The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ($\ |y\ | < 0.5$) are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of $\rm{K}^{0}_{S}$, $\Lambda$, $\Xi$, and $\Omega$ increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$\Lambda+\bar{\Lambda}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
A study of strange hadron production associated with hard scattering processes and with the underlying event is conducted to investigate the origin of the enhanced production of strange hadrons in small collision systems characterised by large charged-particle multiplicities. For this purpose, the production of the single-strange meson ${\rm K^0_S}$ and the double-strange baryon $\Xi^{\pm}$ is measured, in each event, in the azimuthal direction of the highest-$p_{\rm T}$ particle (``trigger" particle), related to hard scattering processes, and in the direction transverse to it in azimuth, associated with the underlying event, in pp collisions at $\sqrt{s}=5.02$ TeV and $\sqrt{s}=13$ TeV using the ALICE detector at the LHC. The per-trigger yields of ${\rm K^0_S}$ and $\Xi^{\pm}$ are dominated by the transverse-to-leading production (i.e., in the direction transverse to the trigger particle), whose contribution relative to the toward-leading production is observed to increase with the event charged-particle multiplicity. The transverse-to-leading and the toward-leading $\Xi^{\pm}$/${\rm K^0_S}$ yield ratios increase with the multiplicity of charged particles, suggesting that strangeness enhancement with multiplicity is associated with both hard scattering processes and the underlying event. The relative production of $\Xi^{\pm}$ with respect to ${\rm K^0_S}$ is higher in transverse-to-leading processes over the whole multiplicity interval covered by the measurement. The ${\rm K}^{0}_{\rm{S}}$ and $\Xi^{\pm}$ per-trigger yields and yield ratios are compared with predictions of three different phenomenological models, namely PYTHIA 8.2 with the Monash tune, PYTHIA 8.2 with ropes and EPOS LHC. The comparison shows that none of them can quantitatively describe either the transverse-to-leading or the toward-leading yields of ${\rm K}^{0}_{\rm{S}}$ and $\Xi^{\pm}$.
Yields of $\rm K^{0}_\rm{S}$ per trigger particle per unit $\Delta\eta\Delta\varphi$ area in pp collisions at $\sqrt{s}=13$ TeV, as a function of the $\rm K^{0}_\rm{S}$ $p_\rm{T}$. Trigger particles are charged particles with $p_\rm{T}>3$ GeV/c. The trigger-particle-$\rm K^{0}_\rm{S}$ correlation is integrated in the ranges $-1.2<\Delta\eta<1.2$ and $-\pi/2<\Delta\varphi<3/2\pi$.
Transverse-to-leading yields of $\rm K^{0}_\rm{S}$ per trigger particle per unit $\Delta\eta\Delta\varphi$ area in pp collisions at $\sqrt{s}=13$ TeV, as a function of the $\rm K^{0}_\rm{S}$ $p_\rm{T}$. Trigger particles are charged particles with $p_\rm{T}>3$ GeV/c. The trigger-particle-$\rm K^{0}_\rm{S}$ correlation is integrated in the ranges $0.86<|\Delta\eta|<1.2$ and $0.96<\Delta\varphi<1.8$.
Toward-leading yields of $\rm K^{0}_\rm{S}$ per trigger particle per unit $\Delta\eta\Delta\varphi$ area in pp collisions at $\sqrt{s}=13$ TeV, as a function of the $\rm K^{0}_\rm{S}$ $p_\rm{T}$. Trigger particles are charged particles with $p_\rm{T}>3$ GeV/c. The trigger-particle-$\rm K^{0}_\rm{S}$ correlation is integrated in the ranges $|\Delta\eta|<0.86$ and $|\Delta\varphi|<1.1$.
The production of (multi-)strange hadrons is measured at midrapidity in proton-proton (pp) 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 (ZDC). The latter provides information on the effective energy available for particle production in the collision once subtracted from the centre-of-mass energy. The yields of ${\rm K}^{0}_{\rm{S}}$, $\Lambda+\overline{\Lambda}$, 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).
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