New results from pp annihilation at rest

Nuclear Physics B (Proc Suppl ) 8 (1989) 179-192 179 North-Holland, Amsterdam

NEW RESULTS FROM ~p ANNIHILATION AT REST

RolfLANDUA

CERN, EP Division, 1211 Geneva 23, Switzerland

New results on ~p annihilation at rest in P-wave are reviewed. The two-body branching ratios of n~z, KK, rip, and nf2(1270) show a strong dependence on the angular momentum and the isospin of the initial state. The suppression of n+n-E(1420) production in P-wave suggests JPC(E) = O- +

1. INTRODUCTION

Needless to repeat it: the standard model is in good shape. QED predicts electromagnetic

processes practically to arbitrary precision, the intermediate vector boson sector has well sustained

crucial tests at the percent level [1], and perturbative QCD is becoming a valuable tool to calculate

quark-gluon interactions at high energy. But the picture would not be complete without the black

sheep of the family: strong interactions in the "confinement region".

The hadronic mass spectrum is surprisingly simple, but it has remained a mystery why the non-

relativistic quark model reproduces it so well. The wisdom acquired over the last 30 years (e.g.

SU(6), vector dominance, chiral dynamics, bag models, meson- and baryon exchange, duality, the

OZI rule) obscures the fact that a comprehensive theory of low energy hadronic processes

(Ecm < 3 GeV) does not exist.

The experimental status of the field is also unsatisfactory. When high energy physics moved on

to the new frontiers of charm and beauty physics, many questions were left unanswered. This was

due to the low statistics, the limitations of the detectors, or controversial results. Doubtful

measurements led to confusion among theoreticians ("narrow baryonium"), stimulating much

theoretical work and a series of experiments with negative outcome. Finally, doubts were raised

about the productivity of hadronic physics at low energy.

The improvement of experimental techniques and the commissioning of LEAR revived the

interest in low energy antiproton physics. A valuable tool to study the unperturbed proton wave

function is ~p annihilation at rest, in the past mainly explored by bubble chamber experiments. Since

annihilation at rest proceeds via the formation of a ffp atom, the initial state has well-defined quantum

numbers, and selection rules severely constrain the number of partial waves contributing to a gwen

final state. In liquid hydrogen, annihilation takes place mainly in atomic S-states due to the Day-

Snow-Sucher effect [2].

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

180 R Landua / New results from pp annihilation ,at re,~t

New results on ~p annihilations at rest in P-wave have been obtained by the ASTER1X

experiment (PS 171) at LEAR. Antiprotons are stopped m a hydrogen gas target at NTP, where the

lower target density leads to an increase of P-wave annihilation. Annihdations from the 2p level are

selected by the coincident observation of an L X-ray transition. The ASTERIX detector and the

technique to select P-wave annlhflauon are descnbed in detad m ref. 3. My review concentrates on

results allowing to explore the dependence of the ~p anmhilatlon on the mmal angular momentum. S-

and P-wave annihilation will be compared for the rcrc and KK, 7t+Tt-rt o, and rt+rt-E(1420) final

states.

2. ANNIHILATIONS INTO ~rc and KK

The theoretical understanding of the ~p annihilation is still fragmentary. As the proton and

antiproton wave funcnons overlap, the symmetries of the underlying quark-gluon and gluon-gluon

interactions are expected to affect the transmon amplitudes m a charactensuc way. Experimental

studies of two-body branching rauos and their dependence on the quantum numbers of the inmal

states are carried out with the hope to find striking patterns, w~tnessing the quark structure of

hadronic matter. The complexity of the multi-body system (6 quarks plus an unknown number of

gluons) implies the invesugation of as many annihilation channels as possible to distinguish between

different theoretical approaches and their prediction s.

The study of the reaction ~p ~ nrc, KK has several advantages: the signals are clean, the

background is relatwely low, and selection rules strongly restrict the number of possible mitxal

states. Table 1 summarizes the transitions allowed from S- and P-states.

