Inclusive Scattering of 500-MeV Protons and Pionic Enhancement of the Nuclear Sea-Quark Distribution

Abstract

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 ...

Self-consistent Green's function method for nuclei and nuclear matter

“…50. The effects of including the SRC is shown in the lower plots, where the correlation functions (163) have been computed with both the Reid-Soft-Core and Bonn-A potentials. Figure 50, also shows the plots labeled 'No SRC' that correspond to neglecting the correlation functions and computing the spectral functions with Eq.…”

“…In the lower plots these SRC are incorporated by the defect functions computed the Reid and the Bonn-A potential (Eqs. (163) and (165) ). to first order in the correlation function,…”

“…One may consider the salient difference of the ratio of spin-longitudinal and spin-transverse response function in nuclear matter and finite nuclei as another indication of the relevance of this suggestion [162]. We also like to point out that experimental information of these response functions [163] - [165] suggests no characteristic enhancement of the (pionic) spin-longitudinal response as expected on the basis of nuclear matter calculations.…”

Saturation of Nuclear Matter and Short-Range Correlations

“…One may consider the salient difference of the ratio of spinlongitudinal and spin-transverse response function in nuclear matter and finite nuclei as another indication of the relevance of our suggestion [44]. We also like to point out that experimental information of these response functions [45,46,47] suggests no characteristic enhancement of the (pionic) spin-longitudinal response as expected on the basis of nuclear matter calculations.…”

Polarization transfer measurements forC12(p→,nDozono1, Wakasa2, Ihara3 et al. 2009Phys. Rev. C5000View full textAdd to dashboardCite

“…scattering [15,16,17,18,19] and ( p, n) reactions [20,21,22,23,24] on various targets at intermediate energies. However, these experimental data did not show the theoretically expected enhancements.…”