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