732
D. Froidevaux
measuring muon momenta have shaped the experiment more than any other physics
consideration (see also Sect. 16.2.1).
As discussed already in Sect. 16.2.2, the choice of magnet was motivated by
the method which would be used for the measurement of muons with momenta
up to ∼ TeV scales. ATLAS has thus opted for a high-resolution, stand-alone
measurement independently of the rest of the sub-detectors, resulting in a very
large volume, with low material density, over which the muon measurement takes
place. The ATLAS toroidal magnetic field provides a momentum resolution which
is essentially independent of pseudorapidity up to a value of 2.7.
This section reviews the main features of the muon spectrometer system and
discusses a few of the challenges encountered. A few examples of the overall
performance expected in the actual configuration of the experiment are presented
in Sect. 16.8.3, where it is also compared to the expected performance of the CMS
muon system.
16.5.1 General Considerations
The physics signatures that give rise to muons are numerous and varied. At the
highest momenta, they include muons from new high-mass (multi-TeV) resonances
such as heavy neutral gauge bosons, Z , as well as decays from heavy Higgs bosons.
At the lowest end of the spectrum, B-physics relies on the reconstruction of muons
with momentum down to a few GeV. The resulting requirements are:
• Resolution: the ‘golden’ decay of the Standard Model Higgs boson into four
muons, H → ZZ → 4 μ, requires the ability to reconstruct the momentum and
thus mass of a narrow two-muon state with a precision at the level of 1%. At the
upper end of the spectrum, the goal is to achieve a 10% momentum resolution
for 1 TeV muons.
• Wide rapidity coverage: almost two-thirds of the decays of an intermediate-mass
Higgs boson to four muons have at least one muon in the region |η| > 1.4. A
hermetic system, which measures muons up to |η| ∼ 2.5, has turned out to be the
best compromise.
• Identification inside dense environments, e.g. hadronic jets or regions with high
backgrounds.
• Trigger: the ability to measure the momenta of muons online on a stand-alone
basis, i.e. without reference to any other detector system, and to select events
with muons above 5–10 GeV momentum is of paramount importance.
There are also the requirements which result from the 25 ns spacing in time
between successive beam crossings and from the neutron radiation environment of
the experimental halls. Good timing resolution and the ability to identify the bunchcrossing in question, as well as redundancy in the measurements, are therefore
also demanded of the muon detectors, which represent by far the largest and most
difficult system to install in the experiment.
D. Froidevaux
measuring muon momenta have shaped the experiment more than any other physics
consideration (see also Sect. 16.2.1).
As discussed already in Sect. 16.2.2, the choice of magnet was motivated by
the method which would be used for the measurement of muons with momenta
up to ∼ TeV scales. ATLAS has thus opted for a high-resolution, stand-alone
measurement independently of the rest of the sub-detectors, resulting in a very
large volume, with low material density, over which the muon measurement takes
place. The ATLAS toroidal magnetic field provides a momentum resolution which
is essentially independent of pseudorapidity up to a value of 2.7.
This section reviews the main features of the muon spectrometer system and
discusses a few of the challenges encountered. A few examples of the overall
performance expected in the actual configuration of the experiment are presented
in Sect. 16.8.3, where it is also compared to the expected performance of the CMS
muon system.
16.5.1 General Considerations
The physics signatures that give rise to muons are numerous and varied. At the
highest momenta, they include muons from new high-mass (multi-TeV) resonances
such as heavy neutral gauge bosons, Z , as well as decays from heavy Higgs bosons.
At the lowest end of the spectrum, B-physics relies on the reconstruction of muons
with momentum down to a few GeV. The resulting requirements are:
• Resolution: the ‘golden’ decay of the Standard Model Higgs boson into four
muons, H → ZZ → 4 μ, requires the ability to reconstruct the momentum and
thus mass of a narrow two-muon state with a precision at the level of 1%. At the
upper end of the spectrum, the goal is to achieve a 10% momentum resolution
for 1 TeV muons.
• Wide rapidity coverage: almost two-thirds of the decays of an intermediate-mass
Higgs boson to four muons have at least one muon in the region |η| > 1.4. A
hermetic system, which measures muons up to |η| ∼ 2.5, has turned out to be the
best compromise.
• Identification inside dense environments, e.g. hadronic jets or regions with high
backgrounds.
• Trigger: the ability to measure the momenta of muons online on a stand-alone
basis, i.e. without reference to any other detector system, and to select events
with muons above 5–10 GeV momentum is of paramount importance.
There are also the requirements which result from the 25 ns spacing in time
between successive beam crossings and from the neutron radiation environment of
the experimental halls. Good timing resolution and the ability to identify the bunchcrossing in question, as well as redundancy in the measurements, are therefore
also demanded of the muon detectors, which represent by far the largest and most
difficult system to install in the experiment.