The atomic nucleus is a bound state of protons and neutrons (so called nucleons). The A4 collaboration is investigating the structure of these nucleons. In a simple picture, nucleons are made up of three elementary constituents, two up- and one down-quark. Today's view of the nucleon is more complex: Beside the valence quarks also gluons, the force carrier of QCD, and quark-antiquark pairs known as sea quarks contribute to the properties of proton and neutron. In the quark sea also flavors occur which are not present in the valence quarks. The A4 collaboration is aiming to measure such flavor contribution, especially those of the strange quarks since they are the lightest non-valence quarks.
In the experiment, polarized electrons are scattered off unpolarized protons and detected in a lead fluoride calorimeter. Depending on the polarization state there are tiny differences in the interaction strength due to the parity violation (PV) in the weak interaction. Consequently, the number of scattered electrons vary for the two polarization states. These numbers can be determined by the measurement of the PV asymmetry. This asymmetry reveals the distribution of strange quarks within the nucleon.
The A4 collaboration measures small asymmetries in the cross section of elastic scattering of polarized electrons off an unpolarized target, basically hydrogen or deuterium. The momentum transfers achieved either in forward angle- oder backward angle-configuration of the detector vary between 0.1 GeV2 and 0.6 GeV2. There are two main physics goals:
- Parity violating electron scattering asymmetries are measured with a longitudinally polarized electron beam. Using input from the Standard Model the contribution of strange sea quarks to the vector form factors of the nucleon are determined. The combination of measurements on hydrogen and deuterium allow for an additional determination of the axial form factor of the proton (referring to the nuclear anapole moment).
- Using a transversely polarized electron beam, the observed asymmetries arise at leading order from the interference of the one- and the two-photon-exchange amplitude. These asymmetries are sensitive to excited intermediate states of the nucleon. The imaginary part of the two-photon exchange amplitude can be determined.
A high power liquid hydrogen target of 10 cm or 20 cm length and an electron beam of I=20 µA lead to luminosities in the order of L=1038 cm-2s-1 The scattered electrons are measured by a total absorbing, segmented lead fluoride calorimeter which deals with event rates of about 100 MHz. The degree of polarization of the electron beam is measured by a laser Compton backscatter polarimeter simultanously to the main experiment. With this apparatus, the internal cavity concept is realized for the first time.