Measurement of the relative $B_c^{\pm }/B^{\pm }$ production cross section with the ATLAS detector at $\sqrt{s}=8\mathrm{TeV}$

The total cross section and differential cross sections for the production of $B_c^{\pm }$ mesons, times their branching fraction to $J/\psi {\pi }^{\pm }$, are measured relative to those for the production of $B^{\pm }$ mesons, times their branching fraction to $J/\psi K^{\pm }$. The data used for this study correspond to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ of $pp$ collisions recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a center-of-mass energy of $\sqrt{s}=8\mathrm{TeV}$. The measurement is performed differentially in bins of transverse momentum $p_{\mathrm{T}}$ for $13\mathrm{GeV}<p_{\mathrm{T}}(B_c^{\pm })<22\mathrm{GeV}$ and $p_{\mathrm{T}}(B_c^{\pm })>22\mathrm{GeV}$ and in bins of rapidity $y$ for $|y|<0.75$ and $0.75<|y|<2.3$. The relative cross section times branching fraction for the full range $p_{\mathrm{T}}>13\mathrm{GeV}$ and $|y|<2.3$ is $(0.34\pm 0.04_{\mathrm{stat}}{{}_{-0.02}^{+0.06}}_{\mathrm{sys}}\pm 0.01_{\text{lifetime}})\%$. The differential measurements suggest that the production cross section of the $B_c^{\pm }$ decreases faster with $p_{\mathrm{T}}$ than the production cross section of the $B^{\pm }$, while no significant dependence on rapidity is observed.

Figure 1

The projection of the fit of the invariant mass distribution for the $B^{\pm }$ meson. “Signal” stands for the signal component of the fit, “Comb. bkg” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and all background components. The fit for the low transverse momentum bin ($13\mathrm{GeV}<p_{\mathrm{T}}(B^{\pm })<22\mathrm{GeV}$) is shown at the top, and that for the high transverse momentum bin ($p_{\mathrm{T}}(B^{\pm })>22\mathrm{GeV}$) is shown at the bottom. The rapidity requirement is $|y(B^{\pm })|<2.3$. The fits are used to extract $N^{\mathrm{reco}}(B^{+})+N^{\mathrm{reco}}(B^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 2

The projection of the fit of the invariant mass distribution for the $B^{\pm }$ meson. “Signal” stands for the signal component of the fit, “Comb. bkg” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and all background components. The fit for the inner rapidity bin ($|y(B^{\pm })|<0.75$) is shown at the top, and that for the outer rapidity bin ($0.75<|y(B^{\pm })|<2.3$) is shown at the bottom. The $p_{\mathrm{T}}$ requirement is $p_{\mathrm{T}}(B^{\pm })>13\mathrm{GeV}$. The fits are used to extract $N^{\mathrm{reco}}(B^{+})+N^{\mathrm{reco}}(B^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 3

The projection of the fit of the invariant mass distribution for the $B^{\pm }$ meson using the complete dataset ($p_{\mathrm{T}}(B^{\pm })>13\mathrm{GeV}$, $|y(B^{\pm })|<2.3$). “Signal” stands for the signal component of the fit, “Comb. bkg” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and all background components. The fit is used to extract $N^{\mathrm{reco}}(B^{+})+N^{\mathrm{reco}}(B^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 4

The projection of the fit of the invariant mass distribution for the $B_c^{\pm }$ meson. “Signal” stands for the signal component of the fit, “Background” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and background components. The fit for the low transverse momentum bin ($13\mathrm{GeV}<p_{\mathrm{T}}(B_c^{\pm })<22\mathrm{GeV}$) is shown at the top, and that for the high transverse momentum bin ($p_{\mathrm{T}}(B_c^{\pm })>22\mathrm{GeV}$) at the bottom. The rapidity requirement is $|y(B_c^{\pm })|<2.3$. The fits are used to extract $N^{\mathrm{reco}}(B_c^{+})+N^{\mathrm{reco}}(B_c^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 5

The projection of the fit of the invariant mass distribution for the $B_c^{\pm }$ meson. “Signal” stands for the signal component of the fit, “Background” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and background components. The fit for the inner rapidity bin ($|y(B_c^{\pm })|<0.75$) is shown at the top, and that for the outer rapidity bin ($0.75<|y(B_c^{\pm })|<2.3$) at the bottom. The $p_{\mathrm{T}}$ requirement is $p_{\mathrm{T}}(B_c^{\pm })>13\mathrm{GeV}$. The fits are used to extract $N^{\mathrm{reco}}(B_c^{+})+N^{\mathrm{reco}}(B_c^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 6

The projection of the fit of the invariant mass distribution for the $B_c^{\pm }$ meson using the complete dataset ($p_{\mathrm{T}}(B_c^{\pm })>13\mathrm{GeV}$, $|y(B_c^{\pm })|<2.3$). “Signal” stands for the signal component of the fit, “Background” stands for the combinatorial background component of the fit, and “Total fit” stands for the fit to the sum of the signal and background components. The fit is used to extract $N^{\mathrm{reco}}(B_c^{+})+N^{\mathrm{reco}}(B_c^{-})$ and its uncertainty in each bin. The inset below each plot shows the bin-by-bin difference between the data point and the value obtained from the fit function divided by the quadrature sum of the statistical uncertainty and the systematic uncertainties obtained from varying the fit model.

Figure 7

Summary of the cross-section measurement presented in this article. The left figure shows the production cross section for the $B_c^{\pm }$ relative to the $B^{\pm }$ (times the corresponding branching fractions) for two bins in $p_{\mathrm{T}}$ (black data points) and for the inclusive bin (horizontal band). The right figure shows the measurement binned in rapidity (black data points). On each point the horizontal bar indicates the bin width. The vertical inner error bars on the data points indicate the statistical uncertainty. The outer error bars indicate the size of the quadrature sum of uncertainties from all sources: statistical, systematic, and lifetime. The double hatched error band indicates statistical uncertainty, while the single hatched bands indicate the quadrature sum of uncertainties from all sources.