φ production in pp annihilation at rest

Physics Letters B 267 ( 1991 ) 299-308 North-Holland

production in

PHYSICS LETTERS B

annihilation at rest

ASTERIX Collaboration J. Reifenrfither ~, K. Beuchert, K.D. Duch 2, H. Kalinowsky, E. Klempt, B. May, P. Weidenauer Institut fiir Physik, Johannes-Gutenberg-Universitdt, W-6500 Mainz, FRG

U. Gastaldi, R. Landua CERN, CH-1211 Geneva, Switzerland

W. Dahme 3 Sektion Physik, Ludwig-Maximilians-Universitgit, W-8OOOMunich, FRG

J.C. Bizot, B. Delcourt Laboratoire de l'Acc~l&ateur Lin~aire, Universit~ de Paris-Sud, F-91405 Orsay, France

B.L. White Department of Physics, University of British Columbia, Vancouver, B.C., Canada V6T2A6

C. Amsler, M. Doser 4, j. Riedlberger 5, U. Straumann and P. Tru61 Physik lnstitut der Universit?it Zfirich, CH-8001 Ziirich, Switzerland Received 3 June 1991

We report on measurements of the branching ratios for t~ production in ~p annihilation at rest in H: gas. Branching ratios for ~n °, 0rl, t~p°, tlr,o and ~n+n- are determined for two data sets with different contributions of annihilations from S and P states of the ~p system. The branching ratios are compared to corresponding annihilation modes where the ~ is replaced by an to meson. We conclude that ~ production is in most reactions enhanced with respect to the expectation based on the OZI rule. In annihilations from the spin triplet ground state of antiprotonic hydrogen into ¢pnthe OZI rule is violated dramatically.

P r o d u c t i o n of¢~ m e s o n s in u a n d d q u a r k s in the initial and interesting phenomena. nearly ideally m i x e d ; t h e

2 3 4 5

hadronic reactions with state m a y i n d i c a t e n e w T h e v e c t o r m e s o n s are quadratic Gell-Mann-

The analysis presented here is part of the Ph.D. thesis of J, Reifenr6ther. Present address: Schott Glaswerke, W-6500 Mainz/Wiesbaden, FRG. Present address: LeCroy Research Systems, CH-1211 Geneva, Switzerland. Present address: CERN, CH- 1211 Geneva, Switzerland. Present address: IMP-ETH, CH-5234 Villigen, PSI, Switzerland.

O k u b o m a s s f o r m u l a suggests a d e v i a t i o n f r o m the ideal m i x i n g angle o f 6 = 3.7°; for the l i n e a r m a s s form u l a the d e v i a t i o n is e v e n smaller. H e n c e a c0/O p r o d u c t i o n ratio o f 1 / t a n 2 6 = 240 o r h i g h e r s h o u l d be expected. But ¢ p r o d u c t i o n is o f t e n e n h a n c e d w i t h respect to this v a l u e [ 1 ]. H i g h ¢ p r o d u c t i o n rates m a y be related to the p r o d u c t i o n o f g l u o n i c states [ 2 ], to a c o n t r i b u t i o n f r o m s~qO f o u r q u a r k states [ 3,4 ] o r the p r e s e n c e o f sea q u a r k s in p r o t o n s [ 5 ]. A l r e a d y d a t a on d i m u o n p r o d u c t i o n in inelastic n e u t r i n o - p r o t o n i n t e r a c t i o n s h a d s h o w n t h a t the proton structure function contains a considerable c o n t r i b u t i o n o f strange q u a r k s at low v a l u e s o f the

0370-2693/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.

