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Volume 187, number 1,2 PHYSICS LETTERS B 19 March 1987

D I R E C T OBSERVATION AND P A R T I A L - W I D T H M E A S U R E M E N T OF 73' DECAY OF C H A R M O N I U M STATES

Annecy ( L A P P ) - C E R N - G e n o a - L y o n ( I P N ) - O s l o - R o m e - S t r a s b o u r g - T u r i n Collaboration

C. BAGLIN a, S. BAIRD b, G. BASSOMPIERRE a, G. BORREANI c, J.-C. BRIENT a, C. BROLL a,l, J.-M. BROM d, L. B U G G E e, T. B U R A N ~, J.-P. BURQ f A. BUSSI~RE a, A. B U Z Z O g, R. CESTER ¢, M. C H E M A R I N f, M. CHEVALLIER f B. ESCOUBES a, j . FAY f, S. F E R R O N I s, V. G R A C C O s, J.-P. G U I L L A U D b, E. K H A N - A R O N S E N e, B. ILLE f K. KIRSEBOM ¢, M. LAMBERT f, L. LEISTAM b, A. L U N D B Y b, M. MACRI b, F. M A R C H E T T O c, E. M E N I C H E T T I c, Ch. M6rch ~, B. M O U E L L I C b, N. PASTRONE c, L. P E T R I L L O h, M.G. PIA ~, J. POOLE b, M. P O U L E T ~, G. R I N A U D O ~, A. S A N T R O N I b, M. SEVERI h, G. S K J E V L I N G ~ and B. S T U G U ~

Lapp, B P 909, F-74019 Annecy le Vwux Cedex, France b CERN, CH-1211 Geneva 23, Swttzerland

Umverstty o f Turm, 1-10125 Turm, Italy d CRN, F-67037 Strasbourg Cedex, France

Unlverstty o f Oslo, N-1000 Oslo 1, Norway f IPN Lyon, F-69622 Vtlleurbanne Cedex, France g Umverstty and INFN, 1-16126 Genoa, Italy h Umverstty o f Rome, 1-00185 Rome, Italy

Received 19 December 1986

As part of the charmonlum formation experiment at the CERN Intersecting Storage Rings, we stud~ed the reactmn plY--ce--.7"r m an antiproton momentum scan through the ~lc, X ~, and X2 regions. We report events observed m the n¢ and X2 regions, whilst no event was observed in the X z region, as expected for a spin-1 state.

1. I n t r o d u c t i o n

The interest in studying the 77 decays o f cha rmon- lUre states is well known. Potential models [ l ], as well as Q C D sum rules [ 2], can be tested by meas- uring these decays. In addition, since the ratio F ( R - , 7 ' / ) / F ( R - - ~ h a d r o n s ) (where R is the resonant state) is directly related to the running coupling constant a s ( Q 2) of QCD, the measurement of this ratio allows the validity o f per turbat ive Q C D calcu- lations to be checked in this range o f Q2.

When this exper iment began to take data, only upper limits for this decay mode of the ~c and X2 states were available [ 3]. Meanwhile, new experimental results have been published: in pho- t on -pho ton interactions at PETRA, the P L U T O

Deceased, 21 July 1984.

0370-2693/87/$ 03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division )

Collaboration [4] has observed a b u m p in the KOK_+x :F mass spectrum, with a mass about 3 GeV. A natural interpretat ion of this bump is given by the reaction 7Y ~]c-~K°K+-x ~:. Using the 1984 Particle Data Group branching ratio BR(~coK°K-+Tt :~ ), they obtained the value of the partial width F (TI¢~ ' / 7 ) - - (14_+I2 ) keV. This gives an experimental indication of a coupling of the I]~ to 77, but the partial width is still quite uncertain, later, the TASSO Collaboration [ 5 ] observed a signal in the same process, giving F 0 ] ~ 7 7 ) = (50_+40) keV. More recently, at PEP, the M A R K II Collaborat ion [ 6 ] observed four events above background in the same channel. The value derived for F(Tl¢-~77) is (8.0_+ 5.0-+ 2.0) keV. The Crystal Ball Collaboration at SPEAR [ 7 ] have observed the ' /7 decays of the go and X2 states, and give the partial width

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F(X2--,~'t) = (2.8 + 2.0) keV. We report the results obtained by the R704 Collab-

oration at the CERN Intersecting Storage Rings (ISR) for the reaction

Pf)--)VT (1)

studied in the mass region of the ~c, )C ~, and )C2 states of the (cO) system.

