Multiplicity of identified particles in e+e− at the Z 0

Abstract

The production rates as a function of Ξ p for the identified hadrons π, K and p have been measured by DELPHI in inclusive hadronic Z 0 decays, as well as separately in decays into light and heavy flavors. The RICH detector (Ring Imaging Čerenkov detector) of DELPHI was used to identify charged hadrons. The vertex detector was used to tag high-impurity samples of light and heavy quark flavour events. The results, based on a data sample of 1 400 000 Z 0 decays collected by DELPHI during 1994, have been compared with the predictions from the JETSET parton shower model and the HERWIG cluster fragmentation model. In general, JETSET describes the measured spectra better. In particular, HERWIG seems to fail in the case of the proton spectra in all flavor samples; overestimates the spectra for all charged particles, pions and kaons in Z 0 → b b events and underestimates the corresponding spectra in Z 0 → u u , d d , s s events. The DELPHI results on pions, kaons and protons in the inclusive flavor sample agree well with the results from the ALEPH and SLD collaborations, whereas the proton spectrum in the low Ξ p region is in disagreement with the results from the OPAL collaboration. The particle multiplicities in Z 0 → q q , Z 0 → b b , and Z 0 → u u , d d , s s events have been measured; improved results were obtained for kaons and protons in the inclusive and b samples and new results were obtained on the multiplicity of pions, kaons and protons in Z 0 → u u , d d , s s events and on the multiplicity of pions in Z 0 → b b events.