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CMS-PAS-HIN-21-017
Multiplicity and transverse momentum dependence of charge balance function in pPb and PbPb collisions
Abstract: We report the charge-dependent two-particle differential correlation function in PbPb at sNN = 5.02 TeV and pPb collisions at sNN = 8.16 TeV. The correlation function is used to study the charge creation mechanism in high-energy heavy-ion collisions. The charge balance function is constructed using the like- and unlike-charge pairs to quantify the correlation of the emitted particles, such as the development of the collectivity and time the charges are created. The multiplicity dependence of balance function is observed both in relative pseudorapidity (Δη) and relative azimuthal angle (Δφ). The charge-dependent study of the correlation function shows the width of the balance function decreases towards the high multiplicity collisions in the low transverse momentum region (pT< 2 GeV/c) for both systems. Neither HYDJET nor HIJING can reproduce the multiplicity dependence, while AMPT is consistent with the experimental findings. No significant multiplicity dependency is observed at larger transverse momentum.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
The balance function as a function of Δη and Δφ in PbPb collisions at sNN = 5.02 TeV. Left to right the 70-80%, 30-40%, 0-10% centrality classes. The trigger and associated particles satisfy the condition 0.5 <pT,asso<pT, trig< 2 GeV/c.

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Figure 1-a:
The balance function as a function of Δη and Δφ in PbPb collisions at sNN = 5.02 TeV. Left to right the 70-80%, 30-40%, 0-10% centrality classes. The trigger and associated particles satisfy the condition 0.5 <pT,asso<pT, trig< 2 GeV/c.

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Figure 1-b:
The balance function as a function of Δη and Δφ in PbPb collisions at sNN = 5.02 TeV. Left to right the 70-80%, 30-40%, 0-10% centrality classes. The trigger and associated particles satisfy the condition 0.5 <pT,asso<pT, trig< 2 GeV/c.

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Figure 1-c:
The balance function as a function of Δη and Δφ in PbPb collisions at sNN = 5.02 TeV. Left to right the 70-80%, 30-40%, 0-10% centrality classes. The trigger and associated particles satisfy the condition 0.5 <pT,asso<pT, trig< 2 GeV/c.

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Figure 2:
The balance function as a function of Δη and Δφ in pPb collisions at sNN = 8.16 TeV. Left to right the 0-40 to 270-300 Nofflinetrk multiplicity classes. The trigger and associated particles satisfy the condition 0.4 <pT,asso<pT, trig< 2 GeV/c.

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Figure 2-a:
The balance function as a function of Δη and Δφ in pPb collisions at sNN = 8.16 TeV. Left to right the 0-40 to 270-300 Nofflinetrk multiplicity classes. The trigger and associated particles satisfy the condition 0.4 <pT,asso<pT, trig< 2 GeV/c.

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Figure 2-b:
The balance function as a function of Δη and Δφ in pPb collisions at sNN = 8.16 TeV. Left to right the 0-40 to 270-300 Nofflinetrk multiplicity classes. The trigger and associated particles satisfy the condition 0.4 <pT,asso<pT, trig< 2 GeV/c.

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Figure 2-c:
The balance function as a function of Δη and Δφ in pPb collisions at sNN = 8.16 TeV. Left to right the 0-40 to 270-300 Nofflinetrk multiplicity classes. The trigger and associated particles satisfy the condition 0.4 <pT,asso<pT, trig< 2 GeV/c.

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Figure 3:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions).

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Figure 3-a:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions).

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Figure 3-b:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions).

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Figure 3-c:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions).

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Figure 3-d:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions).

