Cross section for $b\overline{b}$ production via dielectrons in $d$ + Au collisions at $\sqrt{s_{NN}}=200$ GeV

We report a measurement of $e^{+}{e}^{-}$ pairs from semileptonic heavy-flavor decays in $d+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. By exploring the mass and transverse-momentum dependence of the yield, the bottom decay contribution can be isolated from charm, and quantified by comparison to pythia and mc@nlo simulations. The resulting $b\overline{b}$-production cross section is ${\sigma }_{b\overline{b}}^{d\mathrm{Au}}=1.37\pm 0.28\left(\mathrm{stat}\right)\pm 0.46\left(\mathrm{syst}\right)$ mb, which is equivalent to a nucleon-nucleon cross section of ${\sigma }_{bb}^{NN}=3.4\pm 0.8\left(\mathrm{stat}\right)\pm 1.1\left(\mathrm{syst}\right)\mu b$.

Figure 1

Mass distribution for the combinatorial background determined by event mixing $B_{+-}^{\mathrm{comb}}$ and $B_{\pm \pm }^{\mathrm{comb}}\mathrm{as}$ the red and black line, respectively. The shape difference due to the difference in acceptance between like-sign and unlike-sign pairs in PHENIX is clearly visible. Also shown are foreground like-sign pairs $N_{\pm \pm }$(black points) and $N_{\pm \pm }$ corrected for the acceptance difference (red points). The differences between points and lines are the correlated background.

Figure 2

The top panel shows the $e^{+}{e}^{-}$ pair foreground $N_{+-}$, the combinatorial background $B_{+-}^{\mathrm{comb}}$ determined through event mixing, and the difference of the two which is the sum of the signal we are interested in, $S_{+-}$, and the correlated background $B_{+-}^{\mathrm{cor}}$ that still needs to be subtracted. Shown in the middle panel is the estimate of the correlated background $B_{+-}^{\mathrm{cor}}$, which is the difference between the foreground like-sign pairs $N_{\pm \pm }$ corrected for the relative acceptance difference $\alpha$ between $N_{\pm \pm }$and $N_{+-}$[see Fig. 1 and Eq. (6)] and the combinatorial background $B_{+-}^{\mathrm{comb}}$. The bottom panel shows the signal $S_{+-}$ which is calculated as $N_{+-}-\alpha N_{\pm \pm }$. In this plot, the combinatorial background is normalized in a region with minimal correlated background [22].

Figure 3

$p_T$-integrated pair reconstruction efficiency (solid magenta circles), pair trigger efficiency (solid blue squares), and the product of the two (open black circles).

Figure 4

Inclusive $e^{+}{e}^{-}$-pair yield from minimum bias $d+\text{Au}$ collisions as a function of mass. The data are compared to our model of expected sources. The inset shows in detail the mass range up to 4.5 GeV/$c^2$. In the lower panel, the ratio of data to expected sources is shown with systematic uncertainties.

Figure 5

Double-differential $e^{+}{e}^{-}$-pair yield from semileptonic decays of heavy flavor in inelastic $d+\text{Au}$ collisions. Shown are mass projections in slices of $p_T$. The $p_T$ intervals are indicated in each panel. Systematic uncertainties are shown as bars, downward pointing arrows indicate upper limits at 95% confidence limit.

Figure 6

Top panel compares the mass dependence of $e^{+}{e}^{-}$-pair yield with pythia and mc@nlo calculations. The bottom panel shows the comparison for the $p_T$ dependence. The gray panel shown in top panel is not used in the fitting and is excluded in the $p_T$ projection.

Figure 7

Double-differential $e^{+}{e}^{-}$-pair yield from semileptonic decays of heavy flavor as simulated by pythia and mc@nlo. Shown are mass projections in slices of $p_T$. The $p_T$ intervals are indicated in each panel.

Figure 8

Double-differential $e^{+}{e}^{-}$-pair yield from heavy-flavor decays fit to simulated distributions from pythia. The mass region highlighted by the gray band in Fig. 6 is excluded from the fitting.

Figure 9

Double-differential $e^{+}{e}^{-}$-pair yield from heavy-flavor decays fit to simulated distributions from mc@nlo. The mass region highlighted by the gray band in Fig. 6 is excluded from the fitting.

Figure 10

Ratio of the data points (solid magenta points) to the mc@nlo fit (solid red line) shown in Fig. 9.