27t Nuclear Physics B (Proc Suppl ) 8 (1989) 271 27~; North-IIolland, Amsterdam

ISOSPIN EFFECTS IN ~3He ANNIHILATION AT REST

Presented by G. Bendiscioli b

F. Balestra a , R. Barbieri b, Yu.A. Batusov c, G. Bendiscioli b, F.O. Breivik f, S. Bossolasco a , M.P. Bussa a, L. Busso a, I.V. Falomkin c, L. Ferrero a, C. Guaraldo d, A. Haatuftg, A Halsteinslidg, T. Jacobsen f, E. Lodi Rizzini e, A. Maggiora d, K. M,cklebostg, J.M. Olseng, D Panzieri a, G. Piragino a, G.B. Pontecorvo c, A.M. Rozhdestvensky e, A. Rotondi b, P. Salvim b, M.G. Sapozhnikov c, S.O. Sorensen f, F. Tosello a, V.I. Tretyak c, A. Venaglioni b and A Zenoni b

(a) Torino, (b) Pavia, (c) Dubna, (d) Frascatl, (e) Brescia, (f) Oslo, (g) Bergen

Abstract. The analysis of ~3He annihilation events at rest (from the PS 179 experiment at LEAR) gives the value 0.467+0.035 for the ratio between the annihilationsross sections on n and on p. This low value incficates a strong isospin dependence of the NN amplitude in P wave.

The antiproton-neutron (~n) system is in a pure isospln state with I=l and the

antiproton-proton (~p) system is in a mixture of states with I=0 and I=l. The ratio between the ~n

and ~p annihilation cross sections can be written in the form [1]

R a - °a(Pn-~) - 2~sa(1)/(ya(0) (1)

~(~p) l+O'a(1)/o'a(o)

where ~a(0) and ~a(1) are the annihilation cross sections with I=0 and I=1. If the

antinucleon-nucleon (I~N) interaction depends on the isospin, 1.e. oa(0)/oa(1) ;e 1, then R a ¢ 1

(and viceversa).

Eq. (1) holds strictly for annihilation on free nucleons. To consider anmhdation on nucleons

bound in nuclei, we introduce a similar quantity:

Ra b = oab(~n)/o'ab(~p)

where oab are cross sections for annihilation on single nucleons bound in nuclei.

Here we report the result of the measurement of Rab in the annihilation at rest on 3He and

compare it with previous measurements on 2H and 4He.

In a conventional picture, the annihilation process of ~ on 3He nuclei at rest consists of a

p-nucleon annihilation which may be followed by the interaction between the residual nucleons

and the mesons (mostly pions) produced by the annihilation (final state interaction, FSI).

If only annihilation is effective, ~p annihilation produces an even number of charged pions and

a 2H nucleus; ~n annihilation produces an odd number of charged pions and two protons. Hence,

0920-5632/89/$03.50 Q Elsevier Science Publishers B.V (North-Holland Physics Publishing Division)

G. Bendiscioli et al . / Isospin effects 275

the annihilation processes on n and on p are distinguishible both by the number of charged pions

and by the number of heavy prongs.

FSI may break the 2H nucleus or, through the p-nucleon charge exchange reactions (n-p~

non, g°p.~n+n) may change the primary number of charged pions and heavy prongs. As a

consequence of the charge exchange the ~p annihilations may assume the features of pn

annihilations and viceversa, as it affects the relative numbers of heavy and light particles emitted.

Experimentally, we can divide the events into two sets (a) and (b). Set (a) includes ~p

annihilations and ~n annihilations which look like ~p annihilations due to pion charge exchange.

Set (b) includes ~n annihdations and ~p annihilations that look like ~n annihilations due to pion

charge exchange. Events belonging to set (b) have two heavy prongs and different numbers of

negative and positive pions. All other events belong to set (a).

We define: Om(~P)=cross section for (a) event production; Om(~n)=cross section for (b) event

production; oa(~p)=cross section for annihilation on one bound p; oa(~n)=cross section for

annihilation on one bound n; oPce(~n)=cross section for annihilation on n plus final charge

exchange on p; Once(~P)=cross section for annihilation on p plus final charge exchange on n;

oat=total annihilation cross section. One finds [1]

r + (r+l) ~ Oce Oce/O~t o~(~n) ot

R b = = 2 (2) oa(pp ) 1 - (r+l) Gee (scffo~t

a

(S t

where r=Om(~n)/Om(~p) and ~ce=2 cPce(~n)-cnce(pp). Oce contains terms which tend to cancel

each others ( n ° p ~ + n ; n - p ~ ° n ) . If Oce/Oat << 1, then Rab=2r. r is a measured quantity, while

Oce/oa t must be estimated.

This can be done in an approximate way as follows. In the ~p annihilation the residual

nucleons are (p+n) and, on the average, the number of the produced pions is

(1.5n++l.5~-+2n°) . In the ~n annihilation the residual nucleons are 2p and the pions are

(ln++2n-+2n°). As the pions have momenta around the baryonic resonance momentum, the

cross sections for the different n-nucleon pairs have approximately the ratios

o(n+p):c(rc°p):o(~-p):oce = o(~'n):o(~°n):o(n+n):Oce=9:4:1:2

All this considered, the probability of 7r-neutron charge exchange after annihilation on p is about

0.11 and the probability of ~-proton charge exchange after annihilation on n is about 0.296.

Assuming oa(pn)/~a(pp)=0.48 (see the final result of this work) and the FSI probability to be

about 0.21% (value obtainable from that for annihilation at rest on 4He where FSI =0.34 [5]),

one finds (sce/Ot =(-0.007). Neglecting this ratio in eq. (2), Ra b is understimated by about

4-5%.

276 G. Bendiscioli et al./Isospin effects

~3He annihilation events were detected using a self-shunted streamer chamber (90x70xt 8

cm 3) in a magnetic field (0.4 T) exposed to an antiproton beam (105 MeV/c) from the LEAR

facility of CERN. The apparatus is described in detail m Ref. 2.

2020 events have been measured following the procedure described m Ref. 3 and 962 events

with 3, 5 and 7 prongs have been identified as belonging to set (a) or to set (b). (All events with

one prong belong only to the set (a)). To check our procedure, we calculated the multlphcit2

distributions of ~- produced in the ~p and ~n annxhdation events separately and compared them to

similar distributions obtained with hydrogen and deuterium m bubble chambers. The agreement is

very good [ 1 ].

The result of our analysis is Rab=2r=0.467+0.035 [1]. This value is close to that found for

4He (0.48+0.03 [4,5]) and remarkably smaller than that found for liquid 2H (0.75+0.02 [6],

0.81+0.03 [7]). Note that the values of Rab obtained in these analyses come out from anmhdatlon

probabilities on n and on p measured in the same experiment. So they are not affected by

normahzation errors as in the cases where these probabilities are measured m different

experiments [ 1 ].

Rab<l indicates that the annihilatxon interaction depends on the ~sospm with o a l < ~ a o.

Rab(2H ) equal to about 0.8 indicates that the annihilation on hquid 2H occurs mainly m S wave.

while Ra b (He) equal to about 0.5 indicates that in the annihilauon on He P and (perhaps) D

waves are ~mportant. These explanations are according to I~N and antiprotomc atom data

analyses[ 1,8-12].

Note that, below 400 MeV/c, current/~N potential models predict that in S wave R a ~s about

0.8 (in agreement with the experimental data on 2H), while in P wave R a = 0.7-0.76 (m

disagreement with the data on He)[13].

REFERENCES

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