  Compared to standard retrieval tasks, passage retrieval for conversational
question answering (CQA) poses new challenges in understanding the current user
question, as each question needs to be interpreted within the dialogue context.
Moreover, it can be expensive to re-train well-established retrievers such as
search engines that are originally developed for non-conversational queries. To
facilitate their use, we develop a query rewriting model CONQRR that rewrites a
conversational question in the context into a standalone question. It is
trained with a novel reward function to directly optimize towards retrieval
using reinforcement learning and can be adapted to any off-the-shelf retriever.
CONQRR achieves state-of-the-art results on a recent open-domain CQA dataset
containing conversations from three different sources, and is effective for two
different off-the-shelf retrievers. Our extensive analysis also shows the
robustness of CONQRR to out-of-domain dialogues as well as to zero query
rewriting supervision.