299

Volume 267, number 2

PHYSICS LETTERSB

Bjorken variable x [ 6 ]. This seems to contradict the constituent quark model in which the proton consists of up and down quarks only. But the constituent quark model does not account for chiral symmetry breaking. Chiral symmetry breaking is related to the socalled pion-nucleon sigma term (27~N) which was determined from phase shift analysis and dispersion relations to 56(8) MeV [7]. This value, which was supported in a recent experiment [ 8 ], needs to be extrapolated from the Cheng-Dashen point [ 9 ] to the physical region giving a result of a~N = 49 (8) MeV [ 10 ]. QCD calculations using SU (3) F breaking effects give a value of Cro= 35 (5) MeV with a~N=a0/ (l--y) (y=2(pls~[p)/(plu~+dalp)). In order to explain the discrepancy between the experimental value a~N and the QCD prediction ao [ 11 ] one needs a strange quark content of y ~ 0.2. Also other experiments seem to require a strange quark content of the proton. Experiments on the elastic neutrino-proton scattering [ 12 ] show evidence for rather large axial vector matrix elements which are related to strange quark components in the proton wave function. The asymmetry of the cross section for inelastic scattering of longitudinally polarized muons off polarized targets for spin parallel or antiparallel [ 13 ] is presently discussed as spin crisis o f the proton by many authors [ 14 ]. The spin dependent structure function does not satisfy the Ellis-Jaffe sum rule [ 15 ] in which the proton is assumed to consist of up and down quarks only. In ref. [ 16 ] the fraction of the proton spin carried by strange quarks and gluons was estimated to z k g = - 0.25 (7). This interpretation is, however, questioned by Lipkin [ 17 ]. The sg-content would lead to a universal enhancement of reactions Niq ~ 0X while the coupling to four quark states would limit the enhancement to certain initial and final states, e.g. to pp spin-triplet ground state annihilations into On. O production has also been treated as a resonant final state interaction of a KI(pair produced in the annihilation process. The ratio of On to o~n production is then related to the ratio of s~ to ufi and da creation [ 18 ] and the OZI rule need not be violated. To clarify the underlying mechanism it is therefore imperative to compare 0 and co-production in as many different channels as possible. In this letter we report on measurements of branching ratios for pp annihilation in H2 gas at normal temperature and pressure into final states con300

12 September 1991

taining 0 mesons. References to previous determinations of branching ratios for 0 production in liquid H2 can be found in refs. [ 1,4 ]. The data presented here were taken at LEAR with the ASTERIX spectrometer [19]. The apparatus consists of a hydrogen gas target, a cylindrical X-ray drift chamber (XDC), seven cylindrical multiwire proportional chambers (MWPC's: C1, C2, Q1, Q2, P1, Q3, P2) and two planar end cap detectors. The C and Q chambers had helical cathode readout. The assembly of target and chambers was mounted inside of a solenoid providing a magnetic field of 0.8 Tesla. The XDC served three purposes: it was used to improve on the momentum measurement and pattern recognition for charged particles, for n / K separation via energy loss measurements and to detect X-rays emitted in the cascade of the pp atom [ 20,21 ]. Most low energy X-rays are due to n D - , 2 P transitions of the pp atom. After emission of a nD-o2P transition X-ray, the pp atom annihilates with a probability of 99% [22 ]. The fraction o f p p atoms emitting such an X-ray is 13% only, their detection efficiency about 50%. We enhanced the fraction of events with X-rays on tape to 25% with a trigger selecting isolated conversion points in the XDC gas. In the data set with two tracks of charged particles (2 prong) we have a fraction of P state annihilations of 59 (5)% (p-GAP data). This data set is scanned for low-energy X-rays leading to a reduced data set (~-LX data) with a fraction of P state annihilations of 92.5 ( 1.0)% [23 ]. For 4-prong data the fraction of P state annihilations is 61 (4)% and 86(6)%, respectively [24]. The reactions ~ p ~ 0n°--, K+ K - n ° ,

(la)

~p~0n~K+K-rl; (q-.neutrals),

(lb)

fgp~On+n-~K+K-n+n - ,

(lc)

pp~On+n-n°--,K+K-n+n-n ° ,

(ld)

were investigated using the K + K - decay mode of the 0 meson ~1. The final state is identified by kinematics fitting to the K + K - n °, K + K - q , K + K - n + n - , K + K - n + n - n ° final state and by d E / d x sampling. Events are selected for which the d E / d x probability at This letter is restricted to a discussion of ~ production. The full analysis coveringother aspects (like K* production) will be presented in a forthcomingpublication.

