Background studies for the measurement of the strangeness vector form factor of the proton by parity-violating electron scattering under backward angles

Within the A4 experiment the contributions of the strange quark to the electromagnetic form factors of the proton are measured. These see-quark effects in low energy observables are very important for the understanding of hadron structure, because they are a direct manifestation of QCD degrees of freedom in the non-perturbative regime. Linear combinations of the strangeness vector form factors of the proton (GEs and GMs) are accessible experimentally by measuring the parity violating asymmetry in the cross section of the elastic scattering of longitudinal polarised electrons off unpolarised nucleons. Two such measurements were published by the A4 collaboration before this work. Both of them were forward angle measurements at the Q2 values of 0.23 and 0.10 (GeV/c)2, respectively. A measurement at backward angle with a beam energy of 315 MeV was performed for separating GEs and GMs at the higher of these Q2 values. In the A4 experiment a longitudinally polarised electron beam scatters on a liquid hydrogen target. Single scattered electrons are counted with a Cherenkov calorimeter. The separation of elastic from inelastic events is achieved by means of calorimetric energy measurement. For the backward angle measurement a plastic scintillator was installed as electron tagger for suppressing the γ background coming from the decay of π0 mesons. In order to make the data analysis possible the energy spectra needed to be studied thoroughly. This was done in this work using detailed simulations of both the scattering processes suffered by beam electrons and of the response of the detectors. A method for handling the remaining background due to γ conversion before the scintillator has been also developed. The simulation results agree with the measured spectra at the 5% level and the strategy for handling the background was shown to be feasible. The asymmetry value obtained by handling the background as proposed in this work was combined with the previous A4 forward angle measurement at the same Q2 for separating GEs and GMs at Q2=0.22∝(GeV/c)2. The results are GMs= -0.14 ± 0.11exp ± 0.11theo and GEs= 0.050 ± 0.038exp ± 0.019theo, where the systematic uncertainty due to the background correction is contained in the experimental error. Conclusions about the influence of strangeness on the static electromagnetic properties of the proton are drawn from these results and are presented at the end of the work. (orig.)