- Email: [email protected]
Protonium annihilation into KL0KL0π0 and KL0KS0η
Ph.',s,cs Letters B 319 (1993) 373-38(I Norlh-I Iolland
PHYSICS LETTERS B
P r o t o n i u m a n n i h i l a t i o n into wowo,,.o • "L"S" and
0 0q KLK.s
Crystal Barrel Collaboration
C. Amsler m, D.S. Armstrong a. I. Augustin sl . C.A. Baker d. B.M. Barnett ), C.J. Batty d. K. Beuchert b. P. Birien a. P. Bliim L R. Bossingham a. K. Braune k, J. BroseJ. D.V. B u g g h. M. B u r c h e l l c.2. T. C a s e a. A. C o o p e r h, K . M . C r o w e a, T. D e g e n e r t,. H . P . D i e t z k.t0. S. y o n D o m b r o w s k i m. M. D o s e r c. W. D i i n n w e b e r I', D. E n g e l h a r d t L M. E n g l e r t k M.A. F a e s s l e r k. C. Felix k. G, Folger k.3. R. Glantz f, R. ! [ a c k m a n n ). R . P . H a d d o c k '. F . H . H e i n s i u s f, N . P . H e s s e y e. P. H i d a s c. P. l l l i n g e r I'. D. J a m n i k ~''t. Z. Jfivorfi c H. K a l i n o w s k y J , B. K t i m m l e f. T . Kiel f. J. K i s i c l k.5 E. K l e m p t ). M. K o b e l c. H. K o c h b. C. K o l o k. K. K o n i g s m a n n k'6. M. K u n ? c b. R. L a n d u a L J. L i a d e m a n n b. I[. M a t t h i i y h. M. M e r k e l J'3. J.P. M e r l o ) . C . A . M e y e r m.:. L. M o n t a n e t ¢. A. N o b l e m F. O u l d - S a a d a m. K. P e t e r s h
('.N. Pinder d. G. Pinter c. S. Ravndal b. J. Salk h. A.H. Sanjari h's, E. Schiifer~, B. Schmid m'~ P. Schmidt f. S. Span ier). (7. St raBburgerJ, U. Strohbusch f. M. Suffert t , D. l.}rner m. C. Volcker t, F. Walter), D. Walther b. t, I. Wiedner f. N. Wintcrg. B. Zou h. J. Zoll ~ a n d ('7. Zupan~.i~ k • ('m~('r~t.~ ~f('ahfi~rma. I.BI.. Berk('le.v. ('..I 94"20. ~'X.I b ('mverwtat B~-hum. 1)-447,~¢0 B~ahum. Germany .4( adem.v o t Scwnce. !1-1525 Budal~'St. llungarr d Rutherford Apph'ton Lal~rator 3. ('briton. Ihdcot O X I I OQX. I'K ¢ ('ERN. (71-1211 Gcneve. Swttzcrland f ~'ntversttiJt llamhurg. 1)-22"61 llumt,urg. (h'rmanv ('m~erwtdt Kurl.sruhe. D . ' 6 3 4 4 harlsruhc. (h'rman)" h Queen lfarv and Westficld ('olleg. e. L , m h m !; 1 4VS. ( ' K ' Untrers~tv of('ahforn~a. ~ ..Ineclcs. C I 90024. ('X.I J I m~ersttiit Matn2. l)-53099 .$fatnz. (h'rmunr k Umter~ttat Mi, nchen. D-85"4,~' Mi, nchen. German.v t Centre tie Recherche~ .Vucli'utrc~. 1'-6"03" StraM'our.e. Frantv m I "mverwtdt Zurwh. ('!1.1¢001 /urtch. S~ttzerland
Recep,'ed 28 October 1993 Editor. L. Montanet
k.o "'s" A 0 _ 0 and A.d "0 was m~cshgated v.lth the ('Dstal Barrel detector at LEAR. The anmhilat,on of,~p at rest into "'t t hsq "lhc,s¢ final states v,lth ncgattsc ('-panty are dommatcd b~, o and K°" resonances. I hc rahos of the branching rahos are B(~p~¢,n°)/B{~p ,oq) = 8.3t 2.1 andB(fip .KOK"u" + - K - r ' K n ' - - K ~ K• ~ n ° U B ~ p ~ o n ° . k t oA'o -, I. - s . n o~, = ..04::1:0..
' Nov."at University, ofSiegen. German.~. -~ Now at Unr.crslt) of Kent• ('anterbu O. [;K. Nov, at CERN. (.;en~'se. Swm, erland. "~ ()n Ica~e of absence from the [,mvcrslty of Ljubljana. I.jubljana, Slovcma. 5 ()n leave of abscncc from Ihc [ [rex cr~lt.~ of Stlc~m. Katoy,ice. Poland.
6
Now at Max Planck Inst,lut. Iletdelberg. Gcrman.~. Now at Carncg,c Mcllon [;mvcrs~t). Pittsburgh. PA. lWS.~ Now at Stale Umvcrslt} of Nov. York. Ston~., Brook. NY. USA. '~ Nov, at Umvcrstt~, of Carhforma. I~me. ('A. USA. Jo This ~ork is part of thc Ph D thesis of H.P. Dtctz.
0370-2693/93/$06.0(I 'i:.)1993-Else,,acr~acncc Pubhshcrs B.V. All rights rt'scr,.cd ~hl)l 0370-2693( 93 Ik 14¢,5-'t
373
Volume 319. number 1.2.3
PHYSICS I.ETTERS B
The stud) of tip annih,lation into final states w,th open or h,dden strangeness ,s of special interest. Since. accord,ng to the Na,ve Quark Model [ I ]. the .~ and p contain only fi-fd and uud const,tuent quarks, the s and ~ quarks in the final-state mesons must be created dur, ng the reaction. Alternat,vel.,,, ,n a less naive model, they can be included as ~'s sea quark pairs ,n the wave function of the ~ and p. Thus. one question behind the special interest is how much .is pairs contribute to the (ant,)baD'on wave function at hadronic scales relevant to annihilation at rest (low Q : ) [21. A related quest,on ,s how well the OZI rule [3] applies to specific channels such as ~ p ~ V m ° or ~p--epn°n °. Indeed. the large size of the branching rat,o -~p--a~n ° [4.5]. ~h,ch seems to violate the OZ! rule qmte strongly, has been considered to rod,care either a large amount of ~s ,n the proton wave function [2]. or the existence of a qqss cDpto-cxotic intermediate state [6.7] or. more mundanel), finalstate interaction between K ° and X~ produced via an OZl-allowed intermediate state [8]. Two channels wh,ch have never been studied before arc
--
"0
,O
O
(1)
~p-x°,. K.°,l.
(2)
pp--Kt.Ksn and
In the present experiment the) are detected via the "zero-prong" final states where onl} photons are detected, i.e. via the K ° - - 2 n ° - - 4 ) ' and q-~27 decay modes. The K ° escaping detectton with a probabflit)' of about 50% is ident,fied by missing mass. We l-0 i/0_0 wdl show that ,.l,..sn is dominated by the twobody intermediate states K°'X~u. ~ ' K ° and ,bn°. and K~JK°q is dominated by ,pq. The latter has not been measured before in hquid hydrogen, but the K°-K°'+-K-°K°" and ,=,no branching ratms are alread)' fairly well known from charged final stales [4.5.9.10]. The measurement of absolute branching ratios reqmrcs a precise knowledge of the interact,on probabdit~ of K ° m the calorimeter. An analysis is in progress to de/ermine this quantit~ and wdl Ix" published in a forthcoming paper, together with absolute branching rat,os into ~arious channels containing K°'s. The present investigation is restricted to obtain374
23 Dc~'cmtx'r 1q93
mg relative branching rallos of K°K-u" + X~IK°'. ,',n°. and ¢,q with small stat,stical and systematic errors. The CDstal Barrel detector [ I I ] has been .set up at the Los,,-Energ.~ Ant,proton Ring (LEAR) at CERN to investigate pp annihilations at rest and up to about 2 GeV/c i n e o m m g ~ momentum. It has been designed for high-statist,cs measurements of almost all possible final states and has access to man) channels which have not been investigated before. For channels which are already known it can reduce the present statisucal and s)stemattc uncertainties. The detector co~crs 98% of the full solid angle and is scnsit,~e both to charged part=ties and photons. Charged particles are observed and measured m two proportmnal w,re chambers ( PW('s ) and a t.) hndrical jet drift chamber [JD('). Photons. for example those produced in the decays of n ° - - 7 7 , r/ "77. etc.. are measured in the electromagnetic (e.m.) calorimeter ~hich surrounds the drift chamber. As the results presented here primarily depend on the photon calorimeter. ~e briefl) describe its relevant characteristics. A detailed description of the detector can be found elsewhere [I 1 ]. The calorimeter consists of 1380 CsI(TI ) cr)stals. 16 radiation lengths thick, read out by photod,t.~les. It covers polar angles # between 12 ° and 168 ° and has complete coverage m az,muth. Photons which h,t the cr)stals closest to the acceptance holes (0 < 12 ° and 0 > 168 ° ) of the detector are rejected in the analysis (see below). This reduces the solid angle for photon detect,on, leading to an acceptance of 95% of 4n. The energy cahbration of the cD'stals ts achieved using the 2)' deca) mode of the n °. abundantly produced m ~p annihilatmns, by imposing the standard mass of 134.974 MeV/c: on the reconstructed n °. The photon cnerg) resolution is o~.
