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Beyond Hagedorn: A Harmonic Approach to Deformation
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 -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 deformation. The spectral decomposition provides a numerically stable and efficient method to compute the partition function at finite values of the deformation parameter , allowing us to clearly resolve the analytic structure of the partition function as a function of . 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 .
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References (48)
The partition function depends on both and its complex conjugate . Following the mathematics literature, we write instead of . This convention applies to all modular functions throughout the Letter.
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This is also consistent with Ref. [21]. We believe there is an error in the final result of Eq. (4.8). After correcting this error, the large-energy tail with integrand (while the final result in [21] corresponds to the integral of ) is finite but non-analytic at and divergent for , consistent with our result in this work.
Note that even though there are infinitely many branch points in the range , they are not dense. The location of branch points can be written as where , are coprime integers, and it is then easy to show that there are finitely many small than for any .
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