Index saddle for the D1-D5-P black string and its decoupling limit
Abstract
Boruch, Emparan, Iliesiu, and Murthy recently discussed index saddles for 5d black strings, showing that the black string saddle admits a decoupling limit to a complex, finite-temperature BTZ \times S^2 saddle that computes the index of the dual CFT. In this paper, we pursue an analogous construction for the D1-D5-P black string. We construct a four-charge index saddle in the four-dimensional STU model as the BPS limit of the non-extremal four-charge black hole, and show that it exhibits the new form of attraction. We then uplift it successively to five and six dimensions, via the 4D-5D connection and a chain of string dualities, to obtain the gravitational index saddle for the D1-D5-P black string. We take a systematic decoupling limit of this index saddle and obtain the BTZ \times S^3 saddle that computes the index of the D1-D5 CFT.
Versions
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BibTeX
@misc{nanda2026index,
title = {{Index saddle for the D1-D5-P black string and its decoupling limit}},
author = {Kanhu Kishore Nanda and P Shanmugapriya and Amitabh Virmani},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2606.27093},
eprint = {2606.27093},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.27093},
}
RIS
TY - UNPB
TI - Index saddle for the D1-D5-P black string and its decoupling limit
AU - Kanhu Kishore Nanda
AU - P Shanmugapriya
AU - Amitabh Virmani
PY - 2026
DA - 2026/08/26
PB - arXiv
DO - 10.48550/arxiv.2606.27093
UR - https://arxiv.org/abs/2606.27093
AB - Boruch, Emparan, Iliesiu, and Murthy recently discussed index saddles for 5d black strings, showing that the black string saddle admits a decoupling limit to a complex, finite-temperature BTZ \times S^2 saddle that computes the index of the dual CFT. In this paper, we pursue an analogous construction for the D1-D5-P black string. We construct a four-charge index saddle in the four-dimensional STU model as the BPS limit of the non-extremal four-charge black hole, and show that it exhibits the new form of attraction. We then uplift it successively to five and six dimensions, via the 4D-5D connection and a chain of string dualities, to obtain the gravitational index saddle for the D1-D5-P black string. We take a systematic decoupling limit of this index saddle and obtain the BTZ \times S^3 saddle that computes the index of the D1-D5 CFT.
ER -