2.45%
I-
( i-,'/(;eV )t 4
for the data u ~ d m this work. The angular resolution is a = 2(1 mrad m the polar and a/mlulhal angles for isolated h,gh-energy showers. The trigger for an incident antiproton ,s provided b) a sdicon beam counter placed just before the liquidhydrogen target. This trigger is combined wtth a veto against annihdations producing charged par~tcles, usmg the on-hne reformation from the PWCs. The ~.suiting ~zcro-prong" trigger permits a large increase
Volume 319. number 1.2.3
PIIYSIf_'S I.ETrERS B
in the data-acquisition rate for a n n i h i l a t i o n s ,nto final states with only neutral particles, which represent about 4% of all ann,hilattons. In this way several milhon zero-prong events have been a c c u m u l a t e d during several running pcr,ods. The starting sample for the analysts o f the reactions ( I ) and (2) c o n t a i n s 4.5 10' zero-prong events corresponding to 107 million ~p a n n i h i l a t i o n s at rest. First, 4 • 10 ~ e v e n t s are rejected which show charged tracks in the J D C after off-line reconstruction, having escaped o n - h n e P W C detection. ( ,-% vers small P W C ,nefficiency. ;" conversions, and charged K ° decays are the causes o f these events. ) Next. all events not having exactly six well-~solated e.m. showers with a ram,m u m energy o f 20 MeV are rejected. T h i s cut r e m o v e s about 80% o f the data sample. It a l ~ reduces the detection efficiency since not ever,) photon will manilest itself as one separated e.m. sho~,er: s o m e o f the photons escape through the holes o f the detector: others merge with a n o t h e r photon or, as result o f shower fluctuations, a p p e a r as two separated energy deposits. Since a m o n g the products o f reactions ( I ) and (2). only n °. q. and K.~ are detected whde K ° ~s unobse~'ed, the events o f interest are c h a r a c t e r i r e d by missing energy. Therefore, a cut on the total energy depos~t in the c a l o r i m e t e r is i m p o s e d ~,h~ch corresponds to a w i n d o w around the range o f visible energ.~ in these reactions• O f the 8 . 8 . 10 ~ events, kept up to th~s point, all events are rejected which do not fulfill the c o n d i t i o n 6
900 MeV < Z
I-, < 1400 M e V .
lint
In this way the bulk o f background events such as .6;' is r e m o v e d , leaving 58 000 events. In o r d e r to r e m o v e further background from e v e n t s with higher photon m u l t i p h c , t i e s where one or m o r e photons are lost. events with m,ssing mom e n t u m p o i n t i n g towards one o f the detector holes are rejected. F u r t h e r m o r e we d e m a n d that an e.m. shower does not spread ,ts energ.~ a m o n g adjacent crystals in such a wa~ that the m a x i m u m energy fraction is deposited in a crystal on the b o r d e r o f one o f the holes. Applying these cuts reduces the n u m b e r o f events to 37 270.
~ p ~ 3 r t ° ~ 6 ) , or ~ p - - 2 n ° q
The qualit.~ o f the data o b t a i n e d at th~s level ~s dlustrated by fig. l a. which shows the distribution o f
23 l.K-cember 1993
-z
.,
: ,.!
,,:-
.
•L - .
t_
. L
/ [-'.
~
~-,-~'
2.,,"
. . . . . . . . . . . . .
.
i
r'l
j,
"-'"
.
/
.
•
d~.
ii.i.
eWl
Idrw
ql I
FIg. I In,.anant mass d~stribution. (a) m,ssing mass of events w,th 6 e.m. showers after a cut on the total v~s~ble energy,; (b) n°n ° mas~ after the IC k,nematic fit for events with three n °. (el and (d) ;,'}' mass after the 4C kinematic fit. the miss,ng mass squared
(
:n~,
6 =
2my-
6
2
!-',
-
linK
2
p,
,
m
where a K ° signal shows up clearl.v.
fip--K~6;' permits a one-constraint ( I C ) k i n e m a t i c fit which is s u r v i v e d by 10 305 events w~th a confidence level ( C L ) larger than 5%. At this stage there ts clear evidence for a K.° signal. This is illustrated by fig. lb which is o b t a i n e d for a subsample o f the 10305 events where the six photons c o m b i n e to give three neutral ptons: the invariant mass spectrum o f two of thc'se pions ,s shown 13 entries per event ). In the next step a 4C k i n e m a t i c fit is applied to the 10305 e~ents according to the hypothesis oJ. p- - p - - K o t0 K.,,.. }.
(K.O~ .n°n°) .
T h i s fit ~ields 4395 events with ( ' L > I%. Fig. Ic shows the invariant mass distribution o f the remaining two photons• .~ p r o m i n e n t n ° signal and a small 375
Volume 319. n u m b e r 1•2.3
P H Y S I C S I.FTTERS H
r/signal can be seen, which c o r r e s p o n d to the reactions ( I ) and ( 2 ). F m a l l ) . two 5C k,nematnc fits are applned to these events in order to .separate reactions ( I ) and ( 2 ). m a k i n g the appro~,nmatnon that K.~' decays at the pr,mar3, vertex, in o r d e r to c o m p e n s a t e for this app r o x i m a l , o n the errors on the photon d~rectnon mere appropriatel) increased nn the fit. The data are spl,t into two subsamples: 11): 2834 events wxth ( ' L > 10% for reaction ( I ) and CI° < 1% for reaction (2). (11): 72 events with C L > 10% for reaction (2) and CL < I% for reaction ( 1 ). 1 he p r o b a b i h l ) d,stribut,ons for these fits are flat abo~c 10%. showing that the quality of the final event samples is acceptable and the m e a s u r e m e n t errors on the kinematical variables are correct. It was checked by M o n t e Carlo simulat,on that the feed-through from react,on ( I ) Io 12) is 4 ± 2 e~ents while the feedthrough from (2) to ( I ) is less than one e~ent The n u m b e r o f background events ,n these t ~ o subsamples us e s t i m a t e d from the n u m b e r s o f entries m the m~ariant mass distribut,on o f the two r e m a i n , n g photons, excludnng the n o and the r/sngnals. This g,ves 120 ± 7 background e~ ents ,n the KOK.On ° subsample and 26+ 8 background events in the K°K.°~ I subsample. The K ° and K ° energ) distributions o f the events outside the n ° and ~I sngnal suggest that the background events are h o m o g e n e o u s l ) d~stributed o v e r the allowed phase space• The overall efficlenclt.'s (e) to reconstruct and identff.~ reactions ( I ) and (2) in the neutral deca) mode. a s s u m i n g no K~ interaction and including all cuts and k , n e m a l i c fits• were calculated wnlh a complete M o n t e Carlo s,mulatnon based on (;E.'~NT [ 12 ]. Theyare~ = (21.3...0.5)%forreaction(I)and+ = (22•0 ~ 2.0)% for react,on (2)• T h e i r relative ~arialion A~/+ o v e r the full phase space ns less than 5%. Figs. --"~a "~d sho~ the ^. .t o~,..n. o D a h t z plot and ,Is three projections for the event .sample II) using the m o m e n t u m values from the k i n e m a t i c fit. This Daht7 plot exhibits the follos,,ing p r o m i n e n t features: an a c c u m u l a t i o n o f events in the two K ' ( 8 9 2 ) reg,ons and a sharp peak,rig o f events for which the ( K, Ot K - 0~ ) effective mass is in the ,>(11320) region. We observe a slight as.~mmelr3, between the m 2 ( K ~ n °) and m'-(K.°n °) project,ons. This asymmetr3.' ,s due to a small systematic error in our analysis method. There is a threefold c o m b i n a t o r , a l ambiguity as to which two o f the three n°'s form the K~. The am.~7h
23 i ) c t e m b e r I~q ~,
7
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Ft 8. 2. (a) I)ahtz plot and ( b t - t d l projections for the reaction PI~ • A t. A.~n .o . I')ta.'a.~cosine distribulions (el nn the 2 ~ ~ I.I GcV21 and (f~ ,n the rt'st rest frame of ,:, ImA?A., f r a m e ot" K °" 10.7 -,:- nl- u o ~< 0 9
Ge~'I;
the p+:ak at
ALA S
I) 4 IS d u e tO the cro~xmg w i t h the o t h e r K o" b a n d
o 0 ]t-q] blgutty affects the h'°n '~ and the I,,t,,.s (but not the KUn('l in~ariant mass• This combxnatorml effect s,,as studied v.llh M o n t e Carlo simulations. F r o m this study we d e t e r m i n e d that (4.8 * 0.6%) o f the ,.',no events and (7•8 - 0•7%) o f the K.~K °" events give ~ r o n g n ' n ° comb,nations. This explains the different heights o f the two K °' peaks. In figs• 2e and 2 f t h c deca~o cos,he distribution o f o - ] tJ'' ,°l r,,]t-o ,.. and K °" .K'~n ~' are shown by selecting events with m 2 ( K-tOA ..sq ) < I.I G e V : and (1.7 < m - ' ( K 0 n aJ ) < 0.9 (;eV". The) both exh,b,t the angular d,stribut,on expected fi,r vector resonances p r o d u c e d from the ffp initml state ~S, (see belo~ ). Figs• 3a and 3b show the K['K.°PI Dalitz plol o f the
Volume 319. number 1.2.3
PHYSI( ."Y,LFI'I ERS B
Here. the symbols K ; and K,~ m e a n that the invariant mass of n r' and K'] or K~' is m the region o f the K"" ( 892 ) resonance. We have K7 = ( K '~" + X=U' I/~,'~ and K,~ = (K"" - ~ " )/~ "~. The r e m u s sign between the two terms of eq. (4) projects the part ,,,,lib zero strangeness out o f the final-state wave function cont a m i n g K~K; and K~K~. The angular distribution of the ,:,-deca.~ is gl~en b.~
i [ •i
"-
"
.
