=Paper= {{Paper |id=Vol-2023/212-216-paper-33 |storemode=property |title=Performance of the ATLAS muon trigger in run 2 |pdfUrl=https://ceur-ws.org/Vol-2023/212-216-paper-33.pdf |volume=Vol-2023 |authors=Marcus Morgenstern }} ==Performance of the ATLAS muon trigger in run 2== https://ceur-ws.org/Vol-2023/212-216-paper-33.pdf

Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017

PERFORMANCE OF THE ATLAS MUON TRIGGER IN
RUN 2
M.M. Morgenstern
On behalf of the ATLAS collaboration

Nikhef, National institute for subatomic physics, Amsterdam, The Netherlands

E-mail: a marcus.matthias.morgenstern@cern.ch

The ATLAS trigger system is essential for fulfilling the physics program of the ATLAS experiment. It
consists of two steps, a hardware-based Level-1 trigger and a software-based high-level trigger to
select events of interest at a suitable recording rate. Both stages underwent upgrades to cope with the
challenges of LHC Run 2 data-taking at a centre-of-mass energy of 13 TeV and instantaneous
luminosities up to 2×1034 cm-2s-1. The design of the ATLAS muon trigger and its performance in
proton-proton collisions at 13 TeV are presented.

Keywords: ATLAS, LHC, Trigger, Muon, TDAQ

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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017

1. Introduction
The trigger system of the ATLAS experiment [1] is an essential component of it and is
designed to efficiently select events of high interest for physics analyses. Prompt muons are of crucial
importance in many physics analyses at the LHC, ranging from precise Standard Model measurements
and top physics to Higgs physics and searches for new particles. Thus an efficient muon trigger is
essential to provide the necessary inputs for the analyses.
The ATLAS trigger system consists of a hardware-based Level-1 (L1) trigger and a software-
based High-Level trigger (HLT). The Level-1 trigger decision is formed by the central trigger
processor (CTP) based on the inputs provided by the L1 muon and calorimeter triggers as well as from
several other subsystems. Events accepted at L1 are passed to the HLT, which runs algorithms close to
the offline reconstruction and takes the final acceptance decision.
The increased rate of collision data at the higher instantaneous luminosities achieved at the
LHC and the increased center-of-mass energy provide enormous physics potential, but set new
challenges for the trigger system. To cope with these challenges the muon trigger system was
upgraded.
An overview of the trigger design and recent upgrades to the muon trigger as well as its
performance measured in 2017 proton-proton collision data will be outlined.

2. The ATLAS muon trigger
Muon triggers are based on the information provided by the muon spectrometer (MS) and the
inner detector (ID) of the ATLAS detector. The muon spectrometer is based on three large air-core
superconducting toroids. The field integral of the toroids ranges between 2.0 and 6.0 Tm across most
of the detector. Several detector technologies are used
to provide fast response for triggering purposes as
well as precision tracking. In the central region (|| <
1.05) three layers of resistive plate chambers (RPCs)
and in the end-cap regions (1.05 < || < 2.4) three
layers of thin gap chambers (TGCs) are installed. To
detect the deflection of the muon trajectory in the
toroid magnetic field three layers of monitored drift
tube chambers (MDTs) are installed covering || < 2.
In the region 2.0 < || < 2.7 two layers of MDTs are
combined with one layer of cathode strip chambers Figure 1. Schematic drawing of one quarter
cross-section of the muon system of the
(CSCs). The ID provides track measurements based
ATLAS detector [2]
on hits in the pixel, semiconductor tracker (SCT) and
transition radiation tracker (TRT), arranged in successive layers, with the pixel detector closest to the
interaction point, surrounded by a 2T magnetic field provided by a solenoid. It covers a fiducial region
up to || = 2.5.

