Charge-dependent two- and three-particle correlations measured in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV are presented. Results are obtained for charged particles in the pseudorapidity range $|\eta|<~0.8$ and transverse momentum interval $0.2 \le p_{\rm T}<~5.0$ GeV/$c$ for different collision centralities. The three-particle correlator ${\gamma_{\alpha\beta}} \equiv {\langle \cos(\varphi_\alpha + \varphi_\beta - 2{\Psi_{\rm 2}}) \rangle}$, calculated for different combinations of charge sign $\alpha$ and $\beta$, is expected to be sensitive to the presence of the Chiral Magnetic Effect (CME). Its magnitude is similar to the one observed in Pb-Pb collisions in contrast to a smaller CME signal in Xe-Xe collisions than in Pb-Pb collisions predicted by Monte Carlo (MC) calculations including a magnetic field induced by the spectator protons. These observations point to a large non-CME contribution to the correlator. Furthermore, the charge dependence of ${\gamma_{\alpha\beta}}$ can be described by a blast wave model calculation that incorporates background effects and by the Anomalous Viscous Fluid Dynamics model with values of the CME signal consistent with zero. The Xe-Xe and Pb-Pb results are combined with the expected CME signal dependence on the system size from the MC calculations including a magnetic field to obtain the fraction of CME contribution in ${\gamma_{\alpha\beta}}$, $f_{\rm CME}$. The CME fraction is compatible with zero for the 30% most central events in both systems and then becomes positive; averaging over the 0-70% centrality interval yields an upper limit of 2% (3%) and 25% (32%) at 95% (99.7%) confidence level for the CME signal contribution to ${\gamma_{\alpha\beta}}$ in Xe-Xe and Pb-Pb collisions, respectively.
Submitted to: PLB
e-Print: arXiv:2210.15383 | PDF | inSPIRE
CERN-EP-2022-220
Figure group
Figure 1
The \dc{} (top panels) and \gc{} (bottom panels) correlators as a function of centrality (left panels) and charged-particle density (right panels) for pairs of particles with same (closed markers) and opposite (open markers) charges from Xe--Xe collisions at $\sqrt{s_{\rm NN}}=5.44$ TeV (red circles) compared to Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV (black squares) . The Pb--Pb points are slightly shifted along the horizontal axis for better visibility in the left panels. Bars (boxes) denote statistical (systematic) uncertainties. |  |
Figure 2
The dependence of \gc{} on the pseudorapidity difference $|\eta_{\alpha} - \eta_{\beta}|$ (left panel), the transverse momentum difference $|p_{{\rm T}_{\alpha}} - p_{{\rm T}_{\beta}}|$ (middle panel), and the average transverse momentum $(p_{{\rm T}_{\alpha}} + p_{{\rm T}_{\beta}})/2$ (right panel) for pairs of particles with same (closed markers) and opposite (open markers) charges from 20--30\% Xe--Xe collisions at $\sqrt{s_{\rm NN}}=5.44$ TeV (red circles) compared to results from 30--40\% Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV (black squares) . The Pb--Pb and Xe--Xe same-charge points are slightly shifted along the horizontal axis for better visibility in all panels. Bars (boxes) denote statistical (systematic) uncertainties. |  |
Figure 3
Centrality evolution of the difference between same- and opposite-charge pair correlations for \dc{} (top panel) and \gc{} (bottom panel) compared to model calculations: blast wave (BW) parameterisation coupled to local charge conservation (LCC) effects (blue curves) and Anomalous Viscous Fluid Dynamics (AVFD) (green curves). The BW+LCC model is tuned to reproduce the centrality dependence of $\Delta\delta_{\alpha\beta}$, while AVFD is tuned to describe simultaneously the centrality dependence of $\Delta\delta_{\alpha\beta}$ and $\Delta\gamma_{\alpha\beta}$. Bars (boxes) denote statistical (systematic) uncertainties on the data points, while the thickness of the curves represents the uncertainties on the model calculations. |  |
Figure 4
Difference between opposite- and same-charge pair correlations for \gc{} divided by $v_2$ as a function of centrality (left) and charged-particle density (right) compared to results from Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV . The Pb--Pb points are slightly shifted along the horizontal axis for better visibility in the left panel. Bars (boxes) denote statistical (systematic) uncertainties. |  |
Figure 6
Centrality dependence of the CME fraction extracted using Eq. \ref{eq:fcme} with the expected CME signal from MC Glauber (closed markers) and T$_{\rm R}$ENTo (open markers) models (see text for details). The T$_{\rm R}$ENTo points are slightly shifted along the horizontal axis for better visibility. |  |