.
.
.
.
+ . . . . . .
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23 I)¢¢cmber 1993
.;
I-~g. 3 (a) l)ahtz plot and (b) projet'tton onto the K°K ° I .'~ m~,artant ma~s for the reactmn PI' -'K~'KOsq• event sample (11) and its m Z l K t K~) projection (fitted q u a n m z c s ) . The ¢, szgnal =s p r o m i n e n t . ++hile the o t h e r regions o f the Dalitz plot arc sparsel.~ populated - mainl.~ b.~ b a c k g r o u n d events. As the final state has C-parit} = I. ~t ~s r e a ~ n able to assume that the production is d o m i n a t e d b> the ~1, initial state ~S~. We fit the K~iK°n t' l)alitz plot w~th tranmtion a m p l i t u d e s o f the channels ,~p(~S~) .¢,n u and ~p{~S~) . K " ' K using the max~m u m hkelihood methtnl. An intensit.~ d l s t r i b u h o n o f the form
Z(':')
= qA
" PAA.
v, herc qA ~s the 3 - m o m e n t u m o f the h'~. in the K~K~ rest frame and PAA tS the 3 - m o m e n t u m of the A'~.K.~ system in the laborator3.. The angular distributions of the K ° ' - d e c a ~ s are described by 11 h'°')
= qA " P A . ,
where qA is the 3 - m o m e n t u m of the K~.(h'~) in the K~int'(K°~n°~ rest frame and PA, ts the 3 - m o m e n t u m o f the K~'nt'(K.°n"l s~stem m the laboratory. l ' h e relat~vtstlc Bre~l-W~gner factors for the o and the g °" resonances are !"1 m ) -
[|'(p).
II'(q)
R:
q.p
mo/o
mz-m
o + imol'(m)"
i = I,~A(~S~.,;,II020)) + ,~..A(~S~.K°'(892)~ " ~lth is assumed with c o m p l e x parameters ,t,. The transition a m p l i t u d e s A(+S~.,.',(1020)) and A(~S~. h °" (892 ~) conmst o f a function X which desertbes the decay dzstributzon of the resonances, satzsf~ ing total angular m o m e n t u m and parity eonser~ ation, and a relam, zstzc B r e i t - W i g n e r factor I . ( m ~. The decay distribution functions I are constructed accordzng to the m e t h o d d e v e l o p e d b) Z e m a c h [131. where the three scalar amplztudes correspondzng to the three magnetze substales o f the retrial J = I p r o t o m u m state are represented b~, Ih¢ three c o m p o n e n t s o f a vector a m p l i t u d e A. We have A(3S~,o(1020) ) = Z(~)
t"+m(K~]K°)) •
I'(m)
= L'~ mr. q I I " ( q ) m qo 1|'2(qo) '
~ h e r e m . . E, are the n o m | n a l mass and width o f the resonances, q = q ts the decay m o m e n t u m in the resonance rest frame (q = q,, f f , t = m0), and p = [p ts the l a b o r a t o ~ m o m e n t u m o f the resonance. The function I | ' ( q ) is the B l a t t - W e i s s k o p f d a m p i n g factor for angular m o m e n t u m t = 1 [141. (The angular m o m e n t u m is one unit between the rt.'sonances and the reeothng parttcle, as well as Ix'tween the decay particles o f the resonance. ) Therefore. I1" is
(3) l l ' ( q ) = (q
T h e a m p l i t u d e A ( ~S~. K °" (892) ) c o n t a i n s two terms. each c o r r e s p o n d i n g to an ezgenstate o f charge conjugation with (" = - I :
A(aSI.KO'1892)) = x(K~) - ~r(K.~) • k ( m ( K ~ n ) ) .
"l-'(m(K~'n))
(4)
R)
(
I +
' ) (q.Rl'-
I 2
~ i t h R = (0.197327 G c V ) -t ( = I F e r m i ) . The c o m p l e x parameters ,~, = %,[ exp(i,l,, ) are left flee m the fit. The phase of,~z ts arbitrat 3 and therefore fixed at 0. Sznec the natural width o f the ,t,resonance. ~ = 4.4 MeV, is nearl~ o f the .same magnitude as the experimental resolution a = 2.5 MeV. 377
Volume 319. number 1.2.3
PHYSI(..'S LETTERS B
23 December 1993
Table I Amphtudes and phases of the fit. Initial
Intcrnlc'diale
Stale
Slate
~,
=
~s e x p ( t , p , )
amphtude
phase '.'h = 0 o., = 1 6 4 = 0 . 1 1
3S I
C,(1020)
I"1 = 0.57 .':-0.01
~SI
K ° ' ( 892}
"2. = 0.82 + 0.01
the intensity distribution I is c o n v o l u t e d with a Gaussian curve which represents the resolution along the m: ( K ° K t . s ) ax~s. A constant intens~t) is a d d e d in order to account for the 120 background events mentioned above. The results o f the fit are given in table 1. The n o r m a l i z a t i o n is such that I,,tl: + l't:[ 2 = I. In o r d e r to test the consistency o f our analysis m c t h o d we generated a M o n t e Carlo data set o f K 0t.,,sn 1 - o _ o events with a Dalitz plot distribution according to the fit result given in table I. These M o n t e Carlo data were then anal.~sed exatal) like the real data. as described above. The spt~tra that result from this analysis are s u p e r i m p o s e d o n t o the spectra of the real data in the figs. 2a-2f. T h e a g r e e m e n t between the real data and the s i m u l a t i o n is satisfactor3. A Z"test was applied to test the c o m p a t i b i l i t y o f this simulation and the data. The X " per degree o f f r e e d o m is 1.2. Since the data are well r e p r o d u c e d b) the set of a m p l i t u d e s that we have assumed, we c o n c l u d e that •.-0 ~ ' 0 0 the final state At ^ s n is p r o d u c e d almost entirely from the ~S~ state. We do not need an) substantial c o n t r i b u t i o n from the ~P~ initial state. -% priori, the latter cannot be excluded if we assume that it feeds the resonance both via an / = 0 and an / = 2 trans~lion. H o w e v e r . it would take an ~mprobable accident o f nature for the ~Pt state to produce an angular distribution as close to sin20 as we observe it. Estimates, based on our own data. o f the probabdit~ P,,t that the Kt° does not interact or the interaction is not detected in the calorimeter, ~,,leld P,,t 50 : 10%. Th~s n u m b e r has a relatively large systematic error but it cancels in the ratios of branching ratios unless P,,t d e p e n d s on the K ° m o m e n t u m . Such a d e p e n d e n c e would show up in a systematic d e v i a t i o n between the data and the fit ~ h i c h assumes P,,, to be constant as a function o f the K°n ° invariant mass (or the K~ m o m e n t u m , alternati~ cl~ ). In fact. the data are consistent with no m o m e n t u m d e p e n d e n c e within the
378
Ratio (%) = I(lOb~
32.9J I 0 67.1 t 10
statistical errors and the relevant m o m e n t u m range. The) allow the e~tractlon o f upper limits for the mom e n t u m d e p e n d e n c e o f P,nt : these are taken into account as additional s ) s t e m a t i c unceraamty of 7.5% for the ratio B(-I~p--K"'-~ j + - ~ ' K ° ) / B ( ~ p .,pn °) and of 10% for the ratio B ( ~ i p - - , ~ n " ) / B ( ~ p - - o q ) . We define the n u m b e r ot'ort ° events as the product o f I,,al" and the n u m b e r o f detected ^t.^.s . . . , - u n u events. This )ields 893 ~n ° c~ents c o m p a r e d to 737 events if we stmpl) count the n u m b e r o f events in the o peak. The latter n u m b e r is significantly lower, firstl) because e~ents in the high mass tail o f the O-re~amance are not counted, secondl.,, because events arc lost from the peak due to wrong n ° n C ' c o m b i n a t i o n s as explained above. The ratio B lfip .K°-K °" + -~J K °" ) / B (ffp .Oft ° ) is obtained
as Ill: ->t':
Volume 319. number 1.2.3
B(fP--'~n°)
B (~p -o~/)
I)H~'SI( .'S LET I ERS B
- 8.3 Z 1.7~tax .t. 1 . 3 ~ .
