Multiplicity and transverse momentum dependence of charge-balance functions in pPb and PbPb collisions at LHC energies

A bstract Measurements of the charge-dependent two-particle angular correlation function in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of $$ \sqrt{s_{\textrm{NN}}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mi>NN</mml:mi> </mml:msub> </mml:msqrt> </mml:math> = 8 . 16 TeV and lead-lead (PbPb) collisions at $$ \sqrt{s_{\textrm{NN}}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mi>NN</mml:mi> </mml:msub> </mml:msqrt> </mml:math> = 5 . 02 TeV are reported. The pPb and PbPb data sets correspond to integrated luminosities of 186 nb − 1 and 0.607 nb − 1 , respectively, and were collected using the CMS detector at the CERN LHC. The charge-dependent correlations are characterized by balance functions of same- and opposite-sign particle pairs. The balance functions, which contain information about the creation time of charged particle pairs and the development of collectivity, are studied as functions of relative pseudorapidity (∆ η ) and relative azimuthal angle (∆ ϕ ), for various multiplicity and transverse momentum ( p T ) intervals. A multiplicity dependence of the balance function is observed in ∆ η and ∆ ϕ for both systems. The width of the balance functions decreases towards high-multiplicity collisions in the momentum region &lt; 2 GeV, for pPb and PbPb results. Integrals of the balance functions are presented in both systems, and a mild dependence of the charge-balancing fractions on multiplicity is observed. No multiplicity dependence is observed at higher transverse momentum. The data are compared with hydjet , hijing , and ampt generator predictions, none of which capture completely the multiplicity dependence seen in the data. The comparison of results with different center-of-mass energies suggests that the balance functions become narrower at higher energies, which is consistent with the idea of delayed hadronization and the effect of radial flow.

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