In order to check how efficient is PID method, the calculation of the sample purity are made for all considered particle species. Purity of the sample of particles P can be expressed as:
P = 1 − C. (11)
Dependence of purity on transverse momentum are presented in the Fig. (4-19). Purity inte-grated over pT range for all particle species used in this analysis are listed in table (4.1). It can be seen that used PID method for pions and protons gives over 99% pure sample and for kaons gives over 98% pure sample.
Figure 4-19: Purity plot for Pb-Pb collisions at√
sN N = 5.02TeV, calculated using data from MC AMPT generator.
Particle Purity π+ 0.9994 π− 0.9994 K+ 0.9838 K− 0.9829
p 0.9962
p 0.9946
Table 4.1: Purity integrated over pT range for all particles used in the analysis in Pb-Pb collisions at
√sN N = 5.02TeV.
5 RESULTS
5 Results
In this section there are presented obtained correlation functions for Pb-Pb collisions data at
√sN N = 5.02 TeV. Due to similarity between correlation functions in all calculated centrality ranges, only plots made for centrality range 10-20% are shown in paragraphs (5.1) and (5.2).
Dependence of collision centrality range on the shape of correlation functions is presented in appendix B. This analysis was performed for different particle pairs, which are listed in the table (5.2).
Like-sign pairs Unlike-sign pairs π+π++ π−π− π+π− K+K++ K−K− K+K−
pp + pp pp
Table 5.2: Pairs of particles that were used in this analysis.
5.1 ∆η∆ϕcorrelation functions without corrections Correlation functions for Pb-Pb collisions at √
sN N = 5.02 TeV obtained in this analysis are presented in Fig. (5-20). These correlation functions are raw. Final version of ∆η∆ϕ functions will be obtained after applying efficiency corrections.
Figure 5-20: ∆η∆ϕ functions before corrections for Pb-Pb collisions data at√
sN N = 5.02TeV. Central-ity 10-20%
5.2 ∆η∆ϕcorrelation functions with corrections
In aim to increase accuracy of ∆η∆ϕ functions there were applied corrections, which were mentioned at paragraph (4.4.3). Obtained correlation functions are presented in Fig. (5-21). It can be observed that results have similar structures around area ∆η∆ϕ = (0, 0) as correlation functions from pp collisions at√
s = 7 TeV, but in this case the most of correlation structures are covered by cos(2∆ϕ) shape caused by the elliptic flow effect. Moreover, there is also visible anti-correlation shape in proton-proton correlation function. In unlike-sign protons correlation function appears a decrease in ∆η∆ϕ = (0, 0) bin. This kind of structure is caused by annihila-tion process between protons and anti-protons.
Figure 5-21: ∆η∆ϕ like-sign pairs functions after corrections for Pb-Pb collisions data at√
sN N = 5.02 TeV. Centrality 10-20%.
5.3 ∆η∆ϕcorrelation functions from MC
In Fig. (5-22) ∆η∆ϕ functions for MC truth data are presented. These plots show correla-tions, which are predicted by AMPT model. MC truth data results are different to real data.
Difference is especially visible in proton-proton correlation function. In real data there is visible anti-correlation shape, but in MC truth data it is not.
In the Fig. (5-23) are presented correlation functions for MC reconstructed data after cor-rections. In analysis of this data there were used the same parameters as for the real data.
After applying corrections plots should be the same as for MC truth data, but there are still dif-ferences. To perform accuracy of corrections there was made MC closure test, in which ∆η∆ϕ functions from MC reconstructed are divided by MC truth. Results from MC closure test are
5 RESULTS
presented in the Fig. (5-24).
Figure 5-22: ∆η∆ϕ functions Pb-Pb collisions data at√
sN N = 5.02TeV for MC truth data from AMPT generator.
Figure 5-23: ∆η∆ϕ functions Pb-Pb collisions data at √
sN N = 5.02 TeV for MC reconstructed data from AMPT generator.
Figure 5-24: MC closure test for Pb-Pb collisions data at√
sN N = 5.02TeV for MC reconstructed and MC truth data from AMPT generator.
In the Monte Carlo closure test the biggest difference appears in the unlike-sign kaons and protons. Although there were applied corrections, the difference is < 6%. The smallest dif-ference between MC reconstructed and MC truth is for all pairs of pions, where is less than 1%.
6 SYSTEMATIC UNCERTAINTIES
6 Systematic uncertainties
Many factors like track selection method, PID method or another applied cuts have an influence on uncertainty of two-particle correlation functions. In this section there are presented some systematic check to determine relative uncertainty. Histograms of ∆η∆ϕ functions presented in section (5.1.1) will be compared with other, which were analysed with different parameter change. Comparison is performed as ∆ϕ projection of these pairs of histograms and relative systematic uncertainty plots are calculated as follows:
R = A − B A
where R is relative uncertainty, A is original histogram from section (5.2) and B is adequate histogram with changed parameter in the analysis.
6.1 Comparison of different track selection method
In aim to investigate the differences between various track reconstruction, the analysis was additionally made separately on following track selection cuts:
TPC-only tracks - the track reconstruction is based only on the TPC data.
Hybrid tracks - gaps of the information from the ITS detector in Global Tracks are sup-plemented by the information from TPC-only tracks.
The differences between TPC-only and Global tracks are presented in Fig. (6-25). The most visible difference in almost all presented plots appears in ∆ϕ = 0 bin. The relative difference for like-sign pairs is as follows: for pions 0.04%, for kaons 0.07% and for protons 0.37%. The highest difference in unlike-sign pairs histograms of different track selection is observable for protons is 0.17%, for pions uncertainty is less than 0.05% and for kaons 0.1%.
Fig. (6-26) present results of comparison Hybrid and Global tracks. In all uncertainty plots are visible statistic fluctuations, what proves that these two track selection methods are quite similar. Only in pairs of like-sign and unlike-sign pions are visible structures. However, the uncertainties are much lower than 0.01% of correlation functions.
Figure 6-25: Comparison of ∆ϕ projections with TPC only and Global tracks for Pb-Pb collisions data at√
sN N = 5.02TeV. Centrality 10-20%
Figure 6-26: Comparison of ∆ϕ projections with Hybrid and Global tracks for Pb-Pb collisions data at
√sN N = 5.02TeV. Centrality 10-20%
6 SYSTEMATIC UNCERTAINTIES