CMSPASHIN19008  
Search for strong electromagnetic fields in PbPb collisions at 5.02 TeV via azimuthal anisotropy of $\mathrm{D^0}$ and ${\mathrm{\overline{D}}{}^0}$ mesons  
CMS Collaboration  
November 2019  
Abstract: Motivated by the search for strong electromagnetic fields created in PbPb collisions, the first measurement of the $v_2$ difference ($\Delta v_2$) between $\mathrm{D^0}$ and ${\mathrm{\overline{D}}{}^0}$ is presented as a function of rapidity. The result for the rapidityaveraged $v_2$ difference is found to be $ < \Delta v_2 > = $ 0.001 $\pm$ 0.001 (stat) $\pm$ 0.003 (syst), consistent with zero within experimental uncertainties. Comparisons with models may help to directly constrain the electric conductivity of the hot and dense medium formed in these collisions. Measurements of flow harmonics of $\mathrm{D^0}$ ($\bar{u}c$) and ${\mathrm{\overline{D}}{}^0}$ ($u\bar{c}$) mesons are presented as functions of rapidity ($y$), transverse momentum ($p_{\mathrm{T}}$), and collision centrality, for PbPb collisions at 5.02 TeV, using data collected by the CMS experiment during the 2018 LHC run. The results improve previous ones published by CMS, by extending the $p_{\mathrm{T}}$ coverage and providing more differential information. A clear centrality dependence of prompt $\mathrm{D^0}$ $v_2$ is observed, while $v_3$ is largely independent of centrality. The trend is consistent with expectations of flow driven by the initialstate geometry. No significant rapidity dependence of prompt ${\mathrm{\overline{D}}{}^0}$ $v_2$ and $v_3$ is observed.  
Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, PLB 816 (2021) 136253. The superseded preliminary plots can be found here. 
Figures  
png pdf 
Figure 1:
Simultaneous fit on mass spectrum and $v_2$ ($\Delta v_2$) as function of invariant mass for 3.0 $ < p_{\mathrm{T}} < $ 3.5 GeV/$c$, centrality 2070% and $0.6 < y < 0.0$. 
png pdf 
Figure 2:
Prompt $ {\mathrm{D^0}} $ meson $v_2$ (top) and $v_3$ (bottom) coefficients at midrapidity ($y < 1$) for the centrality classes 010% (left), 1030% (middle), and 3050% (right). The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. Theoretical calculations for $v_n$ coefficient of prompt $ {\mathrm{D^0}} $ mesons are also plotted for comparison. 
png pdf 
Figure 3:
Prompt $ {\mathrm{D^0}} $ meson $v_2$ (top) and $v_3$ (bottom) coefficients at midrapidity ($y < 1$) and forward rapidity ($ 1 < y < 2$) for the centrality classes 010% (left), 1030% (middle), and 3050% (right). The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. 
png pdf 
Figure 4:
Prompt $ {\mathrm{D^0}} $ meson $v_2$ and $v_3$ as functions of centrality, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for rapidity ranges $y < $ 1 and 1 $ < y < $ 2 (left). Prompt $ {\mathrm{D^0}} $ $v_2$ and $v_3$ as function of rapidity, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for centrality 2070% (right). The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. 
png pdf 
Figure 4a:
Prompt $ {\mathrm{D^0}} $ meson $v_2$ and $v_3$ as functions of centrality, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for rapidity ranges $y < $ 1 and 1 $ < y < $ 2. The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. 
png pdf 
Figure 4b:
Prompt $ {\mathrm{D^0}} $ $v_2$ and $v_3$ as function of rapidity, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and for centrality 2070%. The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. 
png pdf 
Figure 5:
Prompt $ {\mathrm{D^0}} $ meson $\Delta v_2$ as a function of rapidity, for 2.0 $ < {p_{\mathrm {T}}} < $ 8.0 GeV/$c$ and centrality 2070%. The vertical bars represent statistical uncertainties and open boxes represent the systematic uncertainties. 
