This was done in a paper by E. Witten: http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.cmp/1104202513

The paper was written before the arxiv came to be, so unfortunately it is not on the arxiv.

Another paper by Witten may be relevant here: http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.2933v4.pdf but I haven't studied it in detail.

This was done in a paper by E. Witten: Link

The paper was written before the arxiv came to be, so unfortunately it is not on the arxiv.

Another paper by Witten may be relevant here: http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.2933v4.pdf but I haven't studied it in detail.