Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at $\sqrt{\it{s}_{\mathrm{NN}}}$ = 2.76 and 5.02 TeV

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Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at $\sqrt{\it{s}_\mathrm{{NN}}} = $ 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity ($\eta$) and transverse momentum ($p_{\rm{T}}$) ranges $\left|\eta \right| <~ 0.8$ and $0.2 <~ p_{\mathrm{T}} <~ 5$ GeV/$c$. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators $\gamma_{1,1} = \langle \cos (\varphi_{\alpha} +\varphi_{\beta} - 2\Psi_{2}) \rangle$ and $\gamma_{1,-3} = \langle \cos (\varphi_{\alpha} -3\varphi_{\beta} + 2\Psi_{2}) \rangle$. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured $\gamma_{1,1}$. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form $\gamma_{1,2} = \langle \cos (\varphi_{\alpha} +2\varphi_{\beta} - 3\Psi_{3}) \rangle$, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of $\gamma_{1,1}$.

 

J. High Energ. Phys. 2020, 160 (2020)
HEP Data
e-Print: arXiv:2005.14640 | PDF | inSPIRE
CERN-EP-2020-098

Figure 1

(Left panel): The centrality dependence of $\delta_1$, $\delta_2$, $\delta_3$, and $\delta_4$ for pairs of particles of opposite (OS) and same (SS) sign measured in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV. (Right panel): The charge-dependent differences, $\Delta \delta_{\mathrm{n}}$ for $n$~=~1,~2,~3~and~4, as a function of collision centrality. The statistical uncertainties for some data points are smaller than the marker size. The systematic uncertainties of each data point are represented by the shaded boxes.

Figure 2

The dependence of $\delta_1$, $\delta_2$, $\delta_3$ and $\delta_4$ on the transverse momentum difference $\Delta p_{\mathrm{T}} = |p_{\mathrm{T},\alpha} - p_{\mathrm{T},\beta}|$ (left panel), the average transverse momentum $\overline{p}_{\mathrm{T}} = (p_{\mathrm{T},\alpha} + p_{\mathrm{T},\beta})/2$ (middle panel) and the pseudorapidity difference $\Delta\eta = |\eta_{\alpha} - \eta_{\beta}|$ (right panel) of the pair. The results for both opposite (circles) and same sign (squares) particle pairs are reported for one indicative centrality interval (30--40$\%$) of Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV.

Figure 3

(Left panel): The centrality dependence of $\gamma_{1,1}$, $\gamma_{1,-3}$, $\gamma_{1,2}$ and $\gamma_{2,2}$ for pairs of particles of opposite (OS) and same (SS) sign measured in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV. (Right panel): The charge-dependent differences $\Delta \gamma_{1,1}$, $\Delta \gamma_{1,-3}$, $\Delta \gamma_{1,2}$ and $\Delta\gamma_{2,2}$ as a function of collision centrality.

Figure 4

The dependence of $\gamma_{1,1}$, $\gamma_{1,-3}$, $\gamma_{1,2}$ and $\gamma_{2,2}$ on the transverse momentum difference $\Delta p_{\mathrm{T}} = |p_{\mathrm{T},\alpha} - p_{\mathrm{T},\beta}|$ (left panel), the average transverse momentum $\overline{p}_{\mathrm{T}} = (p_{\mathrm{T},\alpha} + p_{\mathrm{T},\beta})/2$ (middle panel) and the pseudorapidity difference $\Delta\eta = |\eta_{\alpha} - \eta_{\beta}|$ (right panel) of the pair. The results for both opposite and same sign particle pairs are reported for one indicative centrality interval (30--40$\%$) of Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV.

Figure 5

The dependence of $\Delta \gamma_{1,1}$, $\Delta \gamma_{1,2}$ and $\Delta \gamma_{2,2}$ on centrality, measured in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 2.76$~and 5.02~TeV. The data points for $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV are shifted along the horizontal axis for better visibility.

Figure 6

(Left) The centrality dependence of $\Delta \delta_1$ measured in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV. The curve (denoted as BW-LCC) presents the blast-wave parametrization coupled to local charge conservation effects The model is tuned to reproduce the measured values of $\Delta \delta_1$ (see text for details). (Right) The comparison of the centrality dependence of the CME-sensitive correlator $\Delta \gamma_{1,1}$ with expectations from the BW-LCC model.

Figure 7

(Left panel) The centrality dependence of $\Delta\gamma_{1,1}/v_2$ and $\Delta\gamma_{1,2}/v_3$ for Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 2.76$~TeV according to the AMPT and BW-LCC model. (Right panel) The differences between $\Delta\gamma_{1,1}/v_2$ and $\Delta\gamma_{1,2}/v_3$ in AMPT and BW-LCC, denoted as $\Delta(\Delta\gamma/v_{\mathrm{n}})$ The solid and dotted line is the result of a fit with a constant function to AMPT and BW-LCC result respectively.

Figure 8

(Left panel) The CME fraction extracted in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 2.76$~TeV. (Right panel) The CME fraction extracted in Pb--Pb collisions at $\sqrt{s_\mathrm{{\rm NN}}} = 5.02$~TeV. The systematic uncertainty is shown as hatched band at zero line around the centrality value of 60\% The solid blue lines correspond to fit with a constant function to the data points. See text for details.