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

TL;DR

This study investigates the properties of the underlying event in proton-proton collisions at 13 TeV using strange hadrons, revealing discrepancies between data and existing models, and providing insights into hadronization processes.

Contribution

First measurement of underlying-event observables with strange hadrons in 13 TeV pp collisions, highlighting limitations of current models in describing the data.

Findings

Models fail to describe the full kinematic range of data.

The ratio N(Λ + Λ̄)/N(K_S^0) varies little with charged particle multiplicity.

Discrepancies suggest need for improved hadronization modeling.

Abstract

Properties of the underlying-event in $pp$ interactions are investigated primarily via the strange hadrons $K_{S}^{0}$, $\Lambda$ and $\bar\Lambda$, 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 $K_{S}^{0}\rightarrow\pi^+\pi^-$, $\Lambda\rightarrow\pi^-p$ and $\bar\Lambda\rightarrow\pi^+\bar{p}$. 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 $10 < p_T \leq 40$ GeV are studied using the number of prompt charged particles in the transverse region. The ratio $N(\Lambda + \bar\Lambda)/N(K_{S}^{0})$ 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.