News topic detection and categorization
I'm not an expert in this field but you should take a look at the work of Bhargav Srinivasa Desikan, a gensim contributor, who works a lot with topic modelling.

He has a couple of notebooks on his github account which could interest you, especially this one (should be pretty much your use case if I understand your problem correctly).

The aforementioned notebook evaluates Latent Semantic Indexing, the Hierarchical Dirichlet Process as well as the Latent Dirichlet Allocation for identifying topics.