New correlator to detect and characterize the chiral magnetic effect

Abstract

A charge-sensitive in-event correlator is proposed and tested for its efficacy to detect and characterize charge separation associated with the Chiral Magnetic Effect (CME) in heavy ion collisions. Tests, performed with the aid of two reaction models, indicate discernible responses for backgroundand CME-driven charge separation, relative to the second-(Ψ2) and third-order (Ψ3) event planes, which could serve to identify the CME. The tests also indicate a degree of sensitivity which would enable robust characterization of the CME via Anomalous Viscous Fluid Dynamics (AVFD) model comparisons. 25.75.Gz, 25.75.Ld High-energy nuclear collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) can result in the creation of a plasma composed of strongly coupled chiral quarks and gluons or the Quark-Gluon Plasma (QGP). Topological transitions such as sphalerons [1,2], which occur frequently in the QGP [3,4], can induce a net axial charge asymmetry of the chiral quarks which fluctuate from event to event. In the presence of the strong electromagnetic B-fields created in the same collisions, this chiral anomaly is predicted to convert into an electric current which produces a finalstate charge separation known as the Chiral Magnetic Effect (CME) [5][6][7][8][9][10]. For recent reviews, see e.g. [11][12][13].The electric current J Q , created along the B-field, stems from anomalous chiral transport of the chiral fermions in the QGP:where σ 5 is the chiral magnetic conductivity, µ 5 is the chiral chemical potential that quantifies the axial charge asymmetry or imbalance between right-handed and lefthanded quarks in the plasma, and Q is the quark electric charge [8,[14][15][16]. Thus, experimental observation of its associated charge separation, could provide crucial insights on anomalous transport and the interplay of chiral symmetry restoration, axial anomaly, and gluonic topology in the QGP. The B-field, which is strongly time-dependent [17][18][19], is generated perpendicular to the reaction plane (Ψ RP ) defined by the impact parameter and the beam axis. Consequently, CME-driven charge separation can be identified and characterized via the first P -odd sine term (a 1 ) in a Fourier decomposition of the chargedparticle azimuthal distribution [20]: dN ch dφ ∝ [1 + 2 n v n cos(n∆φ) + a n sin(n∆φ) + ...] (2) where ∆φ = φ − Ψ RP gives the particle azimuthal angle with respect to the reaction plane angle, and v n and a n denote the coefficients of P -even and P -odd Fourier terms, respectively. The second-order event plane, Ψ 2 , determined by the maximal particle density in the elliptic azimuthal anisotropy and the beam axis, is usually employed as a proxy for Ψ RP in experimental measurements. Here, it is noteworthy that the third-order event plane, Ψ 3 , can not be used to detect CME-driven charge separation, since there is little, if any, correlation between Ψ RP and Ψ 3 . The event-by-event fluctuations contribute to an event-wise de-correlation between the magnetic field direction imp...

Experimental searches for the chiral magnetic effect in heavy-ion collisions

“…Among various observables [108,109,110,111,112], a commonly used observable to measure the CMEinduced charge separation in heavy-ion collisions is the three-point correlator [113]. In non-central heavy-ion collisions, the overlap interaction region is of an almond shape.…”

“…For events with CME signals, charge separation along the magnetic field gives sin φ ± ≈ 1 and a maximal difference sin φ + − sin φ − ≈ ±2. The ∆S distribution would therefore become wider than its reference distribution, which can be constructed by randomizing the particle charges and by rotating the events by π/2 in azimuth [109,110]. The ratio of real event distribution to the reference distribution, R(∆S), would thus be concave [109,110,201].…”

“…The ∆S distribution would therefore become wider than its reference distribution, which can be constructed by randomizing the particle charges and by rotating the events by π/2 in azimuth [109,110]. The ratio of real event distribution to the reference distribution, R(∆S), would thus be concave [109,110,201]. For flow-induced background, the initial expectation was that the R(∆S) curve would be convex [109].…”

Sensitivity analysis for observables of the chiral magnetic effect using a multiphase transport model

“…Recently, a new observable, namely the shape of R Ψm , has been proposed to be a more sensitive probe to search for the CME signal. Many studies of the R Ψm observable have been reported [30][31][32][33]. For examples, some studies show that the shape of R Ψm dustribution is convex due to background but concave due to the CME [31,32], but another study shows that R Ψm could be also concave due to the background only [33].…”

“…Many studies of the R Ψm observable have been reported [30][31][32][33]. For examples, some studies show that the shape of R Ψm dustribution is convex due to background but concave due to the CME [31,32], but another study shows that R Ψm could be also concave due to the background only [33]. On the other hand, because the lifetime of magnetic field may be quite short due to the limited conductivity of QGP [34][35][36], it is questionable whether the CME signal formed in the early stage can survive from strong final state interactions since relativistic heavy-ion collisions actually involves many final dynamic evolution stages.…”