Pion excess, nuclear correlations, and the interpretation of(p→,n→)spin transfer experiments

Abstract

Conventional theories of nuclear interactions predict a net increase in the distribution of virtual pions in nuclei relative to free nucleons. Analysis of data from several nuclear experiments has led to claims of evidence against such a pion excess. These conclusions are usually based on a collective theory (RPA) of the pions, which may be inadequate. The issue is the energy dependence of the nuclear response, which differs for theories with strong NN correlations from the RPA predictions. In the present paper, information about the energy dependence is extracted from sum rules, which are calculated for such a correlated, noncollective nuclear theory. The results lead to much reduced sensitivity of nuclear reactions to the correlations that are responsible for the pion excess. The primary example is ( p, n) spin transfer, for which the expected effects are found to be smaller than the experimental uncertainties. The analysis has consequences for Deep Inelastic Scattering (DIS) experiments as well.

Distorted wave impulse approximation analysis for spin observables in nucleon quasielastic scattering and enhancement of the spin longitudinal response

“…The nucleon number k = 0 denotes the incident or exit nucleon. Then the polarization, the analyzing power and the polarization transfer coefficients are given respectively by 12) where i, j = x, y, z or q, n, p. The DWIA calculation is usually carried out in the [x,ŷ,ẑ] frame, while the [q,n,p] frame is sometimes more convenient for the theoretical analysis. The relation between D ij (i, j = q, n, p) and D ij (i, j = x, y, z) is given by 13) where θ p is the angle betweenp andẑ.…”

“…From these results, it was concluded [10,11] that there is no enhancement of R L , namely no collective enhancement of the pionic modes, which was interpreted as evidence against the pion excess in the nucleus. With help of sum rules, Koltun [12] analyzed the data by means of a correlated nuclear theory, in which the two-nucleon correlation is dominating, and concluded that there is no disagreement between the data and the model for R L , but R T is not explained. He claimed that there is no collective enhancement of the pionic modes.…”

Nuclear deep-inelastic lepton scattering and coherence phenomena

“…[243,244,245,246]. These studies include calculations within the delta-hole model [245], the role of NN correlations and the energy dependence of nuclear response functions [246], possible effects of "Brown-Rho scaling" on nuclear structure functions [243], and implications of low-energy pion-nucleus scattering data for nuclear deep-inelastic scattering and Drell-Yan production [244].…”

Nuclear parton distributions and the Drell-Yan process

“…The nucleon light-cone distribution functions are driven by the nuclear spectral function, which defines the energy-momentum distribution of bound nucleons [15][16][17][18][19][20][21][22]. The calculation of mesonic correction is less certain and model-dependent [34][35][36][37][38][39][40][41][42].…”