Performance of the ATLAS Trigger System in 2015

TL;DR

The ATLAS trigger system underwent extensive LS1 upgrades to prepare for Run 2, enabling data-taking at $\sqrt{s}=13$ TeV with high rates and pile-up. The hardware and software upgrades span Level-1 calorimeter and muon triggers, the unification of the Level-2/EF workflow into a single HLT, enhanced data streaming, and the planned Fast TracKer integration, all aimed at sustaining a $\sim$1 kHz physics output within a $40$ MHz collision rate. The paper presents a comprehensive performance study of trigger signatures (electrons/photons, muons, jets, tau leptons, MET, $b$-jets, and B-physics) using 2015 data, showing close data–MC agreement, efficient track and calorimeter reconstruction, and effective rate management across operational luminosities. These results demonstrate the Run 2 trigger system's capability to support ATLAS physics programs at high energy, high luminosity, and significant pile-up, with flexible menus and advanced reconstruction closely aligned to offline algorithms. The improvements have broad significance for real-time event selection in high-luminosity hadron colliders and set a benchmark for future trigger-level analyses and rapid data-driven performance assessments.

Abstract

During 2015 the ATLAS experiment recorded 3.8 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of 13 TeV. The ATLAS trigger system is a crucial component of the experiment, responsible for selecting events of interest at a recording rate of approximately 1 kHz from up to 40 MHz of collisions. This paper presents a short overview of the changes to the trigger and data acquisition systems during the first long shutdown of the LHC and shows the performance of the trigger system and its components based on the 2015 proton--proton collision data.

Figures (50)

• Figure 1: The ATLAS TDAQ system in Run 2 with emphasis on the components relevant for triggering. L1Topo and FTK were being commissioned during 2015 and not used for the results shown here.
• Figure 2: Schematic view of the trigger towers used as input to the L1Calo trigger algorithms.
• Figure 3: The per-bunch trigger rate for the L1 missing transverse momentum trigger with a threshold of 50 (L1_XE50__) as a function of the instantaneous luminosity per bunch. The rates are shown with and without pedestal correction applied.
• Figure 4: A schematic view of the muon spectrometer with lines indicating various pseudorapidity regions. The curved arrow shows an example of a trajectory from slow particles generated at the beam pipe around $z\sim10\m$. Triggers due to events of this type are mitigated by requiring an additional coincidence with the TGC-FI chambers in the region $1.3 < |\eta| < 1.9$.
• Figure 5: (a) Number of events with an L1 muon trigger with transverse momentum () above 15 (L1_MU15__) as a function of the muon trigger $\eta$ coordinate, requiring a coincidence with the TGC-FI chambers (open histogram) and not requiring it (cross-hatched histogram), together with the fractional event rate reduction in the bottom plot. The event rate reduction in the regions with no TGC-FI chambers is consistent with zero within the uncertainty. (b) Efficiency of L1_MU15__ in the end-cap region, as a function of the of the offline muon measured via a tag-and-probe method (see Section \ref{['sec:sigperf']}) using $Z\to \mu\mu$ events with (open dots) and without (filled dots) the TGC-FI coincidence, together with the ratio in the bottom panel.