The ATLAS Run-2 Trigger Menu for Higher Luminosities: Design, Performance and Operational Aspects

The ATLAS experiment aims at recording about 1 kHz of physics collisions, starting with an LHC design bunch crossing rate of 40 MHz. To reduce the massive background rate while maintaining a high selection efficiency for rare physics events (such as beyond the Standard Model physics), a two-level trigger system is used. Events are selected based on physics signatures such as the presence of energetic leptons, photons, jets or large missing energy. The trigger system exploits geometrical information on candidate objects, as well as multi-variate methods to carry out the necessary physics filtering. In total, the ATLAS online selection consists of thousands of different individual triggers. A trigger menu is a compilation of these triggers which specifies the physics algorithms to be used during data taking and the bandwidth a given trigger is allocated to. Trigger menus reflect not only the physics goals of the collaboration for a given run, but also take into consideration the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout and offline processing farm. For the 2017 run, the ATLAS trigger has been enhanced to be able to handle higher instantaneous luminosities (up to 2.0×1034 cm−2s−1) and to ensure the selection robustness against higher average multiple interactions per bunch crossing. In these proceedings, we describe the design criteria for the trigger menus used for Run 2 at the LHC. We discuss several aspects of the process, from the validation of the algorithms, the fine-tuning of the prescales, and the monitoring tools that ensure the smooth operation of the trigger during data taking. We also report on the physics performance of a few trigger algorithms.


ATLAS Run-2 Trigger and Data Acquisition
In Run-2, ATLAS uses a two level trigger system to efficiently select interesting events and reduce the interaction rate of 40 MHz to 1 kHz: • Hardware-based Level-1 (L1) trigger: • Level-

Level-1 Topological Trigger
• Level-1 Topological Trigger module combines calorimeter and muon information at Level-1 and applies topological selections to reduce the rate (e.g., angular distances, di-object invariant mass, transverse mass) Not reviewed, for internal circulation studies using L1Topo is given in the following references: [5,6]. Throughout this note we assume the 16 centre-of-mass energy of 14TeV, luminosity of 2⇥10 34 cm 2 s 1 , bunch-spacing of 25 ns and pile-up of 17 50 interactions per bunch crossing.

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The document is structured as follows. Next section describes L1Topo input data formats and hard- • electron/photon and tau: up to 120 objects for electron/photon candidates and tau candidates sep-  Trigger Level Analysis (TLA) • Novel idea to circumvent the bandwidth limitation using partial event building (< 5% standard event size recorded) • Prescale factors normally applied to the HLT jet triggers in the standard stream

ATLAS Trigger software validation
ATL-DAQ-PROC-2016-040 by production of output metrics for validation. Once the data has been processed, the software validation expert then makes available the output metrics to the signature experts and solicits their feedback. If sign-o↵ is given by every signature group then the software release can be deployed for running in ATLAS, if not then new bug tickets are created and the process begins anew. Overall this cycle typically takes between three and seven days.  l-1 trigger e ciency loss and rate reduction applying the new medium isolation on the electromagnetic s with E T > 22 GeV and E T > 24 GeV with respect to the default isolation used in 2016 data taking. fault) isolation is applied for EM clusters with E T < 50 GeV, where the transverse energy in an alorimeter towers around the EM candidate relative to the EM cluster E T is required to be less than E T /8 1.8 GeV} (max{1 GeV, E T /8 2.0 GeV}). The e ciency is measured with respect to the o✏ine electron candidates satisfying a likelihood-based tight identification and with E T at least 5 GeV above rigger threshold. The e ciencies are measured with a tag-and-probe method using Z ! ee decays in gger reprocessings. The rate predictions are obtained with a trigger reprocessing of enhanced bias data to a luminosity of 2 ⇥ 10 34 cm 2 s 1 . New Level-1 EM medium isolation cuts have been implemented rate of the lowest unprescaled Level-1 triggers while keeping the e ciency loss as low as possible, to e increasing luminosity in 2017, and are compared with the default isolation cuts used for 2016 data • Lowest unprescaled single-muon triggers HLT mu26 ivarmedium || HLT mu60 (triggers seeded by L1 MU20) • HLT track-based isolation applied • Efficiency with respect to offline isolated medium muons using Z → µ µ Tag  • Jet trimming procedure: anti-k t R = 1.0 algorithm used for large-R jets -Within a jet, recluster the constituents into subjets with radius R sub = 0.2 -Discard subjets if p T,i < f cut · Λ hard (f cut = 0.05 used) -Remaining subjets assembled into the trimmed jet

Prospects
• The upgrades of the detectors and trigger system will be essential in the coming