Reading Trigger Prescales


for section Reading trigger prescales per event from D3PD's: knowledge of D3PDMaker ntuple format


The Atlas trigger system consist of three levels. Level 1(L1), Level 2(L2) and the EventFilter(EF). Level 1 is hardware based. L2 and the EF are the High Level Trigger(HLT). The HLT is software based. Trigger rates reduce from input to output of each level. 40MHz bunch crossing rate to 75KHz L1 output rate to ~200 Hz HLT output rate.

Particle triggers Tau, Muon, Electron are seeded at L1 by localized "Regions of Interest(RoI's)" in the detector. RoI's are localized regions which contain traces from a particle candidate. If the RoI passes the criteria to fire the one or more of the associated L1 triggers (essentailly L1 trigger Items) the L1 trigger fires and the data from the Region of Interest(RoI) is sent to the software farm of the HLT.

  • L1 items seed 1 or more HLT chains
In the HLT the L2 chains associated with the L1 item that the RoI passed run on the RoI. Algorithms connected to the L2 chain process the RoI. If the RoI passes the algorithms of the L2 chain, the associated EF chain runs. If RoI passes the EF chain algorithms, then the EF chain and the associated physics trigger fires.

For physics trigger to fire for event the requirement is one or more RoI's fire(pass) the trigger.

Prescales reduce trigger output rate for given stage (trigger chain or item). Prescale is random reset of passed chain(or item) to fail. Prescaled chain passes 1 of (prescale number) events passed by unprescaled trigger.

Physics trigger prescale = (prescale of Level 1 item) x (prescale of Level 2 chain) x (prescale of Event Filter chain)

Further Information: Trigger for Dummies

Reading Trigger Prescales from web monitoring tools.

Reading Trigger Configuration

From: TauWG good run list select run number

Note Trigger prescales listed are lowest prescales in run;

Reading Trigger Chain Prescales

From: TauWG good run list select run number

Reading trigger prescales per event from D3PD's

Requires that sequence EF chain->L2 chain->L1 item is known (This info is not available in D3PD): Ref Reading Trigger Configuration

Execute following sequence per event in tauPerf tree .

  • access entry in metadata that corresponds to current event in tauPerf tree.
    • per event of tauPerf, match trig_DB_HLTPSK, trig_DB_L1PSK, and trig_DB_SMK -> HLTPSK, L1PSK, and SMK of event of trigtauMeta/TrigConfTree.
  • from metadata calculate prescale
    • match chain names to chain ids
      • EF chain name -> EF chain id from HLTNameMap (map of chain names to chain ids)
      • L2 chain name -> L2 chain id from HLTNameMap
      • L1 item name -> L1 item id from LVL1NameMap (map of L1 item names to item ids)
    • match chain ids to prescales
  • Trigger prescale for event = (EF prescale)x(L2 prescale)x(L1 prescale)

Further reference and example code: TriggerD3PDMaker twiki

Further Info

Introductory level description of atlas trigger system(partially outdated)Trigger for Dummies

Description w example code of D3PD ntuple event level trigger content from generic trigger tools TriggerD3PDMaker twiki

unofficial software based brief Intro to HLT processing

semi expert software based description of HLT: HLTExtendedGlossary

-- PtacekE - 23-Aug-2010

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