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

Using a silicon-microstrip detector array to identify secondary vertices, we have observed $b\to J/\psi \to {\mu }^{+}{\mu }^{-}$ decays in $800\mathrm{GeV}/c$ proton-gold interactions. The doubly differential cross section for $J/\psi$ mesons originating from $b$-quark decays, assuming linear dependence on nucleon number, is $d^2\sigma /{\mathrm{dx}}_F{\mathrm{dp}}_T^2=107\mathrm{}\pm 28\pm 19[\mathrm{pb}/(\mathrm{GeV}/c{)}^2]$/nucleon at $x_F=0.05\mathrm{}$ and $p_T=1\mathrm{GeV}/c$. 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 $\sigma (\mathrm{pN}\to b\overline{b}+X)=5.7\mathrm{}\pm 1.5\pm 1.3$ nb/nucleon.