Strangeness production as a function of charged-particle multiplicity in proton-proton collisions at ${\bf \sqrt{s}~=~5.02}$ TeV

TL;DR

The paper analyzes strangeness production in pp collisions at $\sqrt{s}=5.02$ TeV as a function of midrapidity charged-particle multiplicity, extending previous energy studies. Using the ALICE detector, it reconstructs ${\rm K}^{0}_{\rm S}$, $\Lambda$, $\Xi$, and $\Omega$ via weak decays and corrects yields for acceptance, efficiency, and feed-down, with spectra extrapolated to $p_T=0$ via Lévy-Tsallis fits. The results show linear multiplicity scaling for ${\rm K}^{0}_{\rm S}$, $\Lambda$, and $\Xi$, but a faster-than-linear rise for $\Omega$, and a multiplicity-driven hardening of the $p_T$ spectra, including a shifting $\Lambda/{\rm K}^{0}_{\rm S}$ peak. Comparisons to MC models indicate that incorporating collectivity-like effects or modified hadronization (CR+Ropes in PYTHIA, core-corona in EPOS4, and DCCI dynamics) is necessary to describe the observed trends, highlighting the role of partonic-density dynamics in small collision systems.

Abstract

(Multi-)strange particle production rates and transverse momentum distributions are measured at midrapidity ($|y| < 0.5$) as a function of the charged-particle multiplicity density by the ALICE experiment at the LHC, using proton-proton collisions at a center-of-mass energy of ${\bf \sqrt{s}~=~5.02}$~TeV. This study extends similar studies performed at ${\bf \sqrt{s}~=~7}$~TeV and ${\bf \sqrt{s}~=~13}$~TeV to a lower energy regime, improving the statistical precision and extending the measurement to previously unexplored low-multiplicity regions. While $K_S^0$, $Λ$, and $Ξ$ yields can be described with a linear multiplicity dependence within uncertainties, the $Ω$ yields follow a significantly faster than linear increasing trend. For all analyzed particles, the overall production rate is consistent with those observed at higher energy and at similar multiplicity densities. Transverse momentum distributions are observed to evolve with multiplicity. Several state-of-the-art QCD-inspired Monte Carlo models have been compared to the data, testing some recently introduced features to address the findings at higher energies. Models can qualitatively describe the transverse momentum spectra and the $Λ/K_S^0$ spectral ratio only if collectivity is introduced in the evolution of the system.

Figures (6)

• Figure 1: $p_{\rm T}$ distributions at midrapidity for ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$, and $\Omega$ for multiplicity classes and ${\rm INEL>0}$, as indicated. Statistical uncertainties are shown as error bars, the systematic ones as boxes. The bottom panels present the ratio of all spectra to the ${\rm INEL>0}$ one. The dashed curves show the Levy-Tsallis Tsallis:1988 fit to the $p_{\rm T}$ distributions.
• Figure 2: $\langle p_{\rm T}\xspace\rangle$ in pp collisions at different center-of-mass energies and midrapidity as a function of the event multiplicity for ${\rm K}^{0}_{\rm{S}}$ and $\Lambda$ (top panels), $\Xi$ and $\Omega$ (bottom panels). $\sqrt{s}~=~5.02$ TeemV results are shown as black dots, $\sqrt{s}~=~7$ TeemV results ALICEstrPP7 as green squares and $\sqrt{s}~=~13$ TeemV results ALICEstrPP13 as red diamonds. Statistical uncertainties are reported as error bars, systematic uncertainties as boxes whose width represent the systematic error on the multiplicity determination.
• Figure 3: $p_{\rm T}$ distribution of $\Lambda$/${\rm K}^{0}_{\rm{S}}$ ratio in pp collisions at $\sqrt{s}~=~5.02$ TeemV and midrapidity for different multiplicity classes are shown with different colours. The lowest, highest and the ${\rm INEL>0}$ classes are highlighted.
• Figure 4: $p_{\rm T}$-integrated yields in pp collisions at different center-of-mass energies and midrapidity as a function of the event multiplicity for ${\rm K}^{0}_{\rm{S}}$ and $\Lambda$ (top panels), $\Xi$ and $\Omega$ (bottom panels). $\sqrt{s}~=~5.02$ TeemV results are shown as black dots, $\sqrt{s}~=~7$ TeemV results ALICEstrPP7 as green squares and $\sqrt{s}~=~13$ TeemV results ALICEstrPP13 as red diamonds. Statistical uncertainties are represented as error bars and systematic uncertainties as boxes whose width represent the systematic error on the multiplicity determination. Linear combined fits are shown as dashed lines, while the quadratic fit to the $\Omega$ progression is shown as a dotted line.
• Figure 5: $p_{\rm T}$-integrated yields (left column) and $\langle p_{\rm T}\xspace\rangle$ (right column) at $\sqrt{s}$ = 5.02 TeV for ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$, and $\Omega$ compared with MC generators PYTHIA 8 Monash 2013 (pythia8cr, dashed line), PYTHIA 8 QCD-CR+Ropes (bierlich2015effects, long-dashed line), DCCI (DCCIcoco, dotted line) and EPOS 4 (EPOS4strang, dash-dotted line).