S Wave

P Wave

~+~- ~o~o K+K - KsKs + KsKL

KLKL

1So (0- + ) X{P) ~(P) ~(P} ~(P) X~(C,P)

3S 1 (1 - - ) J )~ (C) t~ X (C) 1/

1P l (1 +" ) X (P) ~ (P) ~ {P) ~ ~ {C,P) X (P)

3P o (0 ++) , / / ¢ ' ,,¢ X cc;

3p 1 (1 ++) ~ ( C , P ) ~{C,P) ~ ( C , P ) ~ {P) ~ ( C , P )

3P2 (2++) ~g ~g ~ ~,g ~ (C)

TABLE 1 Selection rules for ~p ---) ~+Tt-, K+K -, KsK s and KsK L

(re=allowed; X= forbidden by C-panty or Panty conservauon)

R. Landua / New results from pp annihilation at rest 181

The branching ratios of xx and KK for annihilations in liquid hydrogen (mainly S-wave) have

been measured by several experiments [4-6]. With the new results obtained by the ASTERIX

collaboration [7,8], the branching ratios for pure S- and P-wave annihilation can be derived:

x+rc - x'°m O xn K+K" KsK L KsKs+ K°F, ° K

KLKL

S wave

P wave

31.9+2.0 forbidden 31.9+2.0 10.8+0.5 8.3_+0.5 forbi&[en 8.3+0.5 19.1+0.7

48.1+4.9 24.1+2.51 72.2+5.5 2.87_+0.51 forbirhfen 0.88+0.23 0.88_+0.23 3.7+0.6

TABLE 2. Summary of results from ~p --> n+x -, K+K -, KsKs+KLK L, and KsKL at rest

(Branching ratios in units of 10"4; data from ref. 4-8)

The branching ratios show striking differences between initial states with L=0 and L=I: the

~x+n - branching ratio increases by 50 % (from 32 to 48-10-4). The K+K - ratio drops by a factor of

3.8 (from 10.8 to 2.9.10-4), and KOK o is suppressed by one order of magnitude (from 8.3 to

0.88.10-4). While the total branching ratio for nn increases by a factor of 2.3, for KK it decreases

by a factor of 5.1.

Why does a change of the initial angular momentum affect the annihilation in such a remarkable

way? Several promising theoretical approaches based on non-perturbative QCD have been proposed

in the last years [10], but they do not - yet - reproduce all results quantitatively. Self-critical reviews

of the theoretical situation have been presented at this conference [11,12]. It has been noted earlier

that the KK suppression cannot be reconciled easily with traditional meson exchange models [13].

Many authors agree that ffp annihilation is in lowest order approximated by the annihilation of one q~

pair and the rearrangement of the qq~-~ intermediate state (Fig. la), or the annihilation of two q~ pairs

followed by the creation of a new pair (Fig. lb). However, the quantum numbers of the q~ vertex are

controversial: if 1-gluon exchange dominates, the q~ pair must be in a 3S 1 state (jPC = 1--); if many

gluons participate, the assignment of vacuum quantum numbers (0 ++) might be a better choice (in

analogy to Pomeron exchange), requiring the q~pair to be in a 3Po state.

IThe ~'°r~° branching rauo of~p anmhdations m hquid hydrogen has recently been re-measured with higher precimon [9]. Since n°n ° is forbidden from S-wave, its branching ratio is a direct measure of the P-wave contribution to the annihilation. With the knowledge of its P-wave branching ratio, it has been found that 8.6+1.2 % of all annihilations in liquid hydrogen proceed from P-states [7].

182 R Landua/New results from pp anmhilatum at re,~t

pp --) qqqq _~ MIM 2 + gluon[sl

a. Rearrangement graph

4 4

pp --~ qq _~ MIM 2 + (]loons

b Annihilation graph

p~ __.> only _). MI M2 gluons

c OZI rule forbidden graph

Fig 1 Quark-hne graphs contributing to Op anmhdatlon

(gluon producUon mchcated m lowest order)

For the KK final state, only the annihilation graph (Fig. lb) contributes. The suppression of thts

graph in P wave - and therefore KK suppression - can easily be explained by the 3P o model, since

for initial P wave the created sgpair must have an even angular momentum (1=0,2,..), owing to parity

conservation. Assuming a 3Po vertex (1=1), this is forbidden. However , advocates of the 3S 1 model

argue that the KK suppression could also be due to a different mixing of the I=0 and I=l initial state

components in S- and P-states.

