Eur. Phys. J. C 22, 423-430 (2001)
DOI: 10.1007/s100520100818
Global positioning of spin GPS scheme for half-spin massive spinors
S. Jadach1, 2, B.F.L. Ward1, 3, 4 and Z. Was1, 21 CERN, Theory Division, 1211 Geneva 23, Switzerland
2 Institute of Nuclear Physics, ul. Kawiory 26a, Kraków, Poland
3 Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996-1200, USA
4 SLAC, Stanford University, Stanford, CA 94309, USA
(Received: 5 September 2001 / Published online: 23 November 2001 - © Springer-Verlag / Società Italiana di Fisica 2001 )
Abstract
We present a simple and flexible method of keeping track of the complex
phases and spin quantisation axes for half-spin initial- and final-state
Weyl spinors in scattering amplitudes of Standard Model high energy physics
processes. Both cases of massless and massive spinors are discussed.
The method is demonstrated and checked numerically for spin
correlations in
production and decay.
Its application is in our
work of combining effects due to
multiple photon emission (exponentiation) and spin, embodied in the
Monte Carlo event generators for production and decay of unstable fermions
such as the
lepton, t-quark and hypothetical new heavy
particles.
In particular, the recurrent problem of combining, for such unstable
fermions, one author's calculation of production and another author's
calculation of decay, in the presence or absence
of multiple photon effects, is there given a practical solution, both
for Weyl spinor methods and for the traditional
Jacob-Wick helicity methods. Moreover, for massive fermions
we give a simple representation of the amplitude for
emission
ideally suited for numerical evaluation.
No other method is known to us which for arbitrary
n has been realized numerically for
unstable, massive fermions.
Our paper can contribute also, to the discussion on
design principles of the phenomenology work for the future accelerators such
as LHC or NLC.
© Società Italiana di Fisica, Springer-Verlag 2001