Measurement of inclusive π⁰ production in hadronic Z ⁰ decays

Abstract

An analysis is presented of inclusive π⁰ production in Z ⁰ decays measured with the DELPHI detector. At low energies, π⁰ decays are reconstructed by using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to $x_p={2cdot p≪/{sqrt s}=0.75}$) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for qq̅ and bb̅ events. The number of π⁰’s per hadronic Z ⁰ event is $N(≪^0)/Z_{had} ^0=9.2pm 0.2({⤪ stat})pm 1.0 ({⤪ syst})$ and for bb̅ events the number of π⁰’s is ${⤪ N}(≪^0)/{⤪ b⋏r b}=10.1pm 0.4({⤪ stat})pm 1.1 ({⤪ syst})$. The ratio of the number of π⁰’s in bb̅ events to hadronic Z ⁰ events is less affected by the systematic errors and is found to be 1.09 ±0.05 ±0.01. The measured π⁰ cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the $xi_{⤪ p}={⤪ ln}(1/{⤪ x_p})$ distribution is $xi_p^{⋆ar}=3.90_{-0.14}^{+0.24}.$ The average number of π⁰’s from the decay of primary B hadrons is found to be N(B → π⁰ X)/B hadron = 2.78 ± 0.15(stat) ± 0.60(syst).