Small System Collectivity in Relativistic Hadron and Nuclear Collisions

Abstract

The bulk motion of nuclear matter at the ultra-high temperatures created in heavy-ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider is well described in terms of nearly inviscid hydrodynamics, thereby establishing this system of quarks and gluons as the most perfect fluid in nature. A revolution in the field is underway, spearheaded by the discovery of similar collective, fluid-like phenomena in much smaller systems including p+p, p+A, d+Au, and ${}^3$He$+$Au collisions. We review these exciting new observations and their implications.

33 pages, 8 figures, 156 references. Invited review prepared for Annual Review of Nuclear and Particle Science 68 (2018). Version 2: Improved figures, references, corrected typos. Definitions of many quantities are now provided in margins

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• Effect of flow fluctuations and nonflow on elliptic flow methods
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• Eccentric protons? Sensitivity of flow to system size and shape in p+p, p+Pb and Pb+Pb collisions
• Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s[NN]) = 2.76 TeV
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• Shape and flow fluctuations in ultra-central Pb+Pb collisions at the LHC
• Measurements of azimuthal anisotropy and charged-particle multiplicity in $d$$+$Au collisions at $\sqrt{s_{{}_{NN}}}=$200, 62.4, 39, and 19.6 GeV
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