3. Level-1 muon trigger

At Level-1 muons are identified by spatial and
temporal coincidence requirements on the hits
provided by the RPCs or TGCs. To estimate the pT of
the muons, the degree of deviation from the hit pattern
of an infinite momentum assumption is used to define
six L1 thresholds. Information on Regions of Interest
(RoIs) is provided by the central trigger based on the
Figure 2. L1 muon trigger efficiency in one  slice
as a function of the muon candidate pseudo-
rapidity 

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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017

L1 decision, and passed to the HLT. The typical size of a RoI in x is 0.1 x 0.1 (0.03 x 0.03) in
the barrel (end-cap) region. In the barrel region for the three highest pT thresholds hits in all three
layers are required, while for the
remaining lower pT thresholds the
hit requirement is loosened to
two coincidence hits. In the end-
cap
regions the coincidence in three
layers is required for all
thresholds. To increase the
coverage, additional RPC
chambers have been installed in
2015 in the regions close to the
feet that support the ATLAS Figure 3. Left: The pseudo-rapidity (η) distributions of the L1 muon trigger
detector. Figure 2 shows the L1 with the pT threshold of 20 GeV (L1_MU20) and the rate reduction effect by
muon trigger efficiency in one a coincidence with small-wheel TGCs in end-cap inner (EI) and forward inner
slice of the azimuthal angle with (FI) chambers. Right: L1_MU20 trigger rate as a function of instantaneous
the new chambers versus . In luminosity in 2016 (black) and 2017 (red) data-taking [4]
green the effect of the additional
RPC chambers can be seen.

For the end-cap regions an additional coincidence of the small-wheel TGCs in the forward
inner (FI) and end-cap inner (EI) chambers is required to reduce fake muon contributions from
particles not originating from the interaction point. As shown in Figure 3 (left) fake muons can be
significantly suppressed. In 2017, the overlap region at the barrel feet region and look-up table in the
end-cap region have been optimized using 2016 data, the resulting rate reduction is presented in Figure
3 (right).

4. High-Level muon trigger
The muon reconstruction at the HLT is split into a fast and a precision reconstruction stage. In
the fast reconstruction stage the MDT precision hits are used to refine the muon candidate provided by
the L1. First, MS-only muon candidates are built by performing a track fit based on the precision
measurement of drift times and positions in the MDT chambers. The transverse momentum is assigned
via look-up tables. Subsequently, the MS track is back-extrapolated to the interaction point and
combined with tracks reconstructed in the inner detector, forming combined muon candidates with
refined track parameters. In the regions equipped with CSC chambers, an additional measurement
outside the magnetic field is used to back-extrapolate to hits in the CSC layer to improve pT resolution
(see Figure 4). Selection criteria defined for each muon trigger chain are applied to these muon
candidates to allow early rejection of fake muons.
If the fast reconstruction step is passed
successfully, the muon candidate enters the precision
stage. Starting from the refined RoI provided by the
fast reconstruction, segment and track reconstruction
exploiting the information from all MS detectors is
performed. As for the fast stage, MS-only muon
candidates are built first and are subsequently
combined with ID tracks to form combined muons.
In addition to the back-extrapolation from the MS to
the ID (inside-out), there exists another algorithm
that extrapolates ID tracks to the MS in case the
Figure 4. Impact of the CSC hit measurement on the
transverse momentum resolution of the fast muon
reconstruction as a function of the pT of the
reconstructed muon [4]
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017

