CMS-HIN-16-022

CMS-HIN-16-022 ; CERN-EP-2017-220
Observation of correlated azimuthal anisotropy Fourier harmonics in pp and pPb collisions at the LHC
CMS Collaboration
26 September 2017
Phys. Rev. Lett. 120 (2018) 092301
Abstract: The azimuthal anisotropy Fourier coefficients ( $v_n$ ) in 8.16 TeV pPb data are extracted via long-range two-particle correlations as a function of event multiplicity and compared to corresponding results in pp and PbPb collisions. Using a four-particle cumulant technique, $v_n$ correlations are measured for the first time in pp and pPb collisions. The $v_2$ and $v_4$ coefficients are found to be positively correlated in all collision systems. For high multiplicity pPb collisions an anticorrelation of $v_2$ and $v_3$ is observed, with a similar correlation strength as in PbPb data at the same multiplicity. The new correlation results strengthen the case for a common origin of the collectivity seen in pPb and PbPb collisions in the measured multiplicity range.
Links: e-print arXiv:1709.09189 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $v_2$ , $v_3$ [25], and $v_4$ coefficients from long-range two-particle correlations for 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ as a function of ${N_\text {trk}^\text {offline}}$ in ${{\mathrm{p}} {\mathrm{p}}}$ at ${\sqrt{\smash[b]{s}}} =$ 13 TeV (a), pPb at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV [32] and 8.16 TeV (b), and PbPb at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV collisions (c). The results corrected by low-multiplicity subtraction are denoted as $v_{n}^\text {sub}$ . The lines show the $v_{n}$ results before subtraction of low-multiplicity correlations. The gray boxes represent systematic uncertainties.
png pdf	Figure 2: The SCs for the second and third coefficient (red points) and the second and fourth coefficient (blue points) as a function of ${N_\text {trk}^\text {offline}}$ in pp collisions at ${\sqrt{\smash[b]{s}}} =$ 13 TeV (a), pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 and 8.16 TeV (b), and PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV (c). The gray boxes represent systematic uncertainties.
png pdf	Figure 3: The SCs for the second and third coefficients (left) and the second and fourth coefficients (right) normalized by $\left < v_2^{2}\right > \left < v_3^{2}\right >$ and $\left < v_2^{2}\right > \left < v_4^{2}\right >$ from two-particle correlations. The track ${p_{\mathrm {T}}}$ range is 0.3 $< {p_{\mathrm {T}}} <$ 3 GeV/ $c$ and the results are shown as a function of ${N_\text {trk}^\text {offline}}$ in pp collisions at ${\sqrt{\smash[b]{s}}} =$ 13 TeV, pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV, and PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV.

Summary

In summary, the first measurements of azimuthal anisotropy Fourier coefficients and correlations of different coefficients in pPb collisions at

$\sqrt{\smash[b]{s_{_\mathrm{NN}}}} =$ 8.16 TeV are presented based on data collected by the CMS experiment at the LHC. The

$v_2$ ,

$v_3$ , and

$v_4$ Fourier coefficients are extracted from long-range two-particle correlations in classes of event multiplicity, and are found to be consistent with pPb data at

$\sqrt{\smash[b]{s_{_\mathrm{NN}}}} =$ 5.02 TeV. The pPb results are compared to those in pp at

$\sqrt{\smash[b]{s}} =$ 13 TeV and PbPb at

$\sqrt{\smash[b]{s_{_\mathrm{NN}}}} =$ 5.02 TeV. Using a four-particle cumulant technique, correlations of different coefficient orders are obtained, where a negative (positive) correlation is observed between

$v_2$ and

$v_3$ (

$v_4$ ) in pPb collisions. This behavior is similar to what is observed in the PbPb system, where the result is attributed to the hydrodynamic flow of a strongly interacting medium. Normalized correlation coefficients for

$v_2$ and

$v_3$ are found to be quantitatively similar between pPb and PbPb, while for

$v_2$ and

$v_4$ the results are larger in PbPb than in PbPb. The corresponding result in pp collisions shows a similar trend at high multiplicity but the statistical uncertainties are too large to make a quantitative statement. The results presented in this Letter provide further evidence of a similar origin of collectivity observed in small and large hadronic systems and impose constraints on theoretical model calculations.

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