2. Experimental set-up A detailed description of the experimental tech-

nique and of the apparatus has been given in previous publications [8]. The proton-antiproton

annihilations were produced, in ring 2 of the ISR, by intersecting a cooled coasting antiproton beam with a molecular hydrogen jet.

The detector (fig. 1 ) wa a non-magnetic spectrom- eter with two detection arms set symmetrically rela- tive to the beam axis. Each arm had a charged-particle tracking section, including sets of scintillation hodo- scopes, a threshold ~erenkov counter for tagging electrons, and a transversally and longitudinally segmented electromagnetic calorimeter. For 77 detection, the calorimeter is the essential part of the apparatus and has to provide a high 7:°/7 rejection factor in order to be able to isolate the ~7 signal over

/ / v - X--sONT ,o s s , _ . .

$

Fig. 1. Schematic view of the experimental apparatus.

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the large multineutral hadronic background. The calorimeter was set up to optimize

(i) the efficiency of detection of the low-energy

33'S, (ii) the 71/spatial resolution,

(iii) the energy resolution. Each calorimeter had four parts: (1) The 1/ converter, or precalorimeter. This was

made of lead/scintillator sandwiches amounting to a total length of 4.7Xo. The transverse segmentation consisted of 29 vertical strips and 14 horizontal wedge-shaped strips.

(2) The analog chambers. These consisted of four identical (X, Y) chambers with analog readout of the the cathodic strips (10 mm pitch).

(3) The shower hodoscope. This consisted of 14 vertical scintillator strips (NEI 10, 1 cm thick) used for triggering.

(4) The lead-glass wall was made of 66 blocks of F2 glass with the dimensions 15 × 15 × 30.5 cm 3.

The length of the blocks (30.5 cm or 10Xo) ensured full shower containment.

A system of guard counters completely sur-

200 o= 27 MeV

160

120

~0

I I i i 100 200 300 t¢00 500

M('('KI IMeVl

Fig. 2. Effective mass spectrum for two photons m the same arm.

rounded the two arms, covering most of the remain- ing solid angle. This guard system was composed of scintillation counters followed by segmented lead /scintillator sandwich counters (4.7Xo). Its aim was to detect charged particles and/or "/-rays outside the detection arms and therefore to set multiparticle or multi-33 veto conditions.

The fast trigger (200 ns) for the 33y final state was implemented using the guard system and shower hodoscope information.

A slow trigger (450 ~ts) made use of a fast proces- sor, which read the calorimeter fast ADCs, calcu- lated the energy deposited in the precalorimeter and the lead glass, and performed a cut on the total energy deposited. The trigger rate was a few events per sec- ond. Triggering for the ~ono final state was also obtained at the same time. The ability of the detector to reconstruct n°'s is shown in fig. 2, which displays the reconstructed mass for the 11 pairs.

3. Data taking The energy scan was done around the qc, X z, and

X2 regions. The energies and luminosities are given in table 1. The X~ scan was performed to constrain the background level in the X2 sample.

Owing to the presence of the multi-y veto condi- tion, the largest background (from hadronic multi- neutrals) is coming from the x°x° channel, which simulates an exclusive 33y final state from symmetric n ° decays inside the angular resolution or from very asymmetric n° decays where the low-energy y escapes detection. In order to evaluate the multi-33 background to reaction (1), we have also studied the reaction

p p ~ n ° x ° (2)

in the same regions.

Table 1 Candidate events, mass range, and lumlnoszty

Mass range No. o f events Luminosity (MeV) (nb - j )

2963-2969 0 96 2975-2987 12 511 3023-3025 1 98 3509-3516 0 494 3552-3564 6 821

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4. Data analysis The selection of T7 final-state candidates in reac-

tion (1) was based on the single y-shower topology, the energy deposit, and the two-body kinematics.

As afirst step we used the following criteria in order to obtain an intermediate sample of a reasonable size suitable for a detailed study of the final analysis cuts: - no charged particle entering the detector; - total energy deposited in the calorimeters, > 0.8Etot; - only one electromagnetic shower per arm, with E > 150 MeV; - an effective mass of these two showers > 75% of the centre-of mass energy (i.e. > 2.2 GeV for the q¢); - extra energy deposited in the detector, < 50 MeV.

