Clarify HLT2 line rate guidelines
In the past (slide 7) we've advised tuning HLT2 line selections to meet the 'signal rate' of the signal process, but this advice can be too coarse to be useful.
There are some cases when one might tune to different criteria:
- If the signal rate is very low, below 1 Hz, then tuning to O(1 Hz) can still be reasonable. (We expect the total HLT2 output rate to be O(10 kHz), up to 100 kHz, so can afford lots of O(1 Hz) lines).
- This type of hand-waving is not obvious to a lot of folks!
- If large sideband regions are required for proper signal modelling, or if the line will select several closely-related decays, the target rate is not an exclusive signal rate.
- More?
There's also the general issue of what to do if one cannot see a way of reducing a rate further to meet the guidelines. It would be useful to offer some strategies here (e.g. checking if all intermediate filters/combinations are producing far too many candidates).
We want to avoid giving broad limits like "anything below 100 Hz is OK". This can lead to many lines barely making the cut, and once we have a full trigger menu we will have many lines needing a lot of attention. Offering plain, helpful advice on line tuning has the potential to save us some work down the line.
I'll start working on a short page that outlines some of these ideas. Input is very welcome, here or in the coming MR.