3. Measurement of transverse momentum spectra 57
3.4. Trigger and event selection
3.4.3. z-Vertex selection
The η-acceptance of a detector depends on the z-position of the primary vertex as shown in Figure 3.3. This is simply the effect of the detector geometry, and most pronounced for the detectors at small radii (SPD), while being negligible for the TPC. An example of the distribution of reconstructed tracks in the (Vz,η) plane is shown in Figure 3.4.
The acceptance cut ofη < 0.8motivates the selection of events which have a reconstructed primary vertex within±10cm around the nominal interaction point (center of the detector), for full ITS acceptance. This choice of the acceptance is indicated in Figure 3.3 and contains the vast majority of reconstructed tracks as illustrated in Figure 3.4. Furthermore, the cut on z-Vertex removes unwanted collisions from satellite bunches.
For the Pb–Pb data this selection of events is also part of the centrality determination, which implies that no reliable centrality is assigned to events with vertex position outside of this window.
Distributions of the reconstructedz-vertex positions are shown in Figure 3.5 for all energies and collision systems. The fraction of events that pass the z-Vertex selection criterion is also indicated and ranges from 85-90%.
64 3. Measurement of transverse momentum spectra
(cm) Vz
-20 -10 0 10 20
η
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Figure 3.4.: Distribution of reconstructed tracks in the (Vz, η)-plane as observed in pp colli-sions at p
s = 7 TeV. The black rectangle shows the acceptance (|Vz| < 10 cm,
|η|<0.8) selected for the analysis.
The resolution of the primary vertex reconstruction was studied in Monte Carlo simulations.
Figure 3.6 shows the difference observed between the generated and reconstructed primary vertex (∆Vz) in pp collisions atp
s =7TeV for different track multiplicities Ncv (number of tracks contributing to the vertex reconstruction). Even for small number of tracks most vertices are reconstructed with less than 1 mm difference. The accuracy of the z-vertex reconstruction improves if a larger number of tracks contribute, and the non-Gaussian tails at large residuals are greatly reduced. The resolution of the primary vertex reconstruction σV z is extracted from fits of a Gaussian function to the distribution of residuals∆Vz and shown in Figure 3.7 as a function of the number of contributing tracks for all pp data sets. Average values ofNcvand Ncv are also indicated in Figure 3.7. In p–Pb and Pb–Pb collisions the average multiplicities are much larger than in pp leading to an even better primary vertex resolution.
In general the efficiency of the z-Vertex reconstruction depends on the position of the primary vertex as shown in Figure 3.8 for pp collisions. Within the selected range of|Vz|<10 cm the efficiency is approximately independent of the vertex position. The dependence on the event multiplicity is shown in Figure 3.9, revealing a deficiency for low multiplicity events with 1-2 particles produced in the acceptance. This has an impact also on thepTspectra, as discussed in section 3.10.5. For events with at least one reconstructed track in the acceptance the vertex re-construction is fully efficient. The vertex rere-construction algorithm uses the average interaction profile measured over many events as a constraint of the primary vertex position. This allows to reconstruct thez-position of the vertex also for events that have only a single reconstructed track.
3.4. Trigger and event selection 65
(cm) Vz
-30 -20 -10 0 10 20 30
evtN
102
103
104
105
88.24%
= 0.9 TeV s pp,
this analysis
(a) pp,p
s= 0.9 TeV.
(cm) Vz
-30 -20 -10 0 10 20 30
evtN
103
104
105
106
89.68%
= 2.76 TeV s pp,
this analysis
(b) pp,p
s= 2.76 TeV
(cm) Vz
-30 -20 -10 0 10 20 30
evtN
104
105
106
107
89.11%
= 7 TeV s pp,
this analysis
(c) pp,p
s = 7 TeV
(cm) Vz
-30 -20 -10 0 10 20 30
evtN
102
103
104
105
86.45%
= 5.02 TeV sNN
p-Pp,
this analysis (2012 data)
(d) p-Pb,p
s= 5.02 TeV Vz (cm)
-30 -20 -10 0 10 20 30
evtN
102
103
104
105
106
107
87.06%
= 2.76 TeV sNN
Pb-Pb, this analysis
(e) Pb-Pb, 2.76 TeV
Figure 3.5.: Distribution of the z-position of the primary vertex for the data sets used in this analysis. The 10 cm range used to measure the pT spectra is indicated as well as the fraction of events selected (out of all events that have a reconstructed vertex).
