  In ultrarelativistic heavy-ion collisions, the event-by-event variation of
the elliptic flow $v_2$ reflects fluctuations in the shape of the initial state
of the system. This allows to select events with the same centrality but
different initial geometry. This selection technique, Event Shape Engineering,
has been used in the analysis of charge-dependent two- and three-particle
correlations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} =2.76$ TeV. The
two-particle correlator $\langle \cos(\varphi_\alpha - \varphi_\beta) \rangle$,
calculated for different combinations of charges $\alpha$ and $\beta$, is
almost independent of $v_2$ (for a given centrality), while the three-particle
correlator $\langle \cos(\varphi_\alpha + \varphi_\beta - 2\Psi_2) \rangle$
scales almost linearly both with the event $v_2$ and charged-particle
pseudorapidity density. The charge dependence of the three-particle correlator
is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity
violating effect of the strong interaction. However, its measured dependence on
$v_2$ points to a large non-CME contribution to the correlator. Comparing the
results with Monte Carlo calculations including a magnetic field due to the
spectators, the upper limit of the CME signal contribution to the
three-particle correlator in the 10-50% centrality interval is found to be
26-33% at 95% confidence level.
