  Future linear colliders plan to collide polarised beams and the planned
physics reach requires knowledge of the state of polarisation as precisely as
possible. The polarised beams can undergo depolarisation due to various
mechanisms. In order to quantify the uncertainty due to depolarisation, spin
tracking simulations in the International Linear Collider (ILC) Beam Delivery
System (BDS) and at the Interaction Point (IP) have been performed. Spin
tracking in the BDS was achieved using the BMAD subroutine library, and the
CAIN program was used to do spin tracking through the beam-beam collision.
Assuming initially aligned beamline elements in the BDS, a ground motion model
was applied to obtain realistic random misalignments over various time scales.
Depolarisation at the level of 0.1% occurs within a day of ground motion at a
noisy site. Depolarisation at the IP also exceeds 0.1% for the nominal
parameter sets for both the ILC and for the Compact Linear Collider (CLIC).
Theoretical work is underway to include radiative corrections in the
depolarisation processes and simulation of the depolarisation through the
entire collider is envisaged.
