In string theory I came across the fact that there are difficulties in describin

Question

In string theory I came across the fact that there are difficulties in describing the coupling of R-R fields with world sheets in RNS formalism and it can be done in GS formalism only. Can someone explain the reason(s) behind that?

Also, I think if you have a space time where you can quantize strings and you have non zero R-R fields, then you need to use GS formalism to quantize strings there. It seems to me that these two are related and again my question is- why is that? Do we always need to use GS formalism there?

Explanation / Answer

In the RNS formalism the vertex operators for Ramond fields are not polynomials in the worldsheet fields. If you want to include them as part of the background, you'd have to deal with non-polynomial actions on the worldsheet. This is technically difficult.

One way to deal with this difficulty is to use a different method to quantize the worldsheet of the string, like the GS method or the Berkovits method. These are all equivalent, but some things that are difficult in one language are easier in another (and vice versa). In the quantization methods where spacetime SUSY is manifest on the worldsheet, RR fields are simpler, but seems to me many other things are more difficult, but maybe I am biased by my own education here.

For an example how such quantization works, take a look at this paper by Berkovits-Vafa-Witten.

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