Volume 267, number 2

PHYSICS LETTERS B

for the presence of two kaons is larger than the d E / dx probability for the all-pion hypotheses. In the case of four charged particles the largest (of four) d E / d x probabilities is used to define the correct configuration. It was checked that rejected events contain no visible (~ nor K* signal and accepted events show no K ° signal. But the data on reaction ( l d ) is still contaminated by K -+K°n ~ nn ° events. In order to reduce this background we demand that the d E / d x probability for the one kaon and three pion hypothesis is smaller by a factor of two than the probability for the two kaon and two pion hypothesis. The factor of two gives the best background rejection without significant losses of ¢ mesons. The data sets with 2 and 4 prongs are each split into two subsamples. For reactions ( l a ) , ( l b ) we required in the trigger two hits in any of the chambers C1, C2, Q2, P1; for reactions ( l c ) , ( l d ) four hits leading to 3.6× l 0 6 and 9 . 0 × 1 0 6 events, respectively. The reconstruction program yields 2.4X 106 two-prong and 5.6× 106 four-prong events with a common vertex and with d E / d x information for all tracks. For reactions ( 1a), ( 1b) with relatively high particle momenta we asked for two long tracks (reaching Q3). This reduces the two-prong data sample to 1.2 X 106. The momenta in the four-prong data set are lower on average and we took the full data sample of 5.6 × 106. Fig. 1 shows the K + K - invariant mass spectra for reactions ( 1a ) - ( I d) and for the p-GAP and 1)-LX data sets. An expanded view of the K + K - invariant mass in the ¢ mass region is presented as insets. The spectra for the ~-GAP data sample are shown in the left part, for the p-LX data in the right part of the i figures. The spectra were fitted with a gaussian and a phenomenological background function. The fitted mass of the ¢ was found to lie in the range from 1018.2 MeV (fig. l a ' ) to 1019.9 MeV (fig. la). The observed width of the ¢ is typically 10 MeV ( F W H M ) . The number o f ¢ in each spectrum is given in the fig-

12 September 1991

Entries/SMeV f

Entries/SMeV ~oo

s

qbrf°

0 |O0

50

b

54(s) i

1000 r

,

+

, ,

T

1400 1

+

f l~50 O0

•

60

0

0

200

80

,

Cn.',..n.- fro

I00

40~

0

d Fig. 1. K + K - invariant mass spectra for 13p--*K+K-n° ( l a ) , Ib K+K-I1 ( l b ) , K + K - n + n - ( l c ) , K + K - n + n - n ° ( l d ) . The spectra on the left side are p-GAP data ( ( a ) - ( d ) ) , the spectra on the right side 15-LX data ( ( a ' ) - ( d ' ) ). Expanded views of the K + K mass spectra in the ¢ mass region are shown as insets. The mass scale in the insets ranges from 0.98 to 1.08 G e V / c 2. The number of C-mesons resulting from fits is given in the figures (see text).

0 0.9:

.gB d"

kJs -O:!* 0 1.750,96 1.35

m(k*K-)/OeV

1°7

1.35

1.75

m(,K"K-)/GeV 301

Volume 267, number 2

PHYSICS LETTERS B

ures. The second peak in figs. ld, ld' is due to o~K+K - phase space events. We determine the branching ratio for the reaction pp~C~X by BR(OX) = N, xdref B R ( r e f ) ,

12 September 1991

o

c~ ~q q) tLaJ

(2)

/Vreft/,X

where Nref is the number of events in a reference channel, N~x the number of 9 mesons in the reaction and dre f and d~x the corresponding detection efficiencies. The detection efficiencies are determined by a Monte Carlo program which takes into account the geometry of the detector, multiple scattering and energy loss in the chambers, the detection efficiencies of the chambers C and Q and the measured resolution for reconstructed space points in the XDC [ 19 ]. For the 2 prong data we use the annihilation p p ~ n + n - n ° as reference channel, with B R ( r e f ) = 5 . 2 ( 4 ) % and 4.9(5)% for lb-GAP and p-LX data, respectively. Pions are more energetic and the decay probability is smaller, hence the detection efficiency for the n+n-r~ ° final state is larger by a factor of 1.6 than for the reaction ( I a) with d~o = 13 ( 1.3)%. The branching ratios into ~+~-~o for our 2 track data sample were determined in ref. [23 ]. In the p-GAP data set we observe a clear ~ peak of 131 (26) ~ mesons. These are due to reaction pp--,~Tt°; the most likely background would be ~p--, Cn°Tt°. Monte Carlo simulations show that this background is negligible. The O-LX data set consists of a fraction of 24% of the p-GAP data set. Hence we might expect 31 (6) ¢~no events in the lb-LX data set. Fig. la' shows that this is evidently not the case. From the number of ~ mesons and (2) we determine the branching ratios B R ( g n ° ) = 1 9 ( 5 ) × 1 0 -5 , ~ - G A P d a t a ,

(3a)

BR(~n °) = 3(3) × 10 -5 , O-LX data.

