Date: 201902
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: Regional Jet 900 (CRJ900)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: A320
Reporter organization: Government
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Time Pressure; Situational Awareness; Human-Machine Interface; Distraction; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Human Factors; Staffing
Primary problem: Human Factors

Narrative: First; some background information. My area is currently at a staffing level that affects safety on a daily basis. We consistently do not have enough people to safely staff a shift but are forced to work around it and do our jobs as best we can. Most of us; myself included; are working 6-day workweeks on a regular basis. This week for me was no exception. My first shift of the week; [Day 1]; was short staffed. [Day 2] was even worse. I made two requests for earlier shifts on [Day 2] in order to get more rest on my quick turn; among other reasons; but both were denied. During one rotation; I went over 2 hours on position. I was worn out and burned out. [Day 2] evening I went home and my child was ill. [Day 2] night; I put in a request for one hour of leave for [Day 3] morning in order to help care for my sick child. I didn't sleep well and kept checking web schedule overnight to see if it had been approved.It was addressed around [late]; at which point I was already leaving for work. The request was denied and I was frustrated. Lately it is difficult to maintain family life with the inability to change shifts or get leave. This was just one more example on a heap of hundreds of examples in the last year. I arrived to work and sat on position for about 1 hour and 55 minutes my first round; which did not help to curb my frustrations at the situation. When I got to work; we had two supervisors and both were plugged in. They were using one of our controllers to staff CIC (Controller in Charge). All of this to say that during the time of the event I was distracted by a year's worth of staffing issues; being tired; and my child being sick. I took sector XX at XA:07. I knew when I took the position there were several items needing addressed right away: I had around 7 departures to transition; ZZZ spacing to achieve; a slow TBM needing routing and a climb; a slow ZZZ1 arrival underneath a faster ZZZ2 arrival that needed swapped out; etc. However I didn't have time to address any of this because literally 1 second after I gave my initials; I was told via landline call from [another] sector that they were handing me off a medical emergency aircraft; Aircraft X returning to [Departure airport] and descending to 28;000 feet. The aircraft was in a terrible spot; right on top of a slew of departures. I got control from the sector. At this point; I had trouble coming up with a solid plan whereas usually I would be able to easily make a plan. I attribute this to the aforementioned background influences leading up to this event. Aircraft X checked on and requested a turn for [the Airport]. Because I didn't have a better plan yet; I issued a westbound heading of 250 so the aircraft at least wouldn't be going in the complete opposite direction of [the airport]. I descended the aircraft to 24;000 feet and began coordinating with approach control. My supervisor was aware of the emergency (I believe another sector controller told him). He was standing next to the sector from when I took the handoff on the emergency; until after the event took place; to my knowledge. Aircraft X had an aircraft below him; Aircraft Y; a departure climbing to 21;000 feet. My priority was to get the emergency aircraft lower so I attempted to stop another aircraft; Aircraft Z; at 18;000 feet so that when separation was achieved between Aircraft X and Aircraft Y via the Aircraft X's heading; I could safely descend Aircraft X to 19;000 feet. However; I didn't have communication on Aircraft Z so I had to coordinate yet again and change the plan. I descended Aircraft X to 22;000 feet (he read it back correctly); and handed him off to approach control with their control reference Aircraft Y underneath. I switched communication at XA:11; 4 minutes after taking the position and the handoff on the aircraft. I didn't get a chance to put the new assigned altitude in the data block and by the time I realized it; approach control had the handoff and my workload was too heavy.While this was all going on; the amount of workload for the sector had gotten to a crazy level and I remember wondering why the Supervisor didn't give me an assist Controller to do all the coordinating I was doing; but I didn't have a moment to stop and ask. Especially because while I was trying to keep airplanes separated and manage the sector; the Supervisor was in my other ear continually asking me questions and trying to get me to get more information about the emergency from the pilot. I truly felt like I didn't have a second to spare; and I got the information I could and then switched the Aircraft X to approach control.The Supervisor had been bugging me to ask if the passenger was a male or a female and at that point; I just couldn't handle any more distractions so I switched the aircraft thinking approach control could get more information when there was time; whereas I didn't have the time. I was frustrated at the Supervisor for adding a distraction of persistent questioning when really I needed help; but I tried to push past this and continue to do my job. Just before the event; the complexity of the sector involved: multiple aircraft checking on and stepping on each other; a ZZZ1 departure in confliction with ZZZ1 arrivals; a ZZZ1 arrival in confliction with an ZZZ2 arrival and another faster ZZZ1 arrival behind; and other aircraft needing attention. I remember seeing the Aircraft X data block reflecting the aircraft being at 22;000 feet; the assigned altitude. At XA:11:58 the AIRCRAFT X showed a hit of 21300 feet. I saw this hit a few seconds later. At this time the AIRCRAFT X was 1.78 miles and 300 feet from the Aircraft Y according to replay. At the time just before this hit; I had 4-6 aircraft checking on at once and a phone call that I was attempting to sort out. My brain when it saw the first hit did not comprehend what it was showing. I irrationally thought maybe it was a bad hit or maybe approach control was using visual separation (yes I know this is not possible at this altitude; but as I said; my initial thoughts were irrational). While my brain was trying to process what I was seeing; I switched an aircraft to another frequency; kind of on auto-pilot. Around 5 seconds after I noticed the first hit; the second hit showed the Aircraft X at 20;600 feet and that's when it hit me that the aircraft was actually descending. On this hit; the aircraft were .72 miles and 400 feet according to Falcon. It bothers me that it took me those 5 seconds to process the event. I should have called traffic immediately to the Aircraft Y. By the time I had processed; it was too late and the Aircraft X was already showing 400 feet below the Aircraft Y and in a rapid descent. I called approach control to ask what the AIRCRAFT X was doing and the controller said the Aircraft X said he had been assigned 20;000 feet; which was not the case according to the replay review. At the time of the event; I was looking at approximately 20 data blocks on my scope. After the event took place; The Supervisor who had witnessed the event; asked if I wanted to be pulled off position. This was well-intentioned but yet another distraction. I was too busy to give the sector to another controller for about 10 more minutes; when I was more caught up and could give a position relief briefing. The Supervisor  reviewed the replay with me afterwards. He said they would be asking him why there was no Assist Controller and he wanted me to agree with him that the sector had not been busy and that I hadn't needed one until the event and by that time it was too late; or something to that effect. I couldn't agree with him; but I don't know if he would have gotten one there in time had I had the chance to ask for one; as the whole event occurred in less than 5 minutes after taking the position.We need better staffing. We need more overtime budget allowed because every day we have less staffing than we can safely manage in our area and are told thatmore controllers will not be called in on overtime. We need supervisors that can recognize a bad situation and staff the sectors appropriately. We need less time on position so we are not constantly burned out and fatigued. As for me; I need to recognize better when I'm not as rested as I could be. I need to take the time to ask for an Assist when things get critical. I need to remember to issue a safety alert. I need to not let distractions get in the way of doing my job. I am haunted by this event and will take many lessons from it. I don't know or understand why TCAS didn't go off for the Aircraft Y. That is supposed to be our fail-safe. I don't know if it went off for the Aircraft X. I don't remember if Conflict Alert activated.




Synopsis: Center Controller reported an aircraft descended through its assigned altitude which resulted in an NMAC.