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, at LHC energies, the observed charge asymmetry dependence of $v_2$ in heavy ion collisions arises from a chiral magnetic wave.
Highlights
Observing macroscopic phenomena arising from quantum anomalies is a subject of interest for a wide range of physics communities, from magnetized relativistic matter in threedimensional Dirac and Weyl materials [1,2,3] to hot plasma in the early universe or formed in relativistic heavy ion collisions [4,5,6]
The main physics observable of interest in this analysis is the slope parameter extracted by fitting a linear function to the normalized vn differences,/(vn− + vn+), as a function of the true event charge asymmetry value, Atcrhue, obtained by correcting Aochbs for the detector acceptance and
Atcrhue increases is observed for both pPb and PbPb collisions with an approximately linear dependence
Summary
Observing macroscopic phenomena arising from quantum anomalies is a subject of interest for a wide range of physics communities, from magnetized relativistic matter in threedimensional Dirac and Weyl materials [1,2,3] to hot plasma in the early universe or formed in relativistic heavy ion collisions [4,5,6]. If approximate chiral symmetry is restored, the interactions of chiral quarks with these gluon fields can produce a chirality imbalance, violating the local P and CP symmetries [9,10] This anomalous chiral effect can manifest itself as an electric current along or opposite to a strong magnetic field [11,12,13]. Following a hydrodynamic evolution of the medium formed in AA collisions, this electric quadruple moment is expected to result in a charge-dependent variation of the secondorder anisotropy coefficient (v2) in the Fourier expansion of the final-state particle azimuthal distribution. In the presence of a CMW, the difference of v2 values between positively and negatively charged particles will be proportional to Ach. Similar charge-dependent effects from the CMW are not expected for the third-order anisotropy coefficient (v3) [13].
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