The cross section and the proton structure function F2 for neutral current deep inelastic e+p scattering have been measured with the ZEUS detector at HERA using an integrated luminosity of 30 pb-1. The data were collected in 1996 and 1997 at a centre-of-mass energy of 300 GeV. They cover the kinematic range 2.7 < Q^2 < 30000 GeV2 and 6.10^-5 < x < 0.65. The variation of F2 with x and Q2 is well described by next-to-leading-order perturbative QCD as implemented in the DGLAP evolution equations.
The electromagnetic structure function, F2(C=EM), in NC DIS scattering at Q**2 from 2.7 to 30000 GeV**2.
The corrections to the structure function, F2(C=EM), in NC DIS scattering at Q**2 from 2.7 to 30000 GeV**2.
The relative uncertainties in the reduced cross section. See text of paper for more details. There is an additional 2 PCT overall normalization error not included, andan addtional uncertainty of 1 PCT at low Q**2.. DUNC - Uncorrelated systematic error. Correlated Systematic Errors:. D1 - positron finding and efficiency. D2 - positron scattering angle - A. D3 - positron scattering angle - B. D4 - positron energy scale. D5 - hadronic energy measurment - FCAL. D6 - hadronic energy measurment - BCAL. D7 - hadronic energy measurment - RCAL. D8 - hadronic energy flow - A. D9 - background subtractions. D10 - hadronic energy flow - B.
Measurements of the individual multiplicities of pi+, pi- and pi0 produced in the deep-inelastic scattering of 27.5 GeV positrons on hydrogen are presented. The average charged pion multiplicity is the same as for neutral pions, up to approximately z= 0.7, where z is the fraction of the energy transferred in the scattering process carried by the pion. This result (below z= 0.7) is consistent with isospin invariance. The total energy fraction associated with charged and neutral pions is 0.51 +/- 0.01 (stat.) +/- 0.08 (syst.) and 0.26 +/- 0.01 (stat.) +/- 0.04 (syst.), respectively. For fixed z, the measured multiplicities depend on both the negative squared four momentum transfer Q^2 and the Bjorken variable x. The observed dependence on Q^2 agrees qualitatively with the expected behaviour based on NLO-QCD evolution, while the dependence on x is consistent with that of previous data after corrections have been made for the expected Q^2-dependence.
The measured PI0 multiplicity. Additional 9 PCT systematic error.
The measured multiplicity for charged pions, individually and the average. Additional 7 PCT systematic error.
The charged pion multiplicity as a function of x for four different z regions.
Dijet production has been studied in neutral current deep inelastic e+p scattering for 470 < Q**2 < 20000 GeV**2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb**{-1}. Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q**2 allows both a precise determination of alpha_s(M_Z) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of alpha_s(M_Z), as determined from the QCD fit, is alpha_s(M_Z) = 0.1166 +- 0.0019 (stat.) {+ 0.0024}_{-0.0033} (exp.)} {+ 0.0057}_{- 0.0044} (th.).
The differential dijet cross section dsig/dZP1.
The differential dijet cross section dsig/dlog10(x).
The differential dijet cross section dsig/dlog10(xi).
We have measured the charge asymmetry in like-sign dilepton yields from B^0 B^0-bar meson decays using the CLEO detector at the Cornell Electron Storage Ring. We find a_ll = [N(l+l+) - N(l-l-)]/[N(l+l+) + N[l-l-)] = +0.013 +/- 0.050 +/- 0.005 . We combine this result with a previous, independent measurement and obtain Re(epsilon_B)/(1+|epsilon_B|^2) = +0.0035 +/- 0.0103 +/- 0.0015 (uncertainties are statistical and systematic, respectively) for the CP impurity parameter, epsilon_B.
CONST(NAME=EPSILON) is CP impurity parameter.
Open charm production in gamma-gamma collisions is studied with data collected at e+e- centre-of-mass energies from 189 GeV to 202 GeV corresponding to a total integrated luminosity of 410 pb-1. The charm cross section sigma(gamma gamma ---> c c~ X) is measured for the first time as a function of the two-photon centre-of-mass energy in the interval from 5 GeV to 70 GeV and is compared to NLO QCD calculations.
The total cross section for the process E+ E- --> E+ E- CQ CQBAR X.
The total cross section for the process GAMMA GAMMA --> CQ CQBAR X.
Correlation matrix of the data after unfolding.
The production of c and b quarks in gamma-gamma collisions is studied with the L3 detector at LEP with 410 pb^-1 of data, collected at centre-of-mass energies from 189 GeV to 202 GeV. Hadronic final states containing c and b quarks are identified by detecting electrons or muons from their semileptonic decays. The cross sections sigma(e+e- -> e+e- c c~ X) and sigma(e+e- -> e+e- b b~ X) are measured and compared to next-to-leading order perturbative QCD calculations. The cross section of b production is measured in gamma-gamma collisions for the first time. It is in excess of the QCD prediction by a factor of three.
Total cross section for charm production.
Total cross section for beauty production.
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.
Jet production in charged and neutral current events in the kinematic range of Q^2 from 640 to 35000 GeV^2 is studied in deep-inelastic positron-proton scattering at HERA. The measured rate of multi-jet events and distributions of jet polar angle, transverse energy, dijet mass, and other dijet variables are presented. Using parton densities derived from inclusive DIS cross sections, perturbative QCD calculations in NLO are found to give a consistent description of both the neutral and charged current dijet production. A direct, model independent comparison of the jet distributions in charged and neutral current events confirms that the QCD dynamics of the hadronic final state is independent of the underlying electroweak scattering process.
Rates of charged current events as a function of Q**2.
Rates of neutral current events as a function of Q**2.
Normalised distribution in Y2 for NC and CC dijet events. Y2 is the smallest scaled value of KT (KTJET**2/W**2) given by the combination of (2+1) jets. The +1 refers to the proton remnant jet.
The dependence on Q^2 (the negative square of the 4-momentum of the exchanged virtual photon) of the generalised Gerasimov-Drell-Hearn integral for the proton has been measured in the range 1.2 GeV^2 < Q^2 < 12 GeV^2 by scattering longitudinally polarised positrons on a longitudinally polarised hydrogen gas target. The contributions of the nucleon-resonance and deep-inelastic regions to this integral have been evaluated separately. The latter has been found to dominate for Q^2 > 3 GeV^2, while both contributions are important at low Q^2. The total integral shows no significant deviation from a 1/Q^2 behaviour in the measured Q^2 range, and thus no sign of large effects due to either nucleon-resonance excitations or non-leading twist.
The GDH integral as a function of Q2 in the resonance region (W**2 = 1 to 4.2 GeV**2), the measured region (W**2=4.2 to 45 GeV**2), and the total region (W**2= 1 to 45 GeV**2).
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