K.Long, C.Foudas, A.Jamdagni

The HERA collider at the DESY laboratory in Hamburg collides electrons or positrons of energy 27.5 GeV with protons of energy 920 GeV. The ZEUS Collaboration studies events from these collisions, reconstructing the particles thrown out when the proton is broken up in order to measure the structure of the proton and search for new particles. In scattering high energy electrons or positrons off protons the quarks and gluons that make up the proton can be studied down to distances of 10-18 m.

After a successful eight-year running period the HERA accelerator was upgraded in 2001 - 2002 in order to deliver higher luminosity in a second running period. In addition to higher luminosity the lepton beams were longitudinally polarised, allowing tests of the chiral structure of the Standard Model to be made for the first time in electron-proton collisions. The main goal of the ZEUS experiment during this new run is to test the Standard Model of electroweak interactions at the highest possible momentum transfer.

The Imperial group has made several key contributions to the ZEUS detector and physics analysis, which are summarised below.

The Central Tracking Detector

The ZEUS Central Tracking Detector (CTD) is a large cylindrical drift chamber, situated at the centre of the ZEUS detector. The CTD measures tracks from charged particles produced in electron-proton collisions. From these tracks the momentum and species of each particle can be determined and the vertex position of the original electron-proton collision and subsequent secondary vertices from particle decays can be measured. The Imperial group designed and built the high-voltage system for the CTD.

The HERA Transverse Polarimeter

The HERA transverse polarimeter (TPOL) measures the transverse polarisation of the HERA lepton beam by shining a circularly polarised laser beam on the lepton beam and analysing the backscattered Compton photons. As part of the HERA upgrade in 2001 - 2002 the TPOL was upgraded to provide a high precision bunch-by-bunch measurement of the polarisation. Part of this upgrade was the development and installation of a silicon strip detector to improve the accuracy of the measurement. The Imperial group developed, tested and installed the silicon strip detector and later commissioned the detector and analysed the data.

High-Q2 physics analysis

The main physics activity of the Imperial group is measuring the cross sections for charged current (CC) and neutral current (NC) deep inelastic scattering (DIS) interactions at the highest possible momentum-transfer (Q2). The high luminosity running with longitudinally polarised lepton beams has allowed us to make the first ever measurement of the polarisation dependence of the cross section for charged current DIS, which is shown in Figure 1. We plan also to measure the dependence of the neutral current cross section and to use these measurements to search for physics not described by the Standard Model. The high-precision measurement of the polarisation by the TPOL will be of crucial importance in these future measurements.

charged current DIS cross sections as a function of the longitudinal polarisation of the lepton beam
Figure 1: The cross sections for e-p and e+p charged current DIS as a function of the longitudinal polarisation of the lepton beam