Measurement of the Bottom-Quark Production Cross Section in 800 GeV/<i>c</i>Proton-Gold Collisions

Using a silicon-microstrip detector array to identify secondary vertices, we have observed $b\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decays in $800\mathrm{GeV}/c$ proton-gold interactions. The doubly differential cross section for $J/\ensuremath{\psi}$ mesons originating from $b$-quark decays, assuming linear dependence on nucleon number, is ${d}^{2}\ensuremath{\sigma}/{\mathrm{dx}}_{F}{\mathrm{dp}}_{T}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}107\ifmmode\pm\else\textpm\fi{}28\ifmmode\pm\else\textpm\fi{}19[\mathrm{pb}/(\mathrm{GeV}/c{)}^{2}]$/nucleon at ${x}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.05$ and ${p}_{T}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\phantom{\rule{0ex}{0ex}}\mathrm{GeV}/c$. This measurement is compared to next-to-leading-order QCD predictions. The integrated $b$-quark production cross section, obtained by extrapolation over all ${x}_{F}$ and ${p}_{T}$, is $\ensuremath{\sigma}(\mathrm{pN}\ensuremath{\rightarrow}b\overline{b}+X)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5.7\ifmmode\pm\else\textpm\fi{}1.5\ifmmode\pm\else\textpm\fi{}1.3$ nb/nucleon.