Measurement of the Cross Section for Inclusive Isolated-Photon Production in pp Collisions at √s̅ = 13 TeV Using the ATLAS Detector

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in pp collisions at √s̅ = 5.02 and 13 TeV and in p + Pb collisions at √{S_{NN}} = 5.02 TeV, and compared to the results obtained for low-multiplicity Pb + Pb collisions at √{S_{NN}} = 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 p + Pb and low-multiplicity Pb + Pb collisions. On the other hand, the pp 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 Pb + Pb, smaller in p + Pb and smallest in pp collisions. The pp results show no dependence on the collision energy, nor on the multiplicity.