Additional Study Information: A randomized, placebo-controlled, investigator- and participant-blinded study to evaluate the efficacy, safety, tolerability, and pharmacokinetics of HSY244 in participants with AF. Participants will be randomized and assigned to one of the following 2 treatment arms in a ratio of 1:1. Arm 1: HSY244 intravenous infusion; Arm 2: Placebo intravenous infusion. The study consists of a screening period of up to 3 days and a treatment period of 4 days. After confirming eligibility on Day 1 and pre-dose assessments are completed, study administration will occur and participants will be monitored for cardioversion to sinus rhythm. During the treatment period, participants will be evaluated for efficacy, safety, tolerability, and pharmacokinetics.