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| CMS-HIN-19-008 ; CERN-EP-2020-155 | ||
| Search for strong electric fields in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV using azimuthal anisotropy of prompt ${\mathrm{D^0}}$ and ${\mathrm{\overline{D}}{}^0}$ mesons | ||
| CMS Collaboration | ||
| 26 September 2020 | ||
| Phys. Lett. B 816 (2021) 136253 | ||
| Abstract: The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapidity-dependent difference ($\Delta v_{2}$) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, $v_{2}$) between ${\mathrm{D^0}}$ ($\mathrm{\bar{u}}\mathrm{c}$) and ${\mathrm{\overline{D}}{}^0}$ ($\mathrm{u}\mathrm{\bar{c}}$) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of $\Delta v_{2}$. The rapidity-averaged value is found to be $<\Delta v_{2} > = $ 0.001 $\pm$ 0.001 (stat) $\pm$ 0.003 (syst) in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. In addition, the influence of the collision geometry is explored by measuring the ${\mathrm{D^0}}$ and ${\mathrm{\overline{D}}{}^0}$ mesons $v_{2}$ and triangular flow coefficient ($v_{3}$) as functions of rapidity, transverse momentum (${p_{\mathrm{T}}}$), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt ${\mathrm{D^0}}$ meson $v_{2}$ values is observed, while the $v_{3}$ is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry. | ||
| Links: e-print arXiv:2009.12628 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Simultaneous fit of the $\pi \mathrm{K} $ invariant mass (left) and $ {v_{2}} $ ($\Delta {v_{2}} $) as function of invariant mass (right) for 3.0 $ < {p_{\mathrm {T}}} < $ 3.5 GeV/$c$, centrality 20-70%, and $-0.6 < y < 0.0$. |
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Figure 1-a:
$\pi \mathrm{K} $ invariant mass $ {v_{2}} $ for 3.0 $ < {p_{\mathrm {T}}} < $ 3.5 GeV/$c$, centrality 20-70%, and $-0.6 < y < 0.0$, with superimposed result of the simultaneous fit. |
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Figure 1-b:
$ {v_{2}} $ ($\Delta {v_{2}} $) as function of invariant mass (right) for 3.0 $ < {p_{\mathrm {T}}} < $ 3.5 GeV/$c$, centrality 20-70%, and $-0.6 < y < 0.0$, with superimposed result of the simultaneous fit. |
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Figure 2:
Prompt ${\mathrm{D^0}}$ meson flow coefficients $ {v_{2}} $ (upper) and $ {v_{3}} $ (lower) at midrapidity ($ {| y |} < $ 1.0) for the centrality classes 0-10% (left), 10-30% (middle), and 30-50% (right). The vertical bars and open boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars represent the width of each ${p_{\mathrm {T}}}$ bin. Theoretical calculations for $ {v_{\mathrm {n}}} $ coefficients of prompt ${\mathrm{D^0}}$ mesons are also plotted for comparison (LBT [32], CUJET 3.0 [33], SUBATECH [34], TAMU [35], and PHSD [15]). |
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Figure 3:
Prompt ${\mathrm{D^0}}$ meson flow coefficients $ {v_{2}} $ (upper) and $ {v_{3}} $ (lower) at midrapidity ($ {| y |} < $ 1, red open circles) and forward rapidity (1 $ < {| y |} < $ 2, blue open diamonds) for the centrality classes 0-10% (left), 10-30% (middle), and 30-50% (right). The vertical bars and open boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars represent the width of each ${p_{\mathrm {T}}}$ bin. |
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Figure 4:
Prompt ${\mathrm{D^0}}$ meson $ {v_{2}} $ and $ {v_{3}} $ as functions of centrality, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for rapidity ranges $ {| y |} < $ 1 and 1 $ < {| y |} < $ 2 (left). Prompt ${\mathrm{D^0}}$ $ {v_{2}} $ and $ {v_{3}} $ as functions of rapidity, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for centrality 20-70% (right). The vertical bars represent statistical uncertainties and open boxes represent systematic uncertainties. The horizontal bars represent the width of each bin. |
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Figure 5:
Prompt ${\mathrm{D^0}}$ meson $\Delta {v_{2}} $ as a function of rapidity, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and centrality 20-70%. The vertical bars represent statistical uncertainties and open boxes represent systematic uncertainties. The horizontal bars represent the width of each bin. |
| Tables | |
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Table 1:
Summary of systematic uncertainties in absolute values for $ {v_{2}} $, $ {v_{3}} $, and $\Delta {v_{2}} $. Ranges of the variation of uncertainties for all the bins are presented. The cells filled with "---'' refer to the cases where the uncertainty cancels out. |
| Summary |
|
Measurements of the elliptic ($v_{2}$) and triangular ($v_{3}$) flow coefficients of prompt ${\mathrm{D^0}}$ mesons are presented as functions of transverse momentum (${p_{\mathrm{T}}}$), rapidity, and collision centrality, in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The results improve previously published CMS data by extending the ${p_{\mathrm{T}}}$ and rapidity coverage and by providing more differential information in ${p_{\mathrm{T}}}$, rapidity, and centrality. A clear centrality dependence of prompt ${\mathrm{D^0}}$ meson $v_{2}$ is observed, while $v_{3}$ is largely centrality independent. These trends are consistent with the expectation that $v_{2}$ and $v_{3}$ are driven by initial-state geometry. A weak rapidity dependence of prompt ${\mathrm{D^0}}$ meson $v_{2}$ and $v_{3}$ is observed. When comparing various theoretical calculations to the data at midrapidity, no model is able to describe the data over the full centrality and ${p_{\mathrm{T}}}$ ranges. Motivated by the search for evidence of the strong electric field expected in PbPb collisions, a first measurement of the $v_{2}$ flow coefficient difference ($\Delta v_{2}$) between ${\mathrm{D^0}}$ and ${\mathrm{\overline{D}}{}^0}$ mesons as a function of rapidity is presented. The rapidity-averaged $v_{2}$ difference is measured to be $<\Delta v_{2} > = $ 0.001 $\pm$ 0.001 (stat) $\pm$ 0.003 (syst). This indicates that there is no evidence that charm hadron collective flow is affected by the strong Coulomb field created in ultrarelativistic heavy ion collisions. Future comparisons of theoretical models with these results may provide constraints on the electric conductivity of the quark-gluon plasma. |
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Compact Muon Solenoid LHC, CERN |
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