One effective old technique for ranking is to capture the search-to-click relationship by real users. It's basically the training data by human mapping the search terms they entered to the links they clicked. With just a few of clicks, the ranking relevance goes way up.
May be feeding the data into a neural net would help ranking. It becomes a classification problem - given these terms, which links have higher probabilities being clicked. More people clicking on a link for a term would strengthening the weights.