Eur. Phys. J. C 27, 19-32 (2003)
DOI: 10.1140/epjc/s2002-01085-2

Electric charge screening effect in single- W production with the KoralW Monte Carlo

S. Jadach¹, W. Placzek^{2, 3}, M. Skrzypek^{1, 3}, B.F.L. Ward^{3, 4, 5} and Z. Was<sup>1, 3

1</sup>  Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Cracow, Poland
²  Institute of Computer Science, Jagellonian University, ul. Nawojki 11, Cracow, Poland
³  CERN, Theory Division, 1211 Geneva 23, Switzerland
⁴  Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996-1200, USA
⁵  Werner-Heisenberg-Institut, Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany

(Received: 24 September 2002 / Revised version: 30 October 2002 / Published online: 20 December 2002)

Abstract
Any Monte Carlo event generator in which only initial state radiation (ISR) is implemented, or ISR is simulated independently of the final state radiation (FSR), may feature too many photons with large transverse momenta, which deform the topology of events and result in too strong an overall energy loss due to ISR. This overproduction of ISR photons happens in the presence of the final state particle close to the beam particle of the same electric charge. It is often said that the lack of the electric charge screening effect between ISR and FSR is responsible for the above pathology in ISR. We present an elegant approximate method of curing the above problem, without actually reinstalling FSR. The method provides theoretical predictions of modest precision: . It is, however, sufficient for the current 1W data analysis at the LEP2 collider. Contrary to alternative methods implemented in other MC programs, our method works for the ISR multiphotons with finite p_T . Although this method is not an exact implementation of the complete/exact ISR, FSR and their interference, it is very closely modelled on it. We present a variety of numerical results obtained with the newest version of the KoralW Monte Carlo, in which this method is already implemented.

© Società Italiana di Fisica, Springer-Verlag 2003