Parity Violation in Proton-Proton Scattering

Measurements of parity-violating longitudinal analyzing powers (normalized asymmetries) in polarized proton-proton scattering provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Kernforschungsanlage Jülich) at 230 MeV and near 1.3 GeV. These experiments are intended to provide stringent constraints on the set of six effective weak mesonnucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV is unique in that it selects a single transition amplitude (P2 D2) and consequently constrains the weak meson-nucleon coupling constant h ρ . The TRIUMF 221 MeV proton-proton parity violation experiment is described in some detail. A preliminary result for the longitudinal analyzing power is Az = (1.1 ± 0.4 ± 0.4) × 10. Further proton-proton parity violation experiments are commented on. The anomaly at 6 GeV/c requires that a new multi-GeV proton-proton parity violation experiment be performed. One of the more promising ways to study the neutral weak current interaction in hadronic systems is through measurements of parity violation in nucleon-nucleon (N-N) scattering. In the low-energy region, where meson exchange models provide an adequate description of the strong N-N interaction, an extension can be made to include the weak interaction. Pictorially, the exchanged meson (π, ρ, ω) is emitted at a weak interaction vertex and absorbed at a strong interaction vertex, or vice versa. The weak interaction vertex is calculated from the Weinberg-Salam model with Wand Z-bosons exchanged between intermediate mesons and constituent quarks, treating strong interaction effects in renormalization group theory in the regime of nonperturbative QCD. In a seminal paper following the above approach, restricted to one-boson exchanges, Desplanques, Donoghue and Holstein (DDH) [1] have calculated a set of six weak meson-nucleon coupling constants (a seventh was found to be rather small and is usually neglected). These six weak meson-nucleon coupling constants are denoted: f 1 π , h 0 ρ, h 1 ρ, h 2 ρ, h 0 ω, h 1 ω; where the subscript indicates the exchanged meson and the superscript the isospin change. DDH tabulated “best guess values” and “reasonable ranges”. As shown in Table 1, the “reasonable ranges of values” indicate uncertainties with respect to the “best guess values” of a few hundred percent. Similar calculations have been made by Dubovik and Zenkin (DZ) [2]. Extending the earlier work in the nucleon sector, Feldman, Crawford, Dubach, and Holstein (FCDH) [3] included the weak ∆-nucleon-meson and weak ∆–∆–meson parity violating vertices for π, ρ, and ω mesons. The latter authors also present both “best guess values” and “reasonable ranges” for the weak meson-nucleon coupling constants. Using the expressions of an earlier paper by Desplanques (D) [4] the latter authors (FCDH) present a third set of weak meson-nucleon coupling constants. It is to be noted that the large ranges of possible theoretical values persist. Taking into account the more recent experiments, Desplanques [5] has argued for a reduced range for the weak meson-nucleon coupling constant f 1 π . The weak meson-nucleon coupling constants have also been calculated by Kaiser and Meissner (KM) [6] within the framework of a non-linear chiral effective Lagrangian which includes π, ρ, and ω mesons. In this model f 1 π is considerably smaller than the “best guess value” of DDH or FCDH. Furthermore, a non-zero and non-negligible value for the seventh weak meson-nucleon coupling constant h 1 ρ was found. A complete determination of the six weak meson-nucleon coupling constants requires at least six pieces of independent experimental information. As of to date there do not exist enough experimental constraints of statistical significance to determine the six weak meson-nucleon coupling constants. Consequently, one needs several new precision parity violation measurements. Impressively precise measurements of the longitudinal analyzing power Az in p–p scattering have now been made at 13.6 MeV [Az = (-0.93 ± 0.20 ± 0.05) × 10] at the University of Bonn [7] and at 45 MeV [Az = (-1.57 ± 0.23) × 10] at the Paul Scherrer Institute (PSI) [8]. Here Az is defined as Az = (σ + − σ)/(σ + σ), where σ and σ represent the scattering cross section for polarized incident protons of positive and negative helicity, respectively, integrated over a range of angles determined by the acceptance of the particular experimental apparatus. A non-zero value of Az implies parity violation due to the non-zero pseudo-scalar observable ~σ · ~ p with ~σ the spin and ~p the momentum of the incident proton. From the PSI measurement at 45 MeV and the √ E energy dependence of Az at lower energies, one can extrapolate Az at 13.6 MeV to be Az = (-0.86 ± 0.13) × 10. There exists thus excellent agreement between the above two lower energy measurements. Both results allow pinning down a combination of the effective ρ and ω weak meson-nucleon coupling constants h ρ and h pp ω , with h pp ρ = h 0 ρ + h 1 ρ + h ρ √ 6 and h ω = h0ω + h 1 ω. It should be noted that a measurement of Az in p–p scattering is sensitive only to the short range part of the parity violating interaction (parity violating π exchange would simultaneously imply CP violation and is therefore suppressed). Following the approach of Adelberger and Haxton [9], one can fit the more significant nuclear parity violation data using theoretical constraints by the two parameters f 1 π and (h 0 ρ + 0.6 × h0ω). This leaves the experimental value of f 1 π = (