Measurement of the Sensitivity of Two-Particle Correlations in $pp$ Collisions to the Presence of Hard Scatterings

A key open question in the study of multi-particle production in high-energy$pp$ collisions is the relationship between the "ridge'' - observed azimuthalcorrelations between particles in the underlying event that extend over allrapidities - and hard or semi-hard scattering processes. In particular, it isnot known whether jets or their soft fragments are correlated with particles inthe underlying event. To address this question, two-particle correlations aremeasured in $pp$ collisions at $\sqrt{s}=13$ TeV using data collected by theATLAS experiment at the LHC, with an integrated luminosity of $15.8$$\mathrm{pb}^{-1}$, in two different configurations. In the first case, chargedparticles associated with jets are excluded from the correlation analysis,while in the second case, correlations are measured between particles withinjets and charged particles from the underlying event. Second-order flowcoefficients, $v_2$, are presented as a function of event multiplicity andtransverse momentum. These measurements show that excluding particlesassociated with jets does not affect the measured correlations. Moreover,particles associated with jets do not exhibit any significant azimuthalcorrelations with the underlying event, ruling out hard processes contributingto the ridge.