The difference between K+K - and KOK ° can be translated into different contributions from

amplitudes with isospin I--0 and I=l :

A (K+K -) = 1/3/2 ( A1 - Ao)

A (KOK °) = 1/3/2 ( A1 + Ao)

R. Landua / New results from ~p annihilation at rest 183

In S-wave, K+K - and KOK o are approximately equal, so either I=0 or I=1 dominates. Using

results from ~n ---) KOK - [14], and assuming that S wave annihilation dominates for ~ annihilation in

liquid deuterium, I=1 dominates by a factor 3 over I---0. On the contrary, KO~O is strongly

suppressed compared to K+K - in P wave, which can be explained as the destructive interference of

two isospin amplitudes of approximately equal size. Table 3 gives the isospin contributions and their

relative phase for S-wave annihilation; for P wave, the KOK - branching ratio is not known, therefore

the I--0 and I=1 contribution can not be disentangled. However, the I=1 component must be

sigmflcantly smaller in P-wave than in S-wave.

IA (KOKO)l 2 + IA (K+K-)I 2 =

IA (KOKO)l 2 -IA (K+K-)I 2 =

S wave

IAo12+ IA112 : 19.1+ 0.7

2"IAoI'IA11"cos 0 : -2.5 + 0.7

IAo12 : 4.4 + 2.2

IA112 : 14.7 + 2.1

: 99.1 +3 .5 o

P wave

3.7 + 0.6

-2.0 _+ 0.6

TABLE 3. Isospin contributions [branching ratios in units of 10 -4] and phases in S- and P-

wave [from ref. 8]; Ao, A1, and ~ m S-wave derived from ~n results [14]

At this point it seems appropriate to make a short excursion to ~p annihilation in flight. The ~p

system allows the study of meson-like states in formation experiments. While the total cross-section

measurement provides useful indications of the existence of resonant structures, exclusive channels

are more sensitive to detail. For rc~ and KR~ final states, the same selection rules as at rest apply, and

only a few partial waves contribute. The partial wave analyses of the ffp ~ ~ differential cross-

sections have shown the presence of many resonances in the 2100 ... 2500 MeV region [15]. An

overview of the status of the search for direct channel resonances before 1980 is given in ref. [16].

Recently, the study of the reaction ~p ~ tIxI) in the energy range between 2000 and 2400 MeV has

been proposed [17]. The main interest lies in the study of the intermediate state - likely to be of

purely "gluonic" nature - as a function of energy, since the formation of two s~-mesons in the final

state is only possible via an OZI rule suppressed annihilation graph (fig. lc). The observation of

enhancements in the cross-section and associated activities in one or more partial waves could reveal

the existence of exotic resonances like glueballs, hybrids or other multi-quark systems with

constituent gluons.

For the comparison of branching ratios at rest with cross-sections in flight, the measured cross-

sections for nn and KK [ 18-34] have been normalized (Fig. 2) to the total annihilation cross-section,

approximated by t~ann = 7885 p-0.7 mb ([p] = MeV/c). The points at p=0 indicate the branching

ratios at rest in S- and P-wave.

