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...

We present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from √ sNN = 7.7 to 200 GeV using the STAR detector. The quantity cos(mφ1 + nφ2 − (m + n)φ3) , with φ being the azimuthal angles of the particles is evaluated as a function of √ sNN, collision centrality, transverse momentum, pT , pseudo-rapidity difference, ∆η, and harmonics (m and n). These data provide detailed information on global event properties like the three dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on ∆η is observed for most harmonic combinations which is consistent with breaking of longitudinal boost invariance. An interesting energy dependence is observed when one of the harmonics m, n, or m + n is equal to two, for which the correlators are dominated by the two particle correlations relative to the secondharmonic event-plane. These measurements can be used to constrain models of heavy-ion collisions over a wide range of temperature and baryon chemical potential. Heavy nuclei are collided at facilities like the Relativis-116 tic Heavy Ion Collider (RHIC) and the Large Hadron 117 Collider (LHC) in order to study the emergent proper-118 ties of matter with quarks and gluons as the dominant degrees-of-freedom: a quark-gluon plasma (QGP) [1-4]. 120 The QGP is a form of matter that existed in the early 121 universe when its ambient temperature was more than 155 MeV or 200 thousand times hotter than the center 123 of the sun [5, 6]. As temperatures drop, quarks and glu-124 ons no longer possess the energy necessary to overcome 125 the confining forces of QCD and they become confined 126 into color neutral hadrons and the QGP transitions into a 127 gas of hadrons [7]. This transition occurred in the early 128 universe at about one microsecond after the big bang. 129 Heavy-ion collisions provide the only known method to recreate and study that phase transition in a laboratory 131 setting. 132 To provide the clearest possible picture of this phase 133 transition, a beam energy scan was carried out at RHIC 134 with collision energies ranging from √ s NN =200 GeV 135 down to 7.7 GeV. Lowering the beam energy naturally 136 reduces the initial temperature (T) of the matter cre-137 ated in the collisions, as well as increases the baryon 138 chemical potential µ B , providing information on how the 157 dence of the correlations on the azimuthal angle between 158 particles ∆φ = φ 1 − φ 2 has proven to be particularly in-159 formative. Data have revealed that even when particle 160 pairs are separated by large angles in the longitudinal di-161 rection (large ∆η), they remain strongly correlated in the 162 azimuthal direction. One example of these correlations is 163 a prominent ridge-like structure that can be seen in the 164 two-particle correlations; and this ridge is associated with 165 an enhanced correlation near ∆φ ∼ 0 and π and a long-166 range structure in ∆...

No abstract

The Breit-Wheeler process which produces matter and anti-matter from photon collisions is investigated experimentally through the observation of 6085 exclusive electron-positron pairs in ultraperipheral Au+Au collisions at √ s N N = 200 GeV. The measurements reveal a large fourth-order