CMS-PAS-HIN-16-017

CMS-PAS-HIN-16-017
Charge asymmetry dependence of elliptic and triangular flow in pPb and PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV
CMS Collaboration
February 2017

Abstract: Charge dependent elliptic azimuthal anisotropy ( $v_{\rm 2}$ ) in pPb and both $v_{\rm 2}$ and triangular azimuthal anisotropy ( $v_{\rm 3}$ ) in PbPb collisions are measured at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV with the CMS detector at the LHC. The normalized difference of $v_{\rm 2}$ between positively and negatively charged particles, ( $v_{\rm 2}^{-}-v_{\rm 2}^{+}$ )/( $v_{\rm 2}^{-}+v_{\rm 2}^{+}$ ), shows a linear dependence with respect to the event-charged asymmetry $A_{\rm ch}$ , with comparable slopes observed for pPb and PbPb collisions at similar multiplicities. In PbPb collisions, the slopes are found to be the same for the $v_{\rm 2}$ and $v_{\rm 3}$ coefficients within uncertainties. These observations pose a challenge to the hypothesis that the charge asymmetry dependence of $v_{\rm 2}$ arises from a chiral magnetic wave. However, the results are in qualitative agreement with expectations based on local charge conservation.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRC 100 (2019) 064908. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Elliptic flow $v_2$ for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $v_{2}$ difference, $(v^{-}_{2} - v^{+}_{2})/(v^{-}_{2} + v^{+}_{2})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in PbPb collisions at 5.02 TeV. Statistical uncertainties are indicated by the error bars.
png pdf	Figure 1-a: Elliptic flow $v_2$ for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $v_{2}$ difference, $(v^{-}_{2} - v^{+}_{2})/(v^{-}_{2} + v^{+}_{2})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in pPb collisions at 5.02 TeV. Statistical uncertainties are indicated by the error bars.
png pdf	Figure 1-b: Elliptic flow $v_2$ for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $v_{2}$ difference, $(v^{-}_{2} - v^{+}_{2})/(v^{-}_{2} + v^{+}_{2})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in PbPb collisions at 5.02 TeV. Statistical uncertainties are indicated by the error bars.
png pdf	Figure 2: The event-averaged ${p_{\mathrm {T}}}$ value ( $< {p_{\mathrm {T}}} >$ ) for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $< {p_{\mathrm {T}}} >$ difference, $( < {p_{\mathrm {T}}} >^{-} - < {p_{\mathrm {T}}} >^{+})/( < {p_{\mathrm {T}}} >^{-} + < {p_{\mathrm {T}}} >^{+})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in pPb and PbPb collisions at 5.02 TeV. Statistical uncertainties are covered by the data points.
png pdf	Figure 2-a: The event-averaged ${p_{\mathrm {T}}}$ value ( $< {p_{\mathrm {T}}} >$ ) for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $< {p_{\mathrm {T}}} >$ difference, $( < {p_{\mathrm {T}}} >^{-} - < {p_{\mathrm {T}}} >^{+})/( < {p_{\mathrm {T}}} >^{-} + < {p_{\mathrm {T}}} >^{+})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in pPb collisions at 5.02 TeV. Statistical uncertainties are covered by the data points.
png pdf	Figure 2-b: The event-averaged ${p_{\mathrm {T}}}$ value ( $< {p_{\mathrm {T}}} >$ ) for positive- and negative-charged particles as a function of charge asymmetry (left) and the normalized $< {p_{\mathrm {T}}} >$ difference, $( < {p_{\mathrm {T}}} >^{-} - < {p_{\mathrm {T}}} >^{+})/( < {p_{\mathrm {T}}} >^{-} + < {p_{\mathrm {T}}} >^{+})$ , as a function of corrected event charge asymmetry (right) for multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} <$ 220 in PbPb collisions at 5.02 TeV. Statistical uncertainties are covered by the data points.
png pdf	Figure 3: The linear slope for $v_2$ (left) and $< {p_{\mathrm {T}}} >$ (right) parameters $r^{\rm norm}_{2}$ and $r^{\rm norm}_{ < {p_{\mathrm {T}}} >}$ as a function of event multiplicity in pPb and PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.
png pdf	Figure 3-a: The linear slope for $v_2$ parameter $r^{\rm norm}_{2}$ as a function of event multiplicity in pPb and PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.
png pdf	Figure 3-b: The linear slope for $< {p_{\mathrm {T}}} >$ parameter $r^{\rm norm}_{ < {p_{\mathrm {T}}} >}$ as a function of event multiplicity in pPb and PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.
png pdf	Figure 4: The triangular flow $v_3$ for positive- and negative-charged particles (left) and the normalized difference in $v_{n}$ , $(v^{-}_{n} - v^{+}_{n})/(v^{-}_{n} + v^{+}_{n})$ , for $n=$ 2 and $n=$ 3 (right) as a function of corrected event charge asymmetry for 30-40% centrality of PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. Statistical uncertainties are covered by the data points.
png pdf	Figure 5: The linear slope ( $r^{\rm norm}_{n}$ ) parameter for elliptic ( $n=$ 2) and triangular ( $n=$ 3) flow as a function of centrality for PbPb collisions at $\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.

Summary

The charged-dependent elliptic flow coefficients (

$v_{2}$ ) in pPb and both

$v_{2}$ and triangular flow coefficients (

$v_{3}$ ) in PbPb have been measured as a function of event-charged asymmetry at

$\sqrt{ s_{\text{NN}} } =$ 5.02 TeV. The normalized slope parameters of

$v_{2}$ in pPb and PbPb collisions are found to be similar at the same event-charged particle multiplicities. Moreover, the normalized slope parameters of the

$v_{2}$ and

$v_{3}$ coefficients show similar magnitudes in various centralities. These observations would not be expected if the charge dependence resulted from a Chiral Magnetic Wave mechanism. The data are qualitatively consistent with predictions from the local charge conservation mechanism.

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