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Register a new userStudy of charge multiplicity in hadronic three-jet Z decays at LEP
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V.A. Uvarov
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The mean charge multiplicity in hadronic three-jet Z decays has been measured with the DELPHI detector as a function of the event topology and compared with recent theoretical calculations. The QCD colour factor ratio C_A/C_F was determined, and the gluon contribution to this multiplicity was extracted giving a measurement of the mean charge multiplicity of a two-gluon colour-singlet system as a function of the effective c.m. energy covering the range from 16 to 52 GeV.
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Mass dependences of the total production rates per hadronic Z decay of all light-flavour hadrons measured so far at LEP are extrapolated to the zero mass limit (m=0) using phenomenological laws of hadron production related to the spin, isospin, strangeness content and mass of the particles. The vector-to-pseudoscalar and meson-to-baryon ratios at m=0 are found to be: rho^+ / 3 pi^+ = 1.2 +/- 0.3 and pi^+ / p = 2.9 +/- 0.3, in good agreement with the predictions of quark combinatorics.
Recent results on jet physics and tests of QCD from hadronic final states in $e^+e^-$ annihilation at PETRA and at LEP are reviewed, with special emphasis on hadronic event shapes, charged particle production rates, properties of quark and gluon jets and determinations of $alpha_s$. The data in the entire energy range from PETRA to LEP-2 are in broad agreement with the QCD predictions. The world summary of measurements of $alpha_s$ is updated and a detailed discussion of various methods to determine the overall error of $alpha_s (M_Z)$ is presented. The new world average is $alpha_s (M_Z) = 0.119 pm 0.004$. The size of the error depends on the treatment of correlated uncertainties.
A new phenomenological approach is suggested for determining the strangeness content of light-flavour isoscalars. This approach is based on phenomenological laws of hadron production related to the spin, isospin, strangeness content and mass of the particles. From the total production rates per hadronic Z decay of all light-flavour hadrons, measured so far at LEP, the values of the nonstrange-strange mixing angles are found to be |phi_P| = 42.3^{circ} pm 3.5^{circ}, |phi_V| = 10^{circ} pm 8^{circ}, |phi_T| = 16^{circ} pm 11^{circ} and |phi_S| = 13^{circ} pm 9^{circ}. Our results on the eta-eta^{prime}, omega-phi and f_2-f_2^{prime} isoscalar mixing are consistent with the present experimental evidence. The strangeness content obtained for the f_0(980) scalar/isoscalar is not consistent with the values supported by recent model studies and is discussed further in the framework of our approach and the K-matrix analysis.
An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behaviour to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.
The measurements of hadronic structure functions of the photon based on the reaction ee --> ee gamma^(*)(P^2) gamma^*(Q^2)--> ee hadrons are discussed.
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