Observation of Charge-Dependent Azimuthal Correlations in p−Pb Collisions and Its Implication for the Search for the Chiral Magnetic Effect

Abstract

Charge-dependent azimuthal particle correlations with respect to the second-order event plane in p-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 jηj < 2.4, and a third particle measured in the hadron forward calorimeters (4.4 < jηj < 5). The observed differences between the same and opposite sign correlations, as functions of multiplicity and η gap between the two charged particles, are of similar magnitude in p-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.

Electromagnetic fields in small systems from a multiphase transport model

“…The first measurements are consistent with the CME expectations in both Au + Au collisions at the RHIC energy [16,17] and Pb + Pb collisions at the LHC energy [18], though many background effects can also contribute to the observable γ [19][20][21][22][23][24][25]. However, the recent measurements on small systems from the CMS experiment show similar magnitudes of charge azimuthal correlation in p + Pb collisions to those in Pb + Pb collisions at the LHC energy [26,27], which strongly challenges the CME interpretation due to the following arguments. For A + A collisions, the major axis of eccentricity is almost aligned closely with the magnetic field.…”

“…As a result, the "relevance vs irrelevance" between A + A and p + A collisions implies that the correlator due to the CME should be very different between large and small systems. Therefore, the observed similar magnitude and multiplicity dependence of the three-particle correlator indicate that the dominant contribution of the correlation signal may not be related to the CME [26][27][28].…”

“…In Fig. 6, we show the N track dependencies of the electromagnetic fields at r c and t = 0, where N track is the number of final charged particles, which is an important experimental variable [26]. Here we set |η| < 2. are zero (only B y is shown for clarification here), the average of the absolute value of the magnetic fields is still strong over the whole N track range, which could potentially bring some electromagnetic-field-based effects.…”

Phenomenological Review on Quark–Gluon Plasma: Concepts vs. Observations

“…Some doubts on the prevailing interpretation were cast by the recent observation of charge-dependent azimuthal correlations also in p + Pb collisions at the LHC [156]. Moreover, in the presence of elliptic flow (for its definition, see Section 5.1), practically all conventional two-particle correlations like the local charge conservation [157] may contribute to the reaction-plane-dependent correlation function used to quantify the CME [158].…”

Round table: What can we learn about confinement and anoma-lous effects in QCD using analog systems?

“…Reaching an unambiguous conclusion on the existence of CME in heavy ion collisions however is a challenging task, as there are substantial backgrounds. It should be possible to quantify them further using the data on proton-nucleus collisions recently presented by the CMS Collaboration at the LHC [59], and related analyses at RHIC. The dedicated 2018 isobar run at RHIC (using the nuclei with the same mass number but different electric charge) will allow to clearly distinguished between the background effects driven by nuclear geometry and the effects of magnetic field [60].…”