In fact-based information retrieval, state-of-the-art performance is traditionally achieved by knowledge graphs driven by knowledge bases, as they can represent facts about and capture relationships between entities very well. However, in domains such as medical information retrieval, where addressing specific information needs of complex queries may require understanding query intent by capturing novel associations between potentially latent concepts, these systems can fall short. In this work, we develop a novel, completely unsupervised, neural language model-based ranking approach for semantic tagging of documents, using the document to be tagged as a query into the model to retrieve candidate phrases from top-ranked related documents, thus associating every document with novel related concepts extracted from the text. For this we extend the word embedding-based generalized language model (GLM) due to (Ganguly et al., 2015), to employ phrasal embeddings, and use the semantic tags thus obtained for downstream query expansion, both directly and in feedback loop settings. Our method, evaluated using the TREC 2016 clinical decision support challenge dataset, shows statistically significant improvement not only over various baselines that use standard MeSH terms and UMLS concepts for query expansion, but also over baselines using human expert-assigned concept tags for the queries, on top of a standard Okapi BM25-based document retrieval system.
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