184 R. Landua / New results from ~p annihilation at rest

( to -31

!o

[so -s ]

0.8

0.6

0.4

0.2

|

o.[ o !

e (~+~-)

°o 5;0 ,&o ..v,o I I I I I. I I

1.9 2.e 2.1 2 2 2.3 GoY

f l ( K s K s + K L K L)

50O I

! .9 2.0

1O0Q 1509 HEY/© I i I i

2.1 2.2 2.3 GeY

Do --s] {I ( K + K - )

'[ ]' ,

nl , , , . 0 500 1000 1500 Ide¥/c I I I I I I

1.9 2.0 2. I 2.2 2.3 GeY

[10"3][ ~ (KsK L)

0.8~] ~ L ~

0.5

0.4

0.2

0, 0 1500 MeYfc l l ! J

2.2 2.3 GoY

500 10N" I s I

1.9 2.0 2.1

Fig 2 Relative cross-sect~ons of ~ and KK m flight Points at p=0 from anmhilat~on at rest

H=S-wave (I--) l= P-rave(O++,2 ++)


In general, the branching ratios at rest are not very different from the relative cross-sections at

low momenta. In x+x-, a shoulder around 900 MeV/c ('4s = 2000 - 2100 Mev) is followed by a

significant decrease above 1200 MeV/c. In K+K -, the relative cross section stays relatively constant

(= 10-15-10-4). An enhancement of the K+K - cross-section at p = 500 MeV/c, associated with a

strong backward peak of the angular distribution, had been claimed [35], but was not confu'med by a

more recent experiment with higher statistics [23]. The KsKL results suffer from their low statistics.

Below 1000 MeV/c, the relative cross-section is about 4.10 -4, decreasing to 2.10 -4 at higher

momenta, while this value is 8-10 -4 at rest. In contrast, the low KsKs+KLKL branching ratio

observed at rest (0.8-10 -4) is maintained up to 800 MeV/c, suggesting that the same mechanism

responsible for the KsKs suppression at rest is still active beyond the ~p threshold region. Above

1300 MeV/c, PS 185 has presented at this conference new results [34] obtained in the search for

the ~(2220) resonance, giving a relative KsK s cross-section of =10 -4. In this momentum range and

above, the KsKs+KLKL cross-section agrees with KsKL within the errors, while they differ by a

factor of 10 at rest. At present I am not aware of any theoretical model explaining the dependence of

these two-body cross-sections on the initial angular momentum, the isospin and the total energy.

3. ~p ~ x+x-x °

The three-pion final state occurs frequently in ~p annihilation (6.9 + 0.9 % in S wave). It

contains resonances produced by the emission of a single pion ~ p ~ xX), which decay

subsequently into a di-pion system (X ---> xx). Possible quantum numbers of these resonances are

JP = 0+,1-,2 + ..... hence the production of scalar, vector and tensor mesons is expected. Table 4

lists the resonant intermediate states and their angular distribution for the possible initial states.

Isospin xOp o n+p - x°f2(1270)

n-p+

S Wave 1S o (0- +) 1 X tf(L=l, cos2*) tf(L=2, [cos20-1/3] 2)

3S 1 (1--) 0 tf(L=l, sin20) tf(L=l, sin20) ~'

P Wave 1P1 (1 +- ) 0 t~(L=0, const.) t/(L=0, const.) ~'

3P1 (1 ++) 1 ~ t/(L=0, const.) tfCL=l, cos20+1/3) 3P 2 (2 ++) 1 ~ ~¢(L=2, sin20) ~(L=I, sin20)

TABLE4. Selectaon rules and decay angular distribution for resonant intermediate states of~p --4 n+x-x o

leg=allowed (lowest angular momentum, angular dismbution) ;

~= forbidden by C-parity conservation; 3P o state forbidden by panty conservation]

1~6 R. Landua / New results from 13p anmhilation at r~,,st

In bubble chamber experiments (mainly S-wave), the p~ channel conmbuted about 70 % of the

~+~t-~o final state. Although both I=0 and I=1 initial states are allowed, only I--0 p~ production

was found ("p~ puzzle") [36].

The ASTERIX experiment has analyzed 7,750 events with an L X-ray m coincidence, giving an

event sample containing about 90 % P-wave annihilation [37]. Fig. 3 shows the n+rc~ o Dahtz plot

and its projectxons.

o q~

4',d

2.4

1.6

0.8

4 0 0

N

3 0 0

o

~" 2 0 0

e ,

) ,

t O 0

, , ; . ,.?,~.