(3b)

These numbers are corrected for undetected decay modes of the ~. The branching ratio o f I ~ p ~ r ~ ° is obviously much smaller from P states. This is confirmed in the ~--,K+K - decay angular distribution (fig. 2) which is proportional to s i n 2 0 ~ as expected for annihilations from the 3S~ initial state. Oi~ is defined as the angle of the fast kaon with respect to the momentum in the K + K - rest frame. Annihilations from the ~P~ state would lead to a uniform an302

¢osO~ Fig. 2. The O~ K + K - decay angular distribution in the reaction l~p~On° which includes the K + K - n ° events in the mass range 1009 < rn (K + K - ) < 1019 MeV/c 2. Or,x is the angle of the fast kaon with respect to the ¢ momentum in the K * K - rest frame. The data are represented by dots (with error bars), the histograms are results of a fit described in the text.

gular distribution. The enhancement at small values of Oi~ is due to the background under the ~ peak from two fast pions. The reaction p p - , ~rl, 11--,neutral particles, is treated in the same way. The background under the ~ is estimated by fitting to the hypotheses ~ p - ~ K + K - " r l '' with "11" masses of 450 and 650 MeV. The detection efficiency for the reaction Op--,~rl (rl~neutrals) is 8.5 (9)% which gives the branching ratios B R ( ~ q ) = 3 . 6 ( 1 . 0 ) × 1 0 -5 , O-GAPdata,

(4a)

B R ( ~ r l ) = 4 . 3 ( 2 . 2 ) × 1 0 -5 , p - L X d a t a .

(4b)

The branching ratios for ~n+n- and ~n+n-no are determined by using (2) and 4-prong annihilations as reference [ 24 ]. Monte Carlo simulations give a detection efficiency of dref---8.2(8)% for 4 prong events. This detection efficiency is the weighted mean for events with zero, one, two or three missing n ° events. Their fractional contributions are taken from ref. [25 ]. The Monte Carlo detection efficiencies for final states containing ~ mesons are smaller. There are several reasons for this reduction of the detection efficiency: ~ mesons decay into slow kaons, and kaons with momenta below 100 MeV curl in the magnetic field or they stop and decay giving a secondary pion or muon. These events are rejected by the tracking program. -

Volume 267, number 2

PHYSICS LETTERS B

In-flight decays of kaons give a kink in the track which may therefore be lost. At low momenta multiple scattering can result in badly defined tracks not accepted in the fitting. These losses are also described by Monte Carlo simulations but the losses could be different for real and Monte Carlo data. However, from kinematics fitting we find nearly identical probability distributions for Monte Carlo and real data with differences in the fraction of accepted/rejected events always smaller than 5%. This fact proves that our Monte Carlo program reproduces the performances of the detector very well, but we cannot exclude differences between Monte Carlo and real data also for pattern recognition and for tracking at this 5% level and we assign an error of 10% to the detection efficiencies d,x and dre r. We find detection efficiencies of

12 September 1991

120

-

40

-

d0~÷~_ = 3 6 ( 4 ) × 10 -3 ,

(5a)

d ~ = 2 9 ( 3 ) X 10 -3 ,

(5b)

d,p=18(2)Xl0 -3,

(5c)

d,0,=8(2) × 10 -3

(5d)

•

" ~ 60 ©

2

--

~

C ^-,

I

~75

>

550

m (-n+Tr-)/MeV

825

B R ( ¢ r t + x - ) = 5 4 ( 1 0 ) X I 0 -5,

~-OAPdata,

(6a)

B R ( 0 n + n - ) = 7 7 ( 1 7 ) × 1 0 -5,

p-LXdata.

(6b)

Fig. 3a shows the n + n - invariant mass spectrum and fig. 3b the On -+ invariant mass (two entries per event). Fig. 3. n+n - ( a ) , ~ x -+ (b) invariant mass spectra and Tt+n- (c), II~ K + K - (d) decay angular distribution and the angle between the decay planes of the 0 and the x+~ - system (e) for f~p---,¢x+n-. The spectra on the left side are ~-GAP data ( ( a ) - ( e ) ) , the spectra on the right side lb-LX data ( ( a ' ) - ( e' ) ). The data are represented by dots (with error bars), the histograms are results of a fit described in the text. Or.x (O,.) is the angle of the fast kaon (pion) with respect to the ~ momentum in the K+K - (n+n-) rest frame. In ( b ' ) (dashed line) we also show a possible contribution from the reaction PP--,bl ( 1235 )n, b l ( 1235 ) -~0x.

sso

825

m(Tx÷#-)/MeV

14o

~ 70 .~_ -~ "'

[

I

t I

I

I

I

I

~

_.

