The n-Category Café

My friend Chenchang Zhu, a mathematician, has also been working on these things with two physicists. The abstract only briefly mentions 2-groups, but they play a fundamental role in the paper:

• Chenchang Zhu, Tian Lan and Xiao-Gang Wen, Topological non-linear $\sigma$-model, higher gauge theory, and a realization of all (3+1)d topological orders for boson systems.

Abstract. A discrete non-linear $\sigma$-model is obtained by triangulate both the space-time $M^{d+1}$ and the target space $K$. If the path integral is given by the sum of all the complex homomorphisms $\phi \colon M^{d+1} \to K$, with an partition function that is independent of space-time triangulation, then the corresponding non-linear $\sigma$-model will be called a topological non-linear $\sigma$-model which is exactly soluble. Those exactly soluble models suggest that phase transitions induced by fluctuations with no topological defects (i.e. fluctuations described by homomorphisms $\phi$) usually produce a topologically ordered state and are topological phase transitions, while phase transitions induced by fluctuations with all the topological defects give rise to trivial product states and are not topological phase transitions. If $K$ is a space with only non-trivial first homotopy group $G$ which is finite, those topological non-linear $\sigma$-models can realize all $(3+1)d$ bosonic topological orders without emergent fermions, which are described by Dijkgraaf-Witten theory with gauge group $\pi_1(K)=G$. Here, we show that the $(3+1)d$ bosonic topological orders with emergent fermions can be realized by topological non-linear σ-models with $\pi_1(K) =$ finite groups, $\pi_2(K)=\mathbb{Z}_2$, and $\pi_{n > 2}(K)=0$. A subset of those topological non-linear $\sigma$-models corresponds to 2-gauge theories, which realize and classify bosonic topological orders with emergent fermions that have no emergent Majorana zero modes at triple string intersections. The classification of $(3+1)$d bosonic topological orders may correspond to a classification of unitary fully dualizable fully extended topological quantum field theories in 4-dimensions.

The cobordism hypothesis, too, is getting into the act in the last sentence!