Comment on ‘‘Pion absorption on the diproton’’

Abstract

Comment on "Pion Absorption on the Diproton"In a recent Letter, Piasetzky et al l report a phaseshift analysis of cross-section data for the quasifree absorption (in 3 He) of 62.5-MeV n~ by a ^o proton pair. The analysis produces five discrete possible solutions (for l n -p P ^2), yielding quite different predictions for the polarization P y (6) of the outgoing nucleons. We point out here that analyzing powers A y (9) measured for (ppoN^~) reactions on nuclei may already provide the (quasifree) polarization data needed to distinguish the correct solution.By time-reversal and rotational invariance, P y (0) for n~pp-+py Q \n equals A y (0) for K~ production at the same cm. angle via Ppo\n^ ppn~. The latter reaction, not yet studied with polarized free nucleons, is known 2 to be the dominant process occurring inside nuclei in nearthreshold A(p,n~)A + \ transitions. Recently, we have shown 3 that for continuum K~ production (averaging over many final nuclear states), A y (6) appears independent of target and excitation energy. One is motivated to relate these stable continuum results [see Fig. 1 (a)] to the behavior for free Pvo\n-+ppn~, because analogous (/?poi,7r + ) data closely resemble 3,6 measured 5 A y (0) for the corresponding free processes Ppoip-^ dn + and Ppo\P~^ pn7r + . The K + comparison is illustrated in Fig. 1(b), where p^p-^dn^ data are transformed to the /^-nucleus frame as appropriate 3 for an on-shell twobody reaction with a target proton moving toward the projectile. The growing deviation seen at large angles between the nuclear data and quasifree expectations may reflect the neglect of distortions and the full momentum distribution of struck protons in our simple kinematic transformation.We have now applied this same transformation 3 to the phase-shift predictions in Ref. 1 for comparison to the (ppoh^~) continuum data in Fig. 1(a). Two important assumptions, over and above that of a quasifree twonucleon n~ production mechanism, underlie this comparison. We expect the undetected low-energy proton pair to be constrained to a ^o state, not by nuclear structure (as in the 3 He case), but rather by the shortrange interaction causing the high-momentum-transfer (450-700 MeV/c) (p,n) reaction. 2 [A similar assumption, with regard to the 3 S\ np state, appears successful in the (/?poi,7r + ) comparison.] We further assume that the Ppo\n-^ppn~ predictions for a single bombarding energy (rj ab =412 MeV) in Ref. 1 can be applied without modification over the entire range (325-390 MeV) relevant to the quasifree K~ production in our kinematic transformation. [Over the same range, measured 5 A y (6) for Ppo\p-+ dn + undergoes little change in angular shape.] Only a very strong energy dependence could alter the qualitative conclusions drawn below.Of the five phase-shift solutions obtained, 1 only one (No. 2) yields A y (0) resembling the (ppo\,x~) continuum data. Here, the agreement [ Fig. 1(a)] is excellent at C(p>-) 0 <EX> =-19 MeV <Ex>=-2IMeV LU O Q_ 21 M > _l < 23 < 'C( p> + ) C o<Ex> =*20 MeV •<Ex>=*22MeV...