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Figure 4:
The width of the balance function with Nch in Δη (top left for PbPb and top right for pPb collisions) and the ratios of |Δη|/|Δη|Nch<65 (bottom left) and |Δη|/|Δη|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 4-a:
The width of the balance function with Nch in Δη (top left for PbPb and top right for pPb collisions) and the ratios of |Δη|/|Δη|Nch<65 (bottom left) and |Δη|/|Δη|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 4-b:
The width of the balance function with Nch in Δη (top left for PbPb and top right for pPb collisions) and the ratios of |Δη|/|Δη|Nch<65 (bottom left) and |Δη|/|Δη|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 4-c:
The width of the balance function with Nch in Δη (top left for PbPb and top right for pPb collisions) and the ratios of |Δη|/|Δη|Nch<65 (bottom left) and |Δη|/|Δη|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 4-d:
The width of the balance function with Nch in Δη (top left for PbPb and top right for pPb collisions) and the ratios of |Δη|/|Δη|Nch<65 (bottom left) and |Δη|/|Δη|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 5:
The width of the balance function with Nch in Δφ (top left for PbPb and top right for pPb collisions) and the ratios of |Δφ|/|Δφ|Nch<65 (bottom left) |Δφ|/|Δφ|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 5-a:
The width of the balance function with Nch in Δφ (top left for PbPb and top right for pPb collisions) and the ratios of |Δφ|/|Δφ|Nch<65 (bottom left) |Δφ|/|Δφ|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 5-b:
The width of the balance function with Nch in Δφ (top left for PbPb and top right for pPb collisions) and the ratios of |Δφ|/|Δφ|Nch<65 (bottom left) |Δφ|/|Δφ|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 5-c:
The width of the balance function with Nch in Δφ (top left for PbPb and top right for pPb collisions) and the ratios of |Δφ|/|Δφ|Nch<65 (bottom left) |Δφ|/|Δφ|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 5-d:
The width of the balance function with Nch in Δφ (top left for PbPb and top right for pPb collisions) and the ratios of |Δφ|/|Δφ|Nch<65 (bottom left) |Δφ|/|Δφ|Nch<33 (bottom right) for PbPb and pPb collisions, respectively. The statistical uncertainties of the data points are smaller than the marker size and rectangular boxes indicates the systematic uncertainites of that bin.

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Figure 6:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 2.0 <pT,asso< 3.0 <pT,trig< 4.0.

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Figure 6-a:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 2.0 <pT,asso< 3.0 <pT,trig< 4.0.

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Figure 6-b:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 2.0 <pT,asso< 3.0 <pT,trig< 4.0.

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Figure 6-c:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 2.0 <pT,asso< 3.0 <pT,trig< 4.0.

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Figure 6-d:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 2.0 <pT,asso< 3.0 <pT,trig< 4.0.

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Figure 7:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0. The one dimensional projection is derived for Δη in near-side (π/2<Δφ<π/2) and Δφ (0.3 <|Δη|< 1) region.

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Figure 7-a:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0. The one dimensional projection is derived for Δη in near-side (π/2<Δφ<π/2) and Δφ (0.3 <|Δη|< 1) region.

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Figure 7-b:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0. The one dimensional projection is derived for Δη in near-side (π/2<Δφ<π/2) and Δφ (0.3 <|Δη|< 1) region.

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Figure 7-c:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0. The one dimensional projection is derived for Δη in near-side (π/2<Δφ<π/2) and Δφ (0.3 <|Δη|< 1) region.

png pdf
Figure 7-d:
The projection of the balance function in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0. The one dimensional projection is derived for Δη in near-side (π/2<Δφ<π/2) and Δφ (0.3 <|Δη|< 1) region.

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Figure 8:
The width of the balance function is calculated for different pT in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0.

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Figure 8-a:
The width of the balance function is calculated for different pT in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0.

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Figure 8-b:
The width of the balance function is calculated for different pT in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0.

png pdf
Figure 8-c:
The width of the balance function is calculated for different pT in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0.

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Figure 8-d:
The width of the balance function is calculated for different pT in Δη (top left for PbPb and top right for pPb collisions) and Δφ (bottom left for PbPb collisions and bottom right for pPb collisions) in 3.0 <pT,asso< 8.0 <pT,trig< 15.0.
Tables

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Table 1:
Corrected Nch values calculated for different multiplicities for sNN= 5.02 TeV and for PbPb and sNN= 8.16 TeV for pPb collisions

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Table 2:
Summary of systematic uncertainties calculated in Δη for the balance function

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Table 3:
Summary of systematic uncertainties calculated in Δφ for the balance function
Summary
This note presents the measurement of the charge balance function for non-identified particles in PbPb and pPb collisions using the broad pseudorapidity coverage with the CMS detector. For both systems, the dependence of the balance function in relative pseudorapidity (Δη) and relative azimuthal angle (Δφ) was studied for different multiplicity classes and transverse momentum ranges. We observed the width in Δη and Δφ decreases in PbPb and pPb systems only in low pT (pT< 2 GeV/c). These results are consistent with the scenario of the system possessing a large radial flow, and the charges are created at a later stage of the collision. Multiplicity dependence is weaker for the higher value of transverse momentum, which signifies the balancing charge partners are strongly correlated compared to the low pT. Model comparisons such as HYDJET, AMPT and HIJING cannot reproduce the multiplicity dependence of the width in Δη. On the other hand, AMPT, which incorporates collective effects, can reproduce the narrowing of the width similar to the data results. This measurement can be extended by studying the correlation of identified particles using the wide rapidity coverage. That can be used as a probe to the chemical evolution of the produced system and a deeper investigation of radial flow that affects the balance function width.
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Compact Muon Solenoid
LHC, CERN