Polarization

In most processes, incoming beams are assumed unpolarized. However, especially for $\mathrm{e}^+\mathrm{e}^-$ linear collider studies, polarized beams would provide further important information on many new physics phenomena, and at times help to suppress backgrounds. Therefore a few process cross sections are now available also for polarized incoming beams. The average polarization of the two beams is then set by PARJ(131) and PARJ(132), respectively. In some cases, noted below, MSTP(50) need also be switched on to access the formulae for polarized beams.

Process 25, $\mathrm{W}^+ \mathrm{W}^-$ pair production, allows polarized incoming lepton beam particles. The polarization effects are included both in the production matrix elements and in the angular distribution of the final four fermions. Note that the matrix element used [Mah98] is for on-shell $\mathrm{W}$ production, with a suppression factor added for finite width effects. This polarized cross section expression, evaluated at vanishing polarization, disagrees with the standard unpolarized one, which presumably is the more accurate of the two. The difference can be quite significant below threshold and at very high energies. This can be traced to the simplified description of off-shell $\mathrm{W}$'s in the polarized formulae. Good agreement is obtained either by switching off the $\mathrm{W}$ width with MSTP(42)=0 or by restricting the $\mathrm{W}$ mass ranges (with CKIN(41) - CKIN(44)) to be close to on-shell. It is therefore necessary to set MSTP(50)=1 to switch from the default standard unpolarized formulae to the polarized ones.

Also many SUSY production processes now include the effects from polarization of the incoming fermion beams. This applies for scalar pair production, with the exception of sneutrino pair production and $\mathrm{h}^0 \mathrm{A}^0$ and $\H ^0 \mathrm{A}^0$ production, this omission being an oversight at the time of this release, but easily remedied in the future.

The effect of polarized photons is included in the process $\gamma \gamma \to \mathrm{F}_k \overline{\mathrm{F}}_k$, process 85. Here the array values PARJ(131) and PARJ(132) are used to define the average longitudinal polarization of the two photons.