Further observations on midrapidity E_T distributions with aperture corrected scale

In a previous publication [T. Abbott et al., E802 Collaboration, Phys. Rev. C 63, 064602 (2001); 64, 029901(E) (2001)], measurements of the A dependence and pseudorapidity interval (δη) dependence of midrapidity E _T distributions in a half-azimuth (Δ=π) electromagnetic calorimeter were presented for p + Be, p + Au, O+Cu, Si+Au, and Au+Au collisions at the BNL-AGS. The validity of the "nuclear geometry" characterization versus δη was illustrated by plots of the E _T(δη) distribution in each δη interval in units of the measured 〈E_T(δη)〉_p+Au in the same δη interval for p + Au collisions. These plots, with aperture corrected scale in the physically meaningful units of number of average observed p + Au collisions, were nearly universal as a function of δη, confirming that the reaction dynamics for E_T 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 Δ=π, δη=0.3. A key ingredient in these analyses is the probability P₀ for no signal to be detected in a given aperture δη for the fundamental p + Au collision. In fact the measured 〈E_T(δη)〉1_p+Au is biased and the true 〈E_T(δη)〉_p+Au^true for the detector aperture is the measured value times 1-p₀. The issues and merits of measuring the E_T(δη) distribution in units of 〈E_T(δη)〉_p+Au or 〈E _T(δη)〈_p-Au^true in the same δη interval are presented and discussed. This method has application at RHIC, where p-p data could be used as the reference distribution for two participants. The E_T distributions for B+A collisions, with E _T(δη) scale normalized by 〈E _T(δη)〉_p-p^truein the same aperture for p-p collisions, would then be given directly in the popular unit "per participant-pair" [K. Adcox et al., PHENIX Collaboration, Phys. Rev. Lett. 86, 3500 (2001); I. G. Bearden et al., BRAHMS Collaboration, Phys. Lett. B523, 227 (2001); B. B. Back et al., PHOBOS Collaboration, Phys. Rev. C 65, 031901 (R) (2002); C. Adler et al., STAR Collaboration, Phys. Rev. Lett. 89, 202301 (2002)].