Relation to Other Experiments

HAPPEX

There are a number of differences between the capabilities of the HAPPEX and G⁰ experiments. Most importantly, because of the relatively small acceptance of the Hall A spectrometers, the HAPPEX measurement is restricted to forward angle asymmetries. Therefore, no direct extraction of and is possible in the HAPPEX experiment. Secondly, for the forward angle asymmetries, the statistical precision is limited by the acceptances of the spectrometers. In the case of G⁰, forward angle asymmetries will be measured at seven values of momentum transfer concurrently as shown in Figure 6. The statistical precision shown here is that for the experiment as proposed in 1993 with 49% beam polarization and a 30 day run. For comparison, the published result from the recent HAPPEX run is also shown. The HAPPEX plans for 1999 are to reduce the uncertainties by a factor of two with increased running time and improved polarization. HAPPEX also proposed 4 other forward angle asymmetries at momentum transfers ranging from 0.268 to 1.295 GeV². The momentum transfer of the present measurement is slightly below the maximum in the figure-of-merit which drops off by a % for each of the extreme values of momentum transfer above.

Figure 6. Comparison of the G⁰ and HAPPEX forward angle asymmetries (see text for details).

Even though there is overlap in the approved kinematic ranges of the two experiments, there are clearly significant differences in the instrumentation and techniques for the two measurements. The two measurements are therefore complementary and will provide an important mutual cross check of results.

Relation to Other PV Measurements

Jefferson Lab has approved parity-violation measurements with both ²H and ⁴He targets. The ²H measurement proposed in E91-010 is for forward angles and is focussed on strange quark effects. The ²H measurement proposed for the G⁰ experiment is a single backward angle measurement at GeV² used to determine the contribution of the axial radiative correction to the asymmetries (this will complement the measurement of a similar quantity at low momentum transfer in the deuterium part of the SAMPLE experiment at Bates).

The approved experiments for ⁴He are at low momentum transfer ( GeV² : 91-010) and moderate momentum transfer ( GeV² : 91-004). These experiments will determine in the ⁴He nucleus and are therefore complementary to direct measurements on the proton.

Relation to Photoproduction

There has been some discussion of the sensitivity of, in particular, polarized photoproduction [He92, Ti97, Ti98] to test hypotheses of knocking out pairs from the nucleon (E93-022). Such measurements again provide complementary information as the nucleon matrix elements probed are more complicated overlaps. These also depend to a significant degree on models for their extraction.