# Evidence for transverse momentum and pseudorapidity dependent event plane fluctuations in PbPb and $pPb$ collisions

CMS Collaboration; Khachatryan, V; Sirunyan, A M; Tumasyan, A; Aarrestad, T K; Amsler, C; Canelli, F; Chiochia, V; De Cosa, A; Galloni, C; Hinzmann, A; Hreus, T; Kilminster, B; Lange, C; Ngadiuba, J; Pinna, D; Robmann, P; Ronga, F J; Salerno, D; Taroni, S; Yang, Y; et al (2015). Evidence for transverse momentum and pseudorapidity dependent event plane fluctuations in PbPb and $pPb$ collisions. Physical Review C (Nuclear Physics), 92:034911.

## Abstract

A systematic study of the factorization of long-range azimuthal two-particle correlations into a product of single-particle anisotropies is presented as a function of pt and eta of both particles, and as a function of the particle multiplicity in PbPb and $pPb$ collisions. The data were taken with the CMS detector for $PbPb$ collisions at $\sqrt{s_{NN}} = 2.76 TeV$ and $pPb$ collisions at $\sqrt{s_{NN}} = 5.02 TeV$, covering a very wide range of multiplicity. Factorization is observed to be broken as a function of both particle $pt$ and $\eta$. When measured with particles of different pt, the magnitude of the factorization breakdown for the second Fourier harmonic reaches 20% for very central PbPb collisions but decreases rapidly as the multiplicity decreases. The data are consistent with viscous hydrodynamic predictions, which suggest that the effect of factorization breaking is mainly sensitive to the initial-state conditions rather than to the transport properties (e.g., shear viscosity) of the medium. The factorization breakdown is also computed with particles of different $\eta$. The effect is found to be weakest for mid-central PbPb events but becomes larger for more central or peripheral PbPb collisions, and also for very high-multiplicity $pPb$ collisions. The $\eta$-dependent factorization data provide new insights to the longitudinal evolution of the medium formed in heavy ion collisions.

## Abstract

A systematic study of the factorization of long-range azimuthal two-particle correlations into a product of single-particle anisotropies is presented as a function of pt and eta of both particles, and as a function of the particle multiplicity in PbPb and $pPb$ collisions. The data were taken with the CMS detector for $PbPb$ collisions at $\sqrt{s_{NN}} = 2.76 TeV$ and $pPb$ collisions at $\sqrt{s_{NN}} = 5.02 TeV$, covering a very wide range of multiplicity. Factorization is observed to be broken as a function of both particle $pt$ and $\eta$. When measured with particles of different pt, the magnitude of the factorization breakdown for the second Fourier harmonic reaches 20% for very central PbPb collisions but decreases rapidly as the multiplicity decreases. The data are consistent with viscous hydrodynamic predictions, which suggest that the effect of factorization breaking is mainly sensitive to the initial-state conditions rather than to the transport properties (e.g., shear viscosity) of the medium. The factorization breakdown is also computed with particles of different $\eta$. The effect is found to be weakest for mid-central PbPb events but becomes larger for more central or peripheral PbPb collisions, and also for very high-multiplicity $pPb$ collisions. The $\eta$-dependent factorization data provide new insights to the longitudinal evolution of the medium formed in heavy ion collisions.

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## Additional indexing

Item Type: Journal Article, refereed, original work 07 Faculty of Science > Physics Institute 530 Physics English 5 March 2015 18 Feb 2016 10:51 01 Dec 2019 06:39 American Physical Society 0556-2813 Closed https://doi.org/10.1103/PhysRevC.92.034911 arXiv:1503.01692v2