14362100
Measurement of the Dalitz-decay branching ratio of the π0
The ratio $\mathcal{B}= \Gamma(\pi^0 \to{} e^+ e^- \gamma)/\Gamma(\pi^0 \to\gamma\gamma)$ is measured by comparing the production rates of the π0 in the two decay channels. The analysis is based on approximately four million hadronic decays of the Z boson recorded by the ALEPH detector in the years 1991 to 1995. The value of $\mathcal{B}$ is determined to be $(1.140 \pm0.041)\%$. A recalculation of the Particle Data Group world average gives rise to a slightly improved error, and moves the world average closer to the theoretical value.
122373024
A ‘Ranking’ method for neutral pion selection in high multiplicity hadronic events
The selection of neutral pions with a high purity while maintaining also a high efficiency can be important in the formation of statistically significant mass spectra in the reconstruction of short-lived particles such as the omega meson (ω→π+π−π0). In this study a Ranking method, where pions are ranked according to a pion estimator, is described and detailed results from a Monte-Carlo study are presented. The results show that the Ranking method, when applied to high multiplicity events, yields significant improvements in the purity of selected pion candidates and facilitates the relaxation of standard cuts thereby avoiding some systematic uncertainties.
120780880
Inclusive production of the ρ±(770) meson in hadronic decays of the Z boson
Abstract The inclusive production rate of the ρ ± ( 770 ) vector meson in hadronic Z decays is measured with the ALEPH detector at the LEP collider. A total of 3.2 million hadronic events are selected from data recorded between 1991 and 1995. Decays of ρ ± → π 0 + π ± are reconstructed for x E > 0.05 and x p > 0.05 where x p = p ρ / p beam and x E = E ρ / E beam . The average ρ ± multiplicity per hadronic event is evaluated to be N ( ρ ± ) = 2.59 ± 0.03 ± 0.15 ± 0.04 where the first error is statistical and the second systematic. The third error is from the uncertainty in the extrapolation to x p = x E = 0 . The rates and differential cross-section are compared with Monte Carlo model predictions and OPAL measurements. Residual Bose–Einstein correlations are found to be an important component in the analysis.