Black Hole Entropy in the O(N) Model
D. Kabat, S. H. Shenker, M. J. Strassler
TL;DR
The entropy arises from diagrams which are analogous to those introduced by Susskind and Uglum to explain black hole entropy in string theory, and the interpretation of the {sigma}-model entropy depends on scale.
Abstract
We consider corrections to the entropy of a black hole from an $O(N)$ invariant linear $\s$-model. We obtain the entropy from a $1/N$ expansion of the partition function on a cone. The entropy arises from diagrams which are analogous to those introduced by Susskind and Uglum to explain black hole entropy in string theory. The interpretation of the \sm entropy depends on scale. At short distances, it has a state counting interpretation, as the entropy of entanglement of the $N$ fields $\pa$. In the infrared, the effective theory has a single composite field $\s \sim \pa \pa$, and the state counting interpretation of the entropy is lost.