This first step of the analysis led to a total sample of 1173 TT candidate events out of 2.6 million trig- gers (all data taken at the 11c, Z ~, and ~2 states). Their centre-of-mass cos 0* distributions are shown in fig. 3a (s-~9 GeV 2) and fig. 3b ( s - ~ 12 GeV2). The corresponding CM angular distributions of =o~o events (4 y converted in the calorimeters) are shown in figs. 3c and 3d. A comparison of figs. 3a and 3c indicates

250

~, 200

150 o

100

50

al

~ l , I , "--] 02 01, 06

COS e e

1000

000

600

Z~O0

200

_7

, L Ot , I 02 06

cos O*

that in the q~ region there is an important ~o~o background in the 77 sample, since we expect an isotropic angular distribution for the T1~77 decay. In order to reduce this background, we disregarded for the TI~ state the region cos 0"> 0.35, where the ~o~o. induced background is the highest. For the Z i and Z2 states, where the ~o~o angular distribution flattens and there is no definite expectation for the ~2---~/~/ angular distribution, we have considered data up to cos 0*=0.5 so as to avoid edge effects at the limit of the acceptance.

Thefinal step of the data analysis was based on (i) restricted kinematical constraints using the

shower position measurements given by the analog chambers for which angular resolution was obtained using the exclusive JA¢ sample [8,9] (pp-~J/~t -- ,e+e-);

(ii) longitudinal and transverse shower develop- ment characteristics; these were derived also by the analysis of the exclusive J/V sample;

(iii) no possible 7:o reconstruction using the main shower correlated with a very low energy deposit.

The efficiency of this analysis is about 60%. This has been derived by reanalysing the exclusive J/~¢ sample, disregarding the charged-particle tracking section and ~erenkov information.

The final results of the analysis chain are reported in table 1.

5. Results 5.1 The vc sample. Fig. 4 displays the CM angular

distribution for the 7)' candidates in the qc region. The hatched event (corresponding to a mass of

g t.o '~ 30

20 E

02 Or, 06 COS B e

30

2O

10 I

0 02 OZ, 06 COS e e

Fig. 3. The cos 0* angular dsstributlons for the first-step 77 sample (a) at s-~9 GeV 2 (qc sample), (b) at s = 12 GeV 2 (Z2 sample), and cos 0* angular distribution for n°n°, (c) at s - 9 GeV 2, (d) a t s = 12 GeV 2.

4

3

2 I 1

i

[ ] r/c -~ "V)" events

[ ] Background sample

I L [ 01 02

COS 8 °

, I O3

cos O angular d]stnbut]on for the final "qc sample. Fig. 4. The *

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1 2 ' 1 " "l I . . . . . l " " I 1

II ~oa g

if)

~ 3 o 4

0 2

2 97 2 98 2 99 ,3. 3 01 3 02 3 03

Moss (GeV)

Fig. 5. Excltatmn curve m the r h mass region. The curve is the result of the maximum likehhood procedure descrabed m the text The dashed line is the background contribution parametnzed by the expressmn (rb,~k= ab [ 1--O.0024(pbe~,,--3635)] pb obtained from the study of the n °n sample ( Ob ~S the parameter used m the maximum hkehhood fit;/h,¢~,~ xs m MeV).

3023.5 M e V ) , far f r o m the cen t ra l mass va lue o f the

resonance , is cons ide red as a b a c k g r o u n d event . T h e

exc i t a t ion func t i on in this ~1~ mass reg ion is shown in

fig. 5. The cu rve is the resul t o f a fit o b t a i n e d wi th a

m a x i m u m l ike l ihood m e t h o d , desc r ibed in ref. [ 10].

T h e to ta l w id th Ftot, the mass, the b r a n c h i n g rat ios , and the backg round level were free p a r a m e t e r s :1. An

ene rgy -dependen t backg round was used. T h e best- f i t

parameters are

m ~ ( 2 9 8 2 . 6 +2 ~) M e V , +7 5 _ F t o t = (7 .0_7 o) M e V ,

B R (r)~ ~p13) X BR(q~--)~/y) +o 42 = (0.68_o,3z) × 10 -6 ,

:t If Fro, is fixed al (11 +_4) MeV [ 11 ], the best-fit parameters are: mno=(2982.4_+2 6) MeV, BR(q¢--,pl))×BR(rI:~Ty ) =(0.57_+0.26) -6 +14o X 10 , ab,~k= (90-7o) pb, glwng the follow- mg branching ratm: BR('q:-.77) = (0.5 +_0.2 +-0.3) X 10 -3 and partial width T'( rlc --. t,'}' ) = ( 5.7 +- 2.6 -%_ 3.7 ) keV.

Table 2 Resonance parameters from R704

+2OO aback = ( I 0 0 _ loo)Pb •

This cross sect ion takes in to accoun t the e f f ic iency

and accep tance cor rec t ions ( ~ = 0 . 0 5 ) and corre-

sponds to the va lue o f the backg round at the B~ mass.

