New method for measuring azimuthal distributions in nucleus-nucleus collisions

Abstract

The methods currently used to measure azimuthal distributions of particles in heavy ion collisions assume that all azimuthal correlations between particles result from their correlation with the reaction plane. However, other correlations exist, and it is safe to neglect them only if azimuthal anisotropies are much larger than 1/ √ N , with N the total number of particles emitted in the collision. This condition is not satisfied at ultrarelativistic energies. We propose a new method, based on a cumulant expansion of multiparticle azimuthal correlations, which allows measurements of much smaller values of azimuthal anisotropies, down to 1/N . It is simple to implement and can be used to measure both integrated and differential flow. Furthermore, this method automatically eliminates the major systematic errors, which are due to azimuthal asymmetries in the detector acceptance.

Elliptic flow from minijet production in heavy ion collisions

“…2 we show that the standard determination of flow employing Eq. (10) and the flow extracted from the 2nd cumulant proposed in [32,33,34] should yield the same result for v 2 for a trivial choice of weights. This result was confirmed by an explicit flow analysis using two different methods at STAR giving almost the same result for v 2 [36].…”

“…Therefore the elliptic flow observable v 2 may be potentially sensitive to the nonflow correlations originating from minijet production in the initial conditions. To avoid this problem the authors of [32] (see also [34]) introduced a cumulant approach to flow analysis. In terms of the saturation model contribution of minijets to higher order cumulants defined in [32,34] is suppressed by powers of S ⊥ Q 2 s ∼ N part .…”

“…In terms of the saturation model contribution of minijets to higher order cumulants defined in [32,34] is suppressed by powers of S ⊥ Q 2 s ∼ N part . This led the authors of [32,34] to suggest that the high-cumulant flow analysis would be relatively free of minijet effects compared to standard flow analysis.…”

CMS Physics Technical Design Report: Addendum on High Density QCD with Heavy Ions

“…It was thus proposed to measure flow with multi-particle azimuthal correlations by performing a cumulant expansion where the collective source of correlations can be disentangled from other sources [189,190]. The main advantage of the higher order cumulant analysis lies in the fact that, if the flow is larger than the non-flow correlations, the contribution of the latter to v 2 , extracted from higher order correlators, is suppressed by powers of the particle multiplicity in the event, N 0 .…”

“…Since all particles are correlated with the reaction plane, they are also indirectly correlated with each other [179,189]. In the case when the particle distribution can be well described by an elliptic form, equation (4.3), and there are no other particle correlations besides those due to flow (or other correlations can be neglected), the azimuthal anisotropy coefficient can be determined using the two-particle azimuthal correlator without the event plane angle 0 ,…”