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[en] Events with three and four jets in photoproduction regime are investigated with H1 detector at HERA. Jets are reconstructed in the central part of the detector within the acceptance of the central tracking system. Events are selected if the transverse energy of the leading jet is ET>7 GeV. The present theoretical models fail to describe the previous multi-jet state measurements. One improved description is based on the multiple parton interactions models. The measurements are compared with theoretical calculations. Only when including multiple interactions, the predictions come close to the measurements. (orig.)
[en] The Large Hadron Collider will be able to collide protons at energies up to √(s)=14 TeV and lead ions at energies up to √(sNN)=5.5 TeV. One way to study these collisions is through jet-like correlations. In pp collisions, these correlations provide information about the fragmentation of a high momentum parton into a jet of hadrons. This can be compared to calculations and event simulators, such as PYTHIA and PHOJET, because high momentum partons are calculable in perturbative Quantum Chromodynamics and can be fragmented according to models, such as the Lund String Model. One may also study whether the assumed underlying event, likely composed of particles from non-perturbative soft processes, is truly uncorrelated from the jet. In heavy-ion collisions, the jet is used as a probe of the medium created in these collisions. The medium is expected be a Quark-Gluon Plasma, a deconfined state of quarks in gluons. With measurements in pp collisions and calculations/simulations, jets are a well calibrated probe. These correlations can be used to study how energy is lost in the medium and the interactions of the medium and the jet. I present the status of an analysis of 2-particle jet-like correlations of charged particles measured in the ALICE Time Projection Chamber and discuss the physics results that can be obtained from this analysis.
[en] More than 20 years after its discovery, the investigation of the top-quark and its properties is in a high-precision phase. The LHC plays a central role in this investigation since it provides a large number of t anti t-pairs. Precise top-quark measurements are an excellent tool for precision measurements of the Standard Model and searches for new physics. An important ingredient for this kind of measurements are precise theory predictions. Meanwhile, NNLO QCD predictions for t anti t-pair production at the LHC became available for comparison to data. Inclusive or differential, these predictions have in common that they assume stable top-quarks and use spin-summed matrix-elements. The information about spin-correlations with decay products is therefore lost. I present here the first calculation of amplitudes for production of polarized on-shell t anti t-pairs at NNLO QCD which is a necessary ingredient to incorporate spin-correlated top-decays in Narrow-Width-Approximation. The full calculation including higher order corrections to the decays can be used to extract the top-quark mass parameter more precisely from the examination of differential distributions of decay products.
[en] The preparation of polarized antiproton beams by the filter method as proposed for future experiments at FAIR depletes one spin component of a stored beam due to an assumed spin dependent interaction. An increase of polarisation degree goes on the expense of intensity. Alternative methods to prepare polarized beams could be the antiproton production mechanism itself or the use of antiprotons from anti Λ decay. In the usual hadronic quasifree production p+N→anti p+3N, the antiprotons may have substantial polarization if collected at finite angles. Experimentally the possibility has never been studied and theoretical predictions are missing. Problematic may be depolarization during precooling of the anti p in the collector synchrotron. In the anti Λ production via anti p+p→ anti ΛΛ the hyperons are strongly polarized and decay into strongly polarized anti p and p. In the PS185 experiment - a full acceptance geometry spectrometer - at LEAR the feasibility of experiments with polarized antiprotons via anti Λ production has been demonstrated. With the expected antiproton flux at FAIR this method will certainly work in kinematic regions defined by the source reaction.
[en] The accuracy for most of the measurements planned in the ATLAS experiment at the LHC depends on the correct jet energy scale. This scale is affected on one hand by instrumental imperfections of the calorimeter system, and on the other hand by physics effects, such as parton radiation, multiple interactions, hadronisation effects etc. Various in-situ calibration methods will be used to check and correct for these effects. The calibration procedure using dijet events is discussed in this presentation. The results of performance tests of this method, based on full Geant4 simulations, are presented