I f we use the pub l i shed va lue o f B R 0 q : - ) p l b )

= ( I . I + 0.6) × 1 0 - 3 [ 12 ], we ob ta in the fo l lowing b ranch ing rat io:

_ 10 ~+o.4 + 0 . 4 ) X 10 -3 BR(rh- - 'T~ ' ) - , " " - o 3 -

and par t ia l w id th

t 4 a+3 4 + 2 . 4 ) keV F ( q ¢ - * ~ ' ) = ~ .~-3 7 -

T h e first er rors c o m e f r o m the m a x i m u m likeli-

h o o d fit. T h e last e r rors co r r e spond to the uncer ta in -

ues in the o the r pa ramete rs . These unce r t a in t i e s are c o m p o s e d quadra t ica l ly .

We have checked the s tabi l i ty o f our resul ts wi th respect to the d i f fe ren t cuts used in the analysis. T h e

Resonant Mass Total width BR(R--,pl~) × BR(R--,~) F(R--, yT) BR(R--,~) state [MeV] [MeV] ( × 106 ) [keV] ( x 103)

29 ~':''~+2~ 7 a÷75 0.68 -o3~ -,.-,-~ ~ - ~. 06-o~ +-0.4 a) ~ c v ~ . v - - 2 3 " v - - 7 0 + 0 4 2 A " / ¢ - 3 ¢ - ~ - 9 '4 + 0 4

X2 3556.9+0.4_.+05 b~ 2 6-+] 4 b) +0046 0 099-oos5 2.9_+I 3 + +os o-1 .7 1.1_o4-+0.4~)

a) From ref. [12]' BR(n:-,plb) = (1.1 +__0.6)× 10 -3. b) From ref. [10] c) From ref [ 10]: BR (Z2--,plb) = (0,9- +° ] ) × 10 -4.

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Volume 187, number 1,2

Table 3 Summary of partml widths (m keY).

PHYSICS LETTERS B 19 March 1987

Resonant Parhal width [keV] state

R704 PLUTO [4] TASSO [5] MARK II [6] CRYSTAL BALL [7]

4 a+34"4''~ A 14+12 50+40 80--+5.0+2.0 ~c . ~ 3 7 .2-'/~...t X¢ 2 9-+I 03+ 1.7 2.8-+2.0

results are stable for slight variations of these cuts ~;2.

Table 2 summarizes our results. The value o f the partial width is in agreement with the theoretical expectations [ 2,13 ] and is consistent with the upper limit given by the Crystal Ball [ 7 ] and with the values given by PLUTO [4], TASSO [5], and M A R K II [6 ]. Table 3 shows our results and the present published values for this partial width.

5.2 Thex~ andg2 samples. The Z~ and Z2 samples are given in table 1. No candidate event is found in the Z ~ region

A likelihood maximization has been performed using the two samples. In this case, the mass, the total width of the Z2, and the BR(z2--,p~) are as deter- mined in ref. [ 10]. So we have only two free parameters: the branching ratio B R ( z 2 ~ T ) ' ) and the background cross section. The ~2--~/'/ CM angular distribution is strongly dependent on the helicity amplitudes o f the initial and final states and is not likely to be isotropic. However, in the absence of any experimental result for this angular distribution, we consider it as isotropic and give results in this frame- work. These results are

BR (X 2--,pp ) X BR (Z2--)TT)

+ 0 46 = (0.99_0 35) × 10 -7 ,

0"back < 7 0 pb.

This upper limit (90% confidence level) for the background also takes into account the efficiency and acceptance corrections (~ = 0.07). Using

~2 In particular, the maximum hkehhood method applied to the reduced sample corresponding to cos 0"< 0.3 leads to the following results. No. of events in the tie mass intervals of table l: 0/90/0 rnno = (2980.7s~ 9 t )MeV, Ftot= (7.2- +777)Mev, aback <80 pb (84% CL), BR(rl¢-,yT)=(0.7-+°7_+0.4)×10 -3, F(rk--,Ty ) = ( 5.3-+ 3 o _+ 2.9) keV.

= ( 0 . 9 _ o 3 ) × 1 0 -4 BR (Z2_..p~)) +04

we get

B R ( z 2 ~ T y ) = (1.1 +_°45 +0 .4) × 10 -3 ,

which, for a total width Ftot= (2.6-+I o 4 ) MeV, gives the following width:

F (Z2~3" / ) = (2.9-+I 03 + 1.7) keV.