66 3. Measurement of transverse momentum spectra
(cm) Vz
∆
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
frequency
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
cv = 1 N
cv = 2 N
≤ 9 Ncv
3 ≤
10
cv≥ N = 7 TeV s MC, pp,
Figure 3.6.: Distribution of the difference between the generated and reconstructed vertex po-sitions obtained for different number of tracks contributing to the reconstruction of the primary vertex obtained from Monte Carlo simulations with PYTHIA Peru-gia0 for pp collisions atp
s= 7 TeV together with Gaussian fits to the distribution that are used to extractσV z.
Ncv
0 5 10 15 20 25
(cm)Vzσ
0 0.01 0.02 0.03 0.04 0.05
MC, pp = 0.9 TeV
s 〉
σVz
〈 ,
cv〉 N
〈 = 2.76 TeV
s 〈 Ncv〉, 〈σVz〉 = 7 TeV
s 〈 Ncv〉, 〈σVz〉
Figure 3.7.: The resolution of the z-Vertex reconstructionσV z (calculated from fits of a Gaus-sian distribution to the residuals∆Vz) for pp at all energies as a function of the number of tracks contributing to the vertex (open symbols). The average values of the number of contributers and the average resolution is also indicated (full symbols). Statistical uncertainties are smaller than the size of the symbols.
3.4. Trigger and event selection 67
(cm) Vz
-20 -10 0 10 20
efficiency
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
MC, pp, INEL>0
= 0.9 TeV, trigger s
= 0.9 TeV, vertex s
= 2.76 TeV, trigger s
= 2.76 TeV, vertex s
= 7 TeV, trigger s
= 7 TeV, vertex s
Figure 3.8.: Dependence of the trigger and primary vertex reconstruction efficiencies on thez -position of the primary vertex for events which have at least one produced charge particle in the acceptance. Values were obtained from MC simulations for pp col-lisions at different energies.
> 0.15 GeV/c)
| < 0.8, pT
η
ch (|
N
0 1 2 3 4 5 6
efficiency
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
| < 10 cm MC, pp, INEL, |Vz
= 0.9 TeV, trigger s
= 0.9 TeV, vertex s
= 2.76 TeV, trigger s
= 2.76 TeV, vertex s
= 7 TeV, trigger s
= 7 TeV, vertex s
Figure 3.9.: Trigger and primary vertex reconstruction efficiencies in pp collisions as a function of the number of charged particles in the acceptance, obtained in MC simulations.
68 3. Measurement of transverse momentum spectra
event class event numbers after selection criteria
data MC
pp, 0.9 TeV 5 051 124 4 519 879
pp, 2.76 TeV 50 068 777 2 668 398
pp, 7 TeV 117 429 412 101 238 457
p–Pb, 5.02 TeV, 2012 1 439 810 2 301 174
p–Pb 5.02 TeV, 2013 108 084 501 24 887 125
Pb–Pb, 2.76 TeV, 0-5% central 810 667 8 157
Pb–Pb, 2.76 TeV, 5-10% central 810 896 10 107
Pb–Pb, 2.76 TeV, 10-20% central 1 612 402 22 272 Pb–Pb, 2.76 TeV, 20-30% central 1 618 480 25 384 Pb–Pb, 2.76 TeV, 30-40% central 1 626 091 27 593 Pb–Pb, 2.76 TeV, 40-50% central 1 619 740 28 768 Pb–Pb, 2.76 TeV, 50-60% central 1 619 763 31 209 Pb–Pb, 2.76 TeV, 60-70% central 1 623 225 32 569 Pb–Pb, 2.76 TeV, 70-80% central 1 619 009 33 761
Table 3.4.: Event statistics in data and MC simulations used for corrections. Numbers are given for events which fulfil all event selection criteria including a reconstructed vertex.