Information retrieval (IR) is a fundamental task in many real-world applications such as Web search, question answering systems, and digital libraries. The core of IR is to identify information resources relevant to user's information need. Since there might be more than one relevant resource, the returned result is often organized as a ranked list of documents according to their relevance degree against the information need. The ranking property of IR makes it different from other tasks, and researchers have devoted substantial efforts to develop a variety of ranking models in IR. In...