Most of this section is stolen from Gerhard Raven's presentation. (LHCb SoftwareWeek Thursday, March 19, 2009 )


  • An Hlt decision involves more than a single algorithm
    • one might pick some L0 candidates
    • another (sequence) may do some VELO reconstruction
    • a third may match the above two
    • ....

  • In addition, it has some ‘standard’ pre/post processing:
    • ‘entry’ filters (ODIN, and/or L0, and/or other HLT decision(*))
    • Prescale
    • “The algorithm (sequence) which most people think of as the decision”
    • Postscale

  • And it needs to record the final yes/no result
  • And possibly catch errors in the ‘hosted’ algorithms
  • Let’s call this entire structure an ‘HltLine’

HltLine in Python

HltLines are created by calling some dedicated python code

  • Enforces uniform naming convention
    • might add some additional rules!
  • Makes it easier to write HltLines
    • Can ‘copy-and-modify’ entire lines to easily changes create variations:
      • eg. clone, decrease threshold and increase prescale
        • For an example of cloning a line, see the section on Hlt2Lines.
    • re-use pre-defined sequences (of sequences, of ... ) through ‘bindMembers’
      • Useful, as each line SHOULD BE independent, and SHOULD not rely on the results of other lines.
      • This last comment applies in Hlt2 as well as Hlt1. The decision of an Hlt2Line should be independent of that of any other Hlt2Line.
      • Caveats:
        • It might of course make sense to create an Hlt2 line which only runs on the output of an Hlt1 line, and hence depends on the decision of that Hlt1Line.
        • You may wish to add heavily prescaled 'lines' to monitor the intermediate steps in an Hlt2Line.
      • For an example of bindMembers, see the Hlt2SharedParticles page.
  • Registers the existence of a line -- this is used to actually configure Hlt1/Hlt2.

HltLine in C++

  • Invokes the various stages
    • Each stage is an independent algorithm, HltLine only relies on ‘FilterPassed’
  • Updates an entry in HltDecReports ‘as it goes along’
    • Catches exceptions and errors in the ‘hosted’ algorithms, updates HltDecReport entry accordingly, and recovers (hopefully)
  • As a result, the status is recorded in TES (in HltDecReports) in uniform and reliable way:
    • convention: all configured Hlt1 decisions appear in HltDecReports, regardless of their result
    • at some point we may remove negative decision before conversion to rawbank -- but only after it has been shown that on readback we can reliably recreate the (relevant) missing information from the configuration
  • Accept/Reject is recorded separately from ‘how far we got’: even if the event ‘fails’, it could be accepted, eg. because HltLine caught an exception: a (limited!) number of such errors will/could result in an ‘accept’.

-- StephanNies - 12 Apr 2009

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Topic revision: r3 - 2009-04-18 - StephanNies
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