Date: 201202
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Cessna Citation Mustang (C510)
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Confusion; Communication Breakdown
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: A C510 checked on my frequency descending out of approximately FL290 descending to FL260 from Sector 91. The C510 was landing nearby which required me to get him down to a lower altitude (9;000) and sooner due to the proximity of the airport. [I was] descending the aircraft before he leveled off; which required me to point him out to Sector X; so I promptly completed that coordination and descended the C510 to FL220; the top of Approach airspace. I believe I only said 'descending' and did not specify an altitude because I intended to coordinate with Approach Control to descend the aircraft even lower into their airspace. I called the Departure Sector at Approach and pointed out the aircraft descending to 170 using the beacon code. I thought the Controller approved the point out and I did not hear him reference any traffic. I found out after the fact that he did in fact reference a B737 but all I heard was the similar sounding beacon code. I gave my initials and hung up without saying 'Traffic observed' because I did not hear him reference the traffic. After that; I descended the C510 down to 170 and put an interim altitude in the Data Block. Sector X subsequently noticed the aircraft descending below FL220. Their departure; the B737; was climbing to FL210. As soon as he noticed the two aircraft in conflict we tried to save the deal but it was too late and our actions to correct the situation before minimum separation was lost did not work. I climbed the C510 back up to FL210 and then to expeditiously to FL230 and he tried descending the B737. I think this is a great example of hearing what you want to hear and not what was actually said. In addition; I think this event reinforced the importance of using standard phraseology. While I take full responsibility for not hearing the Approach Controller reference the traffic; me not saying 'traffic observed' because I really didn't observe it had the potential to draw attention to the fact that I was not aware of the traffic being referenced. In addition when I made the initial point out to Sector X; I could have said the altitude that I actually intended to descend the aircraft to instead of simply saying 'descending.' That would have likely caused him to reference the B737 as well.




Synopsis: An Enroute Controller described a loss of separation event listing read back hear back and phraseology non-conformance issues as causal factors.