The multiplicity distributions of charged particles in full phase space and in restricted rapidity intervals for events with a fixed number of jets measured by the DELPHI detector are presented. The data are well reproduced by the Lund Parton Shower model and can also be well described by fitted negative binomial distributions. The properties of these distributions in terms of the clan model are discussed. In symmetric 3-jet events the candidate gluon jet is found not to be significantly different in average multiplicity than the mean of the other two jets, thus supporting previous results of the HRS and OPAL experiments. Similar results hold for events generated according to the LUND PS and to the HERWIG models, when the jets are defined by the JADE jet finding algorithm. The method seems to be insensitive for measuring the color charge ratio between gluons and quarks.
Using the CELLO detector, we have measured cross sections for the processγγ→π+π− in the mass range 0.75–1.9 GeV/c2. A partial wave fit to the data indicates the presence of a sizeableS-wave amplitude with signs of resonant behaviour. Values for the γγ width of thef2(1270) are given, showing the model-dependence of this quantity. At higher dipion masses, the model of Brodsky and Lepage is found to give an order of magnitude description of the data.
We present measurements of the structure function \Ft\ in $e~+p$ scattering at HERA in the range $3.5\;\Gevsq < \qsd < 5000\;\Gevsq$. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At $ \qsd < 35 \;\Gevsq$ the range in $x$ now spans $6.3\cdot 10~{-5} < x < 0.08$ providing overlap with measurements from fixed target experiments. At values of $Q~2$ above 1000 GeV$~2$ the $x$ range extends to 0.5. Systematic errors below 5\perc\ have been achieved for most of the kinematic region. The structure function rises as \x\ decreases; the rise becomes more pronounced as \qsd\ increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations.
We employ data taken by the JADE and OPAL experiments for an integrated QCD study in hadronic e+e- annihilations at c.m.s. energies ranging from 35 GeV through 189 GeV. The study is based on jet-multiplicity related observables. The observables are obtained to high jet resolution scales with the JADE, Durham, Cambridge and cone jet finders, and compared with the predictions of various QCD and Monte Carlo models. The strong coupling strength, alpha_s, is determined at each energy by fits of O(alpha_s^2) calculations, as well as matched O(alpha_s^2) and NLLA predictions, to the data. Matching schemes are compared, and the dependence of the results on the choice of the renormalization scale is investigated. The combination of the results using matched predictions gives alpha_s(MZ)=0.1187+{0.0034}-{0.0019}. The strong coupling is also obtained, at lower precision, from O(alpha_s^2) fits of the c.m.s. energy evolution of some of the observables. A qualitative comparison is made between the data and a recent MLLA prediction for mean jet multiplicities.
We present an analysis of strange particle production frome+e− annihilation into multihadronic final states. The experiment was performed with the CELLO detector at the PETRA storage ring at DESY, the data was taken at a centre of mass energy of 35 GeV with an integrated luminosity of 86 pb−1. The particlesKS0,K*± and Λ have been identified by their characteristic decays, and differential cross sections for their production have been obtained. From a comparison ofKS0 andK*± rates the Lund vector meson suppression parameterV/(V+P)S has been determined.
We have measured the inclusive production of γ, π0 and η ine+e− annihilation at the center of mass energy of 35 GeV. The differential cross sections, extended to the kinematical limit and measured with high accuracy, are found to be in good agreement with previously reported results. Using the measured spectra we determine the average multiplicity for each of these particle species.
We have searched for resonance production in the reaction γγ→Ks0Kπ. No signal was found for theηc and an upper limit for the radiative with\(\Gamma _{\gamma \gamma }^{\eta _c } \) keV (95% c.l.) is obtained. For the glueball candidate η(1440) (previouslyi) the upper limit\(\Gamma _{\gamma \gamma }^{\eta (1440)} B(\eta (1440) \to K\bar K\pi )< 1.2keV(95\% c.l.)\) is derived. In the tagged data sample resonance formation of a spin 1 state at 1420 MeV is observed, which is absent in the untagged data. The mass and width of this state are consistent with those of thef1(1420); an analysis of decay angular distributions favours positive parity.
The inclusive forward jet cross section in deep inelastic $e^+p$ scattering has been measured in the region of $x$--Bjorken, ~$4.5 \cdot 10^{-4}$~ to ~$4.5 \cdot 10^{-2}$. This measurement is motivated by the search for effects of BFKL--like parton shower evolution. The cross section at hadron level as a function of \xbj is compared to cross sections predicted by various Monte Carlo models. An excess of forward jet production at small \xbj is observed, which is not reproduced by models based on DGLAP parton shower evolution. The Colour Dipole model describes the data reasonably well. Predictions of perturbative QCD calculations at the parton level based on BFKL and DGLAP parton evolution are discussed in the context of this measurement.
The e^+p neutral-current deep inelastic scattering differential cross-sections $d\sigma/dQ^2$, for Q^2 > 400 GeV^2, $d\sigma/dx$ and $d\sigma/dy$, for Q^2 > 400, 2500 and 10000 GeV^2, have been measured with the ZEUS detector at HERA. The data sample of 47.7 pb^-1 was collected at a center-of-mass energy of 300 GeV. The cross-section, $d\sigma/dQ^2$, falls by six orders of magnitude between Q^2 = 400 and 40000 GeV^2. The predictions of the Standard Model are in very good agreement with the data. Complementing the observations of time-like Z^0 contributions to fermion-antifermion annihilation, the data provide direct evidence for the presence of Z^0 exchange in the space-like region explored by deep inelastic scattering.
The forward-jet cross section in deep inelastic ep scattering has been measured using the ZEUS detector at HERA with an integrated luminosity of 6.36 pb^-1. The jet cross section is presented as a function of jet transverse energy squared, E(T,jet)^2, and Q^2 in the kinematic ranges 10^-2<E(T,jet)^2/Q^2<10^2 and 2.5 10^-4<x<8.0 10^-2. Since the perturbative QCD predictions for this cross section are sensitive to the treatment of the log(E_T/Q)^2 terms, this measurement provides an important test. The measured cross section is compared to the predictions of a next-to-leading order pQCD calculation as well as to various leading-order Monte Carlo models. Whereas the predictions of all models agree with the measured cross section in the region of small E(T,Jet)^2/Q^2, only one model, which includes a resolved photon component, describes the data over the whole kinematic range.
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