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Scientific Report 2007-2009<br />
Particle physics<br />
P2. Test and commissioning of the Muon Spectrometer of the ATLAS<br />
experiment<br />
The detection and the precision measurement of leptons,<br />
in particular electrons and muons, is crucial for<br />
large part of the ATLAS physics program. The muon<br />
spectrometer of the ATLAS experiment is the part of the<br />
detector aiming to identify muons coming from protonproton<br />
collisions, reconstruct their trajectory with high<br />
precision and measure their momenta. It consists of a<br />
barrel and two end-caps air-core toroids instrumented<br />
with three stations of precision chambers and trigger<br />
chambers. A schematic overview of the spectrometer is<br />
shown in Fig.1, where the names and the positions of the<br />
different detectors are shown. The aim of the spectrometer<br />
is to provide a measurement of the momentum for<br />
muons with transverse momenta between few GeV and 1<br />
TeV. It is designed to cover all azimuthal angles and polar<br />
angles down to about 10 ◦ with respect to the proton<br />
beam direction. The expected momentum resolution of<br />
the muon spectrometer is evaluated by Montecarlo simulations<br />
and is shown in Fig.2.<br />
Contribution to resolution (%)<br />
Total<br />
12<br />
Spectrometer entrance<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
10<br />
Multiple scattering<br />
Chamber Alignment<br />
Tube resolution and autocalibration (stochastic)<br />
Energy loss fluctuations<br />
2<br />
10<br />
3<br />
10<br />
Pt (GeV/c)<br />
Figure 2: Contributions to the expected momentum resolution<br />
for muons reconstructed in the muon spectrometer as a<br />
function of transverse momentum.<br />
Thin gap<br />
chambers<br />
Radiation shield<br />
MDT chambers<br />
End-cap<br />
toroid<br />
Cathode strip<br />
chambers<br />
Resistive plate chambers<br />
Barrel toroid coil<br />
20 18 16 14 12 10 8 6 4 2 m<br />
Figure 1: Side view of the ATLAS muon spectrometer. The<br />
p-p collision point is in the origin of the twp-coordinate system.<br />
The <strong>Sapienza</strong> University group in collaboration with<br />
the INFN Sezione di Roma, has built and tested a significant<br />
fraction of the MDT (Monitored Drift Tubes) chambers<br />
composed by 3 cm diameter cylindrical drift tubes<br />
assembled in layers. These chambers that allow to measure<br />
the muon trajectories with a single-hit resolution of<br />
less than 100 µm have been installed and commissioned<br />
in the ATLAS detector and are now fully operational.<br />
The Muon Spectrometer is completely working and<br />
taking data since 2007. Cosmic ray data allow to monitor<br />
the performance of the detector and to test the calibration<br />
and alignment methods.<br />
As shown in Fig.2, the main contribution to the<br />
spectrometer resolution at high momenta comes from<br />
the single MDT tube resolution and calibration. For the<br />
continuous calibration of the MDT chambers, samples<br />
of muon tracks identified by the trigger are sent to<br />
three calibration centers where a complete calibration<br />
procedure is done. One of these calibration centers is<br />
in our University. In order to convert the raw time of<br />
12 m<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
each single tube to the hit position, one needs first to<br />
determine the t 0 of each tube and then to evaluate the<br />
space-to-time relation between the measured time and<br />
the drift distance. The calibration procedure allows<br />
to obtain the t 0 of all the tubes and the space-to-time<br />
relation for calibration regions where the tubes have<br />
the same properties. The full procedure has been<br />
extensively tested during the cosmic rays data taking<br />
and is ready to be used during the LHC run. At the<br />
same time the calibration data are used to study the<br />
quality of the data and contribute to the global quality<br />
assessment of the data taken.<br />
References<br />
1. G. Aad et al., JINST 3, S08003 (2008).<br />
2. C. Adorisio et al., Nucl. Instr. and Meth. A593 232<br />
(2008).<br />
3. P. Bagnaia et al., Nucl. Phys. Proc. Suppl. 177 269<br />
(2008).<br />
4. C. Adorisio et al., Nucl. Instr. and Meth. A598 400<br />
(2009).<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 />
<strong>Sapienza</strong> Università di Roma 109 Dipartimento di Fisica