The branching ratio B R ( z 2 ~ y ' / ) agrees with the theoretical expectation: 1.2× 10 -3 [14]. In addition, these values of branching ratio and partial width are consistent with the values measured by the Crys- tal Ball Collaboration [ 7 ]. All these results are also summarized in tables 2 and 3.

In the Z 1 region, the above value for aback provides an upper limit for the decay rate Z ~ T T . Using the lower limit 0 .5× 10 -4 (ref. [ 10]), for B R ( z I - , p ~ ) , we obtain BR( Z ~ --'3'T ) < 8 × 10 -4, consistent with the spin- 1 assignment for the Z ~ state.

6. Conclusions In an experiment (R704) performed at the CERN

ISR, we have observed TT decays o f qc and Z2 charmonium states directly formed in pO annihilations. New determinations of total and partial widths are obtained, which are consistent with the values given by previous experiments at e+e - colliders.

This experiment would not have been possible without the support of the CERN Management, in particular that o f professor H. Schopper. The following people are gratefully acknowledged for their tech- nical contributions, o f crucial importance to the success o f this novel type of experiment: H. Aaser, G. Abbrugiati, P. Anzoli, R. Audria, J. Ballansat, G.C. Barisone, C. Benvenuti, M. Berthet, J.-C. BiUy, H. Bonnefon, J.-C. Brunet, R. Calder, E. Clapala,

196


F. Conforti, F. Dalla Santa, G. Dughera, B. Duthion, D. Flakowski, G. Foffano, C.B. Girard , G. Gi raudo , E. Gjot te rud , C. Guil lon, G. Jacquet , D. Kemp, R. Keyser, R. Kiesler, E. Lauper, N. Madjar , P. Martucci , G. Massari , G. Maurell i , M. Moynot , P. Mugnier, R. Mundwiller, F. Nordby, E. Peschardt, C. Petit, D. Ploujoux, P. Poggi, M. Reynaud, T. Risselada, P. Sahuc, E. Sbrissa, J.-M. Schmit t and F. Verkerk. We wish to express our warm thanks to the ISR engineers and technicians who prepared the special machine running condit ions. The work o f L. Mat tera , A. Pozzo, F. Tommas in i and U. Valbusa has been essential in the H 2 target system design and commissioning. We are also indebted to the s taff o f the Ant ip ro ton accumula tor (AA) and Proton Syn- chrotron (PS) machines for their remarkable per- formance in manipula t ing the an t ip ro ton beam.

References

[ 1 ] K.C. K6nigsmann, Proe. 5th Intern. Conf. on Physics in collision (Autun, 1985) (l~dltlOnS Frontt6res, Gif-sur- Yvette, 1986) p. 161, and references thereto.

[2] L.J. Reinders et al., Phys. Rep. 127 (1985) I, and references therein.

[ 3] K.C. K6mgsmann, Proc. 17th Rencontre de Monond (Les Arcs, 1982) Vol. 1 (Quarks, Leptons and Supersymmetry) (l~dltlOnS Frontl6res, Gif-sur-Yvette, 1982), p. 63.

[ 4 ] PLUTO Collab., Ch. Berger et al., Phys. Lett. B 167 (1986 ) 120,

[5] TASSO Collab., H. Kolanoskl, Unlversit~'t Bonn prepnnt BONN-HE-86-08 (1986); talk 21 st Rencontre de Monond (Les Ares, 1986).

[ 6 ] MARK II Collab., G. Gldal et al., Contnb. 23rd Intern. Conf. on High energy physics (Berkeley, 1986).

[ 7] CRYSTAL BALL Collab., R.A. Lee, Stanford Report SLAC 282 (1985).

[ 8 ] C. Bag/in et al., Charmonmm spectroscopy at the ISR using an antiproton beam and a hydrogen gas "jet target", CERN- EP Internal Report 85-01 (1985).

[9] J.M. Brom, Thesis No. 1581, Strasbourg University report CRN/HE 85-04 (1985).

[ I 0 ] C. Bag/in et al., Phys. Lett B 172 (1986) 455. [ 11 ] Particle Data Group, Review of Patrick Properties, Phys.

Lett. B 170 (1986) I. [ 12 ] R.M, Baltrusaitls et al., Phys. Rev. D 33 0986) 629. [ 13 ] R. Barblen et al., Phys. Lett. B 106 ( 1981 ) 497;

L J. Remders et al., Phys. Lett B 113 (1982) 411. [ 14] L. Bergstr6m et al., Z. Phys. C 16 (1983) 263.

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