'0 • "0 II B(~p--h'C~'K r~ + t--';'IJ° I~, ,t~ - - ^- -t' f l ^.,~n I B(ffp .~pn°~K°K.°n o)
= 2.04 .,._0.10~=t --r-_0.19,,~.tt . Here. the lower radices "stat" and "s~st" indicate the statistical and the systematic error. The final s~slemattc error squared of the first (second) ratio is the sum of the squares of the 10% (2%) statistical error of the efficiency, the I1)% (7.5%) uncerlaint~, concerning P,,, variattons, and the 5% (5%) systematic error mentioned last. These ratios provide a useful additional input to ~p annihilation models, in particular since we are dealing with a final state of specific ('-parity I. Th~s reduces the posstblc mittal ~p slates to j e c = I - (s-wave) and j e c = I +- (p-wave). It has been shown that only the I - - state ts needed to descrtbe the Dalitz plot m a salisfacto~ wa~. This result ~s plausible: s-wave anmhflatton ts dominant, in general (for a recent publicatton see ref. [161) and. in addition, dynamical selection rules appt'ar to favour the JP¢ = 1 - - initial state [17 I. The ~sospin ~s I for the ,;'m° channel and 0 for o~/. A complcte ~sospin analysts of the K °" K channel has been carried out m the classic paper on ~p .K-Kn by Barash et al. Iql. Thts analysis shows that in our case the population dens(t5 of the Dalite plot is independent of isospin. which therefore, cannot be determined solel.~ from • "[J • ' 0 I] studying the ^L^.sn channel. In conclusion, we obscr.'e that the branching ratios for the channels ~p -. K}~K.~, K.°K;. and ott o are nearly equal, thus confirming a previous observation that the branching ratio into On is not OZI suppressed. However. in contrast, using the branching rauo B { ~ p . , o n ) = (4.0 t 0.8) I0 -a from ref. [51 and our ratio B(,;,n~/o~ 1) we obtain B(ffp- "~1) = 0.48 + 0.17) • 10 -~. This value is as small, compared to the branching ratio B(-~p 'co~l) = (10.4 -.1.0) 10 -'~ [18] or = (15.1 • 1.2)-10 ~ [ I q ] , a s e x petted from the OZI rule. Tentative explanatmns of this phenomenon were suggested by' several authors. Furu! et al. [81 argued that the kaon from K °" decays rescatters off the directl.~ produced kaon thus forming ,,~ mesons via an OZl-allo~,ed intermediate state. The small ratio for ~ i compared to ~.~I can be
23 December 1993
explained b~, the absence of a strange meson decaying into Ktl. Howe~cr. other final states which can be reached b~, rescattering from OZl-allowed mtermedmte states should be enhanced as well - this does not seem to be the case. Stmdarl.v. a large amount of ~s in ~p cannot explain the peculiarity of the ,;,n/con ratio; it ~ould presumabl.~ stmultaneousl~, affect all branching ratios with a ¢0 meson. Perhaps. the absence of an ()Zl suppression in the particular case of ,,6n° can be explained b.,, the absence of cancellation between various intermediate states as dtscussed b.~ I.Ipkln et al. [20]. Alternatively. ('1o.~ and l.ipkin 161 and Dover and F~shbane [ 7 J associate enhanced on productmn w~th the existence o f a cr3.pto-cxot~c 4quark state. This ~ q state ~s identified by the latter authors v,llh the on resonance ( ' ( 1 4 8 0 ) [21 ]. whtch ~s suppomd to couple to the ffp system llowever, the small branching ratm into ,/al excludes the hypothetical [71 crTpto-exotlc / = 0 partner of the ( ' ( 1 4 8 0 ) m e a n . The prt.~nt s~tuation cerlainl~ asks for more data on o production. An analysis, stmilar to the one descr, bed in thts paper, is tn progress on the channel -:p .K~.K~"/which should pro~ (de interesting new information, when compared with the recentl} published branching ratio ,~p .co;' [22 ].
We would like to thank the technical staff of the LEAR machine group and of all the participating mst~tut~ons for their invaluable contributions to the success of the experiment. We acknowledge financial support from the German Bundesmimsterium fur Forschung und Technologic, the Schweizerischer Nationalfonds. the British Science and Engineering Research Council. and the [JS Department of Energ.,, (contract No. DE-FG03-87ER40323 and D E - A C 0 ~ 76SF00098). J.K. and K.M.C. acknowledge support from the A. yon Humboldt Foundation. We wish to thank K. Stcinberger and A. l.oeseh for the system management on the computer cluster in Garching.
Reference~ Ill J.J.J. Kokkeclee. The Quark Model (Benjamin. New York. 1969). [2] J Flhs. L. Gabathuler and M. Karhner. Phys. Lett. B 217 (1989) 173. 37t;
Volume 319. number 1.2.3
PI IYSI( "~iLET'TI: RS B
[3]S. Okubo, Phys. Left. 5 ~1963) 16.5. (;. Zwms. CERN Report No.8419/TII412 (1964), I. hzuka. Progr. Thcor. Phys. Suppl 37 (19660 21 [4] M. Chlha et al.. Ph.~s. Rc~. D 3X (1088} 2021. Ph~,s. Lett. B 202 l l 9 8 8 ) 447. [5] J. Reffenrothcr cl al. Ph.~s. Lett. B 267 ( 1991 ) 299 [6] F.I-. ('lose and H.J. [.,pkm. Ph~,s Roy. Left. 41 (197~;) 1263. [7] C.B. Dover and P.M. Flshbanc. Ph~,s Res. Lctt. 62 (I989) 2917 ISLS. Furu,, Z. Phys. C 4 6 11990) 621; S. Furm, G. Stub¢l, -~ Facs.,,Icr and R. Vmh Mau, Nucl. Ph)s. A 516 1'19901 643. [9J N Barash c! al.. Ph~,s. Rcs. B 139 (19651 1659. B Conforto et al. Nucl Ph,,s. B 3 (1967) 469 [lOJ :k. Belhm ct al., Nuo,,o Cimcnto 63.", q 19691 1199. [I l i E . A k c r e t a l . Nucl Instrum Methods .'~ 321 c l9921 69.
