Underlying-event studies with strange hadrons in pp collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Properties of the underlying-event in pp interactions are investigated primarily via the strange hadrons ${K}_{S}^{0}$ , $\Lambda $ and $\overline{\Lambda }$ , as reconstructed using the ATLAS detector at the LHC in minimum-bias pp collision data at $\sqrt{s}=13$ TeV. The hadrons are reconstructed via the identification of the displaced two-particle vertices corresponding to the decay modes , $\Lambda \to {\pi }^{-}p$ and $\overline{\Lambda }\to {\pi }^{+}\overline{p}$ . These are used in the construction of underlying-event observables in azimuthal regions computed relative to the leading charged-particle jet in the event. None of the hadronisation and underlying-event physics models considered can describe the data over the full kinematic range considered. Events with a leading charged-particle jet in the range of $10<{p}_{T}\le 40$ GeV are studied using the number of prompt charged particles in the transverse region. The ratio $N\left(\Lambda +\overline{\Lambda }\right)/N\left({K}_{S}^{0}\right)$ as a function of the number of such charged particles varies only slightly over this range. This disagrees with the expectations of some of the considered Monte Carlo models.