Online Expansion of Rare Queries for Sponsored Search is a Yahoo paper which discusses how to expand rare queries for serving matching ads. The emphasis is on efficience. The key intuition is to find related "sentences" by hitting the search engine for the rare queries. The related sentences are extracted from search results snippets. High efficience is achieved by using an offline pre-processing step which aims at creating an offline index of related sentences.
The improvement in precision-recall is relevant, but i wonder why the authors are using human evaluators instead of a direct evaluation based on ads click rate for the expanded query set.
Another relevant result is distribution of rare queries
I wonder where the fresh queries are located here. I assume that "swine flu" was not so popular on the web until few months ago. Examples like "swine flu" are more common than one can imagine at first glance.