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| CMS-HIN-16-009 ; CERN-EP-2016-236 | ||
| Observation of charge-dependent azimuthal correlations in pPb collisions and its implication for the search for the chiral magnetic effect | ||
| CMS Collaboration | ||
| 1 October 2016 | ||
| Phys. Rev. Lett. 118 (2017) 122301 | ||
| Abstract: Charge-dependent azimuthal particle correlations with respect to the second-order event plane in pPb and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV have been studied with the CMS experiment at the LHC. The measurement is performed with a three-particle correlation technique, using two particles with the same or opposite charge within the pseudorapidity range $| \eta | < $ 2.4, and a third particle measured in the hadron forward calorimeters (4.4 $ < | \eta | < $ 5). The observed differences between the same and opposite sign correlations, as functions of multiplicity and $\eta$ gap between the two charged particles, are of similar magnitude in pPb and PbPb collisions at the same multiplicities. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect. | ||
| Links: e-print arXiv:1610.00263 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
The same (SS) and opposite sign (OS) three-particle correlator as a function of $ {\Delta \eta } \equiv |\eta _{\alpha }-\eta _{\beta }|$ for 185 $ \leq {N_\text {trk}^\text {offline}} < $ 220 in (a) pPb and (b) PbPb collisions at $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV. The pPb results obtained with particle $c$ in Pb-going (solid markers) and p-going (open markers) sides are shown separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
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Figure 2:
The same sign (SS) and opposite sign (OS) three-particle correlator averaged over $ {| \eta _{\alpha }-\eta _{\beta } | } < $ 1.6 as a function of ${N_\text {trk}^\text {offline}}$ in pPb and PbPb collisions at $ {\sqrt {\smash [b]{s_{_\mathrm {NN}}}}} = $ 5.02 TeV are shown. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
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Figure 3:
The difference of the opposite sign (OS) and same sign (SS) three-particle correlators (a) as a function of $|\eta _{\alpha }-\eta _{\beta }|$ for 185 $ \leq {N_\text {trk}^\text {offline}} < $ 220 and (b) as a function of ${N_\text {trk}^\text {offline}} $, averaged over $ {| \eta _{\alpha }-\eta _{\beta } | }< $ 1.6, in pPb and PbPb collisions at $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV. The pPb results are obtained with particle $c$ from Pb- and p-going sides separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
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Figure 3-a:
The difference of the opposite sign (OS) and same sign (SS) three-particle correlators as a function of $|\eta _{\alpha }-\eta _{\beta }|$ for 185 $ \leq {N_\text {trk}^\text {offline}} < $ 220, averaged over $ {| \eta _{\alpha }-\eta _{\beta } | }< $ 1.6, in pPb and PbPb collisions at $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV. The pPb results are obtained with particle $c$ from Pb- and p-going sides separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
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Figure 3-b:
The difference of the opposite sign (OS) and same sign (SS) three-particle correlators as a function of ${N_\text {trk}^\text {offline}} $, averaged over $ {| \eta _{\alpha }-\eta _{\beta } | }< $ 1.6, in pPb and PbPb collisions at $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV. The pPb results are obtained with particle $c$ from Pb- and p-going sides separately. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
| Summary |
| In summary, charge-dependent azimuthal correlations of same and opposite sign particles with respect to the second-order event plane have been measured in pPb and PbPb collisions at $ \sqrt{s_{\mathrm{NN}} } = $ 5.02 TeV by the CMS experiment at the LHC. The correlation is extracted via a three particle correlator as functions of particle $ \Delta \eta $ and charged-particle multiplicity of the event. The difference between same and opposite sign particles as functions of $ \Delta \eta $ and multiplicity is found to agree for pPb and PbPb collisions, possibly indicating a common underlying mechanism that generates the observed correlation. These results challenge the CME interpretation for the observed charge-dependent azimuthal correlations in nucleus-nucleus collisions at RHIC and the LHC. |
| Additional Figures | |
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Additional Figure 1:
In (a), the same sign (SS) and opposite sign (OS) three-particle correlator averaged over $|\eta _{\alpha }-\eta _{\beta }|<$ 1.6 as a function of $ {N_\text {trk}^\text {offline}} $ in pPb and PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV are shown. In (b), the same correlation as a function of centrality is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV from CMS, at $ \sqrt{s_{\mathrm {NN}}} =$ 2.76 TeV from ALICE [1], and in AuAu collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 0.2TeV from STAR [2]. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
