Observation of Charge-Dependent Azimuthal Correlations in $p\text{−}\mathrm{Pb}$ Collisions and Its Implication for the Search for the Chiral Magnetic Effect

Charge-dependent azimuthal particle correlations with respect to the second-order event plane in $p\text{−}\mathrm{Pb}$ and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV have been studied with the CMS experiment at the LHC. The measurement is performed with a three-particle correlation technique, using two particles with the same or opposite charge within the pseudorapidity range $|\eta |<2.4$, and a third particle measured in the hadron forward calorimeters ($4.4<|\eta |<5$). The observed differences between the same and opposite sign correlations, as functions of multiplicity and $\eta$ gap between the two charged particles, are of similar magnitude in $p\text{−}\mathrm{Pb}$ and PbPb collisions at the same multiplicities. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

Figure 1

The same and opposite sign three-particle correlator as a function of $|\mathrm{\Delta }\eta |\equiv |{\eta }_{\alpha }-{\eta }_{\beta }|$ for $185\le N_{\mathrm{trk}}^{\text{offline}}<220$ in (a) $p\text{−}\mathrm{Pb}$ and (b) PbPb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The $p\text{−}\mathrm{Pb}$ results obtained with particle $c$ in Pb-going (solid markers) and $p$-going (open markers) sides are shown separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.

Figure 2

The same sign and opposite sign three-particle correlator averaged over $|{\eta }_{\alpha }-{\eta }_{\beta }|<1.6$ as a function of $N_{\mathrm{trk}}^{\text{offline}}$ in $p\text{−}\mathrm{Pb}$ and PbPb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ are shown. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.

Figure 3

The difference of the opposite sign and same sign three-particle correlators (a) as a function of $|{\eta }_{\alpha }-{\eta }_{\beta }|$ for $185\le N_{\mathrm{trk}}^{\text{offline}}<220$ and (b) as a function of $N_{\mathrm{trk}}^{\text{offline}}$, averaged over $|{\eta }_{\alpha }-{\eta }_{\beta }|<1.6$, in $p\text{−}\mathrm{Pb}$ and PbPb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The $p\text{−}\mathrm{Pb}$ results are obtained with particle $c$ from Pb- and $p$-going sides separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.