Isospin Dependence of Pion Absorption onHe3

Abstract

It is now well recognized that unless pion absorption by nuclei is understood on a microscopic level, we will not be able to reach an internally consistent description of pion-nucieus interactions. Pion absorption on the deuteron has been studied extensively and can be described very well with the A-excitation mechanism TTNN-~AN + NN. Because of the restriction of spin and isospin of the deuteron wave function, the rr + d process only probes very limited AN^NN dynamics. For a complete microscopic understanding of pion abosrption it is necessary to investigate heavier nuclei in which a pair of nucleons with different spin and isospin is accessible. of California, Los Angeles, Report No. TEP/10, 1982 (to be published). 4 J. Sapirstein, Phys. Rev. D20, 3246 (1979). 5 M. Creutz, Phys. Rev. Lett. 45, 313 (1980); E. Pietarinen, Nucl. Phys. B190 [ FS3j, 349 (1981). 6 For large B we used the generalization of Eq. (1) that includes the usual nonlinear effects of strong magnetic fields ("quadratic Zeeman effect"). Recent coincidence measurements {TT + ,pp) and(77" ,pn) by Ashery et al. 1 have shown that the twobody irNN-*NA -NN process for a T = 1 pair of nucleons in He target nuclei is strongly suppressed and its strength relative to a T = 0 pair of nucleons does not follow a simple isospin argument. Since nucleons in He are predominantly in the relative s states, the intermediate AN in the 5 S 2 state, which dominates absorption for a T = 0 nucieon pair, is forbidden for a T = 1 pair (see It is shown that the strong isospin dependence observed in pion absorption on 3 He can be described by the two-body mechanism irNN^r AN^?NN.The effect of the three-body mechanism is also calculated but found to be negligibly small. The AN^rNN dynamics consistently determined from NN inelasticities is shown to be essential to understand the isospin dependence.

Critical review ofK−ppnbound states

“…The present theoretical situation is such that the microscopic mechanisms for two nucleon kaon absorption are only available from [17,18]. There is no work done on three nucleon K − absorption, and the scarce theoretical work on three nucleon absorption of pions [57,58] is a reflection of the intrinsic theoretical difficulties that any microscopic evaluation involves. This, together with the enormous amount of physical channels that one would have to evaluate to produce the relative yield of just one of them, describes clearly the horizon of such a goal.…”

Pion Absorption in Nuclei

“…If we now assume that all transitions will be dominated by the delta resonance, we find gl/2 = 0 and the cross section for absorption on the deuteron is four times larger than for absorption on the proton pair. In fact, the suppression due to having only angular momentum L = 1 between the delta and the nucleon was calculated (19) to be about two orders of magnitude! Under these conditions we may expect to gain sensitivity to the T = 0 nondelta channel in the reaction.…”

Meson-Exchange Hamiltonian forNNScattering up to 1 GeV andΔ-Nucleus Dynamics

“…The matrix element of v 1 in our model is then defined as a,',, 1 ' a (K/>M^',* a (i>',/>)] P ari S -[second term of Eq. (5)]^ (6) It is clear from our discussion of Fig. 1 that the second term of Eq.…”