Measurement of multi-particle azimuthal correlations in pp, p + Pb and low-multiplicity Pb + Pb collisions with the ATLAS detector

Abstract

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in collisions at = 5.02 and 13 TeV and in + Pb collisions at = 5.02 TeV, and compared to the results obtained for low-multiplicity collisions at = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in + Pb and low-multiplicity collisions. On the other hand, the results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in , smaller in + Pb and smallest in collisions. The results show no dependence on the collision energy, nor on the multiplicity.

Observation of prompt J/ψ meson elliptic flow in high-multiplicity pPb collisions at sNN=8.16 TeV

“…In particular, within a hydrodynamic picture, the second and third Fourier anisotropy components are known as elliptic (v 2 ) and triangular (v 3 ) flow, respectively, and reflect the QGP medium response to the initial collision geometry and its fluctuations [15][16][17]. In recent years, similar long-range collective azimuthal correlations have also been observed in events with high final-state particle multiplicity in proton-proton (pp) [18][19][20][21], proton-nucleus (pA) [22][23][24][25][26][27][28][29][30], and lighter AA collisions [31][32][33], raising the question of whether a fluid-like QGP is created in these much smaller systems. While experimental measurements in these small systems are consistent with the hydrodynamic expansion of a tiny QGP droplet, alternative scenarios based on gluon saturation in the initial state also claim to capture the main features of the correlation data (recent reviews are provided in Refs.…”

Correlated long-range mixed-harmonic fluctuations measured in pp, p+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detector

“…A more detailed description of the track selection for the 2010 Pb+Pb data and 2013 p+Pb data can be found in Refs. [5,17].…”

Development of heavy-flavour flow-harmonics in high-energy nuclear collisions

“…Notice that, within a kinetic description, in order for a system to behave as a fluid the mean-free-path of its constituents has to be much smaller than the system size, λ mfp L. The above condition is only marginally satisfied with perturbative partonic cross-sections and hence the idea of the formation of a strongly-interacting QGP was proposed. A further surprise came in the last few years from the observation of analogous effects (mass-dependent radial, elliptic and triangular flow) also in small systems, like the ones produced in high-multiplicity deuteron-nucleus, proton-nucleus and even proton-proton collisions [43][44][45][46]: in light of the small size of the medium this makes the hydrodynamic interpretation of the experimental measurements in these events quite challenging and alternative explanations have been proposed (see e.g. [47][48][49] Recently some authors proposed a different paradigm to interpret the above experimental observations.…”