ZLA center controller reported a near midair collision between two air carriers.

Attributes
ACN	1619776
Time
Date	201902
Local Time Of Day	1801-2400
Place
Locale Reference	ZLA.ARTCC
State Reference	CA
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR RNAV
Flight Plan	IFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 10
Events
Anomaly	ATC Issue All Types Conflict NMAC

Narrative:

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.

Original NASA ASRS Text

Title: ZLA Center Controller reported a NMAC between two air carriers.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.