Parity nonconservation in the nucleon–nucleon system: nuclear structure issues

Abstract

I discuss some of the nuclear structure issues that arise in nuclear tests of parity nonconservation Je discute quelques-uns des effets de la structure nucliaire qu Can J Phys 66. 503 (1988) To date we have only one definitive measurement of parity nonconservation (PNC) in the two-nucleon system: the analyzing power in the $ + p system for longitudinally polarized protons (1). Since matrix elements of the weak NN interaction depend on five elementary S-P amplitudes, experimentalists have turned to few-nucleon and nuclear systems to obtain additional constraints. This has the somewhat regrettable consequence that the complications of the nuclear many-body problem must be surmounted for one to be able to relate experimental observables to the underlying two-nucleon amplitudes. I would like to discuss some of the techniques that have been developed for this purpose and point out some of their apparent limitations. A more complete discussion of these matters can be found in the review by Adelberger and Haxton (2).Modem PNC experiments concentrate on special transitions involving parity doublets, closely spaced pairs of states having identical spins but opposite parities. Some examples are shown in Fig. 1. If the separation AE between the doublet states is sufficiently small, the parity admixing with more distant states can be ignored. In Fig. 1 both AE and AE', the next smallest energy denominator governing parity mixing, are given. The two-level mixing approximation greatly simplifies the task of the nuclear theoretician: he is only asked to provide accurate wave functions for doublet states, and perhaps for a third state to which they may decay. Typically, the doublet states are low-lying levels in well studied nuclei, so that electromagnetic decay rates, spectroscopic factors, and other information may provide checks on the nuclear wave functions.An important example is the 0+1 * 0-0 doublet at E,, -1 MeV in "F, as illustrated in Fig. 1. The energy denominator, AE = 39 keV, is favorable. Assuming that a two-state approximation is valid, the weak interaction mixes these states 10-0; E,, = 1.08MeV) + 10-0) + e10+1)where VpNc is the weak NN potential. The 0-and Of levels decay by dipole transitions to the I + ground state of I8F. Because the A1 = 0 E 1 decay of the 0-level is isospin forbidden, this state has a comparatively long lifetime, T-= 27.5 t 1 .9 ps. In contrast, the A1 = 1 M1 decay of the 0+ level is isospin favored and fast, T + = 2.5 t 0.3 fs, rendering it one of the strongest known M1 transitions (10.3 * 1.5 Weisskopf units (W.U.)). Because of this large difference in lifetimes, a small admixture of the 0+ state into the 0-level produces a sizeable circular polarization: i apparaissent dans les tests de non-conservation de la paritC.[Traduit par la revue]As 1 /~ cc I( f lTYli)l2~$, the magnitude of the dipole matrix elements can be determined from the known lifetimes of the 1.04 and 1.08 MeV states, yieldingThe factor (gslTf"ag~~l+)/(gslTfl;~I-) can be regarded as an "amplifier" of the PNC effect. The...