Beam Energy Evolution of HBT Systematics at the AGS

Abstract

We present preliminary results of the first π interferometry (HBT) excitation function at intermediate AGS energies. The beam energy evolution of the correlations' dependence on m T , centrality, and emission angle with respect to the reaction plane are discussed. Comparisons with predictions of the RQMD cascade model are made. Two-particle intensity interferometry (HBT) measurements have long been used to study the geometry and dynamics of heavy ion collisions (see, e.g. [1]). Pion correlation functions are sensitive to the pion source size, shape, decay-time, and long-lived particle (e.g. Λ) production. In addition, dynamic effects such as flow produce space-momentum correlations resulting in dependences of the correlation functions on π momentum.In this paper, we discuss an excitation function (2-8 AGeV) of π − HBT measurements. Studying the evolution of the correlations as a function of E beam is important for two reasons. Firstly, a sudden increase, at some E beam , in the lifetime of the hadronic fireball has long been proposed as a robust signal of the onset of QGP formation [2][3][4]. Secondly, the sensitivity of correlation functions to the underlying physics makes such measurements potent tools to test the dynamics of microscopic models of heavy ion collisions. Many models attempt to extrapolate to the RHIC energies. Confidence in the ability to extrapolate (determined by the correct underlying physics and its evolution with energy) would be enhanced if the model reproduces an excitation function of detailed HBT systematics.Using the large-acceptance EOS Time Projection Chamber [5] the E895 collaboration measured charged particles from Au+Au collisions at 2, 4, 6, and 8 AGeV at the Brookhaven AGS. Good particle identification minimized e − contamination of the π − sample. Momentum resolution, largely due to multiple Coulomb scattering and straggling in the 3% interaction length target, was on the order of 1.5-3%. The experimental correlation functions have been corrected for the momentum resolution with an iterative method similar to that employed by the NA44 collaboration [6]. This correction typically increases the fitted λ parameter by 15%, and the radii by 5%.Track merging and splitting effects were eliminated by a two-track geometrical cut based on the tracking algorithm. As expected, this cut discards some pairs (in the "real" and event-mixed distributions) at low relative momentum, q. However, due to detector geometry, this cut preferentially discards low-q pairs at high p T ; thus, to minimize phasespace bias effects, we restrict our analysis to low p T and use narrow windows in p T .A full Coulomb-wave integration [7,8] over a spatial source of 5-fm Gaussian radius was used to generate the Coulomb correction, which was applied pair-wise to the event-mixed

Tilted pion sources from azimuthally sensitive HBT interferometry

“…The required Φ-binning puts severe demands on the pair statistics. A first observation of azimuthally oscillating HBT radii was reported in [14]. Such measurements require a reasonably accurate determination of the reaction plane; a typical uncertainty of 30 • reduces the first and second harmonics in (3) by ∼ 15% and ∼ 45%, respectively, but these losses can be corrected for [7,15].…”

Laying the groundwork at the AGS: Recent results from experiment E895

“…Until recently, most HBT analyses implicitly assumed a cylindrically symmetric emission region, strictly valid only for |b| = 0 collisions. In this standard framework, E895 π − correlation systematics as a function of beam energy, m T , centrality, and rapidity, have already been reported [9,12].…”

“…We have measured excitation functions in four primary areas: (1) proton elliptic and directed flow [7,8], (2) intensity interferometry [9][10][11][12], (3) charged particle spectra [13,14], and (4) strange particle production [15]. In addition to improving our understanding of hot hadronic systems under extreme conditions, these systematics provide a valuable baseline for measurements at higher energy.…”