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IL NUOV0 CIMENTO VOL. 71 A, N. 4 21 0 t tobro 1982

Observation of the Decay ~--~ ~yy.

( I H E P (*)-IISN (**)-LAPP (***) COLLABORATION)

F. BINON (**), C. BRICMAN (**), V . A . DAVYDOV (*), S . V . DONSKOV (*) J . ])UFOURNAUD (***), ~P. DUTEIL (***), M. GOUAN]~RE (***)~ A . V . INYAKIN (*) V . A . KACHANOV (*), D . B . I~AKAURIDZE (*), G . V . KHAUSTOV (*) A . V . KULIK (*), J . P . LAGNAUX (**), A . A . LEDNEV (*), YU. V. MIKHAILOV (*) J . P . PEIGNEUX (***), YU. D. PROKOSHKIN (*), YU. V. I:[ODNOV (*) S . A . SADOVSKY (*), A . V . SINGOVSKY (*), A . V . STARZEV (*) J . P . STReeT (**) and V . P . SIYGO~YAEV (*)

Jo in t Exper imen t o] I H E P . Serpukhov, U S S R C E R N - Geneva, Switzerland

(ricevuto il 21 Giugno 1982)

In memory of A.V. STARZEV who passed away unexpectedly on June 14, 1981

S u m m a r y . - The rare radiat ive decay ~ - ~ ~ w has been effectively observed and studied at the 70 GeV I H E P accelerator with the hodo- scope spectrometer GAMS-2000. The branching ratio for this decay is found to be B R ( ~ --~rOyy) = (9 .5 :~ 2 .3 ) - 10 -4.

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

A prev ious e x p e r i m e n t a t I H E P has shown (1) t h a t the b r a n c h i n g ra t io for the r a d i a t i v e d e c a y of t he ~ -meson in to n o a n d a p h o t o n p a i r

(1) . .q __~ ~ O y y

(*) Inst i tute for High Energy Physics, Serpukhov, USSR. (**) Inst i tute Interuniversitaire des Sciences Nucl6aires, Brussels, Belgium. (***) Laboratoire d 'Annecy de Physique des Particules, France. (***) CERN, Geneva, Switzerland. (1) F. BINON, C. BRICMAN, V. A. DAVYDOV, S. V. DONSKOV, J. DUFOURNAUD, P. DUTEIL,

497

498 IHEP-IISN-LAPP COLLABORATION

is smaller t han 3 .10 -3, br inging to a~n end a long-last ing cont roversy (2) be tween current theoret ical values lower t han 10 -~ (8,4) and exper imen ta l results up to 10 -~ which cast some doubts on schemes like PCAC, current ~lgebra, etc.

Measurements t h a t have been pursued at the I H E P accelerator h~ve allowed us to observe the decay mode (1) for the first t ime.

A contamina t ion ma in ly due to the intense decay channel

(2) ~ -~ 3n ~

might explain earlier claims for the observat ion of decay (1). The present work is devoted to these new results.

2. - Exper imenta l procedure.

The measuremen t s have been per formed with ~-mesons produced by 30 GeV/c negat ive pions in the charge exchange react ion (5)

(3) T.-p -~ ~n .

The hodoseope ~erenkov spec t romete r GAMS-2000 (for details concerning the set-up see (1,6,7)) has been used to detec t the photons emi t ted by the

