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Beyond Hagedorn: A Harmonic Approach to TT¯ Deformation

Jie Gu1,*, Jue Hou1,3,†, and Yunfeng Jiang1,2,‡

  • *Contact author: jie-gu@seu.edu.cn
  • †Contact author: jue.1.hou@kcl.ac.uk
  • ‡Contact author: jinagyf2008@seu.edu.cn

Phys. Rev. Lett. 137, 081603 – Published 21 August, 2026

DOI: https://doi.org/10.1103/kcs1-7q6n

Abstract

We apply harmonic analysis to study the TT¯-deformed torus partition function. We first express the CFT partition functions in terms of Maass waveforms, including the Eisenstein series and cusp forms. These basis functions turn out to deform in a very simple way under the TT¯ deformation. The spectral decomposition provides a numerically stable and efficient method to compute the partition function at finite values of the deformation parameter λ, allowing us to clearly resolve the analytic structure of the partition function as a function of λ. The resulting deformed partition function exhibits a Hagedorn singularity. Building on the harmonic analysis approach, we propose a natural analytic continuation beyond the Hagedorn singularity, which enables us to compute the full partition function for any value of λ.

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References (48)

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