~1'5om (¢frl..-~o/M 1 7 ~ o .eV ,_) 200

d

Pl~om (,.rrt&5)/M evO

175o

8o

C

~.~ I00

The detection efficiencies for Cq and ¢co refer to their n + n - n ° decay mode. They do not yet include the decay branching ratio into n + n - n °. The fractional error of d,o, is larger in order to allow a larger error of the pattern recognition for very low momenta particles. From the number of O mesons, the detection efficiencies ( 5a ) and (5c) and from the 0p ° and ¢Tt+n - phase space contributions determined below we derive ~ production branching ratios:

i~s

C

I

40

c i,i

°o

0

0,25 0.5 0.75 COSG~

~ 200 c5

llll]llllllllllllll 0.25 0.5 0.75 COSQ,~

d

d,

~o c

100

-

+

¢o

t.J

÷

o

00

0.25 0.5 0.75 COSGttK

0.25

0,S

0,75

cosO,x

,_.200

e" .~ ~ 100 c t.d

0

45 (DNN

go

oollllllllJtlllllll 45

90

303

Volume 267, number 2

PHYSICSLETTERSB

The n + n - and K + K - decay angular distribution and the distribution of the angle ~NN between the two normals to the decay planes of COand n + n - system are presented in figs. 3c, 3d, 3e. The spectra 3a, 3a' are obtained in the following way: for all events in the K + K - n + n - data sample falling into a given n + n invariant mass bin the K+K - invariant mass spectrum is fitted and the number of CO'sis determined. This number is then shown in figs. 3a, 3a' as a function of the n + n - invariant mass. The other spectra of fig. 3 are obtained analogously. By construction these spectra contain no background due to non-0 events. Note that the same type of background subtraction was applied in the experiment which reported the existence of the C (1480) meson [ 26 ]. Production of 0p ° intermediate state is important in both data sets. We do not observe any evidence for the C (1480) in our data, and we quote an upper limit of 3X 10 -s (95% CL) for the combined branching ratio O p e C -+(1480)n ~ with C + (1480)~COn -+. At small On+ masses a shoulder is observed which is more prominent in the p-LX data (fig. 3b' ). Its mass suggests that the enhancement may be due to bl(1235)-~0n decays [27]. It must, however, be stressed that the angular distribution of copOproduction from the 3pl state also produces such a shoulder. In a first step we fitted simultaneously the spectra of fig. 3 with amplitudes from O p - - } O n + n - , 0p ° and bl(1235)n ( b l ( 1 2 3 5 ) ~ 0 n ) . Because of the limited statistics we do not take into account interference between different channels. Annihilations into 0p ° are allowed from the 2 S + ~ L s = ~ S o , 3po, 3p,, 3p2 initial states, those into b~ ( 1235)n from all but the ~So state. Phase space contributions come from all initial states. The P-wave contribution in the p-GAP data set is parameterized by the p-LX data set (which is dominantly P-wave) and amplitudes describing S-wave annihilation. The fit yields a P-wave contribution of 61 (7)% in agreement with the result obtained previously [28 ]. The p-LX data set is fitted by subtracting the S-wave contribution. The fit describes the data rather well but in order to limit the number of free parameters we set to zero all parameters which are compatible with zero in the first fit. The bl ( 1235 ) is then not required although we note that without this contribution the fit of the COnmass spectrum 3b, 3b' is only fair. The n + n - and K+K - decay angular distributions 304

12 September 1991

show good agreement with the fit. The two angular distributions should be identical for 0p ° production. The strong decrease at cos OKK= + 1 in figs. 3d, 3d' is due to losses of slow kaons. That both the n + n and K+K - angular distributions can be described simultaneously confirms that the kaon losses are reasonably well described by the Monte Carlo simulations. The fit gives not only the total 0p ° branching ratio but also the contributions from S- and P-wave annihilations for the p-GAP and p-LX data: BR(0p °) = 16(5) × 10 -5 , BR(COp°)=8(3)X10 -5 ,

S-wave, p-GAP data, (7a) S-wave, p-LX data, (7b)

BR(COp°)=I8(6)×10 - 5 ,

P-wave, p-GAP data,

(7c) BR(0p ° ) = 3 6 ( 1 1 ) × 10 -5 ,

P-wave, ~-LX data. (7d)

For the phase space we obtain BR(

(COn+n -

)PS) = 5 (3) × 10 -5 ,

S-wave, O-GAP data, BR(

(On+n -

(8a)

)PS) = 3(2) × 10 -5 ,

S-wave, p-LX data,

(8b)

BR((con+n-)ps) = 1 5 ( 6 ) p × 10 -5 , P-wave, p-GAP data, BR(

(On+n -

(8c)

)PS) = 3 0 ( 1 2 ) × 10 -5 ,

P-wave, ~-LX data.

