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Scientific Report 2007-2009
Particle physics
P3. Test and commissioning of the muon trigger system of the
ATLAS experiment
The trigger system of the ATLAS experiment at LHC
is organized in three hierarchical levels. The first trigger
level (LVL1) is hardware based, implemented in custom
programmable electronics, directly connected to the
front-end of calorimeters and muon detectors. It uses
coarse granularity data and has to reduce the event rate
from 1 GHz (at the design luminosity of 10 34 cm −2 s −1 )
to 100 kHz within a latency of 2.5 µs. Level-2 (LVL2)
and Event Filter (EF), composing the High Level Trigger
(HLT), are software based and run on the on-line trigger
farms. At LVL2 full granularity data, inside the Region
of Interest (ROI) identified at LVL1 are available. The
LVL2 selection reduces the event rate from 100 kHz to
2 kHz, with a latency time of 10 ms. The Event Filter
makes use of the entire detector data, its total latency is
∼ 2 s and sofisticated algorithms are executed in order
to refine the selection and reduce the data throughput
to the ∼ 200 Hz of the event acquisition rate.
High p T muons are important signatures of many processes
predicted in various new physics scenarios. Moreover
they allow to select Standard Model processes which
are usually exploited for calibration and commissioning
of the esperiment for physics. Therefore, the muon
trigger performance has a strong impact on the physics
reach of the experiment. The LVL1 selection is based on
the definition of allowed geometrical roads, the Coincidence
Windows shown in Fig. 1. Given a track that hits
the middle trigger station (pivot plane), the algorithm
searches for time-correlated hits in the confirm plane, inside
a geometrical region around the η and ϕ of the hit on
the pivot plane: the size of the (η, ϕ) intervals defines a
specific p T threshold for the muons originating from the
Interaction Point. There are two confirm planes: one for
low p T triggers in the inner trigger plane (at a distance of
about 70 cm from the pivot plane), and another located
in the outer station, where hits are required, in addition
to a low-p T trigger, for high p T muons.
The ATLAS group of the Physics Department and
INFN section of the Sapienza Rome University has designed
and built the electronics of the LVL1 muon barrel
trigger. This system is based on about 800 electronic
modules mounted on the RPC chambers where the
trigger algorithms described above are implemented. In
2006 and 2007 the muon stations consisting of a sandwich
of MDT and RPC chambers, with their trigger modules
on, were mounted in the ATLAS experiment. The system
was fully cabled in 2007 and 2008 and it was subsequently
tested and calibrated with cosmic rays. The
LVL1 trigger is now fully operational and it triggered
the first muons coming from proton-proton interaction
at the LHC start-up in Dicember 2009.
The Atlas Rome Group is also working in the LVL2
muon trigger algorithms development; at this stage
high precision data from MDT chambers are used to
identify good muon tracks and refine the p T measurement.
At LVL2 it is possible to combine the Muon
Spectrometer (MS) tracks with the information coming
from other detectors to further reduce the background.
One algorithm combines the MS candidate with the
Inner Detector tracks. The combination increases the
sharpness of the threshold at low-p T and helps to reject
muons from in-flight decays of light mesons (π, K). The
calorimetric information is used by another algorithm in
order to tag isolated muons and increases the robustness
of the standard muon triggers. These algorithms were
developed using simulated data and have been tested
with “real data” with cosmic rays. They proved to work
in a reliable way and were operated in trasparent mode
in the December 2009 LHC data taking and ready to
filter the events in the upcoming LHC data taking.
References
1. G. Aad et al., JINST 3, S08003 (2008).
2. G. Chiodini et al., Nucl.Inst.Meth. A 581, 213 (2007).
3. F. Anulli, et al., JINST 4, P04010 (2009).
4. T. F-Martin et al., J.Phys.Conf.Serv. 119, 022022 (2008).
Authors
F. Anulli 1 , P. Bagnaia, C. Bini, C. Boaretto, R. Caloi,
G. Ciapetti, D. De Pedis 1 , A. De Salvo 1 , G. De Zorzi, A.
Di Domenico, A. Di Girolamo, C. Dionisi, S. Falciano 1 ,
P. Gauzzi, S. Gentile, S. Giagu, F. Lacava, C. Luci, L.
Luminari 1 , F. Marzano 1 , G. Mirabelli 1 , A. Nisati 1 , E.
Pasqualucci 1 , E. Petrolo 1 , L. Pontecorvo 1 , M. Rescigno 1 , S.
Rosati 1 , E. Solfaroli Camillocci, L. Sorrentino Zanello, P.
Valente 1 , R. Vari 1 , S. Veneziano 1
http://www.roma1.infn.it/exp/atlas
Figure 1: Schema of the L1 muon trigger coincidence windows.
Low-p T and high-p T roads are shown.
Sapienza Università di Roma 110 Dipartimento di Fisica