[magicalhippo]

I found this[1] talk about LHCb results and future direction illuminating. He explains the trigger setup during the first few minutes, later on he explains how they're searching for new physics.

For run 2 of LHC they used 50000 CPU cores for their software triggers, after the hardware trigger has reduced the 40 MHz input rate down to 1 MHz. The final output of the software triggers is 12.5 kHz, which is persisted to disk. Keep in mind this is just for the LHCb detector.

For run 3, they're planning to remove the hardware trigger bit, running the software triggers directly from the 40 Mhz signal. This would allow them to reprogram the triggers during the run, in case some new interesting theory comes along which for some reason has a signal their current trigger won't identify.

[1]: http://pirsa.org/16010060

[walrus1066]

Cool stuff.

The computation side of the LHC is really impressive. For a full software trigger, you have 25 nanoseconds in which to load all the raw collision data, reconstruct 100's of particle tracks, calculate their momentum, join them up to figure out their decay vertices etc etc, and then, decide whether to store the event.

I recall LHCb could afford higher trigger-rates than CMS/Atlas, an LHCb event is smaller (~100 kB vs 1 MB for CMS) because the detector only covers 300 milli-radians from the collision-axis, in one direction, whereas CMS/Atlas have full coverage.