CMS-PAS-HIN-17-003

CMS-PAS-HIN-17-003
Elliptic flow of charm and strange hadrons in high-multiplicity pPb collisions at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 8.16 TeV
CMS Collaboration
February 2018

Abstract: First measurements of the elliptic flow of prompt $\mathrm{D^{0}}$ mesons in comparison to $\mathrm{K^0_S}$ , $\Lambda$ , $\Xi^{-}$ , and $\Omega^{-}$ hadrons in pPb collisions at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 8.16 TeV are reported. The results are based on data collected by the CMS experiment at the LHC and are presented for particles at midrapidity with transverse momentum up to 8 GeV/c after correcting for the contribution of back-to-back jet correlations. The results are compared to PbPb collision data at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 5.02 TeV for 30-50% centrality. A significant elliptic flow ( $v_2$ ) signal is observed for all particles, in very high-multiplicity (about 200 charged particles) pPb events. When divided by the number of constituent quarks and studied as function of the transverse kinetic energy per quark, the $\mathrm{D^{0}}$ $v_2$ values are observed to be significantly smaller than those of light flavor particles, an effect not observed in PbPb collisions. These results show a weaker collective flow associated with charm quarks than with light flavor quarks in high-multiplicity pPb collisions, and may indicate a weaker coupling between heavy quarks and the hypothesised hydrodynamic quark-gluon plasma.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 121 (2018) 082301. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the ${p_{{\mathrm {T}}}}$ interval 4.2-5 GeV/ $c$ for events with 185 $\leq {N_{\mathrm {trk}^{\mathrm {offline}}}} <$ 250.
png pdf	Figure 2: Top: the $v_{2}^\text {sub}$ results for prompt ${\mathrm {D^{0}}}$ meson, as well as for strange hadrons as a function of ${p_{{\mathrm {T}}}}$ for pPb collisions at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 8.16 TeV with 185 $\leq {N_{\mathrm {trk}^{\mathrm {offline}}}} <$ 250 (left) and PbPb collisions at $\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 5.02 TeV with centrality between 30 and 50% (right). Bottom: the $n_{\mathrm {q}}$ -scaled $v_{2}^\text {sub}$ results. The ${\mathrm {D^{0}}}$ $v_2$ data in PbPb are taken from Ref. [40]. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties.

Summary

First measurements of elliptic azimuthal anisotropies for prompt

$\mathrm{D^{0}}$ mesons in comparison to

$\mathrm{K^0_S}$ ,

$\Lambda$ ,

$\Xi^{-}$ , and

$\Omega^{-}$ hadrons in high-multiplicity pPb collisions at

$\sqrt{\smash[b]{s_{\mathrm{NN}}}} =$ 8.16TeV are presented. Significant positive

$v_2$ values are observed for

$\mathrm{D^{0}}$ mesons with

${p_{{\mathrm {T}}}} >$ 2 GeV/

$c$ . Comparing to strange hadron results, the

$\mathrm{D^{0}}$

$v_2$ values are found to be smaller at a given

${p_{{\mathrm {T}}}}$ value, or at similar transverse kinetic energy per constituent quark after normalizing

$v_2$ by the number of constituent quarks. This effect is not observed in PbPb collisions with centrality between 30 and 50%. These results may indicate a weaker coupling between heavy quarks and the hypothesised hydrodynamic QGP medium in small collision systems.

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