Study of the Jψ→γφφ decay

Volume 241, n u m b e r 4

PHYSICS LETTERS B

24 May 1990

STUDY OF THE J/~-~),~¢~ DECAY DM2 Collaboration D. BISELLO, G. BUSETTO, A. CASTRO, P. GINI, L. PESCARA, P. SARTORI, L. STANCO Dipartimento di Fisica dell'Universith di Padova and INFN, Sezione di Padova, 1-35131 Padua, Italy

Z. AJALTOUNI, A. FALVARD, J. JOUSSET, B. MICHEL, J.C. MONTRET Laboratoire de Physique Corpusculaire, Universitb de Clermont II, BP 45, F-63170 Aubibre, France

J.-E. AUGUSTIN, G. COSME, F. COUCHOT, F. FULDA, G. GROSDIDIER, B. JEAN-MARIE, V. LEPELTIER and G. SZKLARZ Laboratoire de l'Accklbrateur Linbaire, Universit6 de Paris-Sud, F-91405 Orsay, France Received 14 February 1990

The decay J / ~ - - , 7 9 ~ ? K + K - K s° K°L has been studied from 8 × 106 J/xC's produced in the DM2 apparatus at DCI. The branching ratio BR(J/~l-+T(OO)m~<2.gGev/c2 (3.4_+ 0.8 _+0.6)X 10 -4 agrees with the previous DM2 value from the four-charged-kaon mode. The analysis of both modes gives evidence of a resonant ~ production around 2.24 GeV/c 2, preferably pseudoscalar, which accounts for about one half of the total t 0 production below the qc. =

1. Introduction

The radiative decays of the J/~g have been extensively studied because they are considered a powerful tool to reach exotic states as gluonium foreseen by QCD. In a previous study of the J / ~ , 9 ~ - ~ y K + K - K + K - decay [ 1 ] we measured a substantial ~ production below the qc, which shows an enhancement near 2.2 GeV/c 2 with a clear pseudoscalar assignment. A rich pseudoscalar resonant production has been afterwards observed in the vector-vector mass distribution of J/~-,ymm [2,3] and J / l l / ~ y p p [4,5] by the DM2 and MARK III experiments. At present it is difficult to explain such results in a pure qcl scheme. On the contrary, the three tensor states gv (2100), gv(2200), gT(2360), seen in the n - P - , n 0 9 reaction [6] and considered glueball candidates, are not observed in the JDg radiative decays. No evidence of these states was found in ref. [ 1 ], since the small statistics did not permit to separate possible 2 + + signals

from an incoherent production. Subsequently an analysis of the J/~--*YPPchannel [ 4 ] based on a very large statistics has set a tight limit to the J / ~ ? g x ' s decay. In this paper a study of the J / ~ - - , ? ~ ) ~ 3,K+K-K°K ° decay is presented. Evidence is found of the same enhancement observed in the fourcharged-kaon mode and its spin-parity is analyzed. No search for the gr's states is performed because of the even smaller statistics. The study has been performed from eight million J / ~ ' s ~ produced in the DM2 experiment at DCI, the Orsay e+e - colliding ring.

2. Detector

The DM2 detector [8] is a large solid angle specWe were unable to reanalyze a small ( < 8%) set of data. The total number of J / ~ ' s is calculated from a study of the J/w-,pn decay [ 7 ].

0370-2693/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )

617

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trometer. A 0.5 T field is p r o d u c e d by a 2 m in diameter and 3 m long solenoid with a 1 Xo a l u m i n i u m coil. Inside the magnet, p r o p o r t i o n a l and drift chambers allow the measure o f the charged tracks over 87% × 4g steradians with a m o m e n t u m resolution o f 3.5% at G e V / c . A system o f 36 2 cm thick scintillators covering 80% o f the solid angle measures the time-of-flight with a resolution a = 540 ps, including 440 ps from the b e a m time spread, a n d provides a 3a n / K separation up to 450 M e V / c . The p h o t o n detector barrel (6Xo), d i v i d e d into ocrants, is outside the coil: it mainly consists o f planes o f streamer tubes with delay line interleaved with lead. The barrel covers 7 0 % × 4 g steradians and is fully efficient ( > 9 6 % ) for E~> 100 MeV. The resolution on the p h o t o n direction is 10 m r a d in a z i m u t h and 7 m r a d in polar angle. The p h o t o n energy can be measured by c o m b i n i n g the informations on the n u m b e r o f tube hits and the A D C signals. The resolution scales as 0 . 1 9 / ~ below 300 MeV a n d stays at the level o f 35% above this energy. Matching is p e r f o r m e d between the charged tracks in the central detector and the tracks reconstructed in the barrel in order to distinguish true photons from fake ones induced by n and K interactions in the coil or in the p h o t o n detector itself. Two end-cap p h o t o n detectors, o f 5Xo each, are inside the magnetic field, covering 12% o f the solid angle. Due to its limited angular resolution, this detector is used only as veto to define the requested topology.