3.~1)
23 I )¢ccmbcr I '093
[ 1 2 ] R . B r u n c t al., GEAN'r3. [mcrnal Report CERN DI)!I.I-/84.1 t(.I-.RN. 198,"i. [13JCh. Zcmach. Ph.~s Rc~,. B 140 119651 97. 109. [14] J M. Blatt and V Wc,sskopf. Theoretical Nuclear Ph)slts tWlley. New York. 19521. [I.SlParticlc Data (/roup. K. H,kas.a et al.. Rc,,~cw of p a n , d e properties. Phys. Re,,. I ) 4 5 (19~2) Part II. [16]('..~,mslcr et al.. Ph.~s Lett B 297 (19921 214. [ 17 ] ( . B I.~,,er. 1. Gut~'he, M..Maru)ama and -~. Facs.sler. Prog Part Nucl. Ph~,s. 29 (19q2) 87 [18] L Adlelscl al.. Z. Ph~,s. C 42 (19891 49 [19] ('. -~m.,,Icr el al, Z Ph~,s. C 58 (1993) 17.$ [ 2 0 I H J L,pktn. Nucl. Ph.,,s. B 244 (19841 147: B 291 (19~7) 720 [21]S.I Bit.~ukm, c! al.. Ph.~s. Iclt. B 188 [19147) 383; L ( i I.andslx'rg, ,',k~s. J. Nucl Ph.~s. 55 (19921 1051. [22]('. -~mslcrctal Ph)s Lcl(. B 311 (19931 371
PHYSICS LETTERS B
P r o t o n i u m a n n i h i l a t i o n into wowo,,.o • "L"S" and
0 0q KLK.s
Crystal Barrel Collaboration
C. Amsler m, D.S. Armstrong a. I. Augustin sl . C.A. Baker d. B.M. Barnett ), C.J. Batty d. K. Beuchert b. P. Birien a. P. Bliim L R. Bossingham a. K. Braune k, J. BroseJ. D.V. B u g g h. M. B u r c h e l l c.2. T. C a s e a. A. C o o p e r h, K . M . C r o w e a, T. D e g e n e r t,. H . P . D i e t z k.t0. S. y o n D o m b r o w s k i m. M. D o s e r c. W. D i i n n w e b e r I', D. E n g e l h a r d t L M. E n g l e r t k M.A. F a e s s l e r k. C. Felix k. G, Folger k.3. R. Glantz f, R. ! [ a c k m a n n ). R . P . H a d d o c k '. F . H . H e i n s i u s f, N . P . H e s s e y e. P. H i d a s c. P. l l l i n g e r I'. D. J a m n i k ~''t. Z. Jfivorfi c H. K a l i n o w s k y J , B. K t i m m l e f. T . Kiel f. J. K i s i c l k.5 E. K l e m p t ). M. K o b e l c. H. K o c h b. C. K o l o k. K. K o n i g s m a n n k'6. M. K u n ? c b. R. L a n d u a L J. L i a d e m a n n b. I[. M a t t h i i y h. M. M e r k e l J'3. J.P. M e r l o ) . C . A . M e y e r m.:. L. M o n t a n e t ¢. A. N o b l e m F. O u l d - S a a d a m. K. P e t e r s h
('.N. Pinder d. G. Pinter c. S. Ravndal b. J. Salk h. A.H. Sanjari h's, E. Schiifer~, B. Schmid m'~ P. Schmidt f. S. Span ier). (7. St raBburgerJ, U. Strohbusch f. M. Suffert t , D. l.}rner m. C. Volcker t, F. Walter), D. Walther b. t, I. Wiedner f. N. Wintcrg. B. Zou h. J. Zoll ~ a n d ('7. Zupan~.i~ k • ('m~('r~t.~ ~f('ahfi~rma. I.BI.. Berk('le.v. ('..I 94"20. ~'X.I b ('mverwtat B~-hum. 1)-447,~¢0 B~ahum. Germany .4( adem.v o t Scwnce. !1-1525 Budal~'St. llungarr d Rutherford Apph'ton Lal~rator 3. ('briton. Ihdcot O X I I OQX. I'K ¢ ('ERN. (71-1211 Gcneve. Swttzcrland f ~'ntversttiJt llamhurg. 1)-22"61 llumt,urg. (h'rmanv ('m~erwtdt Kurl.sruhe. D . ' 6 3 4 4 harlsruhc. (h'rman)" h Queen lfarv and Westficld ('olleg. e. L , m h m !; 1 4VS. ( ' K ' Untrers~tv of('ahforn~a. ~ ..Ineclcs. C I 90024. ('X.I J I m~ersttiit Matn2. l)-53099 .$fatnz. (h'rmunr k Umter~ttat Mi, nchen. D-85"4,~' Mi, nchen. German.v t Centre tie Recherche~ .Vucli'utrc~. 1'-6"03" StraM'our.e. Frantv m I "mverwtdt Zurwh. ('!1.1¢001 /urtch. S~ttzerland
Recep,'ed 28 October 1993 Editor. L. Montanet
k.o "'s" A 0 _ 0 and A.d "0 was m~cshgated v.lth the ('Dstal Barrel detector at LEAR. The anmhilat,on of,~p at rest into "'t t hsq "lhc,s¢ final states v,lth ncgattsc ('-panty are dommatcd b~, o and K°" resonances. I hc rahos of the branching rahos are B(~p~¢,n°)/B{~p ,oq) = 8.3t 2.1 andB(fip .KOK"u" + - K - r ' K n ' - - K ~ K• ~ n ° U B ~ p ~ o n ° . k t oA'o -, I. - s . n o~, = ..04::1:0..
' Nov."at University, ofSiegen. German.~. -~ Now at Unr.crslt) of Kent• ('anterbu O. [;K. Nov, at CERN. (.;en~'se. Swm, erland. "~ ()n Ica~e of absence from the [,mvcrslty of Ljubljana. I.jubljana, Slovcma. 5 ()n leave of abscncc from Ihc [ [rex cr~lt.~ of Stlc~m. Katoy,ice. Poland.
6
Now at Max Planck Inst,lut. Iletdelberg. Gcrman.~. Now at Carncg,c Mcllon [;mvcrs~t). Pittsburgh. PA. lWS.~ Now at Stale Umvcrslt} of Nov. York. Ston~., Brook. NY. USA. '~ Nov, at Umvcrstt~, of Carhforma. I~me. ('A. USA. Jo This ~ork is part of thc Ph D thesis of H.P. Dtctz.
0370-2693/93/$06.0(I 'i:.)1993-Else,,acr~acncc Pubhshcrs B.V. All rights rt'scr,.cd ~hl)l 0370-2693( 93 Ik 14¢,5-'t
373
Volume 319. number 1.2.3
PHYSICS I.ETTERS B
The stud) of tip annih,lation into final states w,th open or h,dden strangeness ,s of special interest. Since. accord,ng to the Na,ve Quark Model [ I ]. the .~ and p contain only fi-fd and uud const,tuent quarks, the s and ~ quarks in the final-state mesons must be created dur, ng the reaction. Alternat,vel.,,, ,n a less naive model, they can be included as ~'s sea quark pairs ,n the wave function of the ~ and p. Thus. one question behind the special interest is how much .is pairs contribute to the (ant,)baD'on wave function at hadronic scales relevant to annihilation at rest (low Q : ) [21. A related quest,on ,s how well the OZI rule [3] applies to specific channels such as ~ p ~ V m ° or ~p--epn°n °. Indeed. the large size of the branching rat,o -~p--a~n ° [4.5]. ~h,ch seems to violate the OZ! rule qmte strongly, has been considered to rod,care either a large amount of ~s ,n the proton wave function [2]. or the existence of a qqss cDpto-cxotic intermediate state [6.7] or. more mundanel), finalstate interaction between K ° and X~ produced via an OZl-allowed intermediate state [8]. Two channels wh,ch have never been studied before arc
--
"0
,O
O
(1)
~p-x°,. K.°,l.
(2)
pp--Kt.Ksn and
In the present experiment the) are detected via the "zero-prong" final states where onl} photons are detected, i.e. via the K ° - - 2 n ° - - 4 ) ' and q-~27 decay modes. The K ° escaping detectton with a probabflit)' of about 50% is ident,fied by missing mass. We l-0 i/0_0 wdl show that ,.l,..sn is dominated by the twobody intermediate states K°'X~u. ~ ' K ° and ,bn°. and K~JK°q is dominated by ,pq. The latter has not been measured before in hquid hydrogen, but the K°-K°'+-K-°K°" and ,=,no branching ratms are alread)' fairly well known from charged final stales [4.5.9.10]. The measurement of absolute branching ratios reqmrcs a precise knowledge of the interact,on probabdit~ of K ° m the calorimeter. An analysis is in progress to de/ermine this quantit~ and wdl Ix" published in a forthcoming paper, together with absolute branching rat,os into ~arious channels containing K°'s. The present investigation is restricted to obtain374
23 Dc~'cmtx'r 1q93
mg relative branching rallos of K°K-u" + X~IK°'. ,',n°. and ¢,q with small stat,stical and systematic errors. The CDstal Barrel detector [ I I ] has been .set up at the Los,,-Energ.~ Ant,proton Ring (LEAR) at CERN to investigate pp annihilations at rest and up to about 2 GeV/c i n e o m m g ~ momentum. It has been designed for high-statist,cs measurements of almost all possible final states and has access to man) channels which have not been investigated before. For channels which are already known it can reduce the present statisucal and s)stemattc uncertainties. The detector co~crs 98% of the full solid angle and is scnsit,~e both to charged part=ties and photons. Charged particles are observed and measured m two proportmnal w,re chambers ( PW('s ) and a t.) hndrical jet drift chamber [JD('). Photons. for example those produced in the decays of n ° - - 7 7 , r/ "77. etc.. are measured in the electromagnetic (e.m.) calorimeter ~hich surrounds the drift chamber. As the results presented here primarily depend on the photon calorimeter. ~e briefl) describe its relevant characteristics. A detailed description of the detector can be found elsewhere [I 1 ]. The calorimeter consists of 1380 CsI(TI ) cr)stals. 16 radiation lengths thick, read out by photod,t.~les. It covers polar angles # between 12 ° and 168 ° and has complete coverage m az,muth. Photons which h,t the cr)stals closest to the acceptance holes (0 < 12 ° and 0 > 168 ° ) of the detector are rejected in the analysis (see below). This reduces the solid angle for photon detect,on, leading to an acceptance of 95% of 4n. The energy cahbration of the cD'stals ts achieved using the 2)' deca) mode of the n °. abundantly produced m ~p annihilatmns, by imposing the standard mass of 134.974 MeV/c: on the reconstructed n °. The photon cnerg) resolution is o~.