(8d)

Next we consider reaction ( l d ) . The n + n - n ° recoiling against the 0 meson is shown in fig. 4. The solid line corresponds to a fit of the two dimensional scatterplot in which the K+K - mass is plotted against the n + n - n ° mass. The 0B and cococontributions saturate the total 0 production rate but there is background from K e K ° n ~ n n °, K + K - p ° n °, K+K-rl, K + K-co contributions. The fit gives B R ( 0 o ~ ) = 3 0 ( l l ) × 1 0 -5 , p - G A P d a t a ,

(9a)

BR(COc0)=42(14)×10 -5 , O-LXdata.

(9b)

The branching ratios of CO'qin the 4-prong data sample are given by

Volume 267, n u m b e r 2

PHYSICS LETTERS B

Entries/lOMeV a ~(785)

300

80

Entries/lOMeV O' 60(783)

200

100

-

o,

,oo

(548)

600

(548)

8oo

m(Tr*rr-R°)/MeV

,

oo

8oo

L,

8oo

m(Tr+rr-TxO)/MeV

Fig. 4. The n + n - n ° system recoiling against the K + K - pair in 0 p - ~ K + K - n + n - n °. The spectrum on the left side gives p-GAP data (a), the spectrum on the right side 0-LX data ( a ' ) .

B R ( ¢ r l ) = 4 . 3 ( 2 . 2 ) X 1 0 -5 , ~ - G A P d a t a ,

(10a)

B R ( ¢ ~ q ) = 3 . 8 ( 2 . 4 ) × 1 0 -5 , ~ - L X d a t a .

(10b)

The results for 0q production with 1] decaying into neutral particles or n+n ÷ n o are consistent. From now on we use the mean values B R ( ¢ r l ) = 3 . 7 ( 0 . 9 ) × 1 0 -5 , ~ - G A P d a t a ,

(lla)

BR(¢'q)=4.1(1.6)×I0 -5,

(lib)

~-LXdata.

We summarize the branching ratios for ¢ production in table 1. Our results can be compared with those of other experiments. Those results were all obtained by stopping antiprotons in liquid H2 with a fraction of P-wave annihilation of 8.5 ( 1.5 ) % [ 29 ]. The process pp--. ~n ° has been searched for in the inclusive n o momentum spectrum [30] with two associated charged particles. A two standard deviation ~ peak was observed from which a branching ratio of 30(15) × 10 -5 was derived. The channel 0rl has not been observed earlier with an upper limit of

12 September 1991

< 2 8 0 × 10 -5 [31 ] in liquid hydrogen while Cto has been observed before with a ratio of 6 3 ( 2 3 ) × 10 -5 [32]. For Qn+n - a branching ratio of 46(9) × 10 -5 was given in ref. [32]. The CpO was not determined at all. Fig. 5 shows the branching ratios as function of the P-wave contribution of the data. Branching ratios for S- and P-wave annihilations were determined by straight line fits. Errors of the branching ratios and of the P-wave contributions were taken into account. We imposed that all branching ratios were positive. The quantum numbers 2S+~Lj of the ~p atomic states which contribute to S- or P-wave annihilations in a final state are given by selection rules. Annihilations into ~pO and ~to from S-wave proceed via the 1So state, from the P-wave via one of the states 3po, 3P 1 o r 3P 2. We do not quote individual results for i3p~Op ° from 3po, 3P 1 and 3P 2 but note that the 3P l state gives the largest contribution. Annihilations into Cn° or ~rl proceed via the 3S, or 'P, state of the Op atom. The results of the fit are given in table 2. In this table we list the pp initial states in the spectroscopic notation 2s+ 'L] and by giving the quantum numbers I a ( J Pc) and the quasi-two-body branching ratios for 0 and to production. Annihilation into (on° has been observed in ref. [ 30 ]. The branching ratio into ton was derived in two experiments [30,34] with the results 460(140) X 10 -5 and 1040(+1°°) × 10 -5, respectively. The difference between the experiments can be traced back to different fractions of p°l] and qto assigned to a p/ to peak. Ref. [ 30 ] gives a p°r1branching ratio incompatible with other data [ 33 ] and therefore we use the data of ref. [34]. Branching ratios for annihilation into ton+n - and o)p° have been measured in a bubble chamber experiment [32] and in this experiment [28 ]. We also give the branching ratios for I3n--,tonand pn--,~n- [4] for comparison. We determine the ratios Rx for production of ~X

