In the previous chapters we have dealt with the production of the polarized states that serve as initial states in reactions. Here we turn to the measurement of the state of polarization of an ensemble of particles, i.e. to polarimetry.

In the analysis of the state of polarization we may be dealing with stable or unstable particles. If the particles are stable it may be possible to rely on well-understood reactions, such as those of QED, to achieve the polarization analysis, via, e.g. Coulomb interference or scattering off a laser beam. Or, if this is impracticable, it is sometimes possible to use a double-scattering technique even if the reaction mechanism is unknown. The only assumption needed for this is time-reversal invariance. If the particles are unstable their decay angular distribution gives information on their state of polarization prior to decay. This is not surprising if the decay is electromagnetic, so that the decay amplitudes are precisely known. What is remarkable, however, is that even when the decay mechanism is not known certain decays are ‘magic’ and still provide information on the polarization state of the decaying particle. Examples are p → ππ, ω → γπ, D* → γD,Ψ → pπ, α2 → pπ etc.

For electron beams, where we can rely on QED, it has been possible to construct very accurate and rapidly acting polarimeters.

One of the most interesting challenges at the moment is to construct efficient high energy proton polarimeters for use at RHIC, UNK and possibly at Fermilab.