M. GOUAN~RE, .A.V. INYAKIN, V..A. KACHANOV, D. B. KAKAURIDZE, G. V. KHAUSTOV, A. V. KULIK, J. 1 ~. LAGNAUX, _A_. A. LEDNEV, YU. V. MIKHAILOV, J. i ~. PEIGNEUX, YU. D. PROKOSHKIN, YU. V. RODNOV, S. A. SADOVSKY, A. V. SINGOVSKY, A. V. STARZEV, J. P. STROOT and V. P. SIIGONYAEV: :Fad. •iz., 33, 1534 (1981); Lett. Nuovo Cimento, 32, 45 (1981); see also the l i terature quoted therein. (~) PARTICLE DATA GROUP: .Rev. Mod. Phys., 52, 70 (1980). (s) G. OePo and S. ONEDA: Phys. Bey., 160, 1397 (1967). (4) T. P. CHENG: Phys. l~ev., 162, 1734 (1967). (5) W . D . APEL, K. H. AUGENSTEIN, E. BERTOLUCCI, YU. B. BUSHNIN, S. V. DONSKOV, R. FRANCESCHINI, •. GIVOLETTI, :M. I. GRACHEV, A. V. INYAKIN, R. JOHNSON, V. A. KACHANOV, W. KITTENBERGER, R. N. KRASNOKUTSKY, M. KRUGER, G. LEDER, A. A. LEDNEV, I. MANNELLI, YU. V. MIKHAILOV, H. MOLLER, R. OBERPARLEITER, M. PER- NICKA, G. ~ . PIERAZZINI, YU. D. PROKOSHKIN, 1V[. QUAGLIA, D. SCHINZEL, H. SCHNEIDER, A. SCRIBANO, F. SERGAMPIETRI, R. S. SHUVALOV, G. SIGURDSSON, H. M. STAUDENMAIER, M. STEUER, A. THYS, A. N. TOROPIN and M. L. VINCELLI: ~'a~. ~iz., ").9, 1519 (1979); Nucl. Phys. B, 152, 1 (1979). (6) F. BINON, C. BRICMAN, V. •. DAV2gDOV, S. V. DONSKOV, J. DUFOURNAUD, P. DUTEIL, M. GOUAN:ERE, .A.V. INYAKIN, V. A. KACHANOV, D. B. KAKAURIDZE, G. V. KHAUSTOV, A. V. KULIK, J. P. LAGNAUX, A. A. LEDNEV, YU. V. iV[IKHAILOV, J. P. ])EIGNEUX, YU. D. I~ROKHOSHKIN, YU. V. RODNOV, R. ]{OOSEN, S. ~k. SADOVSKY, D. SILLOU, A. V. STARZEV, J. P. STROOT and V. P. SIIGONYAEV: Nuel. Instrum. Methods, 188, 507 (1981). (7) F. BINON, V. A. DAVYDOV, S. V. DONSKOV, J. DUFOURNAUD, P. DUTEIL, IV[. GOUA- NgRE, A. V. INYAKIN, V. A. KACHANOV, D. B. I{AKAURIDZE, G. V. KHAUSTOV, YU. S.

OBSERVATION OF THE DECAY "~--~7~OT~ 4~9

decay ing ~'s. More detai ls a b o u t the spec t rome te r will be g iven in a for th- coming paper . On ly the modif ica t ions which have been b r o u g h t t o t he experi- m e n t a l se t -up used in t he prev ious w o r k (~) are m e n t i o n e d here.

The dis tance L be tween t he l iqu id -hydrogen t a r g e t (7) und GA~IS has been increased f r o m 9 m to 12 m a nd t he m o m e n t u m of t he r :- b e a m bus been

lowered b y 30 %. The d is tance be tween T-shower impac t s on t he spectro-

me te r is, as a consequence~ increased b y 70 ~o. As a resul t , spurious processes (like (2)), wh ich could be confused wi th decay (1) when showers overlup~ are

be t t e r identified. S imul tuneous ly this has b r o u g h t an i m p r o v e m e n t of the mass resolut ion of GAMS for decay ing par t ic les (aM/M _~ 1 ~ ) (fig. ] ) and easier work ing condi t ions for the shower r econs t ruc t i on p rograms .

2000

iroo i i

490 520 550 580 ~: :(I,I eV/c 2)

3000

N

Fig. 1. - Mass resolution of the spectrometer GAMS-2000 for MO-particles produced in reaction (4) decaying into 3re ~ The peak corresponds to ~-~ 37: ~ the arrow points to the value of the ~ mass given in the tables. The mass resolution of GAMS is charac- terized by a M = 5.1 MeV/c ~, ~M/M = 0.9%. The curve is a Gaussian distribution. I t practically coincides with the distribution of events simulated with real showers (see below).

The ~ - b e a m in t ens i ty has been increased up to 1 .5 .30 T per bu r s t for a

p ro ton spill on the in te rna l t a r g e t of 1.5 s, a t t he cost of a more in tense ir- r ad ia t ion of GAMS. I n order to avoid a g radua l b l acken ing of the lead glass

KHODIREV, A. V. KULIK, J . ~). LAGNAUX, . A . A . LEDNEV, YU. V. ~/~IKHAILOV, J . 1 :). PEIGNEUX, YU. D. PROKOSHKIN, YU. D. I~ODNOV, 1:~. ROOSEN, S, A. SADOVSKY, A. V. STARZEV, J. P. STROO~, V. P. STSGONYAEV and A. E. YAXVTIN: Fad. ~iz., 33, 1244 (1981); Z. Phys. C, 9, 109 (1981).


cells which are nearest to the beam, the central radia tor cells of GA~r have been replaced by other ones made of a special glass 80 t imes more radia t ion res is tant (8).

