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 availabe, 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 → lν and γ * → π + π − 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 γ * 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.

Precision Studies of Observables in pp->W->l nu and pp->gamma,Z->l+l- processes at the LHC

“…The transverse momentum spectrum p Z ⊥ of the lepton pair is the observable that exposes most clearly the differences between the two results. For the purpose of this comparison, the more relevant difference to explain is the difference in shape (and absolute value) for p Z ⊥ ∈ [20,100] GeV, that we will address in the next paragraph. At very high p Z ⊥ , differences are also fairly large, but in that region they can be mostly attributed to the MiNLO scale choice: when p Z ⊥ is large (above M Z ), the MiNLO Sudakov form factor switches off, but the strong coupling is evaluated at p Z ⊥ , whereas in SHERPA NNLO+PS and in the fixed-order calculation it is evaluated at the dilepton invariant mass m ll .…”

“…To gain flexibility of its application, the FORTRAN implementation is being replaced gradually by C++ and instead of HEPEVT, the C++ event structure HepMC [99] is used as the event record. Emission kernel based on complete first order matrix elements for QED final state bremsstrahlung was introduced, following papers [96,100] in [101].…”

News on PHOTOS Monte Carlo: γ * → π + π − (γ) and K ± → π + π − e ± ν(γ)

“…Consider K e4 decay, K ± (p) → π + (q + )+ π − (q − )+ e ± (p e )+ ν(p ν ) . (9) In the framework of Scalar QED but neglecting diagrams with photons emission from hadronic or weak blocks, one can calculate the virtual photon corrections. Note that the electron photon vertex is taken from standard QED.…”