SubmissionNumber#=%=#10 FinalPaperTitle#=%=#Are Large Language Models State-of-the-art Quality Estimators for Machine Translation of User-generated Content? ShortPaperTitle#=%=# NumberOfPages#=%=#11 CopyrightSigned#=%=#Shenbin Qian JobTitle#==# Organization#==# Abstract#==#This paper investigates whether large language models (LLMs) are state-of-the-art quality estimators for machine translation of user-generated content (UGC) that contains emotional expressions, without the use of reference translations. To achieve this, we employ an existing emotion-related dataset with human-annotated errors and calculate quality evaluation scores based on the Multi-dimensional Quality Metrics. We compare the accuracy of several LLMs with that of our fine-tuned baseline models, under in-context learning and parameter-efficient fine-tuning (PEFT) scenarios. We find that PEFT of LLMs leads to better performance in score prediction with human interpretable explanations than fine-tuned models. However, a manual analysis of LLM outputs reveals that they still have problems such as refusal to reply to a prompt and unstable output while evaluating machine translation of UGC. Author{1}{Firstname}#=%=#Shenbin Author{1}{Lastname}#=%=#Qian Author{1}{Username}#=%=#s.qian Author{1}{Email}#=%=#s.qian@surrey.ac.uk Author{1}{Affiliation}#=%=#University of Surrey Author{2}{Firstname}#=%=#Constantin Author{2}{Lastname}#=%=#Orasan Author{2}{Username}#=%=#c.orasan Author{2}{Email}#=%=#c.orasan@surrey.ac.uk Author{2}{Affiliation}#=%=#University of Surrey Author{3}{Firstname}#=%=#Diptesh Author{3}{Lastname}#=%=#Kanojia Author{3}{Username}#=%=#dipteshkanojia Author{3}{Email}#=%=#dipteshkanojia@gmail.com Author{3}{Affiliation}#=%=#University of Surrey Author{4}{Firstname}#=%=#Félix Author{4}{Lastname}#=%=#do Carmo Author{4}{Username}#=%=#felixdocarmo Author{4}{Email}#=%=#f.docarmo@surrey.ac.uk Author{4}{Affiliation}#=%=#University of Surrey ========== èéáğö