MUON IDENTIFICATION AND RECONSTRUCTION IN THE ATLAS DETECTOR AT THE LHC
N. CHR. BENEKOS Max-Planck-Institut fr Physik,
Fohringer Ring 6 80805 Muenchen, Germany E-mail: Nectarios.Benekos@cern.ch
On behalf of the ATLAS Muon/Muid Reconstruction group
The muon detection system of the ATLAS detector is characterized by two high precision tracking systems, namely the Inner Detector and the Muon Spectrom- eter plus a thick calorimeter that ensures a safe hadron absorption filtering with high purity muons with energy above 3 GeV. In order to combine the muon tracks reconstructed in the Inner Detector and the Muon Spectrometer the Muon Iden- tification (Muid) Object-Oriented software package has been developed. In this note the Muid reconstruction procedure is briefly described, followed by a more detailed presentation of its performance. The impact of the missing components of the ATLAS Detector on the performance of the measurements is investigated.
1. Introduction
The ATLAS detector
1, currently being installed at CERN, is designed to make precise measurements of 14 TeV proton-proton collisions at the LHC, starting in 2007. ATLAS consists of four main subdetectors:
• the Inner detector(ID) for the measurement of momentum and im- pact parameter of charged particles.
• the Calorimeter system, for measurement of particle and jet ener- gies
• the Muon Spectrometer(MS) for muon identification and momen- tum measurement, consisting of high precision drift tubes for track- ing(MDT,CSC), and a set of two subsystems of trigger chambers:
resistive plate chambers (RPC) and thin gap chambers (TCG) and
• A magnet system for bending of charged particles for momentum measurements
1
The identification of muons is performed using a combination of high precision tracking detector components, including an ID, housed in a uni- form solenoidal field, and a precision MS housed in toroidal fields. The Spectrometer measures curved tracks in 3 stations in the barrel toroid and straight-lines tracks before and after the endcap toroid. There is a suffi- cient calorimeter depth between them to ensure the absorptions of hadrons before the Spectrometer, yielding high purity muons with momenta above 3 GeV.
2. ATLAS Detector Layout
The Inner Detector
2has been in a state of continuous evolution up to the present time. Engineering developments and cost limitations have neces- sitated changes to the layout geometry and detector materials. Changes since the DC1 layout
3with which the present results are compared include and presented
4.
The Muon Spectrometer
5will not be fully deployed at the beginning of the collision period
6. In particular, the installation of the EE(Endcap) wheel and half of the CSC stations will be postponed. This will lead to a less precise measurement of the first segment on the muon trajectory and thus to a deterioration of the muon momentum resolution at |η| < 2.
Moreover, in the initial layout, the rapidity region between 1 and 1.3 will be characterized by both worsened momentum resolution and reconstruction efficiency as is indeed seen in the performance of the full reconstruction of a first simulation of this layout.
3. Muon Reconstruction Software
MOORE
7identifies track segments using local pattern recognition at the detector module level in each of the stations of the MS and performs a track fit, based on the package developed for the ID, iPatRec
8. The end result of MOORE is a collection of data objects, which describes the reconstructed tracks at the entrance of the MS. MOORE has been developed within the ATLAS ATHENA
9software environment.
4. Combined muon reconstruction in ATLAS
In order to combine the muon tracks reconstructed in the MS and the ID,
the Muonidentification (Muid) Object-Oriented(OO) software package has
been developed.
The combined reconstruction improves the track measurements, giving the best possible momentum resolution and reducing the tails in the mo- mentum resolution distribution of the MS, accounting for the fluctuations in the energy loss in the calorimeter. Moreover it improves: the charge determination for high energy muons by means of the longer lever arm - helps to discriminate muons from secondaries in the MS; to reject decay muons(from Kaons and pions) by requiring tracks to originate from the primary vertex and to resolve track finding ambiguities which occur in the MS from local showering and cavern background.
4.1. Muonidentification-Muid
The reconstructed objects produced by MOORE are tracks whose param- eters are expressed at the first measured point inside the MS.
MuidStandAlone propagates the muon track to the vertex. Muid- StandAlone re-express the track parameters with covariance at the closest approach to the nominal vertex (the centre of the beam intersection region).
Calorimeter Coulomb scattering is taken into account by a parametrization of the width of the simulated broadening of position and angular distribu- tions. A correction is also applied to account for Energy loss. This may be from a parametrization or a direct measurement from the CalloCells according to the isolation criteria. The procedure is a full track refit to the MS measurement to allow for reprocessing with updated calibrations at ESD(DST) level plus extra “measurements” representing the Calorimeters.
The combination between the ID and the MS track is performed by the algorithm MuidComb . Tracks are matched by forming a χ
2with 5 degrees of freedom from the parameter differences and summed covariances.
A schematic sketch of the algorithms and the objects exchanged with the Transient Event Store is shown in Figure 1.
5. Muon Reconstruction Expected Performance
The specification is to reach 10 % ΔP
T/P
Tin the barrel region (|η| < 1) and the endcap toroid region (1 . 5 < |η| < 2 . 7).
At high momentum the resolution is dominated by the intrinsic precision
of the muon chambers and alignment errors; at moderate momentum, the
resolution is limited by the multiple scattering to Δ P
T/ P
T∼ 2%. Finally,
at low momenta, the fluctuations in energy loss in the Calorimeters and
in the MS are such that the purpose is to identify which ID tracks are
muons. They are sufficiently well measured by the ID. These specifications
Figure 1. Muonidentification ATHENA algorithms (left) and data objects exchanged with the Transient Event Store(right)
are sufficient for the possible discovery and study of exotic physics such as new gauge bosons.
The reconstruction performance has been tested with single muon sam- ples in a range of transverse momentum from 3 GeV/c to 1 TeV/c, uni- formly spread over a range of |η| up to 2.7. The full reconstruction chain has been executed, namely: the reconstruction in the MS alone (MOORE), the extrapolation to the vertex of the track found in the MS (MuidStan- dalone), the reconstruction in the ID (iPatRec) and the combination of the track found in the MS and in the ID (MuidComb). The global efficiency and p
Tresolution as a function of |η| is shown in Figure 4.3
6. Conclusions
The ATLAS detector has been designed to have a good muon transverse momentum(p
T) resolution of few % which should be independent of p
Tand
|η| for a wide p
Trange.
A robust muon identification and high precision measurement is crucial to full exploit the physics potential of the LHC. The muon energy of physics interest ranges in a large interval from few GeV, where the B-physics studies dominate the physics program, up to the highest values that could indicate the presence of new physics.
The performance of the Reconstruction program on simulated data is
in agreement with the ATLAS design specifications.
Figure 2. Global Efficiency as a function of|η|for a pT=20 GeV
Figure 3. Global Efficiency as a function of|η|for a pT=100 GeV
Figure 4. resolution on pT as a function of|η|for a pT=20 GeV
Figure 5. resolution on pT as a function of|η|for a pT=100 GeV
