Measurement of positron-proton scattering at Q"2 = 0.25 GeV"2

The feasibility of measuring cross sections at small values of the squared four-momentum transfer Q"2 in positron-proton collisions at HERA is demonstrated. The positrons scattered at small angles (19 mrad<#theta#<34 mrad) are detected by a small tungsten calorimeter with silicon diode readout mounted on the beampipe of the ZEUS detector, hence the name bemapipe calorimeter. The operation of a beampipe calorimeter close to the beams of the HERA storage ring requires special care, in particular the high radiation background needs attention. A transputer based readout system, with a sample and hold architecture was used to take data at a beam crossing frequency of 10.4 MHz. Making use of simple energy thresholds and timing cuts, a first-level trigger rate on the order of 10 Hz was achieved at a luminosity of 10"3"0 cm"-"2 s"-"1. The ep cross section integrated over the kinematical range accessible to the beampipe calorimeter has been determined to #sigma#_e_p=49.5#+-#9.1(stat)+13.3/-17.1(syst)nb at a mean <Q"2>=0.25 GeV"2 and Bjorken-x=7.4.10"-"6. Translating this into a total cross section of virtual photons on protons yields #sigma##gamma#'p=130#+-#24(stat)+35/-45(syst)#mu#b at a photon-proton center of mass energy of W=183 GeV and a mean <Q"2>=0.25 GeV"2. The proton structure function is F_2=0.30#+-#0.12. This value agrees with the prediction of Donnachie-Landshoff within errors. (orig.)86 refs.Available from TIB Hannover: RN 2821(96-03) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman