Measurement of Non-prompt $\rm D^0$-meson Elliptic Flow in Pb-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The elliptic flow ($v_2$) of $\rm D^0$ mesons from beauty-hadron decays (non-prompt $\rm D^0$) was measured in midcentral (30-50%) Pb-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The $\rm D^0$ mesons were reconstructed at midrapidity ($|y|<~0.8$) from their hadronic decay $\mathrm{D^0 \to K^-\pi^+}$, in the transverse momentum interval $2 <~ p_{\rm T} <~ 12$ GeV/$c$. The result indicates a positive $v_2$ for non-prompt $\rm D^0$ mesons with a significance of 2.7$\sigma$. The non-prompt $\rm D^0$-meson $v_2$ is lower than that of prompt non-strange D mesons with 3.2$\sigma$ significance in $2 <~ p_{\rm T} <~ 8$ GeV/$c$, and compatible with the $v_2$ of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.


Eur. Phys. J. C 83 (2023) 1123
HEP Data
e-Print: arXiv:2307.14084 | PDF | inSPIRE
Figure group

Figure 1

Simultaneous fits of the invariant-mass distribution and $v_2^\mathrm{tot}(M_\mathrm{K\pi})$ of $\Dzero$ mesons in $3 < \pt < 4$ $\GeV/c$. Left panel: fits using $\Dzero$-meson candidates with low probability to be a non-prompt $\Dzero$ meson. Right panel: fits using $\Dzero$-meson candidates with high probability to be a non-prompt $\Dzero$ meson. The corresponding BDT score selection for the measured raw yield is reported. The blue lines, the dotted red curves, and the green solid lines represent the total fit function, the combinatorial-background fit function, and the contribution of the reflected signal, respectively.

Figure 2

Left panel: example of the raw-yield distribution as a function of the minimum non-prompt $\Dzero$-meson BDT score threshold to determine the non-prompt $\Dzero$-meson fraction in $3 < \pt < 4$ $\GeV/c$. Right panel: $v_2^{\rm sig}$ as a function of $f_{\rm nonprompt}$ in $3 < \pt < 4$ $\GeV/c$. The blue band represents the 1$\sigma$ confidence interval obtained from the linear fit.

Figure 3

Left panel: Elliptic flow $v_2$ of non-prompt $\Dzero$ mesons (blue points) and average of prompt non-strange D mesons  (red points) as a function of $\pt$ in 30--50% Pb--Pb collisions at $\sqrtsNN=5.02 \TeV$. The symbols are positioned at the average $\pt$ of the reconstructed $\Dzero$ mesons. Statistical uncertainties are shown as vertical lines and systematic uncertainties as boxes. Right panel: non-prompt $\Dzero$-meson $v_2$ compared with model calculations .

Figure 4

Elliptic flow $v_2$ of non-prompt \Dzero mesons (blue points) and electrons from beauty-hadron decays  (red points) as a function of \pt in 30--50\% Pb--Pb collisions at $\sqrtsNN=5.02 \TeV$, compared with the LIDO model predictions .