Table 1 Summary of branching ratios determined in this analysis for 0-GAP and 0-LX data (PS = phasespace). The branching ratios are given in units of 10 -5 .

GAP LX

(gnn),o,

(¢n~)~s

(¢nn)~s

(~po),o,

(gP°)s

(gP°)P

¢~

Cn

Cz°

54(10) 77(17)

5(3) 3(2)

15(6) 30(12)

34(8) 44(12)

16(5) 8(3)

18(6) 36(11)

30(11) 42(14)

3.7(0.9) 4.1(1.6)

19(5) 3(3) 305

Volume 267, number 2

PHYSICS LETTERS B

BR / 10-~

BR / 10 -~

to

BR / 10 -~

BR / 10 -~

b

oo I - O

Cn o

d ~7

5O ~ . . . . . . . ~ . . .

501--

O0

12 September 1991

05

S

10~ P

0.5 S

,

0

P

, 0.5

i

i

,

S

0

P

0.'5

S

P

Fig. 5. Extrapolation to pure S- and P-wave for the branching ratios of: (a) Cn° ( p p ~ K + K - x ° ) , (b) Crl (I~p~K+K-~), (c) Ox+n (solid line) and ¢pO (dashed line) ( l ~ p . K + K - x + x - ), and (d) t~to ( ~ p - - , K + K - n + x - x ° ) . Solid symbols: this experiment, open squares: ref. [30], open triangles: ref. [32]. Table 2 Branching ratios for production of~ and to mesons in ~p annihilations from S and P states of antiprotonic hydrogen. 3P] and J+ + stand for the sum of 3Po, 3P 1 and aP2. The references refer to to production. Initial state

BR (1)p--+0X ) / 10- s

BR (pp ~ toX ) / 10- 4

Ref.

3St=l+(1-- ) 3S~ = 1÷ ( 1- - ) 3S~=0-(1--) tSo= 1 - ( 0 - + ) ~So=0÷ (0 - + ) lbp (S wave)

BR(¢no) = 4 0 ( 8 ) BR(¢~q) = 3.0(3.9) BR(t~p°) = 34(10) BR(t~to) = 53 (22) BR(¢n+n - ) =47( 11 )

BR(tono) = 52(5) a) BR(p°~) = 4 6 ( 5 ) BR(to~)= 104(10) a) BR(cop°) = 191 (37) BR(toto) = 140(60) a) BR(ton+rc - ) =655(68)

[30] [33] [34] [28] [35] [28 ]

~P~=I+(1 + - ) ~P~=0-(I + - ) 3P t= l - ( J + + ) 3Pj=0+(J++) I~P (P wave)

BR(~n°) = 0 ( 3 ) BR(~q)=4.2(2.0) BR(C~p°) = 3 7 ( 9 ) BR(¢to) = 2 9 ( 1 4 ) BR(¢n+n - ) =66(15)

BR(p°rl) = 9.4(5.3)

[33]

BR(top°) =638(130)

[28]

BR (ton+n - ) =705(105)

[28]

l)n

B R ( g n - ) = 5 1 (12) b)

B R ( t o n - ) = 6 0 ( 1 5 ) b)

[4]

a) The branching ratio refers to mixture of 91.5% S-wave and 8.5% P-wave. b) The fraction of S-wave and P-wave is not known, see ref. [29].

and coX from the data of table 2 for S- and P-wave annihilations. For some data (cort°, coq and coco) branching ratios are only known for antiprotons stopping in liquid H2. In these cases we determine the corresponding ¢ production rates for 91.5% Swave and 8.5% P-wave, and list the data under S-wave annihilations. The ratio 2coco/¢cois calculated to account for the factor ½for two identical bosons. We notice that the SU (3) isoscalar coefficients for pp annihilation into cort° and pO~ are the same. This fact allows us to use the suppression of Cno production from P states in a quantitative way: In the comparison of annihilation into core° and pO~qa correction

