Medium Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 10
Distraction; Workload
Conflict NMAC
Flight Crew Took Evasive Action
Human Factors; Weather
Aircraft X checked on; climbed westbound off Atlanta airport. I climbed him to FL230. As I was switching communications; the pilot said that he had a near miss and that he had to pull up the nose to avoid an unknown aircraft at 11;500 in Atlanta Approach airspace. I did not notice the VFR aircraft when Aircraft X was near it; but I did notice the 1200 code after he mentioned the near miss. I was busy with unusual operations; high volume; and weather deviations may have been a factor.I need to keep a better scan going and more overall awareness. Also; the Approach Controller needs to separate the departures from all known traffic before they switches communication.
ZTL Controller reports of an air carrier that reported a near miss. Controller was busy with other duties and did not observe traffic until after the report.