-,7~=~, - - "" ~'" ~:;" i ~ . J ~ r . " 9 , ~ :. - .: ~.~.~ -,::,

. :,.-- -~.. ~....,,~,~- ),,:-.--,, : .

)~ ~ ; . L , ; _ ' ~ _ ~,'~.~ ..-.'~' . . . . . . . . . . . - %':;~Z~9~ ,~''~',""~"."~-" • - , , . , , ~ , ~ . , r',,,',.. :;:..~.-.~-~y~-,,~-,;.

. ' - - : " ,~:c ~2' , [ ' :~ ' ,~ : .~"~ ' . : ' , ' ; , - ' ,~ '~

0.4. 0.8 .2 1.6 2.0 2.4 2.8 m z (~*,(o)

I GeY 2

\ I I I I I I I L ~ I

0.4 0 .8 1.2 ! .6 2.0 2.4 2 .8 GeY 2 m 2 (K*lc-)

4°° f 3oo~-

20()

100

4 0 (

3 0 |

206

1 0 0

f

I I 0.4 0.8 ! .2 1.6 2.0 2.4 2.8 GeY

m z ( ~ ' t ° )

j I

0.4 0.8 1.2 1.6 2.0 2.4 2 .8 Ge'~ m 2 (,+tc ° )

I 2

Fig 3 Dahtz plotand mass projectlorsfor ~p-> ~+~-~o mP-w~v~ anmhllat~onat rest


The Dalitz plot exhibits clearly visible bands due to p+,pO, and f2(1270) production. In the

mass projections, the comparison of the charged and neutral p peaks shows that the charged p states

occur more frequently than the pO, indicating that np production occurs both from I--0 and I=1 in P-

wave. The f2(1270) is produced with a rate comparable to that of the pO. A partial wave analysis

has been performed, based on the Zemach formalism [38], and using relativistic Breit-Wigner

functions to describe resonant states. Amplitudes from rip, nf2, the S-wave background (--- 10 %),

non-resonant 3n production and interference between amplitudes from the same initial state were

taken into account. With these simple assumptions, a satisfactory fit of the Dalitz plot has been

obtained (X2/<Z2>= 1.2). The results of the fit, together with the absolute branching ratios in P

wave, are listed in table 5.

Imual state: 1So 3S 1 1P1 3P1 3P2

rcp (I = 0) -- 4.57 + 0.25 0.93 _+ 0 . 1 8 . . . .

n 9 (I = 1) < 0.14 (2a) . . . . 1.24 + 0.30 0.22 + 0.20

rff2(1270) 0.24 + 0 . 0 7 . . . . 0.69 + 0.12 0.12 + 0.12

3n (non-resonant) 0.84 + 0.31 1.25 + 0.12 all P states: 1.30 + 0.20

TABLE 5. Absolute branching rataos [m %] for mtennedmte resonant states of ~p ---> r~+r~-n ° in S- and P-wave

(interference terms are neglected)

These results can be summarized as follows:

1) There is no "p~ puzzle" in P-wave, since both I=0 and I=1 contribute. Preliminary results

indicating I---1 suppression also in P-wave [39] are not confirmed.

2) The p~ production from I=0 is suppressed by a factor of 5.2 going from S- to P-wave (4.7 %

versus 0.9 %), while it increases by more than one order of magnitude in I=1 (< 0.14 % to

1.44 %).

3) The ~f2(1270) production increases by a factor of 3.4 (from 0.24 to 0.81%), possibly because

of the smaller centrifugal barrier (L=I) in P-wave, compared to L=2 in S-wave.

4) For both S- and P-wave annihilations, a significant non-resonant 3~ production (about 30 %) is

observed. It is unclear whether this is due to direct 3-body annihilations or if very broad resonant

structures in the ~ S-wave simulate this effect.

