Photon polarisation

The photon polarisation is defined as a two component vector normal to the direction of the photon:

$($

$a$1ei φ1

$a$2ei φ2

$)\; =e\phi$_o(

$a$1ei φc

$a$2e-i φc

$)$

where $\phi$_c= (φ₁-φ₂)/2 is called circularity and $\phi$_o= (φ₁+φ₂)/2 is called overall phase. Circularity gives the left- or right-polarisation characteristic of the photon. RICH materials usually do not distinguish between the two polarisations and photons produced by the Cerenkov effect are linearly polarised, that is $\phi$_c=0 . The circularity of the photon is ignored by GEANT.

The overall phase is important in determining interference effects between coherent waves. These are important only in layers of thickness comparable with the wavelength, such as interference filters on mirrors. The effects of such coatings can be accounted for by the empirical reflectivity factor for the surface, and do not require a microscopic simulation. GEANT does not keep track of the overall phase.

Vector polarisation is described by the polarisation angle $tan\psi =\; a$₂/a₁ . Reflection/transmission probabilities are sensitive to the state of linear polarisation, so this has to be taken into account. One parameter is sufficient to describe vector polarisation, but to avoid too many trigonometrical transformations, a unit vector perpendicular to the direction of the photon is used in GEANT.

The polarisation vectors are stored in a special track structure which is lifted at the link LQ(JSTACK-1) when the first Cerenkov photon is stored in the stack.