Matching NLO parton shower matrix element with exact phase space: case of W -> l nu (gamma) and gamma^* -> pi^+pi^-(gamma)

Abstract

The PHOTOS Monte Carlo is often used for simulation of QED effects in decay of intermediate particles and resonances. Momenta are generated in such a way that samples of events cover the whole bremsstrahlung phase space. With the help of selection cuts, experimental acceptance can be then taken into account. The program is based on an exact multiphoton phase space. Crude matrix element is obtained by iteration of a universal multidimensional kernel. It ensures exact distribution in the soft photon region. Algorithm is compatible with exclusive exponentiation. To evaluate the program's precision, it is necessary to control the kernel with the help of perturbative results. If available, kernel is constructed from the exact first order matrix element. This ensures that all terms necessary for non-leading logarithms are taken into account. In the present paper we will focus on the W -> l nu and gamma^* -> pi^+ pi^- decays. The Born level cross sections for both processes approach zero in some points of the phase space. A process dependent compensating weight is constructed to incorporate the exact matrix element, but is recommended for use in tests only. In the hard photon region, where scalar QED is not expected to be reliable, the compensating weight for gamma^* decay can be large. With respect to the total rate, the effect remains at the permille level. It is nonetheless of interest. The terms leading to the effect are analogous to some terms appearing in QCD. The present paper can be understood either as a contribution to discussion on how to match two collinear emission chains resulting from charged sources in a way compatible with the exact and complete phase space, exclusive exponentiation and the first order matrix element of QED (scalar QED), or as the practical study of predictions for accelerator experiments.

24 pages

References in corpus (5)

• Scalar QED, NLO and PHOTOS Monte Carlo
• MC-TESTER: a universal tool for comparisons of Monte Carlo predictions for particle decays in high energy physics
• Soft-photon corrections in multi-body meson decays
• Comparison of SANC with KORALZ and PHOTOS
• Gauge invariant sub-structures of tree-level double-emission exact QCD spin amplitudes

Cited by in corpus (11)

• Measurement of the W -> lnu and Z/gamma* -> ll production cross sections in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector
• Precision Studies of Observables in pp->W->l nu and pp->gamma,Z->l+l- processes at the LHC
• Resonance chiral Lagrangian currents and tau decay Monte Carlo
• QED Bremsstrahlung in decays of electroweak bosons
• Documentation of TauSpinner algorithms -- program for simulating spin effects in tau-lepton production at LHC
• Extra lepton pair emission corrections to Drell-Yan processes in PHOTOS and SANC
• TAUOLA for simulation of tau decay and production: perspectives for precision low energy and LHC applications
• Bremsstrahlung simulation in K to pi l^pm nu_l (gamma) decays
• News on PHOTOS Monte Carlo: gamma^* -> pi^+ pi^-(gamma) and K^\pm -> pi^+ pi^- e^\pm nu (gamma)
• Evaluation of Photos Monte Carlo ambiguities in case of four fermion final states
• New hadronic currents in TAUOLA: for confrontation with the experimental data