Stefan Gieseke

New formalism for QCD parton showers

We present a new formalism for parton shower simulation of QCD jets, which incorporates the following features: covariant kinematics, improved treatment of heavy quark fragmentation, angular-ordered evolution with soft gluon coherence, more accurate soft gluon angular distributions, and better coverage of phase space. It is implemented in the new Herwig++ event generator.

Herwig++ 1.0: an event generator for e+e− annihilation

Results from the new Monte Carlo event generator Herwig++ are pre- sented. This first version simulates Hadron Emission Reactions With Interfering Gluons in electron-positron annihilation. The parton shower evolution is carried out using new evolution variables suited to describing radiation from heavy quarks as well as light partons. The partonic final state is fragmented into hadrons by means of an improved cluster hadronization model. The results are compared with a wide variety of data from LEP and SLC.

NLO corrections to the photon impact factor: Combining real and virtual corrections

In this third part of our calculation of the QCD NLO corrections to the photon impact factor we combine our previous results for the real corrections with the singular pieces of the virtual corrections and present finite analytic expressions for the quark-antiquark-gluon intermediate state inside the photon impact factor. We begin with a list of the infrared singular pieces of the virtual correction, obtained in the first step of our program. We then list the complete results for the real corrections (longitudinal and transverse photon polarization). In the next step we define, for the real corrections, the collinear and soft singular regions and calculate their contributions to the impact factor. We then subtract the contribution due to the central region. Finally, we combine the real corrections with the singular pieces of the virtual corrections and obtain our finite results.

Event generators: New developments.

As in the past, Monte Carlo event generators will play a vital role for the physics analysis of events at present and future hadron colliders like the Tevatron, HERAII and, of course, the LHC. The particular advantages of the simulation of single events with a multi-purpose event generator like HERWIG [1,2] or Pythia [3] are all related to the fact that a fully exclusive final state is obtained. This allows to run the output through the same analysis tools as the data that was measured, thereby allowing to apply e.g. the same physical cuts as those applied to the data after the generation of events. This means that the events generated by the Monte Carlo program are treated in exactly the same way as the data from the measurement.

NLO corrections to the photon impact factor

Abstract We review the program of the calculation of next-to-leading order corrections to the virtual photon impact factor. Following a brief introduction we present some technical aspects for the various contributions. Recently obtained results for transversely polarised virtual photons are discussed and an outline of how infrared divergences are cancelled is given. Implications of the subtraction of leading energy logarithms are discussed.