FAID: Fine-grained AI-generated Text Detection using Multi-task Auxiliary and Multi-level Contrastive Learning. - Minh Ngoc Ta; Dong Cao Van; Duc-Anh Hoang; Minh Le-Anh; Truong Nguyen; My Anh Tran Nguyen; Yuxia Wang; Preslav Nakov; Sang Dinh (CoRR, 2025)