3. Events selection We look [ 9 ] for events with at least one p h o t o n in the barrel a n d four tracks in the central detector with zero total charge, irrespective o f their origin from a c o m m o n vertex in the fiducial volume along the beam axis. N o p h o t o n has to be found in the end-caps. In the first raw selection, tracks belonging to a seco n d a r y vertex, if it exists, have to a p p r o x i m a t e the K ° mass within 75 M e V / c 2 and the two remaining tracks the O mass within 100 M e V / c 2 in the two-pion and two-kaon mass hypothesis respectively. W h e n the secondary vertex is not recognized by the tracking routine for each o f the four possible combinations the quantity 618

24 May 1990

62= (m~-- mK+K-) 2 + 0-2K

(mKo_mn+

n

)2

2

is calculated, where aKK=4 M e V / c 2 a n d a ~ = 6 M e V / c 2 a r e the Monte Carlo mass resolutions at the 0 a n d K ° masses. Events are selected if 62.G<400 and at least one kaon and one pion have T O F s compatible with the mass assignment within 1.4 ns. If more than one combination satisfies the above criteria, that one giving the lowest 62 value is retained. Subsequently, for each p h o t o n we calculate the angle a between the shower axis in the barrel and its direction defined by the vertex of the event and the apex o f the shower. We define as p r o m p t each p h o t o n whose cos a is greater than a value which ranges from 0.90 to 0.98 according to the n u m b e r o f planes hit by the shower (from 2 to 14 respectively). Only one p r o m p t photon has to be present outside a cone o f 25 ° a r o u n d the calculated K ° direction. Finally the events are 2C fit by imposing e n e r g y m o m e n t u m conservation and the K ° mass to the twopion system, the K ° detection not being required, a n d retained if Ev >i 30 MeV and X2~< 10. Energy and mom e n t u m of the p r o m p t photon are used in the fit.

4. Analysis The scatter plot o f mK+K- versus mKOK~gives evidence o f an event accumulation in the two-~ mass range (fig. I a). In order to isolate the 0~ signal, events which satisfy 1002 ~
MeV/c

2 ,

1010~MK+ K- ~< 1030 M e V / c 2 are selected. The a d o p t e d cuts correspond to + 2.5 a on M o n t e Carlo mass resolutions. They are insensitive to the m** value (fig. 2a). The inK+ K- and mKOKo mass distributions when mvovO~s~Lor InK+K- lies within the accepted ranges are reported in fig. l b and fig. lc respectively. A visual inspection indicates that (24 + 8 )% o f the selected events show an interaction pattern in the 25 ° cone a r o u n d the K ° calculated direction. This result agrees with the probability to detect a K ° through its

Volume 241, n u m b e r 4

PHYSICS LETTERS B •

. . . .

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24 May 1990

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t.oa ~04 ~o~

Mr, K- ( C e V / c z)

'

~ toe t.o4 ~.o~ MK*K*L ( G e V / e z)

Fig. 1. (a) Scatter plot of inK+K- versus mKoKoL, (b) InK+K- distribution for 1002 ~ mK~KOL~< 1038 M e V / c 2, (c) rn~0KoL distribution for 1010~mK+K- ~< 1030 M e V / c 2.

1:l

12 o

[::>

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M¢¢ ( G e V / c e) Fig. 2. (a) Monte Carlo 0 - o K + K - and O--,K°K° mass resolution, (b) J/W-+TO¢-q,K+K-Ks°K°L efficiency.

decay or regeneration in the coil or in the barrel, estimated from an inclusive study [7] of the J/~g~ K O l ~ o --0 S J ~ L JL decay. The 0O mass spectrum (fig. 3), obtained from 52 events, shows an enhancement around 2.25 G e V / c 2 and is poorly populated above 2.4 G e V / c 2. Moreover, 6 events are compatible with the expected small qc--'O0 signal :~2 ~2 The analysis for m ~ > 2.9 G e V / c z will be presented elsewhere.

0

~

225

2.5

2.75

3

M¢¢ ( G e V / c e) Fig. 3. Final m ~ mass spectrum, with the estimated background distribution.

