Transition from in-plane to out-of-plane azimuthal enhancement in Au+Au collisions

Abstract

The incident energy at which the azimuthal distributions in semi-central heavy ion collisions change from in-plane to out-of-plane enhancement -E tran is studied as a function of mass of emitted particles, their transverse momentum and centrality for Au+Au collisions. The analysis is performed in a reference frame rotated with the sidewards flow angle (Θ f low ) relative to the beam axis.A systematic decrease of E tran as function of mass of the reaction products, their transverse momentum and collision centrality is evidenced.The predictions of a microscopic transport model (IQMD) are compared with the experimental results.

Azimuthal Dependence of Collective Expansion for Symmetric Heavy-Ion Collisions

“…Figure 2 provides global results: E 0 coll , E max coll (E coll at 90 • ) and ∆E coll as functions of A part for both measured systems. At 90 A·MeV, the in-plane and out-of-plane flow values are very similar (∆E coll ∼0) at all centralities in Au+Au, paralleling the observations for the yields for which a transition from in-plane to out-of-plane enhancement was found [24] as a function of incident energy. At all energies, E 0 coll increases with the centrality corresponding to an increasing baryonic number A of the fireball.…”

“…Nevertheless there are also some advantages in studying less central collisions: i) rotation and shadowing can be used as internal clocks for getting deeper information on the expansion dynamics, ii) the centrality can be used to control the shape and content of the fireball and of the shadowing matter, iii) for a given centrality the passage time of the shadowing objects can be controlled varying the incident energy, iv) the confinement of the spectators becomes more efficient in transverse directions than in the central collisions. Symmetry considerations imply two dominant components in the particle transverse emission: azimuthally symmetric emission and an elliptic modulation of that emission (squeeze-out) which has been predicted by hydrodynamical calculations [12] and extensively studied experimentally [13][14][15][16][17][18][19][20][21][22][23][24]. The squeeze-out has been studied, in particular, as a function of centrality, type of emitted particle, transverse momentum and mass of the colliding systems.…”

“…Detector details can be found elsewhere [25,26,24]. The main component implemented in the FOPI configuration is a Central Drift Chamber (CDC) [26].…”

“…Studies of the squeeze-out phenomena revealed from the very beginning [13] the importance of performing the azimuthal distribution analysis in a reference frame with the polar axis along the sidewards flow direction. The present analysis used a reference frame in which the ellipsoidal pattern of the azimuthal particle distribution at midrapidity maximizes the ratio of the two transverse semiaxes [19,22,24]. The previous comprehensive description of the elliptic modulation of collectivity in the beam range 0.25 -1.15 A·GeV [19] was based on a parameterization of the deuteron (A=2) tranversemass spectra with an expression characteristic of radially symmetric shell expansion.…”

Sensitivity of the transition energy towards mass asymmetry of the colliding nuclei

“…On the other hand, a negative value of Cos2φ denotes preferential out-of-plane emission of the nucleon. Mathematically, it can be written as [9]:…”