Precision measurement with the transverse polarimeter at HERA II

Abstract

The HERA ep–collider provides a longitudinally polarised positron beam to the three experiments HERMES, H1 and ZEUS. The degree of polarisation is determined with two Compton polarimeters, one measuring the transverse polarisation that naturally builds up in a sufficiently planar ring via the Sokolov–Ternov effect, the other one measuring the longitudinal polarisation between the HERMES spin rotators. The future physics program of the collider experiments requires polarimetry with a precision of better than 1%. To meet this goal, the Transverse Polarimeter has been upgraded in 2001 with a new data acquisition system and a silicon strip detector complementing the main calorimeter. A new analysis, which allows an in–situ calibration in parallel to the polarisation measurement without relying on testbeam results obtained under different conditions, is presented in this paper. The new method uses a fit of the double–differential Compton cross section (folded with detector effects) to the measured two–dimensional spectra in energy and position of the scattered photon. In March 2003, HERA succeeded to deliver up to 50% of longitudinal polarisation to all three experiments as well as collisions for H1 and ZEUS. Some first results of the new analysis on polarised HERA II data are shown. PACS: 29.27.Hj Polarized beams – 13.88.+e Polarization in interactions and scattering