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In lecture notes on string theory by David Tong (available for free here http://www.damtp.cam.ac.uk/user/tong/string.html) a brief explanation of the vertex operator for tachyon is given. The main points are:

  1. Because it has to be diffeomorphism-invariant, it is a result of an integration of some quantity over the world-sheet.

  2. Because it is conformal-invariant, the quantity has to correspond to the primary operator with weight $(+1, +1)$ in order to compensate the $d^2z$ measure

  3. Because tachyon is the vacuum of the string, it has to correspond to the highest-weight state of the Verma module. This is fulfilled if

$$ L_n \left| \text{tachyon} \right> = \tilde{L}_n \left| \text{tachyon} \right> = 0, \quad n > 0; $$ $$ L_0 \left| \text{tachyon} \right> = \tilde{L}_0 \left| \text{tachyon} \right> = a \left| \text{tachyon} \right>. $$

Based on these assumptions he identifies the vertex operator with

$$ V = \int d^2 \sigma \: \cdot \, : e^{i k \cdot X } :, $$

where $:...:$ denotes normal-ordering, radial-ordering is implicit and $k$ is on-shell with negative (tachyonic) $m^2$.

$V$ is conformal-invariant. As far as I understand, this can be showed on the quantum level by evaluating the $VT$ OPE (or, equivalently, by evaluating the residues of $VL_n$, where $L_n$ are the Virasoro generators).

I am trying to understand why this is a valid choice. Specifically, it would be great to make sense of this in terms of path integrals of functionals.

It is well known that observables of a quantum theory are expectation values (path integrals weighted by the classical action) of gauge-invariant functionals. These expectations can be evaluated using the Faddeev-Popov procedure which fixes the gauge and ensures that expectations weighted by the new action are correct, but only if the functional of interest is gauge-invariant.

This point of view makes a lot of sense in QFT.

Different ways to fix the gauge exist. These give rise to different actions and (in general) different expectation values. Since the choice of gauge-fixing is arbitrary, only gauge-invariant expectations (which coincide) make sense as physical observables.

Now back to the tachyon vertex operator. I would expect it to correspond to some (classical) gauge-invariant functional over $X^{\mu}$ and the world-sheet metric $g_{\alpha \beta}$.

Such a functional has to be an integral with a diffeomorphism-invariant measure:

$$ \sim \int d^2 \sigma \sqrt{g}. $$

Then there are Weyl transformation. In order to be Weyl-invariant, this integral has to have an inverse world-sheet metric which cancels the square root of the determinant in two dimensions:

$$ \sim \int d^2 \sigma \sqrt{g} g^{\alpha \beta} $$

The last formula contains loose indices which (again, diffeomorphism-invariance) have to be contracted with something. For example,

$$ \sim \int d^2 \sigma \sqrt{g} g^{\alpha \beta} \partial_{\alpha} X^{\mu} \partial_{\beta} X^{\nu} e^{i p \cdot X }, $$

which looks much like the graviton insertion operator.

On the contrary, it can be shown that the tachyon vertex operator is conformal-invariant in some weird quantum sense, by taking its residue with the $L_n$ conformal generator (or just with $T$).

But what about diffeomorphism- and Weyl- invariance, separately? $L_n$s don't generate them. One could expect that, for example, products of $V$ (which correspond to tachyon scattering amplitudes) have different expectations when evaluated on different world-sheet backgrounds related by diffeomorphisms and/or Weyl transformations. Because, again, Faddeev-Popov method gives different expectations of a non-gauge-invariant functional when different gauge-fixings are used.

My question is: what happened to $\sqrt{g}$ and $g^{\alpha \beta}$ in the tachyon vertex operator; why is it considered diffeomorphism- and Weyl- invariant? And is it even possible to represent the Virasoro-Shapiro amplitude, for example, in terms of the expectation of some classical diffeo- and Weyl- invariant functional?

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  • $\begingroup$ As a lazy answer to your first question, not going back to my string theory textbook, I think the factors of $\sqrt{g}$ and $g^{\alpha\beta}$ are missing because he's working in the conformal gauge. In this gauge the metric on the worldsheet is the familiar $ds^2 = dz d\bar{z}$. In a more general gauge they would be present. $\endgroup$ – Surgical Commander Feb 7 '15 at 23:33
  • $\begingroup$ @lurscher, please explain yourself. $\endgroup$ – Prof. Legolasov Feb 10 '15 at 8:47
  • $\begingroup$ @SurgicalCommander I see. But your proposition is based on the fact that there is an diffeo- and Weyl- invariant generalization of the insertion operator. Could you provide one? $\endgroup$ – Prof. Legolasov Feb 10 '15 at 8:51
  • $\begingroup$ @lurscher I was told many times and have seen in different papers that diffeomorphisms are gauge symmetries of the bosonic string. What do you mean by 'not a gauge group of GR'? Why GR, and why are they not even a gauge group of GR? $\endgroup$ – Prof. Legolasov Feb 10 '15 at 8:53
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    $\begingroup$ @Hindsight, here is a pretty good exposition of the argument: mth.kcl.ac.uk/~streater/lostcauses.html#XXII $\endgroup$ – lurscher Feb 10 '15 at 15:23

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