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Add signal removal flags

Timothy Davies requested to merge add-signal-removal-flags into spiir-O4b-review

Description

This MR merges the signal removal changes reviewed internally (!148 (merged)) into a branch off the O4a reviewed commit hash, along with documentation that was added later (!192 (merged) with small updates).

In brief, our FAR calculation was designed under the assumption that our background statistics files do not contain real signals. However, following a signal, we do see substantially increased FARs. This MR adds code to prevent signal-like backgrounds from being added to our background statistics files.

You can see the full review materials & detailed description of the changes here: https://wiki.ligo.org/CBC/Searches/SPIIR/O4bReviewMaterial.

Considerations for review

Note that the review of this MR must be accompanied by a sign-off of our configuration changes, to set the new command line arguments for BBH. Without the new command line arguments, the code should function exactly as in O4a.

As such, if convenient, we could consider approving & merging this MR independently of approving the exact NewSNR threshold, or even before approving the design of signal removal.

Changes

There are a few components to these changes:

  1. Add signal removal for background triggers where any IFO exceeds a NewSNR threshold.
  2. Refactor (non-functional change) to existing background removal.
    1. The existing 'background removal' feature discards some background triggers with only a single bright detector. This corresponds to 'foreground' triggers which are removed following the same criteria.
    2. Note we've considered changes to this existing feature, but for this MR, we will leave the functionality as-is.
    3. Both the existing & new features use the same method to remove backgrounds: setting the coherent SNR to -1.
  3. Add a command line flag to enable/disable the new feature.
  4. Add a command line argument to set the NewSNR threshold for backgrounds to be removed.
  5. Add documentation describing the new flags (see !192 (merged)).

Tests

There were tests for the original feature, but I'll rerun and document them more clearly here.

As this is a somewhat old commit hash, I'm unsure if the Gitlab CI pipeline will run reliably. It should include simple tests for the build & auto-formatters at least.

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