Quantum Chromodynamics (QCD) is generally considered to be the underlying theory of nuclear physics with the gluon as the carrier of the strong force. However, because of the non-perturbative nature of QCD at low and intermediate energies, direct calculations of the nucleon-nucleon (nn) interaction and its implications for nuclei are not yet possible. What is known about QCD at low energies is usually deduced from the symmetries of that theory, such as the symmetry. It has been shown [NaW79] that when the chiral symmetry of QCD with two massless quarks is spontaneously broken, this results in a massless Goldstone boson associated with the conserved axial vector current. Because of the small, but non-zero, masses of the u and d quarks the axial vector current is only partially conserved, and the Goldstone boson resulting from the breaking of chiral symmetry will be a massive, but light, pseudoscalar boson that is the pion. It has also been shown that [WeW81] by employing PCAC (partially conserved axial current) in the case of pseudoscalar coupling of a -pair to the pion at the hadron surface, the form and coupling strength of the vertex is consistent with the phenomenological OPEP (one pion exchange potential).
Given the results of those studies [WeW81] [NaW79] of the effects of chiral symmetry it seems a reasonable extension of QCD, and a more tractable problem, to treat mesons, and in particular pions, as the carriers of the nuclear force. Therefore, the nn interaction, specifically at intermediate and long ranges, is primarily described through meson exchange. Table gives a list of the lightest mesons (and their properties) which are used in meson exchange models.
Table: Light unflavored mesons [PDH96].
If mesons are truly the carriers of the nuclear force, modern calculations based on a model of meson exchange predict that the pion field in the nucleus is enhanced by the collective action of the many nucleons in the nucleus. Details of the model are given in chapter . This enhanced field should have an effect on many experimentally observable properties of the nucleus, particularly those induced by external pion fields [BFS93].
During the past decade a variety of experiments have returned no evidence that this enhancement actually exists. For instance, a measurement of muon pair production in p-nucleus scattering at GeV done at Fermi National Accelerator Lab (FNAL) showed no enhancement in large nuclei compared to for the target-quark momentum fraction range [Ald90]. Since the muon pairs are primarily produced via (the Drell-Yan effect) this lack of enhancement indicates a lack of anti-quarks in the nucleus which, in turn, indicates no enhancement in the pion field [Ald90].
The collective enhancement of the nuclear pion field should also have an effect on the scattering of nucleons from the nucleus. In particular an investigation of the spin-isospin dependent part of the nn interaction should reveal a change in the pion field with respect to the free case. Because the pions' interaction differs on whether the nucleon spin is parallel or normal to the direction of momentum transfer () a measurement of the polarization transfer in a nucleon-nucleus collision should reveal information about the pion field.
In 1982 Alberico, Ericson, and Molinari [AEM82] predicted that there should be a noticeable difference between spin-longitudinal isospin response and the spin-transverse isospin response of the nucleus due to this enhancement in the pion field for momentum transfers between and . The specifics of the prediction will be discussed in chapter .