We present a measurement of the transverse momentum distribution of dielectron pairs with invariant mass near the mass of the Z boson. The data were obtained using the DO detector during the 1994-1995 run of the Tevatron Co!lider at Fermilab. The data used in the measurement corresponds to an integrated luminosity of 108.5 $pb^{-1}$ The measurement is compared to current phenomenology for vector boson production in proton-antiproton interactions, and the results are found to be consistent with expectation from Quantum Chromodynamics (QCD).

We have searched for second generation leptoquark (LQ) pairs in the \mu\mu+jets channel using 94+-5 pb^{-1} of pbar-p collider data collected by the D0 experiment at the Fermilab Tevatron during 1993-1996. No evidence for a signal is observed. These results are combined with those from the \mu\nu+jets and \nu\nu+jets channels to obtain 95% confidence level (C.L.) upper limits on the LQ pair production cross section as a function of mass and $beta, the branching fraction of a LQ decay into a charged lepton and a quark. Lower limits of 200(180) GeV/c^2 for \beta=1(1/2) are set at the 95% C.L. on the mass of scalar LQ. Mass limits are also set on vector leptoquarks as a function of \beta.

This Letter describes a measurement of the muon cross section originating from b quark decay in the forward rapidity range 2.4 < y(mu) < 3.2 in pbarp collisions at sqrt(s) = 1.8 TeV. The data used in this analysis were collected by the D0 experiment at the Fermilab Tevatron. We find that NLO QCD calculations underestimate b quark production by a factor of four in the forward rapidity region. A cross section measurement using muon+jet data has been included in this version of the paper.

We have searched for first generation scalar leptoquark (LQ) pairs in the enu+jets channel using ppbar collider data (integrated luminosity= 115 pb^-1) collected by the DZero experiment at the Fermilab Tevatron during 1992-96. The analysis yields no candidate events. We combine the results with those from the ee+jets and nunu+jets channels to obtain 95% confidence level (CL) upper limits on the LQ pair production cross section as a function of mass and of beta, the branching fraction to a charged lepton. Comparing with the next-to-leading order theory, we set 95% CL lower limits on the LQ mass of 225, 204, and 79 GeV/c^2 for beta=1, 1/2, and 0, respectively.

We present measurements of the b-bbar production cross section and angular correlations using the D0 detector at the Fermilab Tevatron p-pbar Collider operating at sqrt(s) = 1.8 TeV. The b quark production cross section for |y(b)|<1.0 and p_T(b)>6 GeV/c is extracted from single muon and dimuon data samples. The results agree in shape with the next-to-leading order QCD calculation of heavy flavor production but are greater than the central values of these predictions. The angular correlations between b and bbar quarks, measured from the azimuthal opening angle between their decay muons, also agree in shape with the next-to-leading order QCD prediction.

Double-tag events in two-photon collisions are studied using the L3 detector at LEP centre-of-mass energies from root(s)=189 GeV to 209 GeV. The cross sections of the e+e- -> e+e- hadrons and gamma*gamma* -> hadrons processes are measured as a function of the photon virtualities, Q1^2 and Q2^2, of the two-photon mass, W_gammagamma, and of the variable Y=ln(W_gammagamma^2/(Q1 Q2)), for an average photon virtuality &lt;Q2> = 16 GeV2. The results are in agreement with next-to-leading order calculations for the process gamma*gamma* -> q qbar in the interval 2 &lt;= Y &lt;= 5. An excess is observed in the interval 5 &lt; Y &lt;= 7, corresponding to W_gammagamma greater than 40 GeV . This may be interpreted as a sign of resolved photon QCD processes or the onset of BFKL phenomena.

The reaction e+e- -> e+e- gamma* gamma* -> e+e- hadrons for quasi-real photons is studied using data from root(s) = 183 GeV up to 202 GeV. Results on the total cross sections sigma(e+e- -> e+e- hadrons) and sigma(+e- gamma* gamma* -> e+e- hadrons) are given for the two-photon centre-of-mass energies 5 GeV &lt; Wgammagamma &lt; 185 GeV. The total cross section of two real photons is described by a Regge parametrisation. We observe a steeper rise with the two-photon centre-of-mass energy as compared to the hadron-hadron and the photon-proton cross sections. The data are also compared to the expectations of different theoretical models.

Cross sections for the production of two isolated muons up to high di-muon masses are measured in ep collisions at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 71 pb^-1 at a centre of mass energy of sqrt{s} = 319 GeV. The results are in good agreement with Standard Model predictions, the dominant process being photon-photon interactions. Additional muons or electrons are searched for in events with two high transverse momentum muons using the full data sample corresponding to 114 pb^-1, where data at sqrt{s} = 301 GeV and sqrt{s} = 319 GeV are combined. Both the di-lepton sample and the tri-lepton sample agree well with the predictions.

The production of neutrons carrying at least 20% of the proton beam energy ($\xl > 0.2$) in $e^+p$ collisions has been studied with the ZEUS detector at HERA for a wide range of $Q^2$, the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, $e p\to e' X n$, is measured relative to the inclusive cross section, $e p\to e' X$, thereby reducing the systematic uncertainties. For $\xl >$ 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the $\gamma p$ system. For $0.64 < \xl < 0.82$, the rate of neutrons is almost independent of the Bjorken scaling variable $x$ and $Q^2$. However, at lower and higher $\xl$ values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, ${{F}^{\rm\tiny LN(3)}_2}(x,Q^2,\xl)$, rises at low values of $x$ in a way similar to that of the inclusive \ff of the proton. The total $\gamma \pi$ cross section and the structure function of the pion, $F^{\pi}_2(x_\pi,Q^2)$ where $x_\pi = x/(1-\xl)$, have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed $Q^2$, $F^{\pi}_2$ has approximately the same $x$ dependence as $F_2$ of the proton.

Inclusive D^{*+-} production in two-photon collisions is studied with the L3 detector at LEP, using 683 pb^{-1} of data collected at centre-of-mass energies from 183 to 208 GeV. Differential cross sections are determined as functions of the transverse momentum and pseudorapidity of the D^{*+-} mesons in the kinematic region 1 GeV &lt; P_T &lt; 12 GeV and |eta| &lt; 1.4. The cross sections sigma(e^+e^- -> e^+e^-D^{*+-}X) in this kinematical region is measured and the sigma(e^+e^- -> e^+e^- cc{bar}X) cross section is derived. The measurements are compared with next-to-leading order perturbative QCD calculations.