Measurements of the A dependence and pseudorapidity interval [Formula Presented] dependence of midrapidity [Formula Presented] distributions in a half-azimuth (Δφ=π) electromagnetic calorimeter are presented for [Formula Presented] O+Cu, Si+Au, and Au+Au collisions at the BNL-AGS (Alternating-Gradient Synchrotron). The shapes of the upper edges of midrapidity [Formula Presented] distributions as a function of the pseudorapidity interval δη in the range 0.3 to 1.3, roughly centered at midrapidity, are observed to vary with δη, like multiplicity—the upper edges of the distributions flatten as δη is reduced. At the typical fixed upper percentiles of [Formula Presented] distributions used for nuclear geometry characterization by centrality definition—7 percentile, 4 percentile, 2 percentile, 1 percentile, 0.5 percentile—the effect of this variation in shape on the measured projectile [Formula Presented] dependence for [Formula Presented] [Formula Presented]Au projectiles on an Au target is small for the ranges of δη and percentile examined. The [Formula Presented] distributions for [Formula Presented] and [Formula Presented] change in shape with δη
but in each δη interval the shapes of the [Formula Presented] and [Formula Presented] distributions remain indentical with each other—a striking confirmation of the absence of multiple-collision effects at midrapidity at AGS energies. The validity of the nuclear geometry characterization versus δη is illustrated by plots of the [Formula Presented] distribution in each δη interval in units of the measured [Formula Presented] in the same δη interval for [Formula Presented] collisions. These plots, in the physically meaningful units of “number of average [Formula Presented] collisions”, are nearly universal as a function of δη, confirming that the reaction dynamics for [Formula Presented] production at midrapidity at AGS energies is governed by the number of projectile participants and can be well characterized by measurements in apertures as small as Δφ=π, δη=0.3. © 2001 The American Physical Society.