Title: AUGMENTING ZERO-SHOT DENSE RETRIEVERS WITH PLUG-IN MIXTURE-OF-MEMORIES. Abstract: In this paper we improve the zero-shot generalization ability of language models via Mixture-Of-Memory Augmentation (MoMA), a mechanism that retrieves augmentation documents from multiple information corpora (“external memories”), with the option to “plug in” new memory at inference time. We develop a joint learning mechanism that trains the augmentation component with latent labels derived from the end retrieval task, paired with hard negatives from the memory mixture. We instantiate the model in a zero-shot dense retrieval setting by augmenting a strong T5-based retriever with MoMA. Our model, MoMA-DR, obtains strong zero-shot retrieval accuracy on the eighteen tasks included in the standard BEIR benchmark. It outperforms other dense retrieval models of similar scales and achieves comparable accuracy with systems that seek generalization from increased scales in encoder models or vector indices. Our analysis illustrates the necessity of augmenting with mixture-of-memory for robust generalization, the benefits of joint learning, and how MoMA-DR utilizes the plug-in memory at inference time without changing its parameters. We plan to open source our code.