former fails (outside-in). This recovers about 1-5% of muons at low pT.
These baseline algorithms run only for the RoIs provided initially by the L1 trigger. To
recover efficiency losses from the L1 trigger, another algorithm searches for muon candidates in the
full detector (full-scan). As this is very costly in terms of CPU consumption, it is typically considered
for multi-muon triggers where one muon passed the baseline sequence and the second muon is
searched for in the full detector. The full-scan procedure first builds muon candidate tracks in the MS
followed by track reconstruction in the full ID. Subsequently a combined fit is executed.
MS-only triggers are also available. In this case the muon reconstruction is only run in the MS.
While combined muons are suitable for the majority of physics analyses, MS-only triggers are crucial
as well, e.g. in searches for long-lived particles, which do not leave a full trajectory in the ID due to
their displaced decay.
To cope with the rate limitations, additional selections are applied on the muon candidates
built at the HLT stage. First they are required to pass certain pT thresholds. As just applying pT cuts is
not sufficient to get sustainable rates at low pT, additional isolation criteria are required. Isolation
enforces that there is only little activity in the detector around the muon candidate by cutting on the
relative track-pT sum in a cone around the muon candidate with respect to the candidate’s pT. To
reflect the boosted topology of high pT jets, the isolation cone varies as a function of the muon pT.
Applying isolation discards mainly non-prompt muons from heavy flavor, pion and kaon decays,
while accepting nearly 100% prompt muons from Z-boson decays.
Dedicated triggers for di- or multi-muon signatures as well as combinations with other physics
objects, like electrons or hadronically decaying taus are provided as well. In total several hundred
muon chains are included in the trigger menu, while the majority runs prescaled1.

Figure 5. L1 and HLT muon trigger efficiency measured in Z→μμ events as a function of the offline muon p T
(top) and number of reconstructed vertices (bottom). Left: Barrel region. Right: End-cap region. The black dots
represent the L1_MU20 efficiency. In red the combination of the HLT triggers with 60 GeV p T threshold as well
as the isolated trigger with 26 GeV pT requirement is shown. The blue curve shows the HLT efficiency with
respect to the L1 efficiency, i.e. the pure HLT efficiency [4]

1
A prescale of X means that only one out of X accepted events is recorded and the others are discarded.

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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017

5. Efficiency measurement
The performance of the muon triggers is continuously evaluated exploiting a tag-and-probe
method using Z events, which provide a clean signature. Selected events are required to contain
two muons with opposite charge and their invariant mass to be within 10 GeV around the Z mass.
The tag muon is required to be a combined offline reconstructed muon within the fiducial
region of the detector and to pass medium identification, as described in detail in [5]. This selects an
unbiased sample of probe muon candidates used to measure the efficiency, defined as the fraction of
offline reconstructed muon candidates passing several stages of the trigger. Single trigger efficiencies
are shown in Figure 5. In the two plots at the top the efficiency as a function of the transverse
momentum of the offline muon is shown, while in the two plots at the bottom the dependence on the
number of reconstructed vertices, a measure of the amount of pile-up, is shown. Given the different
technologies and acceptances of the MS detectors, efficiencies are evaluated separately in the barrel
(left) and end-cap (right) regions. The black dots represent the efficiency for the L1 trigger with 20
GeV threshold. HLT efficiency for pT thresholds of 60 GeV and 26 GeV with additional isolation
requirement are given by the red dots. In blue the HLT efficiency with respect to the L1 efficiency is
shown. The pure HLT efficiency is nearly 100%. A small degradation at higher numbers of
reconstructed vertices is observed, indicating that the dependence of the efficiency on pile-up is small.

6. Conclusion
Muon triggers are of crucial importance for fulfilling the physics program of the ATLAS experiment.
They have been successfully adapted to cope with the increased challenges during LHC Run 2 data-
taking at a center-of-mass energy of 13 TeV. A high data-taking efficiency has been observed and
measured in Z events. Installation of additional chambers increasing the detector acceptance
yielded higher L1 efficiencies. Additional updates on the muon trigger design have further increased
the efficiency in LHC Run 2.

References
[1] ATLAS Collaboration, The ATLAS Experiment at the CERN Large Hadron Collider, 2008 JINST
3 S08003
[2] ATLAS Collaboration, Performance of the ATLAS muon trigger in pp collisions at √s = 8 TeV,
Eur. Phys. J. C (2015) 75:120
[3] ATLAS Collaboration, Public L1 muon trigger results,
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/L1MuonTriggerPublicResults
[4] ATLAS Collaboration, Public muon trigger results,
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/MuonTriggerPublicResults
[5] ATLAS Collaboration, Muon reconstruction performance of the ATLAS detector in proton-proton
collision data at √s = 13 TeV, Eur. Phys. J. C (2016) 76:292

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