The Longitudinal Polarimeter at HERA

Abstract

The design, construction and operation of a Compton back-scattering laser polarimeter at the HERA storage ring at DESY are described. The device measures the longitudinal polarization of the electron beam between the spin rotators at the HERMES experiment with a fractional systematic uncertainty of 1.6 %. A measurement of the beam polarization to an absolute statistical precision of 0.01 requires typically one minute when the device is operated in the multi-photon mode. The polarimeter also measures the polarization of each individual electron bunch to an absolute statistical precision of 0.06 in approximately five minutes. It was found that colliding and non-colliding bunches can have substantially different polarizations. This information is important to the collider experiments H1 and ZEUS for their future longitudinally polarized electron program because those experiments use the colliding bunches only.

Beam-helicity asymmetry arising from deeply virtual Compton scattering measured with kinematically complete event reconstruction

“…The sign of the beam polarization was reversed three times over the running period. Two Compton backscattering polarimeters [26,27] The commissioning of the recoil detector was completed in 2006 after the switch of the Hera lepton beam from electrons to positrons. Therefore, for the analysis of the beamhelicity asymmetry considered here, data collected with only one lepton beam charge but both beam-helicity states are available.…”

Inclusive dijet cross sections in neutral current deep inelastic scattering at HERA

“…Starting from the year 2004, spin rotators on either side of the ZEUS detector changed the transverse polarisation of the beam into longitudinal polarisation and back again. The electron beam polarisation was measured using two independent polarimeters, the transverse polarimeter (TPOL) [36] and the longitudinal polarimeter (LPOL) [37]. Both devices exploited the spindependent cross section for Compton scattering of circularly polarised photons off electrons to measure the beam polarisation.…”

Measurement of the virtual-photon asymmetry A 2 and the spin-structure function g 2 of the proton

“…The sign of the beam polarization was reversed every few months. The beam polarization was measured by two independent HERA polarimeters [22][23][24]. The average magnitude of the beam polarization was found to be 0.34 ± 0.01.…”