THE ERICSON-ERICSON LORENTZ-LORENZ CORRECTION

The Lorentz-Lorenz correction is briefly reviewed for the electromagnetic system and then the arguments of the Ericsons for the pion-nucleus many-body system are given. A transparent discussion by Torleif Ericson is repeated. The importance of the Ericson-Ericson Lorentz-Lorenz correction is discussed. In particular, it is shown that with the classical value of this correction, multiple scattering of the pions is highly suppressed. Experimental data confirms such a suppression, indicating that in pionic units the value of the EELL parameter cannot be far from the classical (g′0)NΔ = 1 3. No stiffness is seen in the longitudinal response of 208Pb to polarized protons, indicating that (g′0)NN must be substantially greater than 1 3, seeming to require (g′0)NN{greater-than or approximate}0.9. This poses a problem, however, because reanalysis of the Bonn potential in nucleon-nucleon scattering, together with a G-matrix calculation for nuclear matter, does not allow (g′0)NN to be larger than ∼0.525. This value would, however, give pionic contributions which would strongly disagree with EMC and especially, Drell-Yan experiments which measure the change in quark sea in going from deuterium to medium weight nuclei. Some of the discrepancy is removed by noting that the p-meson exchange contribution to (g′0)NN is increased in medium by the factor ( mn mn*)2, where mn nucleon effective mass. Since, however, the proton scattering favors the nuclear surface where mn* is not much smaller than mn, this enhancement is not large for this experiment. It is pointed out that when the decrease with momentum of screening by isobar-hole excitations and the increase in p-exchange tensor contribution with angle is taken into account, (g′0)NN becomes a function of momentum. Its value is estimated to be {greater-than or approximate}0.9 for the momentum transfer k = 1.75 fm-1 where the experiments were performed. Possible reasons why (g′0)NΔ is so much smaller than (g′0)NN are discussed. © 1990.