Multiplicity dependence of light-flavor hadron production in pp collisions at s =7 TeV

Abstract

Comprehensive results on the production of unidentified charged particles, ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}, {\mathrm{K}}^{\ifmmode\pm\else\textpm\fi{}}, {\mathrm{K}}_{\mathrm{S}}^{0}, {\mathrm{K}}^{*}{(892)}^{0}, p, \overline{p}, \ensuremath{\phi}$(1020), $\mathrm{\ensuremath{\Lambda}}, \overline{\mathrm{\ensuremath{\Lambda}}}, {\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}, {\overline{\mathrm{\ensuremath{\Xi}}}}^{+}, {\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}}$, and ${\overline{\mathrm{\ensuremath{\Omega}}}}^{+}$ 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 autocorrelation 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 Large Hadron Collider 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.