The production of (multi-)strange hadrons is measured at midrapidity in proton--proton collisions at $\sqrt{s} = 13$ TeV as a function of the local charged-particle multiplicity in the pseudorapidity interval ${|\eta|<0.5}$ and of the very-forward energy measured by the ALICE Zero-Degree Calorimeters. The latter provides information on the effective energy, i.e. the energy available for particle production in the collision once subtracted from the centre-of-mass energy. The yields of K$^0_{\rm S}$, $\Lambda+{\bar \Lambda}$, and $\Xi^{-}+\overline{\Xi}^{+}$ per charged-particle increase with the effective energy. In addition, this work exploits a multi-differential approach to decouple the roles of local multiplicity and effective energy in such an enhancement. The results presented in this article provide new insights into the interplay between global properties of the collision, such as the initial available energy in the event, and the locally produced final hadronic state, connected to the charged-particle multiplicity at midrapidity. Notably, a strong increase of strange baryon production with effective energy is observed for fixed charged-particle multiplicity at midrapidity. These results are discussed within the context of existing phenomenological models of hadronisation implemented in different tunes of the PYTHIA 8 event generator.
Self-normalised ZN energy as a function of the self-normalised charged-particle-multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV in the standalone classification (V0M event classes).
Self-normalised ZN energy as a function of the self-normalised charged-particle-multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV in the high-local-multiplicity classification (V0M+SPDClusters event classes).
Self-normalised ZN energy as a function of the self-normalised charged-particle-multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV in the low-local-multiplicity classification (V0M+SPDClusters event classes).
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.
Mean multiplicity of $K^{0}_{S}$ per unit $(\eta, \phi)$ in the away region vs. leading-jet $p_{T}$
Statistical covariance between bins of Table 1
Mean multiplicity of $K^{0}_{S}$ per unit $(\eta, \phi)$ in the towards region vs. leading-jet $p_{T}$
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