B737-700
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 10
Human-Machine Interface; Situational Awareness; Confusion
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew FLC complied w / Automation / Advisory
Human Factors; Airspace Structure
I had only been on position for about 1 minute. Aircraft X was already cleared to descend via the arrival. There was no known traffic on my display. I knew nothing was about to happen until suddenly Aircraft X said he was responding to a TCAS alert. At the same moment he transmitted this to me Aircraft Y flashed Conflict Alert on my display. I immediately issued Aircraft X a 40 degree left turn away from Aircraft Y then issued a traffic alert. I watched waiting for the next update to process. I never have had an incident this close and at one point was convinced they would collide. I can only recommend that you possibly look into who was controlling the adjacent sector. I am not sure what the complete story was however the separation responsibility would rest solely with that individual. The adjacent sector Controller is the only person that would have seen these two aircraft the entire time. That Controller would also be the individual who issued the descent clearance that ultimately nearly ended in a mid air collision.
ZLA Center Controller reported a NMAC between two air carriers.