CMS-HIN-17-003

CMS-HIN-17-003 ; CERN-EP-2018-076
Elliptic flow of charm and strange hadrons in high-multiplicity pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV
CMS Collaboration
26 April 2018
Phys. Rev. Lett. 121 (2018) 082301
Abstract: The elliptic azimuthal anisotropy coefficient ( $v_2$ ) is measured for charm ( ${\mathrm{D^0}}$ ) and strange ( ${\mathrm{K^0_S}}$ , $\Lambda$ , $\Xi^-$ , and $\Omega^-$ ) hadrons, using a data sample of pPb collisions collected by the CMS experiment, at a nucleon-nucleon center-of-mass energy ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV. A significant positive $v_2$ signal from long-range azimuthal correlations is observed for all particle species in high-multiplicity pPb collisions. The measurement represents the first observation of possible long-range collectivity for open heavy flavor hadrons in small systems. The results suggest that charm quarks have a smaller $v_2$ than the lighter quarks, probably reflecting a weaker collective behavior. This effect is not seen in the larger PbPb collision system at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV, also presented.
Links: e-print arXiv:1804.09767 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\text {inv}})$ in the ${p_{\mathrm {T}}}$ interval 4.2-5.0 GeV for events with $185 \leq {N_\text {trk}^\text {offline}} < 250$ .
png pdf	Figure 2: Results of elliptic flow ( $v_2$ ) for ${\mathrm {D^0}}$ mesons, as well as ${\mathrm {K^0_S}}$ , $\Lambda$ , ${\Xi ^{-}}$ , and ${\Omega ^-}$ particles, as functions of ${p_{\mathrm {T}}}$ in the multiplicity range 185 $\leq {N_\text {trk}^\text {offline}} <$ 250 in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties.
png pdf	Figure 3: Upper: the ${v_{2}^\text {sub}}$ results for prompt ${\mathrm {D^0}}$ meson, as well as for strange hadrons, as functions of ${p_{\mathrm {T}}}$ in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV with 185 $\leq {N_\text {trk}^\text {offline}} <$ 250 (left) and PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV with centrality between 30 and 50% (right). Lower: the $n_ {\mathrm {q}}$ -scaled ${v_{2}^\text {sub}}$ results. The ${\mathrm {D^0}}$ $v_2$ data in PbPb are taken from Ref. [43]. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties.

Summary

In summary, the first measurements of elliptic azimuthal anisotropies for prompt

${\mathrm{D^0}}$ mesons, as well as

${\mathrm{K^0_S}}$ ,

$\Lambda$ ,

$\Xi^-$ ,

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

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

$v_2$ values are observed for

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

${p_{\mathrm {T}}} >$ 2 GeV. Comparing to strange-hadron results, the

${\mathrm{D^0}}$

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

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

$v_2$ by the number of constituent quarks. The latter effect is not observed in the larger PbPb collision system. It shows that, in high-multiplicity pPb collisions, in contrast to larger nucleus-nucleus collision systems, the collective behavior of charm quarks is weaker than that of the light-flavor quarks.

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