Abstract
Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions – the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in p+Au and d+Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-ion collisions. This bears important implications for the interpretation of the heavy-ion data.
Highlights
In quantum chromodynamics, interactions of massless quarks with fluctuating topological gluon fields are predicted to induce chirality imbalance and parity violation in a local domain [1,2,3]
Our new p + Au and d + Au measurements demonstrate that background contributions could produce magnitudes of the γ correlator comparable to what has been observed in Au + Au data, and offer a possible alternative explanation of the γ measurements in Au + Au collisions without invoking
It is expected that the γ correlator from small-system p + Au and d + Au collisions will be dominated by background correlations, as chiral magnetic effect (CME)-induced contributions would be strongly suppressed due to the random orientations of the magnetic field and the participant plane
Summary
Interactions of massless quarks with fluctuating topological gluon fields are predicted to induce chirality imbalance and parity violation in a local domain [1,2,3]. The commonly used observable to search for charge separation in heavy-ion collisions is the three-point azimuthal correlator [15], γ ≡ cos(φα + φβ − 2ψ),. In heavy-ion collisions, it is called the reaction plane (spanned by the impact parameter direction and the beam). It is often approximated by the second order harmonic participant plane (ψ2) [16,17], constructed experimentally by the event plane measured from final state particle azimuthal distribution. Background correlations, on the other hand, are expected to be present in proton-nucleus collisions These correlations are propagated to the three-particle correlator via correlations with respect to particle c, not directly to the impact parameter or the B direction.
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