2.45%
I-
( i-,'/(;eV )t 4
for the data u ~ d m this work. The angular resolution is a = 2(1 mrad m the polar and a/mlulhal angles for isolated h,gh-energy showers. The trigger for an incident antiproton ,s provided b) a sdicon beam counter placed just before the liquidhydrogen target. This trigger is combined wtth a veto against annihdations producing charged par~tcles, usmg the on-hne reformation from the PWCs. The ~.suiting ~zcro-prong" trigger permits a large increase
Volume 319. number 1.2.3
PIIYSIf_'S I.ETrERS B
in the data-acquisition rate for a n n i h i l a t i o n s ,nto final states with only neutral particles, which represent about 4% of all ann,hilattons. In this way several milhon zero-prong events have been a c c u m u l a t e d during several running pcr,ods. The starting sample for the analysts o f the reactions ( I ) and (2) c o n t a i n s 4.5 10' zero-prong events corresponding to 107 million ~p a n n i h i l a t i o n s at rest. First, 4 • 10 ~ e v e n t s are rejected which show charged tracks in the J D C after off-line reconstruction, having escaped o n - h n e P W C detection. ( ,-% vers small P W C ,nefficiency. ;" conversions, and charged K ° decays are the causes o f these events. ) Next. all events not having exactly six well-~solated e.m. showers with a ram,m u m energy o f 20 MeV are rejected. T h i s cut r e m o v e s about 80% o f the data sample. It a l ~ reduces the detection efficiency since not ever,) photon will manilest itself as one separated e.m. sho~,er: s o m e o f the photons escape through the holes o f the detector: others merge with a n o t h e r photon or, as result o f shower fluctuations, a p p e a r as two separated energy deposits. Since a m o n g the products o f reactions ( I ) and (2). only n °. q. and K.~ are detected whde K ° ~s unobse~'ed, the events o f interest are c h a r a c t e r i r e d by missing energy. Therefore, a cut on the total energy depos~t in the c a l o r i m e t e r is i m p o s e d ~,h~ch corresponds to a w i n d o w around the range o f visible energ.~ in these reactions• O f the 8 . 8 . 10 ~ events, kept up to th~s point, all events are rejected which do not fulfill the c o n d i t i o n 6
900 MeV < Z
I-, < 1400 M e V .
lint
In this way the bulk o f background events such as .6;' is r e m o v e d , leaving 58 000 events. In o r d e r to r e m o v e further background from e v e n t s with higher photon m u l t i p h c , t i e s where one or m o r e photons are lost. events with m,ssing mom e n t u m p o i n t i n g towards one o f the detector holes are rejected. F u r t h e r m o r e we d e m a n d that an e.m. shower does not spread ,ts energ.~ a m o n g adjacent crystals in such a wa~ that the m a x i m u m energy fraction is deposited in a crystal on the b o r d e r o f one o f the holes. Applying these cuts reduces the n u m b e r o f events to 37 270.
~ p ~ 3 r t ° ~ 6 ) , or ~ p - - 2 n ° q
The qualit.~ o f the data o b t a i n e d at th~s level ~s dlustrated by fig. l a. which shows the distribution o f
23 l.K-cember 1993
-z
.,
: ,.!
,,:-
.
•L - .
t_
. L
/ [-'.
~
~-,-~'
2.,,"
. . . . . . . . . . . . .
.
i
r'l
j,
"-'"
.
/
.
•
d~.
ii.i.
eWl
Idrw
ql I
FIg. I In,.anant mass d~stribution. (a) m,ssing mass of events w,th 6 e.m. showers after a cut on the total v~s~ble energy,; (b) n°n ° mas~ after the IC k,nematic fit for events with three n °. (el and (d) ;,'}' mass after the 4C kinematic fit. the miss,ng mass squared
(
:n~,
6 =
2my-
6
2
!-',
-
linK
2
p,
,
m
where a K ° signal shows up clearl.v.
fip--K~6;' permits a one-constraint ( I C ) k i n e m a t i c fit which is s u r v i v e d by 10 305 events w~th a confidence level ( C L ) larger than 5%. At this stage there ts clear evidence for a K.° signal. This is illustrated by fig. lb which is o b t a i n e d for a subsample o f the 10305 events where the six photons c o m b i n e to give three neutral ptons: the invariant mass spectrum o f two of thc'se pions ,s shown 13 entries per event ). In the next step a 4C k i n e m a t i c fit is applied to the 10305 e~ents according to the hypothesis oJ. p- - p - - K o t0 K.,,.. }.
(K.O~ .n°n°) .
T h i s fit ~ields 4395 events with ( ' L > I%. Fig. Ic shows the invariant mass distribution o f the remaining two photons• .~ p r o m i n e n t n ° signal and a small 375
Volume 319. n u m b e r 1•2.3
P H Y S I C S I.FTTERS H
r/signal can be seen, which c o r r e s p o n d to the reactions ( I ) and ( 2 ). F m a l l ) . two 5C k,nematnc fits are applned to these events in order to .separate reactions ( I ) and ( 2 ). m a k i n g the appro~,nmatnon that K.~' decays at the pr,mar3, vertex, in o r d e r to c o m p e n s a t e for this app r o x i m a l , o n the errors on the photon d~rectnon mere appropriatel) increased nn the fit. The data are spl,t into two subsamples: 11): 2834 events wxth ( ' L > 10% for reaction ( I ) and CI° < 1% for reaction (2). (11): 72 events with C L > 10% for reaction (2) and CL < I% for reaction ( 1 ). 1 he p r o b a b i h l ) d,stribut,ons for these fits are flat abo~c 10%. showing that the quality of the final event samples is acceptable and the m e a s u r e m e n t errors on the kinematical variables are correct. It was checked by M o n t e Carlo simulat,on that the feed-through from react,on ( I ) Io 12) is 4 ± 2 e~ents while the feedthrough from (2) to ( I ) is less than one e~ent The n u m b e r o f background events ,n these t ~ o subsamples us e s t i m a t e d from the n u m b e r s o f entries m the m~ariant mass distribut,on o f the two r e m a i n , n g photons, excludnng the n o and the r/sngnals. This g,ves 120 ± 7 background e~ ents ,n the KOK.On ° subsample and 26+ 8 background events in the K°K.°~ I subsample. The K ° and K ° energ) distributions o f the events outside the n ° and ~I sngnal suggest that the background events are h o m o g e n e o u s l ) d~stributed o v e r the allowed phase space• The overall efficlenclt.'s (e) to reconstruct and identff.~ reactions ( I ) and (2) in the neutral deca) mode. a s s u m i n g no K~ interaction and including all cuts and k , n e m a l i c fits• were calculated wnlh a complete M o n t e Carlo s,mulatnon based on (;E.'~NT [ 12 ]. Theyare~ = (21.3...0.5)%forreaction(I)and+ = (22•0 ~ 2.0)% for react,on (2)• T h e i r relative ~arialion A~/+ o v e r the full phase space ns less than 5%. Figs. --"~a "~d sho~ the ^. .t o~,..n. o D a h t z plot and ,Is three projections for the event .sample II) using the m o m e n t u m values from the k i n e m a t i c fit. This Daht7 plot exhibits the follos,,ing p r o m i n e n t features: an a c c u m u l a t i o n o f events in the two K ' ( 8 9 2 ) reg,ons and a sharp peak,rig o f events for which the ( K, Ot K - 0~ ) effective mass is in the ,>(11320) region. We observe a slight as.~mmelr3, between the m 2 ( K ~ n °) and m'-(K.°n °) project,ons. This asymmetr3.' ,s due to a small systematic error in our analysis method. There is a threefold c o m b i n a t o r , a l ambiguity as to which two o f the three n°'s form the K~. The am.~7h
23 i ) c t e m b e r I~q ~,
7
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.
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,,~ I:.
.
.
.,: _
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t
, _ _ I%
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~'(s92)
•,;
~r: o ~ -
c)
II '
.+,t
:v~
:-'I
,....~.
;.+:
,t
l+•-..P
,q.'
+(~o2o)
:' I
d)
-: ,.-,:
tl
i
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:11
.
: ';~L..~ I
?+' ;+
+-Sl" ~
,.
