1. A computer-implemented method for ranking documents, comprising: identifying a plurality of query-document pairs based on clickthrough data for a plurality of documents; building a latent semantic model based on the plurality of query-document pairs, wherein the plurality of query-document pairs comprises a plurality of query-title pairs, and wherein building the latent semantic model comprises building a bilingual topic model by using the plurality of query-title pairs to learn a semantic representation of a query based on a likelihood that the query is a semantics-based translation of each of the plurality of documents; and ranking the plurality of documents for a Web search based on the latent semantic model.