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Polarization Transfer in the 4He(epol,e¢ ppol)3H Reaction (Strauch, Neupane, Berman) Whether nucleons undergo modification of their internal structure when bound in the nuclear medium is a longstanding issue in nuclear physics. Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium, including possible modification of the nucleon form factor and/or spinor. Experiment E93-049 in Hall A measured the polarization-transfer coefficients over the range of Q2 from 0.5 to 2.6 (GeV/c)2, and as a function of missing momentum in the range from 0 to 240 MeV/c, in order to study possible medium modifications of the electric-to-magnetic form-factor ratio for protons bound in the 4He nucleus [Str02]. 4He was selected for study since its relative simplicity allows for realistic microscopic calculations and since its high density enhances any possible medium effects. Also, a variety of calculations indicate that polarization observables for the 4He(e,e¢ p)3H reaction are minimally influenced by reaction-mechanism effects, such as final-state interactions (FSI) and meson-exchange currents (MEC). It is precisely these effects that have so far prevented a clean determination of nucleon medium modifications from unpolarized response functions in (e,e¢ p) experiments. For free electron-nucleon scattering, the ratio of the Sachs form factors GE and GM is given by
where P¢ x and P¢ z are the transverse and longitudinal transferred polarizations, and the other factor is purely kinematic. E93-049 was designed to detect differences between the in-medium polarizations compared with the free values. Thus, the 4He(e,e¢ p)3H polarization ratio is normalized to the hydrogen polarization ratio measured with the identical spectrometer setting. The experimental results are shown in Fig. 19, along with the findings of a similar Mainz experiment [Die02] at Q2 = 0.4 (GeV/c)2, which are consistent with our JLab data. The data are also compared with the results of several theoretical calculations in Fig. 19. A recent calculation by Lu et al. [Lu98], consistent with present constraints on possible medium modifications, suggests a measurable deviation from this free-space ratio over the Q2 range of the E93-049 experiment. Proper interpretation of the results requires accounting for reaction-mechanism effects (e.g., FSI and MEC), that are (partly) included by our acceptance-averaging of relativistic calculations of the Madrid group [Cab98]. At Q2 = 0.52 and 1.0 (GeV/c)2, corresponding to two of our four kinematic settings, the plane-wave impulse-approximation (PWIA) calculation is not able to describe the experimental results for R = (P¢ x / P¢ z)4He/(P¢ x /P¢ z)1H. The relativistic distorted-wave impulse-approximation (RDWIA) calculation gives a smaller value of R, but still over-predicts the data. The inclusion of a medium modification of the proton form factor as predicted by Lu et al. in the RDWIA calculation is in excellent agreement with both settings, as seen in Fig. 19. Thus, these measurements of the double ratio at both Mainz and JLab provide an indication that one should describe nuclei in terms of medium modifications of the proton form factors that are predicted, e.g., by the quark-meson coupling model.
Figure 19. Recent data for the polarization-transfer double ratio R = (P¢ x /P¢ z)4He/(P¢ x /P¢ z)1H from Mainz and JLab (filled symbols) and proposed data of JLab Experiment 03-104 (open circles). The data are compared with the full relativistic calculation of the Madrid group [Cab98], with and without including medium modifications as predicted by a quark-meson coupling model [Lu98]; to calculations by the Gent group [Ryc99, Deb01]; and at Q2 < 0.5 (GeV/c)2 only, to Laget's full calculation, including two-body currents [Lag94]. The present data are considered to be very important, since such a medium modification of the bound-nucleon form factors may carry strong implications for the nuclear EMC effect (see [Mel02]). The data have been reported most recently at the Fourth Conference on Perspectives in Hadronic Physics in Trieste [Str03a], and have been accepted for publication in Physical Review Letters [Str02]. The JLab PAC 24 has just approved our new proposal for high-precision measurements of this 4He double ratio of polarization-transfer coefficients at values for Q2 of 0.8 and 1.3 (GeV/c)2 [Str03b]. This experiment will reduce the statistical uncertainties in the double-polarization ratio at each value of Q2 by over a factor of two compared with the previous measurement, resulting in roughly equal contributions from statistical and systematic uncertainties. These two Q2 values were selected since they lie in a region where theoretical calculations are expected to be most reliable. These data, which we expect to obtain soon, will provide one of the most stringent tests to date of the applicability of conventional meson-nucleon calculations. |
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