Multiplicity dependence of light-flavor hadron production in pp collisions at $\sqrt{s}$ = 7 TeV

Abstract

Comprehensive results on the production of unidentified charged particles, ${\pi }^{\pm }$, ${\mathrm{K}}^{\pm }$, ${\mathrm{K}}_{\mathrm{S}}^0$, $\mathrm{K}$*(892)${}^0$, $\mathrm{p}$, $\overline{\mathrm{p}}$, $\varphi$(1020), $\Lambda$, $\overline{\Lambda }$, ${\Xi }^{-}$, ${\overline{\Xi }}^{+}$, ${\Omega }^{-}$ and ${\overline{\Omega }}^{+}$ hadrons in proton-proton (pp) collisions at $\sqrt{s}$ = 7 TeV at midrapidity ($|y|<0.5$) as a function of charged-particle multiplicity density are presented. In order to avoid auto-correlation biases, the actual transverse momentum ($p_{\mathrm{T}}$) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle to pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at LHC energies. The obtained $p_{\mathrm{T}}$ distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.

45 pages, 15 captioned figures, 12 tables, authors from page 40, published, figures at http://alice-publications.web.cern.ch/node/4516

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