Systematics of midrapidity transverse energy distributions in limited apertures from p+Be to Au+Au collisions at relativistic energies - art. no. 064602

Measurements of the A dependence and pseudorapidity interval (delta eta) de pendence of midrapidity E-T distributions in a half-azimuth (Delta phi = pi ) electromagnetic calorimeter are presented for p + Be, p + Au, O + Cu, Si + Au, and Au + Au collisions at the BNL-AGS (Alternating-Gradient Synchrotr on). The shapes of the upper edges of midrapidity E-T distributions as a fu nction of the pseudorapidity interval delta eta in the range 0.3 to 1.3, ro ughly centered at midrapidity, are observed to vary with delta eta. like mu ltiplicity-the upper edges of the distributions flatten as delta eta is red uced. At the typical fixed upper percentiles of E-T distributions used for nuclear geometry characterization by centrality definition-7 percentile, 4 percentile, 2 percentile, I percentile, 0.5 percentile-the effect of this v ariation in shape on the measured projectile A(p) dependence for O-16, Si-2 8, Au-197 projectiles on an Au target is small for the ranges of delta eta and percentile examined. The E-T distributions for p + Au and p + Be change in shape with delta eta; but in each delta eta interval the shapes of the p + Au and p + Be distributions remain indentical with each other-a strikin g confirmation of the absence of multiple-collision effects at midrapidity at AGS energies. The validity of the nuclear geometry characterization vers us delta eta is illustrated by plots of the E-T(delta eta) distribution in each delta eta interval in units of the measured <E-T(delta eta)> (p + Au) same delta eta interval for p + Au collisions. These plots, in the physical ly meaningful units of "number of average p + Au collisions," are nearly un iversal as a function of delta eta confirming that the reaction dynamics fo r E-T production at midrapidity at AGS energies is governed by the number o f projectile participants and can be well characterized by measurements in apertures as small as Delta phi = pi, delta eta = 0.3.