Tables  
png pdf 
Table 1:
Summary of systematic uncertainties for $v_2$. Ranges of variation of uncertainties for each binning are presented. The cells filled with "$$'' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out. 
png pdf 
Table 2:
Summary of systematic uncertainties for $v_3$. Ranges of variation of uncertainties for each binning are presented. The cells filled with "$$'' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out. 
png pdf 
Table 3:
Summary of systematic uncertainties for $\Delta v_2$. Ranges of variation of uncertainties for each binning are presented. The cells filled with "$$'' refer to the cases where no estimate of uncertainty is required for the source or where the uncertainty cancels out. 
Summary 
New measurements of prompt ${\mathrm{D^0}}$ mesons elliptic ($v_2$) and triangular ($v_3$) flow are presented as a function of ${p_{\mathrm{T}}}$, rapidity and collision centrality, in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The results improve previously published CMS data by extending the ${p_{\mathrm{T}}}$ coverage and by providing more differential information. A clear centrality dependency of prompt ${\mathrm{D^0}}$ $v_2$ is observed, while $v_3$ is largely centrality independent. The trend is consistent with the expectation of a centrality dependency driven by initialstate geometry. No significant rapidity dependency of prompt ${\mathrm{D^0}}$ $v_2$ and $v_3$ is observed, although possible dependency on rapidity cannot be discarded. When comparing against various theoretical calculations at midrapidity, no model is able to describe the data over the full centrality and ${p_{\mathrm{T}}}$ ranges. Motivated by the search for a strong electric field possibly created in PbPb collisions, a first measurement of the $v_2$ difference ($\Delta v_2$) between ${\mathrm{D^0}}$ and ${\mathrm{\overline{D}}{}^0}$ as a function of rapidity is presented. The rapidityaveraged $v_2$ difference is measured to be $<\Delta v_2 > = $ 0.001 $\pm$ 0.001 (stat) $\pm$ 0.003 (syst), consistent with zero within the experimental uncertainties, indicating that no effect of electric field on charm hadron collective flow is observed. Future model comparisons may provide constraints on the electric conductivity of the QGP medium. 
References  
1  BRAHMS Collaboration  Quark gluon plasma and color glass condensate at RHIC? The Perspective from the BRAHMS experiment  NP A 757 (2005) 1  arXiv:nuclex/0410020 
2  PHOBOS Collaboration  The PHOBOS perspective on discoveries at RHIC  NP A 757 (2005) 28  arXiv:nuclex/0410022 
3  STAR Collaboration  Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions  NP A 757 (2005) 102  arXiv:nuclex/0501009 
4  PHENIX Collaboration  Formation of dense partonic matter in relativistic nucleusnucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration  NP A 757 (2005) 184  arXiv:nuclex/0410003 
5  ALICE Collaboration  Elliptic flow of charged particles in PbPb collisions at 2.76 TeV  PRL 105 (2010) 252302  1011.3914 
6  ATLAS Collaboration  Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in leadlead collisions at $ {\sqrt{s_{_{\text{NN}}}}} = $ 2.76 TeV with the ATLAS detector  PLB 707 (2012) 330  1108.6018 
7  CMS Collaboration  Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions at $ {\sqrt{s_{_{\text{NN}}}}} = $ 2.76 TeV  PRC 87 (2013) 014902  CMSHIN10002 1204.1409 
8  J.Y. Ollitrault  Determination of the reaction plane in ultrarelativistic nuclear collisions  PRD 48 (1993) 1132  hepph/9303247 
9  S. Voloshin and Y. Zhang  Flow study in relativistic nuclear collisions by Fourier expansion of azimuthal particle distributions  Z. Phys. C 70 (1994) 665  hepph/9407282 
