 Hagedorn temperature

If a density of states has high-energy growth proportional to a power of energy times $e^{{\beta }_{H}E}$, its canonical Boltzmann sum fails for $\beta <{\beta }_H$. The Hagedorn temperature is the corresponding threshold. The open bosonic string mass spectrum and large-level degeneracy of an open bosonic string give ${\beta }_H=4\pi \sqrt{{\alpha }^{\prime }}$ in units with Boltzmann's constant, the speed of light and Planck's constant set to one. Polynomial prefactors affect behavior at the threshold itself; the growth alone does not distinguish a limiting temperature from a phase transition.

The Hagedorn temperature is a concept in theoretical physics, particularly in the context of string theory and quantum statistical mechanics. It refers to a specific temperature above which a system of particles (or strings) exhibits a phase transition. At or above this temperature, the number of states (or configurations) of the system grows exponentially, leading to a system that behaves in a fundamentally different way from low-temperature scenarios.