Abstract
The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapidity-dependent difference (Δv2) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, v2) between D0 (u‾c) and D‾0 (uc‾) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of Δv2. The rapidity-averaged value is found to be 〈Δv2〉=0.001±0.001(stat)±0.003(syst) in PbPb collisions at sNN=5.02TeV. In addition, the influence of the collision geometry is explored by measuring the D0 and D‾0mesons v2 and triangular flow coefficient (v3) as functions of rapidity, transverse momentum (pT), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt D0 meson v2 values is observed, while the v3 is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry.
Highlights
The observation of a strongly-coupled quark-gluon plasma (QGP), a state of matter composed of deconfined quarks and gluons, was established by experiments investigating ultrarelativistic heavy ion collisions at the BNL RHIC [1,2,3,4] and CERN LHC [5,6]
The analysis presented in this letter uses approximately 4.27 × 109 minimum bias (MB) PbPb collision events collected by the CMS experiment during the 2018 LHC run
The nonprompt D0 meson contamination is taken into account as a systematic uncertainty, by checking that the nonprompt D0 meson fraction is always smaller than 12%. This implies that the central values of vn will not be considerably affected by this component, being compatible within statistical uncertainties. Such a low fraction arises from the use of prompt D0 meson signals in the boosted decision tree (BDT) training, together with variables that are highly correlated with the distance of closest approach (DCA) to the primary vertex, which is defined as the flight distance of the D0 particle times the sine of the pointing angle in three dimensions
Summary
The observation of a strongly-coupled quark-gluon plasma (QGP), a state of matter composed of deconfined quarks and gluons, was established by experiments investigating ultrarelativistic heavy ion collisions at the BNL RHIC [1,2,3,4] and CERN LHC [5,6]. The second(v2) and third- (v3) order Fourier coefficients are referred to as “elliptic” and “triangular” flow harmonics, respectively Measuring these coefficients for particle species with different quark composition provides additional information about this hot and dense medium [13]. Because of their large mass, charm and bottom quarks are produced earlier in the collisions than the light quarks (up and down) [14,15]. In ultrarelativistic heavy ion collisions, very strong and transient (∼10−1 fm/c) magnetic and electric fields are expected to be induced by the collision spectators and participants [19] Such electromagnetic (EM) fields are predicted to produce a difference in the vn harmonics for positively and negatively charged particles [19]. Measurements of the v2 difference between D0 and D0 mesons, v2, as a function of rapidity are presented as a method to probe possible effects originating from the Coulomb fields
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