Isospin Dependence of Pion Absorption on a Pair of Nucleons

Abstract

The relative cross sections for the 2N + TT-2iV reactions were determined where the initial two nucleons are in a relative s state, and coupled either to isospin zero or to one. This information is extracted from two-nucleon coincidence measurements of the (7r + , 2p) and the (7r" f pn) reactions on targets of 3 He and 4 He.PACS numbers: 25.80.+f Pion absorption is usually treated as proceeding through the n +2iV-2iV process. Considerable effort 1 has been devoted to understanding the TT + +d -* pp reaction, in which absorption takes place on a T = 0, S =1 nucleon pair. At bombarding energies in the (3,3) resonance region this reaction is believed to be dominated by the intermediate AN configuration with (J 7r ) AAr =2 + and AAT orbital angular momentum 1,^=0. Much less consideration has been given to the analogous process in which the pion is absorbed by two nucleons in a T =1, S =0 state. Such nucleon pairs are present in all nuclei heavier than the deuteron, and understanding absorption on such pairs is important to the understanding of nuclear pion absorption in general. The inverse of this process certainly does play a role in pion production {N A +N B -N c +N D +TT) and is part of the cross sections designated as a ±1 and a 01 in the traditional parametrization. 2 In pion production, however, several partial waves may contribute, and there is no simple way to select the piece of the cross section in which the (N C N D ) pair is restricted to J ,r = 0 + , T =1.If one assumes A dominance in the absorption process then it follows that the reaction TT + 2N -2N can only go through a T = 1 final state. If only isospin is considered, absorption on an initially T =0 nucleon pair is twice as likely as that on an initially T =1 pair. 3 It should be noted, however, that absorption on an initial J 7r = 0 + , T = 1 pair involves intermediate states that are different from the T = 0 case, as may be seen in Table I. In particular, L^^O, which dominates absorption on a deuteron, is not allowed in the T =1 case. Thus more detailed dynamical con-siderations may very well enter in this ratio.In the present work we report on coincidence measurements of the ir + 2N -* 2N process with targets of 4 He and 3 He. We assume 4 He and 3 He to have simple wave functions with all particles in relative s states, so that a comparison of the (TT + 9 2p) and the (n~ ,pn) reactions will be simply related to the isospin cross sections; the former must start with an initial pair of (np) nucleons that could be in either aT=loraT=0 state, while the latter must start with the initial pair of T =1 protons.The experiment was performed with 165-MeV pion beams from the P 3 channel of the Los Alamos Meson Physics Facility. Cryogenic liquid targets were used, with thicknesses of 343 and 629 mg/ cm 2 for 3 He and 4 He, respectively. 4 Protons were detected with a magnetic spectrometer, 5 with a momentum acceptance Ap/p =± 25% and solid angle of 8 msr. Coincident protons and neutrons were TABLE I. Quantum numbers permitted in the A model for n + (2N) {~*...

Pion Absorption in Nuclei

“…Several studies of this reaction have been carried out in the past few years. Early measurements gave only the ratios between absorption on the p-p pair and the p-n pairs in 3He for pions at rest (82) and for fast pions at some angles (80,83). More recently, fi rst measurements of complete angular distributions were done at 65 MeV (84) and at 65 and 85 MeV (85, 86); preliminary results were presented for 120 MeV (87) and 165 MeV (88).…”

Positive pion absorption on3Heusing modern trinucleon wave functions

“…Experimental cross sections of quasifree two-nucleon absorption of pions on helium isotopes have been obtained from the meson factories of LAMPF [12], TRIUMF [13] and PSI [14], but scarce data exist also for the polarization of outcoming fast protons [15,16]. These are obtained at so called conjugate angles corresponding to kinematics where it is believed that the spectator is not an active participant and does not absorb momentum from the pion.…”

Isospin Dependence of Pion Absorption onHe3

“…(1) that includes the usual nonlinear effects of strong magnetic fields ("quadratic Zeeman effect"). 7 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 Table I 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.…”