Measurement of the $B^{\pm }$ Meson Nuclear Modification Factor in Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$

The differential production cross sections of $B^{\pm }$ mesons are measured via the exclusive decay channels $B^{\pm }\to J/\psi K^{\pm }\to {\mu }^{+}{\mu }^{-}{K}^{\pm }$ as a function of transverse momentum in $p$$p$ and Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ per nucleon pair with the CMS detector at the LHC. The $pp(\mathrm{Pb-Pb})$ data set used for this analysis corresponds to an integrated luminosity of $28.0{\mathrm{pb}}^{-1}$ ($351\mu {\mathrm{b}}^{-1}$). The measurement is performed in the $B^{\pm }$ meson transverse momentum range of 7 to $50\mathrm{GeV}/c$, in the rapidity interval $|y|<2.4$. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from $pp$ reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Figure 1

Invariant mass distributions of $B^{\pm }$ candidates in $pp$ (left) and Pb-Pb (right) collisions measured in $|y|<2.4$ and in the $p_T$ region $10–15\mathrm{GeV}/c$.

Figure 2

The $p_T$ -differential production cross section of $B^{\pm }$ in $pp$ (left) and Pb-Pb (right) collisions at $\sqrt{s}=5.02\mathrm{TeV}$. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The systematic uncertainty boxes here include both the correlated and uncorrelated contributions added in quadrature. The global systematic uncertainty is listed in the legend and not included in the point-to-point uncertainties. For the $p$$p$ cross section, they comprise the uncertainties in the integrated luminosity measurement and in the branching fraction $\mathcal{B}$. For the Pb-Pb cross section, they comprise the uncertainties in $T_{AA}$, $N_{\mathrm{MB}}$, and $\mathcal{B}$. The $pp$ cross section is compared to fixed-order plus next-to-leading logarithm (FONLL) calculations [41, 42, 43] represented by the colored boxes with the heights indicating the theoretical uncertainty.

Figure 3

The $p_T$ dependence of the nuclear modification factor $R_{AA}$ of $B^{\pm }$ measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The global systematic uncertainty, represented as a grey box at $R_{AA}=1$, comprises the uncertainties in the integrated luminosity measurement and $T_{AA}$ value. Four theoretical calculations are also shown for comparison: TAMU [44, 45], Djordjevic [46], cujet3.0 [47, 48, 49], and anti–de Sitter/conformal field theory ($\mathrm{AdS}/\mathrm{CFT}$) HH [50, 51]. The line width of the theoretical calculation from Ref. [44, 45] represents the size of its statistical uncertainty.