+ "[
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. 5
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'- I
:ii5+(......... 'G,. :3+i +
''+
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::+-
j
,
Ft 8. 2. (a) I)ahtz plot and ( b t - t d l projections for the reaction PI~ • A t. A.~n .o . I')ta.'a.~cosine distribulions (el nn the 2 ~ ~ I.I GcV21 and (f~ ,n the rt'st rest frame of ,:, ImA?A., f r a m e ot" K °" 10.7 -,:- nl- u o ~< 0 9
Ge~'I;
the p+:ak at
ALA S
I) 4 IS d u e tO the cro~xmg w i t h the o t h e r K o" b a n d
o 0 ]t-q] blgutty affects the h'°n '~ and the I,,t,,.s (but not the KUn('l in~ariant mass• This combxnatorml effect s,,as studied v.llh M o n t e Carlo simulations. F r o m this study we d e t e r m i n e d that (4.8 * 0.6%) o f the ,.',no events and (7•8 - 0•7%) o f the K.~K °" events give ~ r o n g n ' n ° comb,nations. This explains the different heights o f the two K °' peaks. In figs• 2e and 2 f t h c deca~o cos,he distribution o f o - ] tJ'' ,°l r,,]t-o ,.. and K °" .K'~n ~' are shown by selecting events with m 2 ( K-tOA ..sq ) < I.I G e V : and (1.7 < m - ' ( K 0 n aJ ) < 0.9 (;eV". The) both exh,b,t the angular d,stribut,on expected fi,r vector resonances p r o d u c e d from the ffp initml state ~S, (see belo~ ). Figs• 3a and 3b show the K['K.°PI Dalitz plol o f the
Volume 319. number 1.2.3
PHYSI( ."Y,LFI'I ERS B
Here. the symbols K ; and K,~ m e a n that the invariant mass of n r' and K'] or K~' is m the region o f the K"" ( 892 ) resonance. We have K7 = ( K '~" + X=U' I/~,'~ and K,~ = (K"" - ~ " )/~ "~. The r e m u s sign between the two terms of eq. (4) projects the part ,,,,lib zero strangeness out o f the final-state wave function cont a m i n g K~K; and K~K~. The angular distribution of the ,:,-deca.~ is gl~en b.~
i [ •i
"-
"
.
.
.
.
.
+ . . . . . .
" c
•...
+.+
23 I)¢¢cmber 1993
.;
I-~g. 3 (a) l)ahtz plot and (b) projet'tton onto the K°K ° I .'~ m~,artant ma~s for the reactmn PI' -'K~'KOsq• event sample (11) and its m Z l K t K~) projection (fitted q u a n m z c s ) . The ¢, szgnal =s p r o m i n e n t . ++hile the o t h e r regions o f the Dalitz plot arc sparsel.~ populated - mainl.~ b.~ b a c k g r o u n d events. As the final state has C-parit} = I. ~t ~s r e a ~ n able to assume that the production is d o m i n a t e d b> the ~1, initial state ~S~. We fit the K~iK°n t' l)alitz plot w~th tranmtion a m p l i t u d e s o f the channels ,~p(~S~) .¢,n u and ~p{~S~) . K " ' K using the max~m u m hkelihood methtnl. An intensit.~ d l s t r i b u h o n o f the form
Z(':')
= qA
" PAA.
v, herc qA ~s the 3 - m o m e n t u m o f the h'~. in the K~K~ rest frame and PAA tS the 3 - m o m e n t u m of the A'~.K.~ system in the laborator3.. The angular distributions of the K ° ' - d e c a ~ s are described by 11 h'°')
= qA " P A . ,
where qA is the 3 - m o m e n t u m of the K~.(h'~) in the K~int'(K°~n°~ rest frame and PA, ts the 3 - m o m e n t u m o f the K~'nt'(K.°n"l s~stem m the laboratory. l ' h e relat~vtstlc Bre~l-W~gner factors for the o and the g °" resonances are !"1 m ) -
[|'(p).
II'(q)
R:
q.p
mo/o
mz-m
o + imol'(m)"
i = I,~A(~S~.,;,II020)) + ,~..A(~S~.K°'(892)~ " ~lth is assumed with c o m p l e x parameters ,t,. The transition a m p l i t u d e s A(+S~.,.',(1020)) and A(~S~. h °" (892 ~) conmst o f a function X which desertbes the decay dzstributzon of the resonances, satzsf~ ing total angular m o m e n t u m and parity eonser~ ation, and a relam, zstzc B r e i t - W i g n e r factor I . ( m ~. The decay distribution functions I are constructed accordzng to the m e t h o d d e v e l o p e d b) Z e m a c h [131. where the three scalar amplztudes correspondzng to the three magnetze substales o f the retrial J = I p r o t o m u m state are represented b~, Ih¢ three c o m p o n e n t s o f a vector a m p l i t u d e A. We have A(3S~,o(1020) ) = Z(~)
t"+m(K~]K°)) •
I'(m)
= L'~ mr. q I I " ( q ) m qo 1|'2(qo) '
~ h e r e m . . E, are the n o m | n a l mass and width o f the resonances, q = q ts the decay m o m e n t u m in the resonance rest frame (q = q,, f f , t = m0), and p = [p ts the l a b o r a t o ~ m o m e n t u m o f the resonance. The function I | ' ( q ) is the B l a t t - W e i s s k o p f d a m p i n g factor for angular m o m e n t u m t = 1 [141. (The angular m o m e n t u m is one unit between the rt.'sonances and the reeothng parttcle, as well as Ix'tween the decay particles o f the resonance. ) Therefore. I1" is
(3) l l ' ( q ) = (q
T h e a m p l i t u d e A ( ~S~. K °" (892) ) c o n t a i n s two terms. each c o r r e s p o n d i n g to an ezgenstate o f charge conjugation with (" = - I :
A(aSI.KO'1892)) = x(K~) - ~r(K.~) • k ( m ( K ~ n ) ) .
"l-'(m(K~'n))
(4)
R)
(
I +
' ) (q.Rl'-
I 2
~ i t h R = (0.197327 G c V ) -t ( = I F e r m i ) . The c o m p l e x parameters ,~, = %,[ exp(i,l,, ) are left flee m the fit. The phase of,~z ts arbitrat 3 and therefore fixed at 0. Sznec the natural width o f the ,t,resonance. ~ = 4.4 MeV, is nearl~ o f the .same magnitude as the experimental resolution a = 2.5 MeV. 377
Volume 319. number 1.2.3
PHYSI(..'S LETTERS B
23 December 1993
Table I Amphtudes and phases of the fit. Initial
Intcrnlc'diale
Stale
Slate
~,
=
~s e x p ( t , p , )
amphtude
phase '.'h = 0 o., = 1 6 4 = 0 . 1 1
3S I
C,(1020)
I"1 = 0.57 .':-0.01
~SI
K ° ' ( 892}
"2. = 0.82 + 0.01
the intensity distribution I is c o n v o l u t e d with a Gaussian curve which represents the resolution along the m: ( K ° K t . s ) ax~s. A constant intens~t) is a d d e d in order to account for the 120 background events mentioned above. The results o f the fit are given in table 1. The n o r m a l i z a t i o n is such that I,,tl: + l't:[ 2 = I. In o r d e r to test the consistency o f our analysis m c t h o d we generated a M o n t e Carlo data set o f K 0t.,,sn 1 - o _ o events with a Dalitz plot distribution according to the fit result given in table I. These M o n t e Carlo data were then anal.~sed exatal) like the real data. as described above. The spt~tra that result from this analysis are s u p e r i m p o s e d o n t o the spectra of the real data in the figs. 2a-2f. T h e a g r e e m e n t between the real data and the s i m u l a t i o n is satisfactor3. A Z"test was applied to test the c o m p a t i b i l i t y o f this simulation and the data. The X " per degree o f f r e e d o m is 1.2. Since the data are well r e p r o d u c e d b) the set of a m p l i t u d e s that we have assumed, we c o n c l u d e that •.-0 ~ ' 0 0 the final state At ^ s n is p r o d u c e d almost entirely from the ~S~ state. We do not need an) substantial c o n t r i b u t i o n from the ~P~ initial state. -% priori, the latter cannot be excluded if we assume that it feeds the resonance both via an / = 0 and an / = 2 trans~lion. H o w e v e r . it would take an ~mprobable accident o f nature for the ~Pt state to produce an angular distribution as close to sin20 as we observe it. Estimates, based on our own data. o f the probabdit~ P,,t that the Kt° does not interact or the interaction is not detected in the calorimeter, ~,,leld P,,t 50 : 10%. Th~s n u m b e r has a relatively large systematic error but it cancels in the ratios of branching ratios unless P,,t d e p e n d s on the K ° m o m e n t u m . Such a d e p e n d e n c e would show up in a systematic d e v i a t i o n between the data and the fit ~ h i c h assumes P,,, to be constant as a function o f the K°n ° invariant mass (or the K~ m o m e n t u m , alternati~ cl~ ). In fact. the data are consistent with no m o m e n t u m d e p e n d e n c e within the
378
Ratio (%) = I(lOb~
32.9J I 0 67.1 t 10
statistical errors and the relevant m o m e n t u m range. The) allow the e~tractlon o f upper limits for the mom e n t u m d e p e n d e n c e o f P,nt : these are taken into account as additional s ) s t e m a t i c unceraamty of 7.5% for the ratio B(-I~p--K"'-~ j + - ~ ' K ° ) / B ( ~ p .,pn °) and of 10% for the ratio B ( ~ i p - - , ~ n " ) / B ( ~ p - - o q ) . We define the n u m b e r ot'ort ° events as the product o f I,,al" and the n u m b e r o f detected ^t.^.s . . . , - u n u events. This )ields 893 ~n ° c~ents c o m p a r e d to 737 events if we stmpl) count the n u m b e r o f events in the o peak. The latter n u m b e r is significantly lower, firstl) because e~ents in the high mass tail o f the O-re~amance are not counted, secondl.,, because events arc lost from the peak due to wrong n ° n C ' c o m b i n a t i o n s as explained above. The ratio B lfip .K°-K °" + -~J K °" ) / B (ffp .Oft ° ) is obtained
as Ill: ->t':
Volume 319. number 1.2.3
B(fP--'~n°)
B (~p -o~/)
I)H~'SI( .'S LET I ERS B
- 8.3 Z 1.7~tax .t. 1 . 3 ~ .
