CMS-PAS-HIN-19-008

CMS-PAS-HIN-19-008
Search for strong electromagnetic fields in PbPb collisions at 5.02 TeV via azimuthal anisotropy of $\mathrm{D^0}$ and ${\mathrm{\overline{D}}{}^0}$ mesons
CMS Collaboration
November 2019

Abstract: Motivated by the search for strong electromagnetic fields created in PbPb collisions, the first measurement of the $v_2$ difference ( $\Delta v_2$ ) between $\mathrm{D^0}$ and ${\mathrm{\overline{D}}{}^0}$ is presented as a function of rapidity. The result for the rapidity-averaged $v_2$ difference is found to be $< \Delta v_2 > =$ 0.001 $\pm$ 0.001 (stat) $\pm$ 0.003 (syst), consistent with zero within experimental uncertainties. Comparisons with models may help to directly constrain the electric conductivity of the hot and dense medium formed in these collisions. Measurements of flow harmonics of $\mathrm{D^0}$ ( $\bar{u}c$ ) and ${\mathrm{\overline{D}}{}^0}$ ( $u\bar{c}$ ) mesons are presented as functions of rapidity ( $y$ ), transverse momentum ( $p_{\mathrm{T}}$ ), and collision centrality, for PbPb collisions at 5.02 TeV, using data collected by the CMS experiment during the 2018 LHC run. The results improve previous ones published by CMS, by extending the $p_{\mathrm{T}}$ coverage and providing more differential information. A clear centrality dependence of prompt $\mathrm{D^0}$ $v_2$ is observed, while $v_3$ is largely independent of centrality. The trend is consistent with expectations of flow driven by the initial-state geometry. No significant rapidity dependence of prompt ${\mathrm{\overline{D}}{}^0}$ $v_2$ and $v_3$ is observed.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 816 (2021) 136253. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Simultaneous fit on mass spectrum and $v_2$ ( $\Delta v_2$ ) as function of invariant mass for 3.0 $< p_{\mathrm{T}} <$ 3.5 GeV/ $c$ , centrality 20-70% and $-0.6 < y < 0.0$ .
png pdf	Figure 2: Prompt ${\mathrm{D^0}}$ meson $v_2$ (top) and $v_3$ (bottom) coefficients at midrapidity ( $\|y\| < 1$ ) for the centrality classes 0-10% (left), 10-30% (middle), and 30-50% (right). The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. Theoretical calculations for $v_n$ coefficient of prompt ${\mathrm{D^0}}$ mesons are also plotted for comparison.
png pdf	Figure 3: Prompt ${\mathrm{D^0}}$ meson $v_2$ (top) and $v_3$ (bottom) coefficients at midrapidity ( $\|y\| < 1$ ) and forward rapidity ( $1 < \|y\| < 2$ ) for the centrality classes 0-10% (left), 10-30% (middle), and 30-50% (right). The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties.
png pdf	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 function 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 the systematic uncertainties.
png pdf	Figure 4-a: 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. The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties.
png pdf	Figure 4-b: Prompt ${\mathrm{D^0}}$ $v_2$ and $v_3$ as function of rapidity, for 2.0 $< {p_{\mathrm {T}}} <$ 8.0 GeV/ $c$ and for centrality 20-70%. The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties.
png pdf	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 the systematic uncertainties.

Tables
png pdf	Table 1: Summary of systematic uncertainties for $v_2$ . Ranges of variation of uncertainties for each binning are presented. The cells filled with " $-$ '' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out.
png pdf	Table 2: Summary of systematic uncertainties for $v_3$ . Ranges of variation of uncertainties for each binning are presented. The cells filled with " $-$ '' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out.
png pdf	Table 3: Summary of systematic uncertainties for $\Delta v_2$ . Ranges of variation of uncertainties for each binning are presented. The cells filled with " $-$ '' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out.

Summary

New measurements of prompt

${\mathrm{D^0}}$ mesons elliptic (

$v_2$ ) and triangular (

$v_3$ ) flow are presented as a function of

${p_{\mathrm{T}}}$ , 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

${p_{\mathrm{T}}}$ coverage and by providing more differential information. A clear centrality dependency of prompt

${\mathrm{D^0}}$

$v_2$ is observed, while

$v_3$ is largely centrality independent. The trend is consistent with the expectation of a centrality dependency driven by initial-state geometry. No significant rapidity dependency of prompt

${\mathrm{D^0}}$

$v_2$ and

$v_3$ is observed, although possible dependency on rapidity cannot be discarded. When comparing against various theoretical calculations at midrapidity, no model is able to describe the data over the full centrality and

${p_{\mathrm{T}}}$ ranges. Motivated by the search for a strong electric field possibly created in PbPb collisions, a first measurement of the

$v_2$ difference (

$\Delta v_2$ ) between

${\mathrm{D^0}}$ and

${\mathrm{\overline{D}}{}^0}$ 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), consistent with zero within the experimental uncertainties, indicating that no effect of electric field on charm hadron collective flow is observed. Future model comparisons may provide constraints on the electric conductivity of the QGP medium.

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