4. Object definitions and preselection 43
4.3. Muons
Energy Scale and Resolution
Energy scale correction factors are derived using a large sample of collected Z → ee events and the well known Z-boson mass value, and are applied to reconstructed electrons as final energy calibration in data events. An energy smearing is done for simulated events to match the energy resolution in data. Figure 4.3 shows the reconstructed Z → ee invariant mass in both data and MC after applying the electron energy calibration for the 2011 dataset [115].
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→ee Z ATLAS Preliminary
Figure 4.3.: Resolution of the di-electron invariant mass from Z → ee events after applying all energy corrections in the 2011 data set [117].
4.3. Muons
4.3.1. Definition and Selection
Muon candidates are reconstructed in the ATLAS experiment from track segments in the various layers of the muon spectrometer (MS), and matched with tracks found in the inner detector (ID).
Depending on the reconstruction criteria used for the muon identification, different types of muons are available:
• Stand-alone (SA): the muon trajectory uses information from the MS only. SA muons allow to extend the acceptance range to 2.5<|η|<2.7, which is not covered by the ID.
• Combined (CB): track reconstruction is performed independently in the ID and MS; the ID and MS tracks are later combined into one track. This type has the highest muon purity.
• Segmented-tagged (ST): a track in the ID is classified as a muon if, once extrapolated to the MS, it is associated with at least one local track segment in the MDT or CSC chambers.
It is useful in cases where muons have lowpT, or cases in which muons fall in regions with reduced MS acceptance.
• Calorimeter-tagged (CaloTag): a track in the ID is classified as a muon if it is associated to an energy deposit in the calorimeter compatible with a minimum ionising particle (MIP)5. This type has the lowest purity, but recovers acceptance regions with no MS coverage.
5Relativistic charged particles with minimum energy loss rates via ionisation when traversing a block of material - typically about 1.5 MeV per g/cm2. Typical muons are considered to be MIPs.
4. Object definitions and preselection
For the main measurement presented in this thesis, combined muons are used. The final candidates are refitted using the complete track information from both detector systems, and are required to have an|η|<2.5. Muons are required to have a hit pattern in the inner detector consistent with a well-reconstructed track [118]. The muon track longitudinal impact parameter with respect to the primary vertex, z0, is required to be smaller than 2 mm. Muons are also required to be separated by ∆R >0.4 from any selected jet (the jet selection will be presented in Section 4.4).
Furthermore, muons are required to satisfy apT-dependent track-based isolation requirement that has good performance under high pile-up conditions and in boosted top quark topologies, where the ∆Rdistance between the muon and ab-tagged jet scales asmtop/pT ,top. This isolation, denoted asmini-isolation [119], is defined as the scalar sum of thepT of additional tracks, which have aptrackT >1 GeV and fulfil track quality cuts, in a cone of variable radius ∆R <10 GeV/pTµ around the muon. The mini-isolation is required to be less than 5% of the muonpT. Therefore, for higher muonpT, the cut on the sum of tracks is looser, while the cone radius becomes smaller.
4.3.2. Trigger
The muon objects are required to match the logical OR of two single muon triggers during the 2012 data taking, EF mu24i tight and EF mu36 tight. The former trigger applies apT threshold of 24 GeV and an isolation requirement pcone20T /pT µ ≤0.12, while the latter only applies a pT threshold of 36 GeV. Since the mini-isolation requirement is tighter than the one applied in the first trigger, the trigger isolation has no effect on the analysis. The trigger efficiency is measured in Z → µµ events with the tag-and-probe method. Figure 4.4 shows the efficiencies measured for the logical OR of the two single muon triggers used in the 2012 data set and MC simulation, showing an efficiency in the plateau (pT >25 GeV) of approximately 70% in the barrel region (|η|<1.05) and of approximately 86% in the endcap region.
(probe) [GeV]
Figure 4.4.: Efficiency of the two single muon triggers, mu24i tight and mu36 tight, convoluted as an OR between the two, for (left): the barrel and (right): endcap region. The results are shown for both MC simulation and the full 2012 dataset [120].
4.3.3. Performance
Reconstruction, Identification and Isolation Efficiencies
The muon reconstruction efficiencies are computed in data and MC simulation for each muon type in Z → µµevents using the tag-and-probe method, and corresponding SFs are applied to
48
4.3. Muons
MC simulation to bring it in agreement with the data efficiency. Figure 4.5 shows the muon ID reconstruction efficiency as a function of η and pT in the region |η| <2.5 for the full 2012 data set and simulation. The largest discrepancy between data and simulation efficiencies can be seen in the region, 1.5< η <2. This can be explained by two faulty pixel b-layer (innermost pixel layer) modules that were not disabled correctly in the data reconstruction, causing a lower reconstruction efficiency than that predicted by the simulation. These discrepancies are taken into account with the SFs shown in the lower part of the plots. Other than this region, the overall efficiency is greater than 99%, with very good agreement between data and MC.
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
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0.96 0.97 0.98 0.99 1
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L = 20.3 fb-1
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> 10 GeV pT
η -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 Data / MC0.995
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MC Data ATLAS ID Tracks
L = 20.3 fb-1
= 8 TeV s
| < 2.5 η 0.1 < |
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pT
20 40 60 80 100 120
Data / MC0.995 1 1.005
Figure 4.5.: ID muon reconstruction efficiency as a function of (left): η and (right): pT mea-sured inZ →µµevents for muons with apT >10 GeV. The panel at the bottom shows the ratio between the measured and predicted efficiencies. The green areas show the pure statistical uncertainty, while the orange areas also include system-atic uncertainties [118].
The tag-and-probe method is also used in Z → µµ events to calculate the efficiency of the muon isolation requirement. A dimuon selection is applied, requiring the two muons to have opposite-sign charge, pT > 25 GeV, a distance of ∆R ≥ 0.4 to the closest jet and satisfying 80< m(µµ)<100 GeV. Thetag muon has to pass the isolation requirement, and the isolation efficiency is measured by determining if theprobe muon fulfils the isolation requirement as well.
Since the difference between data and simulation is within 0.5% everywhere, a SF of 1.00 ± 0.5% is assigned for the isolation efficiency.
Energy Scale and Resolution
Due to the large amount of Z → µµ events collected in 2011 and 2012, a very precise cal-culation of the muon momentum scale from the Z peak position is possible. The scales are considered separately for the ID and the MS regions, and the corresponding corrections are at the few per mille level.
A smearing of the momentum of the muons is performed in MC simulation in order to make the momentum resolution in MC agree with data. A separate parametrisation is used for the ID and MS region, and the differences in the momentum resolution measurement between the two are also reflected in the smearing correction factor [121]. Figure 4.6 shows the invariant dimuon mass for data and corrected MC using combined muons withpT >25 GeV.
4. Object definitions and preselection
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Figure 4.6.: Dimuon invariant mass for combined muons in the full 2012 data set and the corrected MC simulation, after applying smearing and scale corrections to the latter [122].