Centrality dependence of longitudinal and transverse baryon distributions in ultrarelativistic nuclear collisions

Inclusive double differential multiplicities ${\mathit{d}}^2$N/dy d${\mathit{p}}_{\mathit{t}}$ and related quantities have been measured for protons and deuterons produced in 14.6A GeV/c Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision ‘‘centrality’’ is determined by measuring ${\mathit{N}}_{\mathit{c}}$, the total charged particle multiplicity in the pseudorapidity range 0.85dN/dy at midrapidity and a corresponding decrease at higher rapidities are observed with increasing ${\mathit{N}}_{\mathit{c}}$. For Si+Pb, Boltzmann slope parameters ${\mathit{T}}_{\mathit{B}}$ increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when σ/${\mathrm{\sigma }}_{\mathrm{geo}}$≲10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution dN/dy is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with ${\mathit{N}}_{\mathit{c}}$. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.