188 I{ Landua / New results from pp annihilation ~t re,~t

4. ~p ~ ~q-~- E (1420)

The E(1420) was discovered m ~p annihilation at rest m hquid hydrogen [40]. Although ~t

celebrated its 25th anniversary recently, considerable confusion persists about its nature. To stay

unprejudiced, I prefer to avoid the new name of the E (" f1(1420) "). Instead of adding another

review to those already existing [41], I only summarize some of the many open quesuons: Is the

E(1420) a single resonance, or are there two or more mass degenerate objects? Is the t (1440),

observed in radiative JAg decays, identical with the E(1420)? Is the spin-parity of the resonance(s) 0-

or 1+? Is there a decay E(1420) --~ rC[ao(980)---~KK]? If yes: since the ao(980) decays more

frequently into rlrc, why is there so little evidence for the decay E ~ rl~n? Is the E(1420) mainly an

s~ member of an SU(3) nonet, as indicated by its strong kaonlc decay modes? If yes, why is it

produced in rcp scattering [42,43] but not in K-p scattering [44]? And finally: is it a normal qq

meson, a glueball, or a mixture of both?

At this conference, new results on the E(1420) have been presented [45]. The ASTERIX

collaboration has studied the production and the decay of about 400 E meson events in ~p

annihtlation at rest in H2 gas in the channel ~p --~ n+~-[E ~ KOK+~ - + c.c.]. Fig. 4a shows the

neutral KKn mass distribution, where the E meson appears with a mass M=1413 + 8 MeV and a

width F = 62 + 16 MeV. At 1285 MeV, a peak with 3 ff significance is observed, which is assigned

to the f1(1285) [the former D meson], a well-established resonance with JPC=I++ known to decay

into KKr~.

8 0

6 0

40

20

i

1100 1100 1300 1400

i00

I I

1500 1600 BeY

Fzg. 4a Mass of KKn after subtraction of non-resonant combination


0.9

-~ 0.B ~5 o 0.7

,,,. 0.6

O5

0.9 3> 3 o.8

0.7 0

0.6

~ 0 5

i i i i I t

0.5 0 7 0 .9

M~(K "n) c h a r g e d GeV 2

Dahtz plot for E decay rata K~:~

m ~p anmhdanon at rest (ASTERIX)

04

03

Fig. 4b F~g. 4c

" 0 . . •- - . . . , • o . , " . : { # . . "

t " ,~, ~ . J l " • . • l " " • . . : , ~ , , , ~ ' . : : : ~ . ' , . • ~ ". , ' .

. ' . '~," . 3 : " & , o . d . - . ~ .g , l~ • •

. . " i ~ : t . ~ . , . . ~ ..l~ ~ . . . ~ . . . . . . .

o - . . o .~ . o " . ~ ; • • ' %1 . . . •

• . * t " I . " • • : ' " . ~ : . ' t - . • ; ~ . . . z . . • " , - : " . . . r . . .

• , . ' . ; . . , 1 ~ ¢ " ; . , . . . . . . . : .;.~;~&." , . . • , . . , ' . ~ ,~ o.

. . ' " • " . . . . " , . : : : . F , ; . . "

• " . , z ' ' t " " "

I I 1 t 0 , 9 0.5 0.7

M2(K "n) neutral OeV 2

Dahtz plot for E decay rata KK~

m pp central produc,on [ref 43]

The decay Dalitz plot for events with M(KK~) = 1370 ... 1480 MeV is shown in fig. 4b. The

distribution shows an accumulation of events in the region of low KK masses (right upper comer),

but no clear K* bands. This agrees well with the Dalitz plot from bubble chamber experiments, but

is strikingly different from the plot obtained, for example, in pp central production (fig. 4c). The

partial wave analysis of the ASTERIX data was performed using the Zemach formalism and a least-

squares fit. The K* was described by a relativistic Breit-Wigner, and the low-mass K_K enhancement

was described by the Flatt6 formula for the ao(980) [46].

The fit results exclude a dominant K*K decay mode, and give a strong contribution from the low

KK mass enhancement Cao(980)"). However, there is uncertainty about how to describe this

structure: is it the ao(980) a resonance limited by phase space, a KK final state interaction, or a Ir~

molecule? Different parametrizations of the ao(980) influence the angular distribution stronger than

the difference stemming from a JPC=0-+ or 1 + assignment of the E(1420). Therefore, no clear

preference is given by the fit for either of the two possibilities.

