Inclusive Jet Production with Virtual Photons in Next-to-Leading Order QCD

We present a next-to-leading order calculation for the virtual photoproduction of one and two jets in $ep$ collisions. Soft and collinear singularities are extracted using the phase space slicing method. The collinear photon initial state singularity depends logarithmically on the mass of the virtual photon and is absorbed into the virtual photon structure function. An $\overline{MS}$ factorization scheme is defined similarly to the real photon case. Numerical results are presented for HERA conditions using the Snowmass jet definition for inclusive single jet and dijet cross sections. We study the dependence of these cross sections on the transverse energies and rapidities of the jets. Finally, we compare the ratio of the experimentally defined resolved and direct cross sections with recent ZEUS data as a function of the photon virtuality $P^2$.Comment: LaTeX, 28 pages, 7 figure

Next-to-leading order QCD calculations with parton showers II: soft singularities

Programs that calculate observables in quantum chromodynamics at next-to-leading order typically generate events that consist of partons rather than hadrons -- and just a few partons at that. These programs would be much more useful if the few partons were turned into parton showers, which could be given to one of the Monte Carlo event generators to produce hadron showers. In a previous paper, we have seen how to generate parton showers related to the final state collinear singularities of the perturbative calculation for the example of e+ + e- --> 3 jets. This paper discusses the treatment of the soft singularities.Comment: 26 pages with 5 figures. This version is close to the version to be publishe

Dosimetry characterization of 32^{32}P intravascular brachytherapy source wires using Monte Carlo codes PENELOPE and GEANT4

Monte Carlo calculations using the codes PENELOPE and GEANT4 have been performed to characterize the dosimetric parameters of the new 20 mm long catheter based $^{32}$P beta source manufactured by Guidant Corporation. The dose distribution along the transverse axis and the two dimensional dose rate table have been calculated. Also, the dose rate at the reference point, the radial dose function and the anisotropy function were evaluated according to the adapted TG-60 formalism for cylindrical sources. PENELOPE and GEANT4 codes were first verified against previous results corresponding to the old 27 mm Guidant $^{32}$P beta source. The dose rate at the reference point for the unsheathed 27 mm source in water was calculated to be $0.215 \pm 0.001$ cGy s$^{-1}$ mCi$^{-1}$, for PENELOPE, and $0.2312 \pm 0.0008$ cGy s$^{-1}$ mCi$^{-1}$, for GEANT4. For the unsheathed 20 mm source these values were $0.2908 \pm 0.0009$ cGy s$^{-1}$ mCi$^{-1}$ and $0.311 \pm 0.001$ cGy s$^{-1}$ mCi$^{-1}$, respectively. Also, a comparison with the limited data available on this new source is shown. We found non negligible differences between the results obtained with PENELOPE and GEANT4.Comment: 13 pages, 7 figures, 7 tables (accepted for publication in Medical Physics

Pleniglacial dynamics in an oceanic central European loess landscape

Loess–palaeosol sequences (LPSs) of the oceanic-influenced European loess belt underwent frequent post-depositional processes induced by surface runoff or periglacial processes. The interpretation of such atypical LPSs is not straightforward, and they cannot be easily used for regional to continental correlations. Within the last few years, however, such sequences gained increased attention, as they are valuable archives for regional landscape dynamics. In this study, the Siersdorf LPS was analysed using a multi-proxy approach using sedimentological, geochemical, and spectrophotometric methods combined with luminescence dating and tentative malacological tests to unravel Pleniglacial dynamics of the Lower Rhine Embayment. A marshy wetland environment for the late Middle Pleniglacial to the early Upper Pleniglacial was shown by colour reflectance and grain size distribution. Age inversions from luminescence dating paired with geochemical and sedimentological data reveal long-lasting erosional processes during the early Upper Pleniglacial, which were constrained to a relatively small catchment with short transport ranges. The upper sequence shows typical marker horizons for the study area and indicate harsh, cold-arid conditions for the late Upper Pleniglacial. In comparison with other terrestrial archives, the Siersdorf LPS shows that the Lower Rhine Embayment was more diverse than previously assumed, regarding not only its geomorphological settings and related processes but also its ecosystems and environments.</p

Has the QCD RG-Improved Parton Content of Virtual Photons been Observed?

It is demonstrated that present $e^+e^-$ and DIS ep data on the structure of the virtual photon can be understood entirely in terms of the standard `naive' quark--parton model box approach. Thus the QCD renormalization group (RG) improved parton distributions of virtual photons, in particular their gluonic component, have not yet been observed. The appropriate kinematical regions for their future observation are pointed out as well as suitable measurements which may demonstrate their relevance.Comment: 24 pages, LaTeX, 5 figure

Combining QCD Matrix Elements at Next-to-Leading Order with Parton Showers in Electroproduction

We present a method to combine next-to-leading order (NLO) matrix elements in QCD with leading logarithmic parton showers by applying a suitably modified version of the phase-space-slicing method. The method consists of subsuming the NLO corrections into a scale-dependent phase-space-slicing parameter, which is then automatically adjusted to cut out the leading order, virtual, soft and collinear contributions in the matrix element calculation. In this way a positive NLO weight is obtained, which can be redistributed by a parton shower algortihm. As an example, we display the method for single-jet inclusive cross sections at O(alpha_s) in electroproduction. We numerically compare the modified version of the phase-space-slicing method with the standard approach and find very good agreement on the percent level.Comment: 21 pages, 2 eps figures. Revised section 2. To appear in PR

Factorization Breaking in Dijet Photoproduction with a Leading Neutron

The production of dijets with a leading neutron in ep-interactions at HERA is calculated in leading order and next-to-leading order of perturbative QCD using a pion-exchange model. Differential cross sections for deep-inelastic scattering (DIS) and photoproduction are presented as a function of several kinematic variables. By comparing the theoretical predictions for DIS dijets to recent H1 data, the pion flux factor together with the parton distribution functions of the pion is determined. The dijet cross sections in photoproduction show factorization breaking if compared to the H1 photoproduction data. The suppression factor is S = 0.48 (0.64) for resolved (global) suppression.Comment: 16 pages, 5 figure

Coherent Parton Showers with Local Recoils

We outline a new formalism for dipole-type parton showers which maintain exact energy-momentum conservation at each step of the evolution. Particular emphasis is put on the coherence properties, the level at which recoil effects do enter and the role of transverse momentum generation from initial state radiation. The formulated algorithm is shown to correctly incorporate coherence for soft gluon radiation. Furthermore, it is well suited for easing matching to next-to-leading order calculations.Comment: 24 pages, 3 figure

HERA Physics Beyond the Standard Model

The prospects of physics beyond the standard model in deep inelastic scattering are reviewed, emphasizing some scenarios which attained attention after the observation of an excess of events with large momentum transfer at HERA.Comment: 8 pages, LaTeX, uses iop style files and axodraw.sty, Talk presented at the 3rd UK Phenomenology Workshop on HERA Physics, September 1998, Durha

Standard Model Large-E_T Processes and Searches for New Physics at HERA

Existing and missing calculations of standard model processes producing large transverse energy in electron-proton interactions at HERA are reviewed. The adequacy of the existing standard model Monte Carlo programs for generic searches of exotic processes is analyzed.Comment: 9 pages, LaTeX, uses iop style files, Contribution to the 3rd UK Phenomenology Workshop on HERA Physics, September 1998, Durha