Further observations on midrapidity ET distributions with aperture corrected scale -: art. no. 034908

In a previous publication [T. Abbott , E802 Collaboration, Phys. Rev. C 63, 064602 (2001); 64, 029901(E) (2001)], measurements of the A dependence and pseudorapidity interval (deltaeta) dependence of midrapidity E-T distributions in a half-azimuth (Deltaphi=pi) 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 deltaeta was illustrated by plots of the E-T(deltaeta) distribution in each deltaeta interval in units of the measured <E-T(deltaeta)>(p+Au) in the same deltaeta 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 deltaeta, 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 Deltaphi=pi,deltaeta=0.3. A key ingredient in these analyses is the probability p(0) for no signal to be detected in a given aperture deltaeta for the fundamental p+Au collision. In fact the measured <E-T(deltaeta)>(p+Au) is biased and the true <E-T(deltaeta)>(true)(p+Au) for the detector aperture is the measured value times 1-p(0). The issues and merits of measuring the E-T(deltaeta) distribution in units of <E-T(deltaeta)>(p+Au) or <E-T(deltaeta)>(true)(p+Au) in the same deltaeta 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(deltaeta) scale normalized by <E-T(deltaeta)>(true)(p-p) in the same aperture for p-p collisions, would then be given directly in the popular unit "per participant-pair" [K. Adcox , PHENIX Collaboration, Phys. Rev. Lett. 86, 3500 (2001); I. G. Bearden , BRAHMS Collaboration, Phys. Lett. B523, 227 (2001); B. B. Back , PHOBOS Collaboration, Phys. Rev. C 65, 031901(R) (2002); C. Adler , STAR Collaboration, Phys. Rev. Lett. 89, 202301 (2002)].