The study of the production ~p ~ ~ + n - E is more conclusive. The data show that xnE

production from P wave is compatible with zero (7.10 -4 versus < 1.10-4). In view of the selection

rules acting in ~p annihilation at rest, this gives information on the spin-parity of the E. The (nx)

system recoiling against the E meson is most likely in a 0 ++ state (since pnn < 180 MeV/c). The

relative angular momentum L between the di-pion and the E is expected to be I.,=0 because of the

limited phase space available. This has been confirmed in S-wave annihilations [47]. When L---0, a

190 R. Landua / New results from pp anmhdatio~l at r ~ t

state with 0 -+ (recoiling agamst a 0 ++ nn pair) is only produced from S-wave (1 So), while a 1 ++

state can only come from P wave (3P1). The f1(1285) meson is a 1 ++ pamcle and follows this

prediction. The E meson is not produced from P states, and hence the 0 -+ assignment ~s the most

probable.

In summary, the new results confirm that a 0 -+ resonance is hkely to exist at 1420 MeV. The

obvious difference in the decay Dahtz plot between the E produced m ~p anmhilatlon and the

resonance produced m pp central production suggest, however, that there are two mass degenerate

resonances, one with 0 -+ and one with 1 ++ .

CONCLUSIONS

The study of ~p annihilation at rest in P-wave into two- and three-body final states has yielded

very interesting results. An increase of the ng branching ratio by a factor 2.3 and the suppression of

KK by a factor 5.2 with respect to S-wave annihilations is observed. The KsKs+KLK L final state

occurs much less frequently than KsKL. This suppression is also observed for annihilation in fhght

below 800 MeV/c. In the np final state, the I=1 and I=0 contribute, m contrast to S-wave

annihilations. The absence of nnE(1420) production from P-wave suggests a spin-parity of 0-.

Recently, the new EMC measurement of deep inelasnc polarized I.tp scattering together with

various theoretical arguments has raised the suspicion that the proton contains a s~gmficant number

of s~- quark pairs. If so, the OZI rule would not apply to ~p annihilation [48], and final states

containing sT mesons ('q', ~, f2'(1525) ... . ) would be produced more frequently than predicted by

calculations based on the vahdity of the OZI suppression. A determination of the relevant branching

ratios in S- and P-wave using the ASTERIX data is in progress. Another consequence of this theory

is, that the observation of a resonance in an OZI forbidden channel - in particular ~p ~ ~ - would

not necessarily mean that it is a gluonium state, since it could also be an excited s~-state.

The continuation of the systematic study of ffp annihilation at low energies with high statistics

and improved detectors is necessary and will be continued at LEAR by several experiments.

ACKNOWLEDGEMENTS

I thank R.v.Frankenberg, J. Franz, and T. Shlbata for their comments.

QUESTIONS AND COMMENTS

I.S.Shapiro (Lebedev Institute, Moscow): What is the total branching rano into anmhllation

channels containing strange parnclesq


Answer: In bubble chamber experiments, i.e. mainly S-wave annihilations, kaonic final states

const i tu ted about 7 % of all annihi la t ions. This number was ca lcula ted f rom the number of

annihilations containing at least one Ks decay, assuming no additional selection rules exist to inhibit

or enhance particular annihilation modes, and that isospin invariance applies. The same analysis for

P-wave annihilation at rest by the ASTERIX collaboration is in progress.

I .S.Shapiro: It would be of interest to have more detai led data on the annihilat ion in flight into

K + K - just near the threshold of the ~p ---) AA reaction.

D. Peaslee (Universi ty o f Maryland): I have a comment on the nature of the ao(980). I f it is a

resonance, its phase 0 will give d0/dE > 0 through the peak; i f it is a potent ial well (molecular)

effect, then d0/dE < 0. The analysis of E771 data from Brookhaven (g--p --~ E + n ~ K K x + n at 8

GeV/c) clearly favours the resonance interpretation.

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New results from pp annihilation at rest