10  A. M. Poskanzer and S. A. Voloshin  Methods for analyzing anisotropic flow in relativistic nuclear collisions  PRC 58 (1998) 1671  arXiv:nuclex/9805001 
11  P. BraunMunzinger  Quarkonium production in ultrarelativistic nuclear collisions: Suppression versus enhancement  JPG 34 (2007) S471  arXiv:nuclth/0701093 
12  F.M. Liu and S.X. Liu  Quarkgluon plasma formation time and direct photons from heavy ion collisions  PRC 89 (2014) 034906  arXiv:nuclth/1212.6587 
13  U. Gursoy et al.  Chargedependent Flow Induced by Magnetic and Electric Fields in Heavy Ion Collisions  PRC 98 (2018) 055201  1806.05288 
14  S. K. Das et al.  Directed Flow of Charm Quarks as a Witness of the Initial Strong Magnetic Field in UltraRelativistic Heavy Ion Collisions  PLB 768 (2017) 260  arXiv:nuclth/1608.02231 
15  S. Chatterjee and P. Bozek  Large directed flow of open charm mesons probes the three dimensional distribution of matter in heavy ion collisions  PRL 120 (2018) 192301  1712.01189 
16  STAR Collaboration  Elliptic flow from two and four particle correlations in Au+Au collisions at s(NN)**(1/2) = 130GeV  PRC 66 (2002) 034904  arXiv:nuclex/0206001 
17  M. Luzum and J.Y. Ollitrault  Eliminating experimental bias in anisotropicflow measurements of highenergy nuclear collisions  PRC 87 (2013) 044907  arXiv:nuclex/1209.2323 
18  A. Hoecker et al.  TMVA: Toolkit for Multivariate Data Analysis  PoS ACAT (2007) 040  physics/0703039 
19  CMS Collaboration  Measurement of prompt $ D^0 $ meson azimuthal anisotropy in PbPb collisions at $ \sqrt{{s}_{NN}} = $ 5.02 TeV  PRL 120 (2018) 202301  CMSHIN16007 1708.03497 
20  CMS Collaboration  Description and performance of track and primaryvertex reconstruction with the CMS tracker  JINST 9 (2014) P10009  CMSTRK11001 1405.6569 
21  CMS Collaboration  The CMS experiment at the CERN LHC  JINST 3 (2008) S08004  CMS00001 
22  CMS Collaboration  Chargedparticle nuclear modification factors in PbPb and pPb collisions at $ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}= $ 5.02 TeV  JHEP 04 (2017) 039  CMSHIN15015 1611.01664 
23  T. Sjostrand et al.  An Introduction to PYTHIA 8.2  CPC 191 (2015) 159  1410.3012 
24  CMS Collaboration  Extraction and validation of a new set of CMS PYTHIA8 tunes from underlyingevent measurements  CMSGEN17001 1903.12179 

25  I. P. Lokhtin and A. M. Snigirev  A Model of jet quenching in ultrarelativistic heavy ion collisions and highp(T) hadron spectra at RHIC  EPJC 45 (2006) 211  hepph/0506189 
26  Particle Data Group Collaboration  Review of Particle Physics  PRD 98 (2018) 030001  
27  NA49 Collaboration  Directed and elliptic flow of charged pions and protons in Pb + Pb collisions at 40AGeV and 158AGeV  PRC 68 (2003) 034903  nuclex/0303001 
28  CMS Collaboration  Azimuthal anisotropy of charged particles with transverse momentum up to 100 GeV/ c in PbPb collisions at $ \sqrt {s}_{{NN}} = $ 5.02 TeV  PLB 776 (2018) 195  CMSHIN15014 1702.00630 
29  S. Cao, T. Luo, G.Y. Qin, and X.N. Wang  Linearized Boltzmann transport model for jet propagation in the quarkgluon plasma: Heavy quark evolution  PRC 94 (2016) 014909  1605.06447 
30  J. Xu, J. Liao, and M. Gyulassy  Bridging SoftHard Transport Properties of QuarkGluon Plasmas with CUJET3.0  JHEP 02 (2016) 169  1508.00552 
31  M. Nahrgang et al.  Elliptic and triangular flow of heavy flavor in heavyion collisions  PRC 91 (2015) 014904  1410.5396 
32  M. He, R. J. Fries, and R. Rapp  Heavy Flavor at the Large Hadron Collider in a Strong Coupling Approach  PLB 735 (2014) 445  1401.3817 
Compact Muon Solenoid LHC, CERN 