306

by a factor 2 for the sg component of the q wave function needs to be applied. The factor -~corresponds to a pseudoscalar mixing angle of - 19.5 ° [ 36 ]. For this comparison we use BR(p°rl) from the 3S~ state of 4 6 ( 5 ) X 1 0 -4 and BR(p°q) from ~P~ state of 9.4(5.3) × 10 -4 [33], respectively. For S-wave annihilations the ratios con°/¢n° and 3p°rl/¢n° give compatible results. We do not observe Op__,¢no from P states and we only find a lower limit for 3p°rl/On° of 47 (27) × 10-5 produced from P states. The error of this limit reflects the uncertainty in the p°rI branching ratio. The phase space for ¢ or co production is different

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Table 3 Ratios of branching ratios of (toX)/( CX ) for S- and P-wave.

~pO

COna)

Cnn

~o

¢pO

¢~

139(36) 107(29)

52(15)

56(20) 172(55) 8(3) 91(29) 54(19) 164(53)

> 160 ¢)

14(3)

> 71 c~

9(2)

> 173 c)

14(3)

~X_~

R~ R~

fpR~ fpR~ fvR~ fvR~

2O~a )

8(2) 50(14)

17(4) >47(27) ¢) 16(4) >46(26) c) 20(5) >56(32) c)

12(4) 9(3) 12(4)

a) The ratio refers to mixture of 91.5% S-wave and 8.5% P-wave. b) The fraction of S-wave and P-wave is not known, see ref. [29]. ¢) The lower la-limit is given.

and therefore the m e a s u r e d ratio o f branching ratios m a y need to be corrected. The two b o d y phase space is given by the decay m o m e n t u m q. At small q values the a m p l i t u d e for a decay into two mesons a and b is p r o p o r t i o n a l to qt giving a total phase space factor fp=q2~+~ for the branching ratio, where l is the angular m o m e n t u m between the two mesons.fp reduces drastically the ratios for pp annihilations into highmass mesons. Vandermeulen [37 ] has i n t r o d u c e d a rather successful m o d e l for PP a n n i h i l a t i o n into 2 mesons assuming d o m i n a n c e o f a n n i h i l a t i o n channels with small m o m e n t u m transfers from the PP system to the mesons. He uses a scaling factor fv=qexp[A(S--Sab)t/2]; S is the invariant mass squared o f the Op system a n d sab= ( m a + mb):. By fitting the data A = - 1.2 is derived. F o r quasi-two-body annihilations we give in table 3 the uncorrected ratios o f branching ratios a n d the ratios corrected by the phase space (fp) and V a n d e r m e u l e n scaling factors (fv), respectively. Corrected by the phase space factor q:t+l, the ratios o f the branching ratios show an extremely large violation o f the O Z I rule for all a n n i h i l a t i o n s for which l = 1. However, this phase space factor is unrealistic [33,37]. The f a c t o r f v modifies the double ratio only slightly. Therefore we use the m e a s u r e d branching ratios for our further discussion. We observe violation o f the O Z I rule in all processes but at two rather different levels. O Z I rule violation is significant for all channels, a n d the o~/0 ratio is about 100 for all but the ~rc channel. F o r 0 p - - , ~ t ° and for 0 n ~ 0 x - the violation o f the O Z I rule is dramatic with an m/O ratio o f about 10.

I f we accept the argument that the branching ratio for ton ° and pOq are related, then this strong O Z I rule violation is restricted to the 3S1 state o f the PP and On systems annihilating into ~n which needs, o f course, c o n f i r m a t i o n in future experiments. This selectivity in favour o f one initial state is easily " e x p l a i n e d " if one assumes that this state mixes with a sgqq state having the same q u a n t u m numbers and a mass close to 2Mp [4]. The weaker O Z I rule violation observed in other channels m a y then be due to a sg c o m p o n e n t o f the proton or due to resonant KI~ interactions. We thank the L E A R crew for their support during the runs. This research was s u p p o r t e d in part by the Deutsches B u n d e s m i n i s t e r i u m f'tir Forschung und Technologie, the Institut N a t i o n a l de Physique Nucl6aire des Particules, the Schweizer Nationalfonds, the Osterreichischer N a t i o n a l f o n d s and the Natural Sciences and Engineering Research Council o f Canada.

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