'0 • "0 II B(~p--h'C~'K r~ + t--';'IJ° I~, ,t~ - - ^- -t' f l ^.,~n I B(ffp .~pn°~K°K.°n o)
= 2.04 .,._0.10~=t --r-_0.19,,~.tt . Here. the lower radices "stat" and "s~st" indicate the statistical and the systematic error. The final s~slemattc error squared of the first (second) ratio is the sum of the squares of the 10% (2%) statistical error of the efficiency, the I1)% (7.5%) uncerlaint~, concerning P,,, variattons, and the 5% (5%) systematic error mentioned last. These ratios provide a useful additional input to ~p annihilation models, in particular since we are dealing with a final state of specific ('-parity I. Th~s reduces the posstblc mittal ~p slates to j e c = I - (s-wave) and j e c = I +- (p-wave). It has been shown that only the I - - state ts needed to descrtbe the Dalitz plot m a salisfacto~ wa~. This result ~s plausible: s-wave anmhflatton ts dominant, in general (for a recent publicatton see ref. [161) and. in addition, dynamical selection rules appt'ar to favour the JP¢ = 1 - - initial state [17 I. The ~sospin ~s I for the ,;'m° channel and 0 for o~/. A complcte ~sospin analysts of the K °" K channel has been carried out m the classic paper on ~p .K-Kn by Barash et al. Iql. Thts analysis shows that in our case the population dens(t5 of the Dalite plot is independent of isospin. which therefore, cannot be determined solel.~ from • "[J • ' 0 I] studying the ^L^.sn channel. In conclusion, we obscr.'e that the branching ratios for the channels ~p -. K}~K.~, K.°K;. and ott o are nearly equal, thus confirming a previous observation that the branching ratio into On is not OZI suppressed. However. in contrast, using the branching rauo B { ~ p . , o n ) = (4.0 t 0.8) I0 -a from ref. [51 and our ratio B(,;,n~/o~ 1) we obtain B(ffp- "~1) = 0.48 + 0.17) • 10 -~. This value is as small, compared to the branching ratio B(-~p 'co~l) = (10.4 -.1.0) 10 -'~ [18] or = (15.1 • 1.2)-10 ~ [ I q ] , a s e x petted from the OZI rule. Tentative explanatmns of this phenomenon were suggested by' several authors. Furu! et al. [81 argued that the kaon from K °" decays rescatters off the directl.~ produced kaon thus forming ,,~ mesons via an OZl-allo~,ed intermediate state. The small ratio for ~ i compared to ~.~I can be
23 December 1993
explained b~, the absence of a strange meson decaying into Ktl. Howe~cr. other final states which can be reached b~, rescattering from OZl-allowed mtermedmte states should be enhanced as well - this does not seem to be the case. Stmdarl.v. a large amount of ~s in ~p cannot explain the peculiarity of the ,;,n/con ratio; it ~ould presumabl.~ stmultaneousl~, affect all branching ratios with a ¢0 meson. Perhaps. the absence of an ()Zl suppression in the particular case of ,,6n° can be explained b.,, the absence of cancellation between various intermediate states as dtscussed b.~ I.Ipkln et al. [20]. Alternatively. ('1o.~ and l.ipkin 161 and Dover and F~shbane [ 7 J associate enhanced on productmn w~th the existence o f a cr3.pto-cxot~c 4quark state. This ~ q state ~s identified by the latter authors v,llh the on resonance ( ' ( 1 4 8 0 ) [21 ]. whtch ~s suppomd to couple to the ffp system llowever, the small branching ratm into ,/al excludes the hypothetical [71 crTpto-exotlc / = 0 partner of the ( ' ( 1 4 8 0 ) m e a n . The prt.~nt s~tuation cerlainl~ asks for more data on o production. An analysis, stmilar to the one descr, bed in thts paper, is tn progress on the channel -:p .K~.K~"/which should pro~ (de interesting new information, when compared with the recentl} published branching ratio ,~p .co;' [22 ].
We would like to thank the technical staff of the LEAR machine group and of all the participating mst~tut~ons for their invaluable contributions to the success of the experiment. We acknowledge financial support from the German Bundesmimsterium fur Forschung und Technologic, the Schweizerischer Nationalfonds. the British Science and Engineering Research Council. and the [JS Department of Energ.,, (contract No. DE-FG03-87ER40323 and D E - A C 0 ~ 76SF00098). J.K. and K.M.C. acknowledge support from the A. yon Humboldt Foundation. We wish to thank K. Stcinberger and A. l.oeseh for the system management on the computer cluster in Garching.
Reference~ Ill J.J.J. Kokkeclee. The Quark Model (Benjamin. New York. 1969). [2] J Flhs. L. Gabathuler and M. Karhner. Phys. Lett. B 217 (1989) 173. 37t;
Volume 319. number 1.2.3
PI IYSI( "~iLET'TI: RS B
[3]S. Okubo, Phys. Left. 5 ~1963) 16.5. (;. Zwms. CERN Report No.8419/TII412 (1964), I. hzuka. Progr. Thcor. Phys. Suppl 37 (19660 21 [4] M. Chlha et al.. Ph.~s. Rc~. D 3X (1088} 2021. Ph~,s. Lett. B 202 l l 9 8 8 ) 447. [5] J. Reffenrothcr cl al. Ph.~s. Lett. B 267 ( 1991 ) 299 [6] F.I-. ('lose and H.J. [.,pkm. Ph~,s Roy. Left. 41 (197~;) 1263. [7] C.B. Dover and P.M. Flshbanc. Ph~,s Res. Lctt. 62 (I989) 2917 ISLS. Furu,, Z. Phys. C 4 6 11990) 621; S. Furm, G. Stub¢l, -~ Facs.,,Icr and R. Vmh Mau, Nucl. Ph)s. A 516 1'19901 643. [9J N Barash c! al.. Ph~,s. Rcs. B 139 (19651 1659. B Conforto et al. Nucl Ph,,s. B 3 (1967) 469 [lOJ :k. Belhm ct al., Nuo,,o Cimcnto 63.", q 19691 1199. [I l i E . A k c r e t a l . Nucl Instrum Methods .'~ 321 c l9921 69.
3.~1)
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[ 1 2 ] R . B r u n c t al., GEAN'r3. [mcrnal Report CERN DI)!I.I-/84.1 t(.I-.RN. 198,"i. [13JCh. Zcmach. Ph.~s Rc~,. B 140 119651 97. 109. [14] J M. Blatt and V Wc,sskopf. Theoretical Nuclear Ph)slts tWlley. New York. 19521. [I.SlParticlc Data (/roup. K. H,kas.a et al.. Rc,,~cw of p a n , d e properties. Phys. Re,,. I ) 4 5 (19~2) Part II. [16]('..~,mslcr et al.. Ph.~s Lett B 297 (19921 214. [ 17 ] ( . B I.~,,er. 1. Gut~'he, M..Maru)ama and -~. Facs.sler. Prog Part Nucl. Ph~,s. 29 (19q2) 87 [18] L Adlelscl al.. Z. Ph~,s. C 42 (19891 49 [19] ('. -~m.,,Icr el al, Z Ph~,s. C 58 (1993) 17.$ [ 2 0 I H J L,pktn. Nucl. Ph.,,s. B 244 (19841 147: B 291 (19~7) 720 [21]S.I Bit.~ukm, c! al.. Ph.~s. Iclt. B 188 [19147) 383; L ( i I.andslx'rg, ,',k~s. J. Nucl Ph.~s. 55 (19921 1051. [22]('. -~mslcrctal Ph)s Lcl(. B 311 (19931 371