Background from JD¢ decays into final states with (~ and K °, viz. J / ~ g ~ 0 K s ° K ° - , K + K - ~ + ¢ - ~ ° ~ ° and J / w ~ 0 K * K - ~ K + K - K s ° K ° ~ °, is found to possibly contribute only for re,o> 2.8 GeV/c 2 and to be in fact negligible. Then contamination from multihadron final states with large branching ratios which could simulate the particular mass distribution is estimated from 0-side bins. This background is evaluated to 12 events for m** < 2.9 G e V / c 2 and is mainly concentrated in the highest part of the ~0 mass spectrum. 619

Volume 241, number 4

PHYSICS LETTERS B

and both give evidence of an enhancement around 2.25 GeV/c 2. The partial branching ratio in this mass range from the K + K - K s ° K ° mode,

The global detection efficiency, estimated for pseudoscalar t t systems, is reported in fig. 2b. This spin-parity choice is justified by the results of the angular analysis reported in the following section. Nevertheless, the efficiency does not vary notably for different angular distributions. The total branching ratio, calculated from (33 _+7) events, is BR ( J / ~ - ' Y 0 t )m**<2 . 9 G e V / c

B R ( J / ~-~ T t t ) 2.2
4,

agrees with the corresponding K + K - K + K - value calculated from the data of ref. [ 1 ] :

2

B R ( Jltl--~ ]ttt )2.2
= ( 3 . 4 + 0 . 8 _ + 0 . 6 ) X 1 0 -4,

= (0.9_+0.3_+0.2)× 10 - 4 .

where the systematic error accounts for normalization (15%) and efficiency ( -~ 6% ) uncertainties. This result fully agrees with what was obtained in the t t - , K + K - K + K - mode [ 1 ]: BR ( J / ~ ¢ ~ ' { t t )m,° < 2 . 9 G e V / c

24 May 1990

Then a spin-parity analysis is performed for the events in this mass range. We recall that such a study gave a preferred 0 + assignment [1] for the K + K - K + K - final state. The experimental distributions of the azimuthal angle Z [ 10] between the two t decay planes, and of the two polar angles, 01 and 02, of the K + and Ks° (K + ) decays in their respective t rest frame relative to the t m o m e n t a in the t t rest frame, fig. 5a and fig. 5b for J / v ~ ' t ' t t - ~ K + K 0 0 KsKc and K + K - K + K - respectively, are also similar. A pseudoscalarity analysis is chosen. This analysis permits to disentangle between a pseudoscalar component and a flat distribution. When a pseudoscalar state decays into two identical vectors, the angle Z,

2

= (3.1 _+0.3 _+0.6) X 10 -4,

5. Discussion The t t mass spectra for the two modes, weighted by their relative inverse efficiencies, are also consistent within the large statistical errors (figs. 4a, 4b)

200

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2.;

2.8

M¢¢ (GeV/c z)

Fig. 4. (a) O0-~K+K-K°K °, (b) 00~K+K-K+K - mass distributions weighted by relative inverse efficiencies. 620

Volume 241, number 4

PHYSICS LETTERS B

24 M a y 1990 10

a

2

a)

r--

2

u9

0 ................ O 8

(p > L~

0

.................. 20 40 6 0

80

.

.

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0

05

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~

,\ ( d e g . )

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OK

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M¢¢

Fig. 5. A n g l e Z and cos OK distributions in the 2 . 2 - 2 . 3 G e V / c 2

mass range for ( a ) m , , - K * K K~K°L, ( b ) m ~ K + K

K*K-.

dN

dz

- 1 - cos 2Z.

The Z angle distribution of the events, by 50 M e V / c 2 mass bin, is fit to the function dN/dz=a+bsin2z in order to evaluate the fraction of pseudoscalar events per mass bin. The results, shown in figs. 6a, 6b for the K + K - K ° K ° and the K + K - K + K - modes respectively, agree with a large pseudoscalar production up to 2.3 GeV/c 2 estimated to (64_+22)% and (85 _+27)% of the signal respectively. The same analysis gives (90_+ 16 )% for pseudoscalar Monte Carlo events. The difference between the two modes is due to a small background ( < 5 events at 95% CL) in the K + K - Ks0 KLo sample. On the contrary the background was completely negligible in the four-chargedkaon m o d e [ I ]. It has to be noted that a pure wave analysis is unable in this case to identify the contributions o f 0 + waves. This result has been checked by a pure wave analysis in which the likelihood distributions of pseudoscalar Monte Carlo samples containing a growing percentage of phase space events

~3 T h e D M 2 angular acceptance is sufficiently flat so that the

integration done to obtain the formula above is valid.

