Guest editorial: special issue on ECIR 2021.

Compared to their ECIR 2021 paper (Salle et al., [5]), which received the ECIR 2021 industry impact award, the special issue paper includes among other things additional experimental results for facets, and additional work on learning from negative user feedback. The paper extends the ECIR 2021 paper (Ferreira et al., [2]) among other things by a new user-study that confirms the need for the conversational search paradigm, and an evaluation of the performance of answer generation. The 43rd European Conference on Information Retrieval, ECIR 2021, was supposed to take place as an in-person conference in Lucca, Italy. [Extracted from the article]