Dependence of two-particle azimuthal correlations on the forward rapidity gap width in pPb collisions at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV

Abstract

One of the most striking features of relativistic heavy ion collisions is the presence of collective flow of thousands of produced particles. This flow can be characterized by the Fourier coefficients (${V_{n\Delta}}$) of the azimuthal angular distributions of charged particles, and its existence can be explained by the formation of a quark gluon plasma, which behaves as a fluid. Surprisingly, the angular distributions of particles from very small systems such as proton-lead (pPb), proton-proton (pp), electron-positron, and photon-proton ($\gamma$p) collisions also exhibit non-zero Fourier coefficients, raising the question of whether collective flow is present. This paper presents measurements of $V_{n\Delta}$ from a sample of pPb events at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV that are enriched in photon-lead ($\gamma$Pb) and pomeron-lead ($\mathrm{{\!I\!P}}$Pb) interactions by requiring no particles in the proton-going region. Measurements are made as a function of the forward rapidity gap width (the rapidity range in which no particles are found), the transverse momentum of the particles, and the multiplicity of particles in the event. The results are compared to previous measurements of pp, pPb, and $\gamma$p+$\mathrm{\!I\!P}$p events as well as modern event generators.