E.6

2.8

(CeV/c e)

Fig. 6. Mass spectrum of the pseudoscalar component for ( a ) 09-*K+K-K°K°,

integrated ~3 over the other angles, takes the form

'

-1£

1---2

Angle

.

o 1l

I/

4

0

(b) 00-,K+K-K+K

-.

are compared to the experimental likelihood for a pseudoscalar hypothesis ~4. The probability to obtain a likelihood worse than the experimental one is zero for no background but grows up to 37% if four background events are assumed (fig.7). ~4 The errors account for the number of experimental events. A large number of random samples of Monte Carlo events, numerically equivalent to the experimental one, were analyzed. ® Experimental

Data

Montecarlo 10

a 0

0 o o

o -10 0

b)

c)

d)

e)

5

"it

-1,5 -20

Fig. 7. Experimental log of likelihood ratio of the pseudoscalar to the phase space hypotheses, compared to the same quantities for pseudoscalar Monte Carlo samples, numerically equivalent to the experimental one, mixed to 0 ( a ) - 4 ( e ) phase space events (2.2
GeV/c2). 621

Volume 241, number 4

PHYSICS LETTERS B

We c o n c l u d e that the e n h a n c e m e n t a r o u n d 2.25 G e V / c 2 is largely c o m p a t i b l e w i t h that o b s e r v e d in the s a m e mass range in the f o u r - c h a r g e d - k a o n m o d e . In o r d e r to e s t i m a t e its p a r a m e t e r s the s u m o f the two c)~ spectra o f figs. 4a, 4b n o r m a l i z e d to the s a m e n u m b e r o f J / ~ ' s , is used. A fit to a B r e i t - W i g n e r c u r v e a d d e d to a q u a d r a t i c f u n c t i o n gives m x = (2238 _+ 7 ) M e V / c 2 ,

24 May 1990

300

E" >.

200

o v

F x = (80_+ 3 0 ) M e V / c 2 ,

100

a n d the f o l l o w i n g b r a n c h i n g ratio: BR(J/~TX

(2238)) × BR(X(2238)~09)

= ( 1 . 7 _ + 0 . 5 _ + 0 . 3 ) ) < 1 0 -4 , w h e r e the statistics e r r o r a c c o u n t s also for the fit uncertainties. As discussed above, the s p i n - p a r i t y o f this e n h a n c e m e n t is c o n s i s t e n t with an i m p o r t a n t pseudoscalar c o m p o n e n t . T h i s signal could indicate a new state or be related to the rich p s e u d o s c a l a r a c t i v i t y o b s e r v e d in the J / ~ r a d i a t i v e decays into v e c t o r meson pairs [ 2 - 4 , 5 ] .

2.2

2.4

2.6

2.8

M~ (GeV/c z) Fig. 8. Sum of the inverse efficiency weighted ~ + K + K- K° K ° and ~ ~ K +K- K + K- mass distributions. The overplotted curve refers to the fit described in the text. h a n c e m e n t is f o u n d c e n t e r e d at 2238 M e V / c 2 a n d -~ 80 M e V / c 2 wide, and to a c c o u n t for a b o u t o n e h a l f o f the total ~¢ p r o d u c t i o n b e l o w the qc (fig. 8).

6. C o n c l u s i o n T h e J / ~ - , 7 O 9 - ~ T K + K - K ° K ° decay is o b s e r v e d and the m e a s u r e d b r a n c h i n g ratio B R ( J / ~ - ~ 7QO)m,,<2.gGeV/c2=(3.4+_0.8-+0.6)×lO -4 agrees w i t h that o b t a i n e d in the f o u r - c h a r g e d - k a o n m o d e . S i m i l a r mass spectra are o b t a i n e d a n d f u r t h e r evid e n c e is f o u n d o f an e n h a n c e m e n t in the 0Q m a s s s p e c t r u m a r o u n d 2.25 G e V / c 2. A clear p s e u d o s c a l a r n a t u r e was m e a s u r e d for this e n h a n c e m e n t in the f o u r - c h a r g e d - k a o n m o d e . In the K + K - K s ° K ° m o d e the angular d i s t r i b u t i o n s are c o m p a t i b l e with this ass i g n m e n t , but the p r e s e n c e o f a residual b a c k g r o u n d and the small statistics do not p e r m i t to i m p r o v e the p r e v i o u s results. F r o m a fit o f the sum o f the 0~ mass spectra, w e i g h t e d by the r e l a t i v e inverse efficiencies, this en-

622

References [ 1] D. Bisello et al., Phys. Len. B 179 (1986) 294. [2] D. Bisello et al., Phys. Len. B 192 (1987) 239. [ 3 ] R.M. Baltrusaitis et al., Phys. Rev. Len. 55 (1986) 1723. [4] D. Bisello et al., Phys. Rev. D 39 (1989) 701. [5] R.M. Baltrusaitis et al., Phys. Rev. D 33 (1986) 1222. [6] R. Etkin et al., Phys. Rev. Lett. 49 (1982) 1620. [7] J. Jousset et al., Phys. Rev. D, in press; and preprint LAL 88-25 (1988). [8] J.E. Augustin et al., Phys. Scr. 23 ( 1981 ) 623. [9] More details can be found in P. Gini, Tesi, Universit~ di Padova (1989), unpublished. [ 10] N.P. Chang and C.T. Nelson, Phys. Rev. Lett. 40 (1978) 1617; T.L. Trueman, Phys. Rev. D 18 (1978) 3423,