Matching exact matrix element with NLO parton shower solution with exact phase space: QED scalar QED and QCD

by Prof. Zbigniew Was (Cracow, INP & CERN)

Thursday 11 Jun 2009, 11:00 → 12:00 Europe/Zurich

1/1-025 (CERN)

In the construction of all Monte Carlo simulation programs for experiments in high energy physcis several questions have to be adressed. One has to control parametrization and generation of the phase space and appropriately take care of collinear and soft singularities. Fully differential distribution, based on matrix element has to be provided. Such matrix element can not be calculated directly, but must be obtained using methods based on reshufling of perturbation expansion and/or approximations. As input one is at best limited to fixed oreder perturbative results. Often such results are not reilable all over the phase space too. In the following we will present how spin amplitudes are used in construction of distribution used in PHOTOS Monte Carlo for QED bremsstrahlung corrections in decays of particles and resonances. Examples based on QED and scalar QED will be explained on example of W,Z,B and gamma* decays. They are interesting because one can explore method derived from QED YFS exponentiation but used also for scalar QED where results of perturbative calculation must be supported by agreement with the data. PHOTOS is not based on conformal symmetry, its algorithm for phase-space is iterative but exact as well, that is why it is compatible with Next to Leading Logarithms and offers good example to study Monte Carlo methods. Program is of practical interest too. It is widely used in applications for LHC and experiments at lower energies.