Azimuthal correlations in photoproduction and deep inelastic $ep$ scattering at HERA

Azimuthal correlations in photoproduction and deep inelastic $ep$ scattering at HERA - Abt, I. et al - arXiv:2106.12377DESY-21-099

: Neutral current deep inelastic scattering.

: Resolved photoproduction.

Schematic example of the evolving collision zone in the transverse plane after the initial scattering in resolved photoproduction. Three MPI centres (shown with solid circles) act as sources of gluons. The possibility of rescattering of partons from separate MPI centres is illustrated with the hatched circle. Rescattering is analogous to colour reconnection in PYTHIA. An initial state with a dominant elliptical eccentricity is shown. Final-state particles are shown with blue spheres.

: Photoproduction.

: NC DIS with $Q^2 > 20 \GeV^{\;2}$.

: \cn{1} versus $Q^2$.

: \cn{2} versus $Q^2$.

Charged particle multiplicity distribution $dN/d\Nch$ in photoproduction ($\gamma p$) compared to PYTHIA expectations for different levels of MPI, which are inversely related to \pTref. The mean value of the ZEUS distribution is 22.5. The mean number of MPI for each value of \pTref is: 8.3 ($\pTref=2 \GeV$), 3.8 ($\pTref=3 \GeV$), and 2.2 ($\pTref=4 \GeV$). The dashed line corresponds to an expectation with colour reconnection (CR) switched off. The integral of the distributions in the range shown are normalised to unity. The statistical uncertainties are shown as vertical lines although they are typically smaller than the marker size. Systematic uncertainties are shown as boxes around the data points. Data points are shown at the bin centre.