A measurement of the spin alignment of charged D^* mesons produced in continuum e^+ e^- \to c \bar{c} events at \sqrt{s}=10.5 GeV is presented. This study using 4.72 fb^{-1} of CLEO II data shows that there is little evidence of any D^* spin alignment.
Systematic errors are not given.
Systematic errors are not given.
Two decay modes of D0 --> K- PI+ and D0 --> K- PI+ PI0 are combined.
The shapes of jets with transverse energies, E_T(jet), up to 45 GeV produced in neutral- and charged-current deep inelastic e+p scattering (DIS) at Q**2 > 100 GeV**2 have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the eta-phi plane with a cone radius of one unit. The jets become narrower as E_T(jet) increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in E_T(jet) and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in e+e- interactions and narrower than those in pbarp collisions for comparable E_T(jet). Since the jets in e+e- interactions and e+p DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
The production of K^0_S mesons and Lambda baryons in quark and gluon jets has been investigated using two complementary techniques. In the first approach, which provides high statistical accuracy, jets were selected using different jet finding algorithms and ordered according to their energy. Production rates were determined taking into account the dependences of quark and gluon compositions as a function of jet energy as predicted by Monte Carlo models. Selecting three-jet events with the k_perp (Durham) jet finder (y_cut = 0.005), the ratios of K^0_S and Lambda production rates in gluon and quark jets relative to the mean charged particle multiplicity were found to be 1.10 +/- 0.02 +/- 0.02 and 1.41 +/- 0.04 +/- 0.04, respectively, where the first uncertainty is statistical and the second is systematic. In the second approach, a new method of identifying quark jets based on the collimation of energy flow around the jet axis is introduced and was used to anti-tag gluon jets in symmetric (Y-shaped) three-jet events. Using the cone jet finding algorithm with a cone size of 30 degrees, the ratios of relative production rates in gluon and quark jets were determined to be 0.94 +/- 0.07 +/- 0.07 for K^0_S and 1.18 +/- 0.10 +/- 0.17 for Lambda. The results of both analyses are compared to the predictions of Monte Carlo models.
Ratios of relative yields.
Ratios of absolute rates.
Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_gamma^OBS) and pomeron (beta^OBS) momentum participating in the production of the dijet system. The observed x_gamma^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d(sigma)/d(beta^OBS) increases as beta^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.
Differential cross section as a function of rapidity of the two highest Et jets in event.
Differential cross section as a function of transverse energy Et of the tw o highest Et jets in event.
Differential cross section as a function of invariant mass of the GAMMA P system.
The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the \pi^0, \eta, \rho(770)+-, \omega(782), \eta'(958) and a_0(980)+-. The particle multiplicities per hadronic Z^0 decay, extrapolated to the full energy range, are: <n_\gamma> = 20.97 +/- 0.02 +/- 1.15, <n_\pi^0> = 9.55 +/- 0.06 +/- 0.75, <n_\eta> = 0.97 +/- 0.03 +/- 0.11, <n_\rho^+-> = 2.40 +/- 0.06 +/- 0.43, <n_\omega> = 1.04 +/- 0.04 +/- 0.14, <n_\eta> = 0.14 +/- 0.01 +/- 0.02, <n_a_0+-> = 0.27 +/- 0.04 +/- 0.10. where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models.
Particle multiplicities per hadronic decay extrapolated to the full energy range.
Photon fragmentation function.
Photon fragmentation function.
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 second systematic (DSYS) error is the correlated systematic error due to the scale uncertainty of the calorimeter.
Fragmentation functions for charged particles in Z -> qq(bar) events have been measured for bottom (b), charm (c) and light (uds) quarks as well as for all flavours together. The results are based on data recorded between 1990 and 1995 using the OPAL detector at LEP. Event samples with different flavour compositions were formed using reconstructed D* mesons and secondary vertices. The \xi_p = ln(1/x_E) distributions and the position of their maxima \xi_max are also presented separately for uds, c and b quark events. The fragmentation function for b quarks is significantly softer than for uds quarks.
Fragmentation function for 'uds-quark' events.
Fragmentation function for 'c-quark' events.
Fragmentation function for 'b-quark' events.
We have searched for the production of a selectron and a squark in $e^+p$ collisions at a center-of-mass energy of 300 GeV using the ZEUS detector at HERA. The selectron and squark are sought in the direct decay into the lightest neutralino in the framework of supersymmetric extensions to the Standard Model which conserve R-parity. No evidence for the production of supersymmetric particles has been found in a data sample corresponding to 46.6~pb$^{-1}$ of integrated luminosity. We express upper limits on the product of the cross section times the decay branching ratios as excluded regions in the parameter space of the Minimal Supersymmetric Standard Model.
No description provided.
The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.
No description provided.
No description provided.
No description provided.
The process e+e- to gamma gamma (gamma) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 56.2 pb-1 taken at a centre-of-mass energy of 183 GeV. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda+ > 233 GeV and Lambda- > 265 GeV as well as a mass limit for an excited electron, M(e*) > 227 GeV assuming equal e*egamma and eegamma couplings. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons.
No description provided.
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