Improved measurement of the $b\overline{b}$ 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.

Figure 1

Plan view of the HERA-B detector.

Figure 2

Invariant mass plots for both $J/\psi$-decay channels: (a) muon sample, (b) electron sample.

Figure 3

Comparison between the distribution of the lepton impact parameter to the wire for real data (points with error bars) and for the prompt $J/\psi$ MC (histogram). (a) $J/\psi \to {\mu }^{+}{\mu }^{-}$ candidates selected in the region $2.95<m_{{\mu }^{+}{\mu }^{-}}<3.25\mathrm{GeV}/c^2$ after sideband subtraction for real data, (b) double BR tagged $J/\psi \to e^{+}{e}^{-}$ candidates selected in the region $2.9<m_{e^{+}{e}^{-}}<3.2\mathrm{GeV}/c^2$ after sideband subtraction for real data. The continuous solid line shows the $b\to J/\psi$ MC sample (arbitrary normalization) while the arrows mark the cut position determined by the optimization procedure.

Figure 4

Dimuon mass distributions after vertex detachment cuts, from the full muon sample. (a) Upstream events (combinatorial background), (b) downstream events. The solid line shows the result of the likelihood fit.

Figure 5

Dilepton mass distribution after vertex detachment cuts, from the full electron sample. (a) Upstream events (combinatorial background), (b) downstream events. The solid line shows the result of the likelihood fit.

Figure 6

Scatter plot of ${\mu }^{+}{\mu }^{-}$ (a) and $e^{+}{e}^{-}$ (b) invariant mass versus the proper time for the detached candidates. The downstream detached candidates are marked with filled circles in the $J/\psi$ regions ($3.0<m_{{\mu }^{+}{\mu }^{-}}<3.2\mathrm{GeV}/c^2$ and $2.9<m_{e^{+}{e}^{-}}<3.2\mathrm{GeV}/c^2$) and with open circles elsewhere. Upstream candidates are shown with open triangle marks. The hatches define the regions excluded from the mean lifetime measurement.

Figure 7

(a) Likelihoods for the ${\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}$ detached candidate fits and joint likelihood as a function of the mean lifetime; (b) distribution of proper times for the detached $J/\psi$ events corrected for selection efficiency.

Figure 8

${\mu }^{+}{\mu }^{-}$ invariant mass for the $J/\psi +h^{\pm }$ analysis for upstream (a) and downstream (b) events. The line in (b) shows the result of the unbinned max likelihood fit. (c) ${\mu }^{+}{\mu }^{-}$ invariant mass as a function of the proper time for the detached candidates.

Figure 9

Likelihoods of the invariant mass fits as a function of $R_{\Delta \sigma }$, shown separately for ${\mu }^{+}{\mu }^{-}$ data (dotted line), $e^{+}{e}^{-}$ data (dashed line), and for the combination of the two (continuous line). The dotted-dashed line shows the likelihood function corresponding to the 2000 data analysis [10] which is used in the final average.

Figure 10

Comparison of the available $\sigma (b\overline{b})$ measurements with the theoretical predictions of R. Bonciani et al. [6] updated with the NNLO parton distribution function in [40] (solid line: central value, dashed lines: upper and lower bounds) and N. Kidonakis et al. [7] (dotted-dashed line: central value; dotted lines: upper and lower bounds). The upper and lower bounds have been defined by changing the renormalization and factorization scales (by a factor 0.5 and 2.0) and the quark mass ($4.5–5.0\mathrm{GeV}/c^2$).