arXiv subjects

Michael Klasen

Publications and source records attributed to Michael Klasen.

94 records · Page 6Linked to original sources

arXiv1998

Three jet cross sections in photoproduction at HERA

We calculate three jet cross sections in photoproduction using exact matrix elements for the direct and resolved contributions. Numerical distributions are presented in a generic, irreducible set of variables that allows to disentangle the dynamics of partonic QCD subprocesses from each other and from pure phase space distributions. The results are compared to preliminary data from the ZEUS collaboration at HERA. It is found that the largest contribution comes from photon-gluon fusion in the mass range 36 GeV $< M_{\rm 3-jet} <$ 80 GeV. The measured leading jet scattering angle distribution is consistent with the $t$-channel exchange of a massless fermion in $2\to 2$ scattering, where the third parton is assumed to arise from soft bremsstrahlung. The data are inconsistent with pure phase space and Rutherford scattering distributions.

hep-ph↗

arXiv1998

Massive Lepton Pairs as a Prompt Photon Surrogate

We discuss the transverse momentum distribution for the production of massive lepton-pairs in hadron reactions at fixed target and collider energies within the context of next-to-leading order perturbative quantum chromodynamics. For values of the transverse momentum $Q_T$ greater than the pair mass $Q$, $Q_T > Q$, we show that the differential cross section is dominated by subprocesses initiated by incident gluons. Massive lepton-pair differential cross sections are an advantageous source of constraints on the gluon density, free from the experimental and theoretical complications of photon isolation that beset studies of prompt photon production. We compare calculations with data and provide predictions for the differential cross section as a function of $Q_T$ in proton-antiproton reactions at center-of-mass energies of 1.8 TeV, and in proton-nucleon reactions at fixed target and LHC energies.

hep-ph↗

arXiv1997

Inclusive Jet Production in $γp$ and $γγ$ Processes: Direct and Resolved Photon Cross Sections in Next-To-Leading Order QCD

The production of jets in low $Q^2$ $ep$ scattering (photoproduction) and in low $Q^2$ $e^+e^-$ scattering ($γγ$ scattering) allows for testing perturbative QCD and for measuring the proton and photon structure functions. This requires exact theoretical predictions for one- and two-jet cross sections. We describe the theoretical formalism, giving sufficient details, for calculating the direct and resolved processes in $γp$ and $γγ$ reactions in next-to-leading order QCD. We present the complete analytical results for the Born terms, the virtual, and the real corrections. To separate singular and regular regions of phase space we use the phase space slicing method with an invariant mass cut-off. In this way, all soft and collinear singularities are either canceled or absorbed into the structure functions. Using a flexible Monte Carlo program, we evaluate the cross sections numerically and perform various tests and comparisons with other calculations. We consider the scale dependence of our results and compare them to data from the experiments H1 and ZEUS at HERA and OPAL at LEP.

hep-ph↗

arXiv1997

Photon Scattering in Muon Collisions

We estimate the benefit of muon colliders for photon physics. We calculate the rate at which photons are emitted from muon beams in different production mechanisms. Bremsstrahlung is reduced, beamstrahlung disappears, and laser backscattering suffers from a bad conversion of the incoming to the outgoing photon beam in addition to requiring very short wavelengths. As a consequence, the cross sections for jet photoproduction in $μp$ and $μ^+μ^-$ collisions are reduced by factors of 2.2 and 5 compared to $ep$ and $e^+e^-$ machines. However, the cross sections remain sizable and measurable giving access to the photon and proton parton densities down to $x$ values of $10^{-3}$ to $10^{-4}$.

hep-ph↗