Studies of Beauty Suppression via Nonprompt $D^0$ Mesons in Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

The transverse momentum spectra of $D^0$ mesons from $b$ hadron decays are measured at midrapidity ($|y|<1$) in $pp$ and Pb-Pb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The $D^0$ mesons from $b$ hadron decays are distinguished from prompt $D^0$ mesons by their decay topologies. In Pb-Pb collisions, the $B\to D^0$ yield is found to be suppressed in the measured $p_{\mathrm{T}}$ range from 2 to $100\mathrm{GeV}/c$ as compared to $pp$ collisions. The suppression is weaker than that of prompt $D^0$ mesons and charged hadrons for $p_{\mathrm{T}}$ around $10\mathrm{GeV}/c$. While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured $B\to D^0$ suppression at higher $p_{\mathrm{T}}$, the data show an indication of larger suppression than the model predictions in the range of $2<p_{\mathrm{T}}<5\mathrm{GeV}/c$.

Figure 1

(a) Example of a three-component invariant mass fit of a $D^0$ DCA bin for $p_{\mathrm{T}}$ of $6–7\mathrm{GeV}/c$ in Pb-Pb collisions. (b) DCA distributions for $D^0$ candidates in the signal invariant mass region and in the sidebands (scaled by the mass range ratio of 0.5) for $D^0$ $p_{\mathrm{T}}$ of $6–7\mathrm{GeV}/c$ in $pp$ collisions. (c) Signal DCA distribution obtained with the invariant mass fit for each DCA bin, and a $\text{prompt}+\text{nonprompt}$ two-component fit to it, for $D^0$ $p_{\mathrm{T}}$ of $6–7\mathrm{GeV}/c$ in Pb-Pb collisions. (d) Signal DCA distribution obtained with the sideband subtraction, and a $\text{prompt}+\text{nonprompt}$ two-component fit to it, for $D^0$ $p_{\mathrm{T}}$ of $6–7\mathrm{GeV}/c$ in $pp$ collisions.

Figure 2

Upper panel: $B\to D^0$ $p_{\mathrm{T}}$-differential cross section in $pp$ collisions and invariant yield in Pb-Pb collisions normalized with $T_{AA}$, at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The vertical bands around the data points represent the bin-by-bin systematic uncertainties. Uncertainties are smaller than the symbols in most cases. The cross section in $pp$ collisions is compared to FONLL calculations [33]. Lower panel: The data/FONLL ratio for the $B\to D^0$ $p_{\mathrm{T}}$ spectra in $pp$ collisions.

Figure 3

The $B\to D^0$ nuclear modification factor $R_{AA}$ for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ (red circles) compared to other particles [16, 18, 19, 24] (upper panel), and to various theoretical predictions [34, 35, 36, 37, 38, 39, 40, 41] (lower panel). The vertical bands around the data points and at unity represent the bin-by-bin and global systematic uncertainties, respectively.