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Anna Phan | USLHC | USA

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Detector monitoring…

Greetings from the LHCb detector control room everybody! For the past few days, I’ve been waking up very early in the morning and cycling here to do my part in keeping the LHCb detector running and recording as much data as possible.

It’s probably been mentioned by other people in previous posts, but the LHC particle physics detectors[*] are constantly monitored, 24 hours a day, 365 days a year[**]. There are various levels of detector and data monitoring: the first level consists of people in the various detector control rooms, called online shifts, the second level consists of people on call, called expert shifts, and the third level consists of people doing remote monitoring of data quality and reconstruction, called offline shifts[***].

Each experiment requires a different number of people at each monitoring level, depending on what is deemed necessary. For example, LHCb has 2 people on shift in the control room here at P8. I believe CMS has 5 people in theirs at P5 while ATLAS has 12 over at P1. These online shifts are 8 hours each, the morning one running from 7am to 3pm, the evening one running from 3pm to 11pm and the night one running from 11pm to 7am. These people are in charge of making sure that the detectors are running smoothly, event selections go as planned and data is getting read out properly and there are no obvious problems.

Online at LHCb, we know we’re doing a good job if our detector efficiency is high. We want to record as many interesting collision events as possible during stable beam periods for our physics analyses. Time lost by detector problems is data lost. Above is a nice pie chart of the detector performance for the year during stable beams. I say nice, as we are approximately 90% efficient; we have been able to record around 90% of the luminosity which LHC has delivered to us this year. Of course it would be better if we were at 100%, but this is not really possible with the time required to get the detector from standby into ready (ramping up the HV on the subdetectors and moving the VELO into position). The other two slices of the pie, related to subdetector and readout issues, we try very hard to reduce.

If things don’t look good in the control room, and we can’t figure out why, we call the people on expert shift to diagnose the problem. Expert shifts usually last a week, and require carrying a phone and being within approximately half an hour of the control room. They also need to attend the daily morning run meetings where the detector plan for the day is laid out. When there are stable beams, the optimal plan is obviously to take as much data as possible, but sometimes stable beams are needed for subdetector scans which are important for understanding how the beam radiation is affecting the electronics. When there aren’t stable beams, then the plans can include subdetector calibration or firmware and software upgrades.

Oooh! The LHC is injecting beam now, I better get back to work[****]!

—————————————-
[*] I apologise for not mentioning ALICE at all, but I don’t know anybody from that collaboration well enough to ask random questions like how their shifts are run.

[**] Okay that was a bit of an exaggeration, the detectors aren’t monitored 365 days a year, they are monitored according to the LHC schedule. For example, we don’t have shifts over the winter shutdown period and the number of shifts is reduced during machine development and technical stop periods.

[***] I’m generalising here, each of the experiments actually calls each of their shifts different names. Random fact: LHCb calls their expert shifts, piquet shifts. As far as I can tell, this is the French word for stake or picket, but I haven’t been able to figure out why this is the word used to describe these shifts.

[****] Guess I should mention that I’m on data manager shift at the moment, so my job is to check the quality of the data we are recording.

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