Editing ATLAS experiment (section)

====Trigger system====
The [[trigger (particle physics)|trigger]] system<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/><ref>{{cite journal|title=ATLAS Trigger and Data Acquisition: Capabilities and commissioning|author=D. A. Scannicchio|journal=Nuclear Instruments and Methods in Physics Research Section A|year=2010|volume=617|issue=1/3|doi=10.1016/j.nima.2009.06.114|pages=306–309|bibcode = 2010NIMPA.617..306S}}</ref> uses fast event reconstruction to identify, in real time, the most interesting [[event (particle physics)|events]] to retain for detailed analysis.  In the second data-taking period of the LHC, Run-2, there were two distinct trigger levels:<ref>{{cite journal|title=ATLAS Run-2 status and performance|author=ATLAS collaboration|journal=Nuclear and Particle Physics Proceedings|year=2016|volume=270|doi=10.1016/j.nuclphysbps.2016.02.002|pages=3–7|bibcode=2016NPPP..270....3P|url=https://cds.cern.ch/record/2048973}}</ref>

# The Level 1 trigger (L1), implemented in custom hardware at the detector site. The decision to save or reject an event data is made in less than 2.5 μs. It uses reduced granularity information from the calorimeters and the muon spectrometer, and reduces the rate of events in the read-out from 40&nbsp;[[Hertz#SI multiples|MHz]] to 100&nbsp;[[Hertz#SI multiples|kHz]]. The L1 rejection factor in therefore equal to 400.
# The High Level Trigger trigger (HLT), implemented in software, uses a computer battery consisting of approximately 40,000&nbsp;[[Central processing unit|CPUs]]. In order to decide which of the 100,000 events per second coming from L1 to save, specific analyses of each collision are carried out in 200 μs. The HLT uses limited regions of the detector, so-called Regions of Interest (RoI), to be reconstructed with the full detector granularity, including tracking, and allows matching of energy deposits to tracks.  The HLT rejection factor is 100: after this step, the rate of events is reduced from 100 to 1&nbsp;[[Hertz#SI multiples|kHz]]. The remaining data, corresponding to about 1,000 events per second, are stored for further analyses.<ref name="CERN">{{cite news |work=[[ATLAS collaboration]] Research News |title=Trigger and Data Acquisition System|url=https://atlas.cern/discover/detector/trigger-daq |date=October 2019}}</ref>