AUEB at BioASQ 7 - Document and Snippet Retrieval.

We present the submissions of AUEB to the BIOASQ 7 document and snippet retrieval tasks (parts of Task 7b, Phase A). Our systems build upon the methods we used in BIOASQ 6. This year we also experimented with models that jointly learn to retrieve documents and snippets, as opposed to using separate pipelined models for document and snippet retrieval. We also experimented with models based on BERT [5]. Our systems obtained the best document and snippet retrieval results for all batches of the challenge that we participated in.