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

The
ALICE
collaboration

JHEP 09 (2020) 160, 2020.

"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 $\gamma_{1,1}$."

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