Multiparton Interactions in Photoproduction at HERA

The high energy photoproduction of jets is being observed at the ep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets per ep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult. Total rates for the production of many (i.e. > 2) jets could provide direct evidence for the presence of multiple interactions, although parton showering and hadronization significantly affect low transverse energy jets.Comment: 21 pages, 8 figures include

Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas

In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with ~100 nm GaN thickness, which suppress the phonon mean free path. However, when the thickness is increased to ~1.2 $\mu$m, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to ~40 mW m$^{-1}$ K$^{-2}$ at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs

Inclusive particle production at HERA: Higher-order QCD corrections to the resolved quasi-real photon contribution

We calculate in next-to-leading order inclusive cross sections of single-particle production via resolved photons in $ep$ collisions at HERA. Transverse-momentum and rapidity distributions are presented and the scale dependence is studied. The results are compared with first experimental data from the H1 Collaboration at HERA.Comment: 11 pages with 15 uuencoded PS figures. Preprint DESY 93-03

Inclusive Dijet Production at HERA: Direct Photon Cross Sections in Next-To-Leading Order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for direct photoproduction in low $Q^2$ $ep$ collisions at HERA. Infrared and collinear singularities in real and virtual contributions are cancelled with the phase space slicing method. Analytical formulas for the different contributions giving the dependence on the slicing parameter are presented. Various one- and two-jet distributions have been computed demonstrating the flexibility of the method.Comment: 30 pages, latex, 11 figures appended as uuencoded fil

Dijet Rapidity Gaps in Photoproduction from Perturbative QCD

By defining dijet rapidity gap events according to interjet energy flow, we treat the photoproduction cross section of two high transverse momentum jets with a large intermediate rapidity region as a factorizable quantity in perturbative QCD. We show that logarithms of soft gluon energy in the interjet region can be resummed to all orders in perturbation theory. The resummed cross section depends on the eigenvalues of a set of soft anomalous dimension matrices, specific to each underlying partonic process, and on the decomposition of the scattering according to the possible patterns of hard color flow. We present a detailed discussion of both. Finally, we evaluate numerically the gap cross section and gap fraction and compare the results with ZEUS data. In the limit of low gap energy, good agreement with experiment is obtained.Comment: 37 pages, Latex, 17 figure

Jet photoproduction and the structure of the photon

Various jet observables in photoproduction are studied and compared to data from HERA. The feasibility of using a dijet sample for constraining the parton distributions in the photon is then studied. For the current data the experimental and theoretical uncertainties are comparable to the variation due to changing the photon parton distribution set.Comment: 20 pages including 11 figures. Latex using revtex and psfig macros. Several references added. Submitted to Phys. Rev.

On the photoproduction of jets at HERA

We discuss the inclusive jet production at HERA in the next-to-leading logarithm approximation. Theoretical uncertainties are considered in some details. We show the importance of the jet rapidity distribution to constrain the parton densities in the photon. A comparison is made with the recent H1 data.Comment: 10 + 7 figures included uuencoded tar-compressed ([email protected]) , ENSLAPP-A-484/94, LPTHE Orsay 94-8

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

Virtual Photon Strucutre Functions and the Parton Content of the Electron

We point out that in processes involving the parton content of the photon the usual effective photon approximation should be modified. The reason is that the parton content of virtual photons is logarithmically suppressed compared to real photons. We describe this suppression using several simple, physically motivated ans\"atze. Although the parton content of the electron in general no longer factorizes into an electron flux function and a photon structure function, it can still be expressed as a single integral. Numerical examples are given for the \eplem\ collider TRISTAN as well as the $ep$ collider HERA.Comment: 16 pages and 3 figures (available from DREES@WSICPHEN as topdraw or PS files); LaTeX with equation.sty; MAD/PH/819, BU 94-0

Diffractive Photoproduction of Jets with a Direct Pomeron Coupling at HERA

We investigate in detail the effect of a direct pomeron coupling to quarks on the production of jets in $ep$ scattering with almost real photons. Jet production via a direct pomeron coupling is compared with the resolved--pomeron mechanism. We consider both direct and resolved photoproduction. Rapidity and transverse momentum distributions are calculated and compared with preliminary H1 and ZEUS data.Comment: 21 pages, Latex file, 7 figures (uuencoded) enclosed, Preprint DESY 94-140, KEK-TH-407 and KEK Preprint 94-7