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We have measured a complete set of polarization-transfer observables in the inclusive scattering of 500-MeV protons from 2 H and Pb at # = 1.75 fm -1 ., Axial longitudinal and transverse response functions derived from these data show no differences between Pb and 2 H. This implies no enhancement of the nuclear pion field in heavy nuclei and consequently that models of the low-* A dependence of certain nuclear structure functions requiring such an enhancement are unlikely to be correct.PACS numbers: 25.40.Ep, 24.70. + sThe recent discovery of significant differences 1 between the F 2 structure functions of 2 H and Fe has aroused much interest in both the nuclear-and particle-physics communities. This so-called European Muon Collaboration (EMC) effect 1 is strong evidence that some constituents of nuclear matter behave quite differently in a large nucleus compared with free (or almost free, as in deuterium) nucleons. The question of what specific quark or quark-cluster phenomena give rise to the enhancement in F\ Q /Ff in the region of the scaling variable x ^ 0.3 has been the subject of numerous calculations. 2 " 10 One possibility, closely connected to current issues in medium-energy physics, is that the enhancement in F\* at small x is due to the nuclear pion field. 2 " 5 For many years it has been conjectured that the pion field might be enhanced in a region of momentum transfer q -2 fm -1 ; extreme enhancements, currently out of favor, would lead to a low critical density for pion condensation. Scattering of a lepton by such an enhanced field naturally leads to increased scattering in the range x ~~ mJm N \ thus pions seem an intuitively appealing candidate for understanding the low-x EMC effect.The most direct probe of the nuclear pion field is the nucleon, which couples to other nucleons via pion exchange. Under suitable approximation (discussed below) the axial-longitudinal coupling of the nucleon to the pion field may be isolated from other couplings by measuring a complete set of polarization-transfer observables 11 " 13 and forming the longitudinal spin-flip probability, S L .We have measured a complete set of polarization-transfer observables for the inclusive excitation of the quasielastic continuum in 2 H and natural Pb with 500-MeV protons. At a momentum transfer of 1.75 fm -1 we find no enhancement of the longitudinal spin-flip probability for Pb compared with 2 H. We believe that this is strong evidence against any enhancement in the nuclear pion field in this range of momentum transfer; this in turn casts serious doubt on explanations of the EMC effect requiring excess pions. [2][3][4][5] Beams of 500-MeV protons alternately polarized normal to the reaction plane (N), longitudinally (along the beam, L), and sideways (S = NxL) were provided by the Clinton P. Anderson Meson Physics Facility (LAMPF). Protons inelastically scattered at 0i ab = 18.5° from natural Pb foils and a liquid deuterium target were momentum analyzed in the high-resolution spectrometer (HRS). Outgoing proton polarizations were ...

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