CMS-HIN-16-017

CMS-HIN-16-017 ; CERN-EP-2017-180
Challenges to the chiral magnetic wave using charge-dependent azimuthal anisotropies in pPb and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV
CMS Collaboration
29 August 2017
Phys. Rev. C 100 (2019) 064908
Abstract: Charge-dependent anisotropy Fourier coefficients ($v_n$) of particle azimuthal distributions are measured in pPb and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV with the CMS detector at the LHC. The normalized difference in the second-order anisotropy coefficients ($v_2$) between positively and negatively charged particles is found to depend linearly on the observed event charge asymmetry with comparable slopes for both pPb and PbPb collisions over a wide range of charged particle multiplicity. In PbPb, the third-order anisotropy coefficient, $v_3$, shows a similar linear dependence with the same slope as seen for $v_2$. The observed similarities between the $v_2$ slopes for pPb and PbPb, as well as the similar slopes for $v_2$ and $v_3$ in PbPb, are compatible with expectations based on local charge conservation in the decay of clusters or resonances, and constitute a challenge to the hypothesis that the observed charge asymmetry dependence of $v_2$ in heavy ion collisions arises from a chiral magnetic wave.
Links: e-print arXiv:1708.08901 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The elliptic anisotropy $v_2$ (top left) and event-averaged ${p_{\mathrm {T}}} $ value ($ < {p_{\mathrm {T}}} > $, top right) for positively ($\mathrm{h}^{+}$) and negatively ($\mathrm{h}^{-}$) charged particles, and their normalized differences (bottom row), as functions of $A^\text {true}_\text {ch}$ for the multiplicity range 185 $ \leq {N_\text {trk}^\text {offline}} < $ 220 of pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. Statistical uncertainties are smaller than the marker size, while systematic uncertainties are not displayed.
png pdf	Figure 2: The linear slope parameters, $r^\text {norm}$, for $v_2$ (filled symbols) and $ < {p_{\mathrm {T}}} >$ (open symbols) as functions of event multiplicity in pPb and PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The two highest multiplicity ranges of PbPb data are selected based on the centrality, while the others are obtained by selecting on $ {N_\text {trk}^\text {offline}} $. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.
png pdf	Figure 3: The $v_3$ coefficient for positively and negatively charged particles (top) and the normalized difference in $v_{n}$, $(v^{-}_{n} - v^{+}_{n})/(v^{-}_{n} + v^{+}_{n})$ (bottom), for $n=$ 2 and 3, as functions of true event charge asymmetry for the 30-40% centrality class in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV.
png pdf	Figure 4: The linear slope parameters, $r^\text {norm}_{2}$ and $r^\text {norm}_{3}$ as functions of the centrality class in PbPb collisions. Average $ {N_\text {trk}^\text {offline}}$ values for each centrality class are indicated on the top axis. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively.

Summary

In summary, the charge-dependent Fourier coefficients of the azimuthal anisotropy have been measured in pPb and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV as functions of the charge asymmetry of the produced hadrons. The normalized differences in the $v_2$ coefficient between positively and negatively charged particles in pPb and PbPb, and that in the $v_3$ coefficient in PbPb collisions, are found to depend linearly on the charge asymmetry. The normalized slope parameters of the $v_2$ coefficient versus charge asymmetry in pPb collisions are found to be significant and similar to those in PbPb collisions over a wide range of charged particle multiplicities. The normalized slope parameters of the $v_2$ and $v_3$ coefficients in PbPb collisions show similar magnitudes for various centrality classes. Significant charged asymmetry dependence is also observed for the event-averaged transverse momenta of positively and negatively charged particles in both pPb and PbPb collisions. None of these observations is expected from the chiral magnetic wave mechanism, but they are qualitatively consistent with predictions based on local charge conservation. New measurements presented here on the charge-dependent azimuthal anisotropy in pPb and PbPb collisions pose challenges to the chiral magnetic wave as its origin.

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