1}\xi_{a_2...a_s})J^{a_1...a_s}=\int \phi J-\int \phi_{a_1...a_s}\partial_m J^{ma_2..a_s}=\int \phi J$$ we see that a statement that is dual to the conservation of $J$ is the gauge invariance of $\phi$.

I have not read the paper yet, but as far as I can see they play with the dimension of $J$ and for $d+s-2$ and $2-s$ it describes a conserved tensor and a gauge field just because of representation theory of the conformal group (decoupling of certain null states). At any given moment of time in the paper $J$ has some fixed dimension and is either a conserved tensor, a gauge field or just a spin-s conformal field of generic dimension $\Delta$.

On the last but one, you are right in that gauge invariance has a little to do with conformallity. The answer is spin and dimension dependent. For $s=0$ there is $m^2$ for which the scalar is conformal. For $s=1$ and certain $m^2$ the Maxwell field is a gauge field but the Maxwell equation is conformal in $d=4$ only. Beyond $d=4$ a gauge spin-one field is not conformal, or a spin-one conformal field is not a gauge field. For $s\geq2$ the situation is even more tricky: in $AdS_4$ the gauge fields are conformal, but in Minkowski space they are not conformal (in terms of gauge potentials $\phi_{\mu_1...\mu_s}$). You may have a look at http://arxiv.org/abs/0707.1085

On the second, first of all the transversality is on the right place in 5.1. Secondly, your confusion (inspired by my answer to another question) is that there are two different classes of fields people are interested in. First is the class of usual particles, where we talk about representations of the Poincare algebra $iso(d-1,1)$ if we are in $d$-dimensional Minkowski space or $so(d-1,2)$ and $so(d,1)$ if we are in anti de Sitter ot de Sitter (there we need harmonicity, tracelessness, transversality). Conformal fields are in the second class. Conformal means that it must be a representation of the conformal group $so(d,2)$ for Minkowski-$d$, note that $iso(d-1,1)\in so(d,2)$. The conformal group of anti de Sitter-$d$ is also $so(d,2)$. Note that the symmetry algebra of AdS-$(d+1)$ is exactly the conformal group of Minkowski-$d$. So when we talk about conformal fields we are interested in reprsetations of $so(d,2)$ (the signature can vary depending on the problem, it is some real form of $so(d+2)$). I would like to stress that conformal fields in d-dimensions are in one-to-one correspondence with usual fields in $AdS_{d+1}$, for the algebra is the same, which is at the core of AdS/CFT correspondence.

For example, a spin-$0$ field in Minkowski space obeys $\square \phi=0$. It gives rise to an irreducible representaion of $iso(d-1,1)$. Coincidentally, the same representation turns out to be an irreducible representation of a bigger algebra, $so(d,2)$, the conformal algebra. It is a coincidence. There exists also a spin-$0$ conformal field of weight $\Delta$, say $\phi_\Delta(x)$. Without imposing any equations it is an irreducible representation of $so(d,2)$. As a representation of its subalgebra $iso(d-1,1)$ it decomposes into an intergral of representations (Fourier) and is highly reducible. There is a special weight $\Delta=(d-2)/2$ for which $\phi_\Delta(x)$ is reducible and the decoupling of null states is achived via $\square \phi=0$ (analogous to the conservation of $J$ above). Note that $J$ above is an irreducible representation of $so(d,2)$ but it is highly reducible under $iso(d-1,1)$. For special weight $d+s-2$ we have to impose the conservation condition in order to project out the null states, but again the conserved tensor is an irreducible of $so(d,2)$ and reducible under $iso(d-1,1)$. So your confusion is because the fields are conformal, these are representations of a bigger algebra, they are more 'fat' and require less equations (even no at all) to project onto an irreducible.

$S^3$ is the analog of Minkowski-$3$ (compactified and Euclidian), then $so(4)$ is the analog of $iso(3,1)$ and they are interested in normalizable functions, these are the spherical harmonics or polynomials depending on coordinates. Then they discuss labelling of these representations using $so(4)\sim su(2)\oplus su(2)$ and proceed to doing some integrals.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
answered Sep 13, 2013 by JohnS (180 points) [ no revision ]
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Thanks to your questions I am motivated enough to read the two papers and then can answer.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
Thanks! May be I will put up this issue of conformal higher-spins as a separate question. BTW, would you know an answer to this representation theory question that I asked here, math.stackexchange.com/questions/479217/…

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user user6818
The answer does not fit as a comment. To be short, as I said conformal fields in $d$ are reps of $so(d,2)$. These can be interpreted as reps of its subalgebra $so(d-1,2)$, which is the symmetry algebra of $AdS_{d}$. That is: conformal field in $d$ equals a number of $AdS_d$ fields. In the case of a $4d$ conformal spin-$s$ field the decomposition gives a number of partially-massless fields in $AdS_4$. This leads to technical simplifications as one can factor the order $2s$ operator into two-derivative operators of partially-massless fields and then apply what people know about $\det \Delta+a$

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
All fields in $AdS_4$ decomposition of a spin-$s$ conformal field are normal fields (not conformal), sometimes they are even massive. But whole set in the decomposition has the action of conformal group on it. As far as I can see this is a technical trick to reduce a problem to something known. Footnote 23 is vague, one of the partially-massless fields is known to be conformal itself, but I see no application of this fact in the paper.They all come together, there is no point to single out this field alone. Recall, that this decomposition was just a technical trick.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
Actually, I did not check explicitly that the decomposition I mentioned holds, could be that this is just a technical trick in a sense of counting of degrees of freedom.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
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Lastly, there are two very different classes of fields: conformal fields and just fields (plane waves basically at free level). Which fields are in the game is specified when we define a theory. There are conformal theories and there are just theories.

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user John
Thanks for the help! Let me look through your references. I find it kind of curious that Arkady's partition function 3.25 seems to have nothing to do with the Fierz-Pauli equations as in equation 5.1 on page 25 of this paper, arxiv.org/pdf/1306.5242v2.pdf - I would have though that there would be a value of "m" in this 5.1 that would tune the spin-s fields to conformality - but it seems that was never the case! right? Arkady's 3.18 has any relation to Klebaniv's 5.1?

This post imported from StackExchange Physics at 2014-03-07 13:47 (UCT), posted by SE-user user6818

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