The interaction of virtual photons is investigated using double tagged gammagamma events with hadronic final states recorded by the ALEPH experiment at e^+e^- centre-of-mass energies between 188 and 209 GeV. The measured cross section is compared to Monte Carlo models, and to next-to-leading-order QCD and BFKL calculations.
Differential cross section as a function of the relative energy of the scattered electrons.
Differential cross section as a function of the polar angle THETA of the scattered electrons.
Differential cross section as a function of the virtuality Q**2 of the photons.
Di-jet producion is studied in collisions of quasi-real photons at e+e- centre- of-mass energies sqrt(s)ee from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k_t clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy Etm(jet) of the two leading jets, and as a functiuon of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, xg, for different regions of Etm (jet). Angular distribution in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of |eta(jet)| and |delta eta (jet)| is presented where eta(jet) is the jet pseudo-rapidity. Different regions of the xg+ -xg- -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.
The di-jet cross section as a function of the angle between the jet and thedirection of the incoming parton in the centre-of-mass frame for the region whe re both X(C=GAMMA+) and X(C=GAMMA-) are > 0.75.
The di-jet cross section as a function of the angle between the jet and thedirection of the incoming parton in the centre-of-mass frame for the region whe re both X(C=GAMMA+) and X(C=GAMMA-) are < 0.75.
The di-jet cross section as a function of the mean transverse energy of thedi-jet system for the full X(C=GAMMA+) and X(C=GAMMA-) region.
A study of b quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be <xe>=0.7193+-0.0016(stat)+0.0036-0.0031(syst)
No description provided.
Correlation matrix of statistical uncertainties.
Correlation matrix of positive systematic uncertainties.
Charged particle momentum distributions are studied in the reaction e+e- -> hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of- mass energy of 201.7 GeV and a total integrated luminosity of 433 pb-1. The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions.
The measured values of the PTIN distribution.
The measured values of the PTOUT distribution.
The measured values of the rapidity, YRAP, distribution.
The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process $e^+p -> e^+ + {dijet} + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 ${\rm pb^{-1}}$. Events with photon-proton centre-of-mass energies, $W_{\gamma p}$, between 134 and 269 GeV and a photon virtuality, Q^2, less than 1 ${\rm GeV^2}$ were selected requiring at least two jets of transverse energy $E_T^{\rm jet1(2)} >7(6)$ GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, $p_T^{\rm rel}$. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum $p_T^{b} > p_T^{\rm min} =$ 5 GeV and pseudorapidity $|\eta^{b}| <$ 2. The extrapolated cross section is $1.6 \pm 0.4 (stat.)^{+0.3}_{-0.5} (syst.) ^{+0.2}_{-0.4} (ext.) {nb}$. The result is compared to a perturbative QCD calculation performed to next-to-leading order.
The differential distribution of PT(C=REL) for heavy quark decays. The second DSYS error is due to the energy scale uncertainty.
The differential distribution of X(C=GAMMA,OBS), the fraction of the photons momentum contributing to the production of the two highest transverse energy jets. The second DSYS error is due to the energy scale uncertainty.
Cross section for beauty production with a prompt electron in the restricted kinetic region.
Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e^+ + p --> e^+ + n + jet + jet + X_r have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb^{-1}. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies E^{jet}_T > 6 GeV, neutron energy E_n > 400 GeV, and neutron production angle theta_n < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model.
The differential dijet cross section as a function of ET for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ET for the neutron-tagged data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ETARAP for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeterenergy scale.
The dependence of the photon structure on the photon virtuality, Q^2, is studied by measuring the reaction e^+p\to e^+ + {\rm jet} + {\rm jet} + {\rm X} at photon-proton centre-of-mass energies 134 < W < 223 GeV. Events have been selected in the Q^2 ranges \approx 0 GeV^2, 0.1-0.55 GeV^2, and 1.5-4.5 GeV^2, having two jets with transverse energy E_T^{jet} > 5.5 GeV in the final state. The dijet cross section has been measured as a function of the fractional momentum of the photon participating in the hard process, x_gamma. The ratio of the dijet cross section with x_gamma < 0.75 to that with x_gamma > 0.75 decreases as Q^2 increases. The data are compared with the predictions of NLO pQCD and leading-order Monte Carlo programs using various parton distribution functions of the photon. The measurements can be interpreted in terms of a resolved photon component that falls with Q^2 but remains present at values of Q^2 up to 4.5 GeV^2. However, none of the models considered gives a good description of the data.
Dijet cross section for the low ET set of cuts.
Dijet cross section for the high ET set of cuts.
Ratio of Dijet cross sections as a function of Q**2 for XOBS(C=GAMMA) less than to greater than 0.75 for the lower ET cuts.
Production of Sigma- and Lambda(1520) in hadronic Z decays has been measured using the DELPHI detector at LEP. The Sigma- is directly reconstructed as a charged track in the DELPHI microvertex detector and is identified by its Sigma -> n pi decay leading to a kink between the Sigma- and pi-track. The reconstruction of the Lambda(1520) resonance relies strongly on the particle identification capabilities of the barrel Ring Imaging Cherenkov detector and on the ionisation loss measurement of the TPC. Inclusive production spectra are measured for both particles. The production rates are measured to be <N_{Sigma-}/N_{Z}^{had}> = 0.081 +/- 0.002 +/- 0.010, <N_{Lambda(1520)}/N_{Z}^{had}> = 0.029 +/- 0.005 +/- 0.005. The production rate of the Lambda(1520) suggests that a large fraction of the stable baryons descend from orbitally excited baryonic states. It is shown that the baryon production rates in Z decays follow a universal phenomenological law related to isospin, strangeness and mass of the particles.
The measured differential cross section for SIGMA- production.
The total production rate of SIGMA-. The second systematic (DSYS) error is due to the extrapolation to the fullx-range.
The measured differential cross section for LAMBDA(1520) production. The first error is the fit error.
We have studied hadronic events from e+e- annihilation data at centre-of-mass energies of sqrt{s}=172, 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb^-1. We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling alpha_s, the mean charged particle multiplicity <nch> and the peak position xi_0 in the xi_p=ln(1/x_p) distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of alpha_s, <nch> and xi_0 are consistent with previous determinations at sqrt{s}=MZ.
Distribution of Thrust.
Distribution of Thrust Major.
Distribution of Thrust Minor.
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.
Forward jet cross section as a function of ET**2/Q**2. The second DSYS error is the uncertainty in the energy scale of the calorimeter.
Measured forward-jet x distribution.
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