16.8 Expected Performance of Installed Detectors .1 Tracker Performance
16.8.3 Muon Performance
Resolution [GeV] [GeV]
40 80
70 60 50 40 30 20 10 0 35
0 200 400 600 QCD jets Susy tt A → ττ
800 1000 1200 ATLAS
CMS
Curve: 0.57 √ΣET Curve: 0.97 √ΣET
1400 ΣET [GeV]
1600 1800 2000
ΣET [GeV]
4000 3500 3000 2500 2000 1500 1000 500 30
25 20 15 10 5 0
Fig. 16.22 For ATLAS (left) and CMS (right), expected precision on the measurement of the missing transverse energy as a function of the total transverse energy,ET, measured in the event (see text)
the CMS energy resolution is approximately 14%. The intrinsic performance of the CMS hadron calorimeter can be improved using charge particle momentum measurements, a technique often referred to as particle flow, which was developed at LEP [23]. Initial studies indicate that the jet energy resolution can be significantly improved at low energies, typically from 17 to 12% for ET = 50 GeV and
|η|<0.3, but such large improvements are not expected for jet transverse energies above 100 GeV or so.
Finally, Fig.16.22illustrates a very important aspect of the overall calorimeter performance, namely the expected precision with which the missing transverse energy in the event can be measured in each experiment as a function of the total transverse energy deposited in the calorimeter. The results for ATLAS are expressed as theσ from Gaussian fits to the (x,y) components of theETmissvector for events from high-pT jet production and also from other possible sources containing several high-pT jets. In the case of CMS, where the distributions are non-Gaussian, the results are expressed as the r.m.s. of the same distributions for events from high-pT
jet production. For transverse momenta of the hard-scattering process ranging from 70 to 700 GeV, the reconstructedET ranges from about 500 GeV to about 2 TeV.
The difference in performance between ATLAS and CMS is a direct consequence of the difference in performance expected for the jet energy resolution.
detail from the real geometries of all the chambers and support structures have been refined repeatedly over the years.
In ATLAS, the quality of the stand-alone muon measurement relies on detailed knowledge of the material distribution in the muon spectrometer, especially for intermediate-momentum muons. Reconstruction of these with high accuracy and without introducing a high rate for fake tracks, has to take into account multiple scattering of the muons and thus the details of the material distribution in the spectrometer. This necessitates a very detailed mapping of the detector and the storage of this map for use by the offline simulation and reconstruction programs.
The corresponding effect in CMS is much smaller, since the amount of iron in between the muon stations dominates by far and the details of the material are necessary only in the boundaries between the iron blocks.
Figures16.23and16.24show the expected resolution on the muon momentum measurement. The expected near-independence of the resolution from the pseu- dorapidity in ATLAS, along with the degradation of the resolution at higherηin CMS are clearly visible. The resolution of the combined measurement in the barrel region is slightly better in CMS due to the higher resolution of the measurement in the tracking system, whereas the reverse is true in the end-cap region due to the better coverage of the ATLAS toroidal system at large rapidities. A summary of the performance of the two muon measurements can be found in Table16.13for muon momenta between 10 and 1000 GeV.
The expected performance matches that expected from the original designs. An interesting demonstration of the robustness of the muon systems comes from the reconstruction of muons in heavy-ion collisions. Whereas neither experiment was specifically designed for very high reconstruction efficiency in the very special conditions of heavy-ion collisions, it turns out that they can yield significant physics signals for a few key signatures such asJ /ψandϒ, ϒproduction [27].
12 11 10 9 8 7 6 5 4 3 2 1 0
Contribution to resolution [%]
pT [GeV]
10 102 103 Tube resolution and autocalibration Chamber alignment Multiple scattering Energy loss flutuations Total
Iηl < 1.5
12 11 10 9 8 7 6 5 4 3 2 1 0
Contribution to resolution [%]
pT [GeV]
10 102 103 Tube resolution and autocalibration Chamber alignment Multiple scattering Energy loss flutuations Total
Iηl > 1.5
pT resolution [%]
pT [GeV]
10 102 103 40
30
20
10
0
Muon spectrometer Inner detector Combined
Fig. 16.23 Expected performance of the ATLAS muon measurement. Left: contributions to the momentum resolution in the muon spectrometer, averaged over|η| <1.5. Centre: same as left for 1.5<|η|<2.7. Right: muon momentum resolution expected from muon spectrometer, inner detector and their combination together as a function of muon transverse momentum
Muon system only Inner Tracker only Full system
Δp/p
10–1 1
10–2
10–3
103 102
10
p [GeV/c]
0.0<η<0.2
Muon system only Inner Tracker only Full system
Δp/p
10–1
10–2
10–3
103 102
10
p [GeV/c]
1.8<η<2.0
Fig. 16.24 Expected performance of the CMS muon measurement. The muon momentum resolution is plotted versus momentum using the muon system only, the inner tracker only, or their combination (full system) for the barrel, with|η| < 0.2 (left), and for the end-caps, with 1.8<|η|<2.0 (right)
Table 16.13 Main parameters of the ATLAS and CMS muon measurement systems as well as a summary of the expected combined and stand-alone performance at two typical pseudorapidity values (averaged over azimuth)
Parameter ATLAS CMS
Pseudorapidity coverage
Muon measurement |η|<2.7 |η|<2.4
Triggering |η|<2.4 |η|<2.1
Dimensions [m]
Innermost (outermost) radius 5.0 (10.0) 3.9 (7.0)
Innermost (outermost) disk (z-point) 7.0 (21–23) 6.0–7.0 (9–10) Segments/super-points per track for barrel (end-caps) 3 (4) 4 (3–4)
Magnetic fieldB[T] 0.5 2
Bending power (BL[Tm]) at|η| ≈0 3 16
Bending power (BL[Tm]) at|η| ≈2.5 8 6
Combined (stand-alone) Momentum resolution at
p=10 GeV/c andη≈0 1.4% (3.9%) 0.8% (8%)
p=10 GeV/c andη≈2 2.4% (6.4%) 2.0% (11%)
p=100 GeV/c andη≈0 2.6% (3.1%) 1.2% (9%)
p=100 GeV/c andη≈2 2.1% (3.1%) 1.7% (18%)
p=1000 GeV/c andη≈0 10.4% (10.5%) 4.5% (13%)
p=1000 GeV/c andη≈2 4.4% (4.6%) 7.0% (35%)