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Additional Figure 2:
The same sign (SS) and opposite sign (OS) three-particle correlator averaged over $|\eta _{\alpha }-\eta _{\beta }|<$ 1.6 as a function of centrality is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV from CMS with $ {| \eta | }<$ 2.4 and $ {| \eta | }< $ 0.8, at $ \sqrt{s_{\mathrm {NN}}} =$ 2.76 TeV from ALICE [1], and in AuAu collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 0.2TeV from STAR [2]. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
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Additional Figure 3:
The same sign (SS) and opposite sign (OS) three-particle correlator averaged over $|\eta _{\alpha }-\eta _{\beta }|<$ 1.6 as a function of $ {N_\text {trk}^\text {offline}} $ in pPb and PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV are shown. Particle $c$ is measured from both $HF_{+}$ and $HF_{-}$. Statistical and systematic uncertainties are indicated by the error bars and shaded regions, respectively. |
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Additional Figure 4:
The same sign (SS) and opposite sign (OS) three-particle correlator as a function of $ {\Delta \eta } $ is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV for 60-70% centrality, with particle $\alpha , \beta $ within $ {| \eta | }<$ 2.4 and particle $c$ between 4.4 $ < {| \eta | }< $ 5.0 as default kinematic range, and shown with particle $\alpha , \beta $ within $ {| \eta | }< $ 0.8 and particle $c$ between 4.5 $ < {| \eta | }< $ 5.0. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
png pdf |
Additional Figure 5:
The same sign (SS) and opposite sign (OS) three-particle correlator as a function of $ {\Delta \eta } $ is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV for 60-70% centrality, with particle $\alpha , \beta $ within $ {| \eta | }<$ 2.4 and particle $c$ between 4.4 $ < {| \eta | }< $ 5.0 as default kinematic range, and shown with particle $\alpha , \beta $ within $ {| \eta | }< $ 0.8 and particle $c$ within $ {| \eta | }< $ 0.8. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
png pdf |
Additional Figure 6:
The difference between the opposite sign (OS) and the same sign (SS) three-particle correlator as a function of $ {\Delta \eta } $ is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV for 60-70% centrality, with particle $\alpha , \beta $ within $ {| \eta | }<$ 2.4 and particle $c$ between 4.4 $ < {| \eta | }< $ 5.0 as default kinematic range, and shown with particle $\alpha , \beta $ within $ {| \eta | }< $ 0.8 and particle $c$ between 4.5 $ < {| \eta | }< $ 5.0. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
png pdf |
Additional Figure 7:
The difference between the opposite sign (OS) and the same sign (SS) three-particle correlator as a function of $ {\Delta \eta } $ is presented in PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV for 60-70% centrality, with particle $\alpha , \beta $ within $ {| \eta | }<$ 2.4 and particle $c$ between 4.4 $ < {| \eta | }< $ 5.0 as default kinematic range, and shown with particle $\alpha , \beta $ within $ {| \eta | }< $ 0.8 and particle $c$ within $ {| \eta | }< $ 0.8. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. |
png pdf |
Additional Figure 8:
The impact parameter distributions from epos lhc simulation of pPb and PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV are presented. For pPb , the events with selection of top 0.006-0.06% of the multiplicity distribution are shown, equvalent to the multiplicity class $185 \leq {N_\text {trk}^\text {offline}} < 220$ in data. For PbPb , the events with centrality selection of 60-70% are shown, which is an equvalent to the multiplicity class 100 $ \leq {N_\text {trk}^\text {offline}} < $ 300 in data. |
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Additional Figure 9:
The cosine of the relative angle between the reaction plane, denoted as $\Psi _{\rm RP}$, and the participant plane, denoted as $\Psi _{\rm PP}$, from MC Glauber simulation of pPb and PbPb collisions at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV are presented. The average of the distributions are also shown for both pPb and PbPb collisions. |
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Additional Figure 10:
Event display of one high-multiplicity pPb event using MC Glauber simulation at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV is shown. The red circle is the proton projectile, the green circles are the participant nucleons, and the blue circles are the spectator nucleons. The red and black arrow refer to the angle of the reaction plane ($\Psi _{\rm RP}$) and the participant plane ($\Psi _{\rm PP}$) in the transverse direction. |
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Additional Figure 11:
Event display of one peripheral PbPb event using MC Glauber simulation at $ \sqrt{s_{\mathrm {NN}}} =$ 5.02 TeV is shown. The green circles are the participant nucleons, and the blue circles are the spectator nucleons. The red and black arrow refer to the angle of the reaction plane ($\Psi _{\rm RP}$) and the participant plane ($\Psi _{\rm PP}$) in the transverse direction. |
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