arXiv subjects
Publications and source records attributed to Michael Krämer.
The theoretical status of MSSM particle production at the hadron colliders Tevatron and LHC is reviewed, including next-to-leading order supersymmetric QCD corrections. The higher-order corrections significantly reduce the theoretical uncertainty and lead to a rise of the lower bounds on supersymmetric particle masses, as demonstrated for the case of top-squark and gaugino pair production at the Tevatron.
I discuss the production of leptoquarks in $p\bar{p}$ collisions at the Fermilab Tevatron. Evaluated on the basis of a recent next-to-leading order calculation, the combined D0 and CDF leptoquark searches lead to a parameter-free lower mass bound of about $\ms \simgt 240$ GeV for scalar leptoquarks or squarks decaying solely to a first generation charged lepton plus quark. These results provide very stringent constraints on any explanation of the recently observed HERA excess events in terms of new particle production.
I discuss the impact of color-octet contributions to inclusive $J/ψ$ photoproduction at HERA. Emphasis is put on resolved photon processes and on $J/ψ$ polarization, which will be experimentally accessible at HERA in the near future. Both analyses provide a powerful test of the NRQCD factorization approach to charmonium production.
We examine the contributions of soft gluons to the Higgs production cross section at the LHC in the Standard Model and its minimal supersymmetric extension. The soft gluon radiation effects of this reaction share many features with the Drell-Yan process, but arise at lowest order from a purely gluonic initial state. We provide an extension of the conventional soft gluon resummation formalism to include a new class of contributions which we argue to be universal, and resum these and the usual Sudakov effects to all orders. The effect of these new terms is striking: only if they are included, does the expansion of the resummed cross section to next-to-leading order reproduce the exact result to within a few percent for the full range of Higgs boson masses. We use our resummed cross section to derive next-to-next-to-leading order results, and their scale dependence. Moreover, we demonstrate the importance of including the novel contributions in the resummed Drell-Yan process.
The associated production of a J/psi and a photon in photon-hadron collisions is considered, and shown to be a good probe of the presence of colour-octet mediated channels in quarkonia production, as predicted by the factorization approach within NRQCD. Total and differential cross sections for photoproduction at fixed target experiments and at HERA are presented. Associated J/psi + gamma production at hadron colliders is briefly discussed.
We work out and review prospects for future quarkonium physics at HERA. We focus on the impact of color-octet contributions and discuss how measurements at HERA can be used to test the picture of quarkonium production as developed in the context of the NRQCD factorization approach.
I discuss the impact of color-octet contributions and higher-order QCD corrections on the cross section for inelastic $J/ψ$ photoproduction. The theoretical predictions are compared with recent experimental data obtained at HERA.
We have calculated the leading color-octet contributions to the production of $J/ψ$ particles in photon-proton collisions. Using the values for the color-octet matrix elements extracted from fits to prompt $J/ψ$ data at the Tevatron, we demonstrate that distinctive color-octet signatures should be visible in $J/ψ$ photoproduction. However, these predictions appear at variance with recent experimental data obtained at HERA, indicating that the phenomenological importance of the color-octet contributions is smaller than expected from theoretical considerations and suggested by the Tevatron fits.
Higgs-strahlung $e^+e^-\to ZH$ and $WW$ fusion $e^+e^-\to\barν_eν_e H$ are the most important mechanisms for the production of Higgs bosons in $e^+e^-$ collisions at LEP2 and future $e^+e^-$ linear colliders. We have calculated the cross sections and energy/angular distributions of the Higgs boson for these production mechanisms. When the $Z$ boson decays into (electron-)neutrinos, the two production amplitudes interfere. In the cross-over region between the two mechanisms the interference term is positive and of the same size as the individual cross sections, thus enhancing the production rate.
In two-photon collisions at LEP2 and a future $e^+e^-$ linear collider heavy quarks (mainly charm) will be pair-produced rather copiously. The production via direct and resolved photons can be distinguished experimentally via a remnant-jet tag. We study correlations of the heavy quarks at next-to-leading order in QCD in the direct channel, which is free from phenomenological parton densities in the photon. These correlations are therefore directly calculable in perturbative QCD and provide a stringent test of the production mechanism.
We present a complete calculation of the higher-order perturbative QCD corrections to inelastic photoproduction of $J/ψ$ particles. A comprehensive analysis of total cross sections and differential distributions for the energy range of the fixed-target experiments and for inelastic $J/ψ$ photoproduction at HERA is performed. The cross section and the $J/ψ$ energy spectrum are compared with the available photoproduction data including first results from HERA. This analysis will not only provide information on the gluon distribution of the proton but appears to be a clean test for the underlying picture of quarkonium production as developed so far in the perturbative QCD sector.
Heavy quarks are copiously produced in two-photon collisions at $e^+e^-$ colliders. The theoretical predictions including QCD radiative corrections are compared to recent experimental data on $γγ$ production of charm quarks at PETRA, PEP, TRISTAN and LEP. Photoproduction of heavy quarks at HERA is an important tool to measure the gluon distribution in the proton. New theoretical results on heavy quark photoproduction at large transverse momenta are discussed and NLO predictions for inelastic $J/ψ$ photoproduction in the HERA energy range are given. The sensitivity of the results to the parametrization of the gluon distribution in the small-$x$ region is demonstrated.
Inelastic photoproduction of $J/ψ$ particles at high energies is one of the processes to determine the gluon distribution in the nucleon. The QCD radiative corrections to the color-singlet model of this reaction have recently been calculated. They are large at moderate photon energies, but decrease with increasing energies. I compare the cross section and the ${J/ψ}$ energy spectrum with the available fixed-target photoproduction data. Predictions for the HERA energy range are given which demonstrate the sensitivity of the result to the parametrization of the gluon distribution in the small-$x$ region. (Talk presented at the Workshop on "Heavy Quark Physics", Bad Honnef, FRG, Dec. 1994)