Improved measurement of thebb¯production cross section in 920 GeV fixed-target proton-nucleus collisions

A new measurement of the $b\overline{b}$ production cross section in 920 GeV proton-nucleus collisions is presented by the HERA-B Collaboration. The $b\overline{b}$ production is tagged via inclusive bottom quark decays into $J/\psi$ mesons by exploiting the longitudinal separation of $J/\psi \to l^{+}{l}^{-}$ decay vertices from the primary proton-nucleus interaction point. Both $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ channels are reconstructed for a total of $83\pm 12$ inclusive $b\to J/\psi X$ events found. The combined analysis yields a $b\overline{b}$ to prompt $J/\psi$ cross section ratio of $\frac{\Delta \sigma (b\overline{b})}{\Delta {\sigma }_{J/\psi }}=0.032\pm {0.005}_{\mathrm{stat}}\pm {0.004}_{\mathrm{sys}}$ measured in the $x_F$ acceptance ($-0.35<x_F<0.15$), extrapolated to $\sigma (b\overline{b})=14.9\pm {2.2}_{\mathrm{stat}}\pm {2.4}_{\mathrm{sys}}\mathrm{\text{nb/nucleon}}$ in the total phase space.