I n these conditions the n u m b e r of events

(4) ~ - p -~ M~ ]---> m y

detected b y the spec t rometer , a f te r selection b y the t r igger system, amoun ted to ~ 300 per beam burs t . Fas t processors have been added to the da ta acqui- sition system. They allow us to compute the to ta l energy release ~ ----- ~ ~ A j and the first radial m o m e n t ~ r ~-- ~ ~Ajrj/L of the showers in GAMS (At is the magni tude of the signal in the j - th counter , rj is its distance to the b e a m axis and ~j is its normal iza t ion coefficient). The ~ values are de te rmined exper imenta l ly during the cal ibrat ion of GAMS in a 25 GeV/e electron beam (~,7) and are stored in the m e m o r y of the processors.

_~1~]~ is propor t iona l to to M ~ -~ p~, where M and PT are the mass and the t ransverse m o m e n t u m of the decaying part icle, respectively. The selection of events hav ing radial m o m e n t a ~ r above ~ 350 MeV eliminates most of pion charge exchange processes = - p - ~ 7 : ~ (~), wi thout affecting the detect ion efficiency for ~-mesons. The n u m b e r of events s tored into the compute r H P 2] 00 A is thus b rought down to ~ 100 events /burs t only.

Dur ing the four days of da ta taking, 3.1011~ - have reached the liquid-

hydrogen t a rge t leading to the product ion of 600000 ~-mesons in the u - p --~ ~u react ion. This n u m b e r is 30 t imes larger than the statist ics in our previous exper iment and in all o ther works which were devoted to the s tudy of decay (1), al together. The background due to ~ dec~ying into 37:0 and other processes, which m a y s imulate decay (1), has been lowered a t the same t ime by more t han one order of magni tude leading to a level sensi t ivi ty <~ 10 -3 for decay (1).

3. - R e c o n s t r u c t i o n a n d s e l e c t i o n o f e v e n t s .

D a t a wri t ten on magnet ic t ape (about 2.5 million triggers) have been pro- cessed with the help of a set of p rograms allowing one to reconst ruct and to make the k inemat ica l analysis of the events (1,7).

(s) A . V . INYAKIN, D. B. KAKAURIDZE, A. A. LEDNEV, YU. D. ]~ROKOSHKIN and A. V. SINGOVSKY: preprint I H E P 82-6, Serpukhov (1982). (9) W . D . AP]~L, K. H. _A-UG]~NST]~IN, ]~. BERTOLUCCI, S. V. DONSKOV, A. V. INYAKIN, R. JOHNSON, V. A. KACHANOV, R. N. KRASNOKUTSKY, ~ . KRUGER, G. LEDER, A. A. LEDNEV, I. ~r YU. V. ~r H. 7-~/[0LL]~R, G. M. PIERAZZINI, YU. D. PROKOSHKIN, ~][. QUAGLIA, D. SCHINZEL, H. SCUNEID~R, A. SCRIBANO, F. S]~RGAM- PH~TRI, R. S. SHUVALOV, G. SmUr~DSSON, M. STEUER, A. N. TOROPIN and M. L. VINC]~LLI: Yad. ~iz., 30, 373 (1979); Nucl. Phys. B, 154, 189 (1979).

OBSERVATION OF TITEDECAY ~__~:Oyy 501

The new reconst ruct ion p rog ram looks for showers and evaluates the co- ordinates and energy of each Y by mak ing use of the informat ion ga thered on shower shape and f luctuations dur ing the cal ibrat ion of the spec t rometer with electrons of 10 GeV and 25 GeV. Even t s are sorted out according to the mult i- pl ici ty m of 7 's emi t ted in react ion (4). I t has been much improved with regard to the version used in our previous work (1), giving a be t t e r separat ion of near- ly ing showers. Classification ambigui t ies are much reduced. Less ~han a few per cent of events need to be t ransfer red be tween ad jacent classes m and re:k1 in the course of the analysis. Technical improvemen t s of the spectro- me te r GAMS have b rought the m i n i m u m energy threshold for y detect ion down to Ew~ . ----0.15 GeV, result ing in be t t e r constraints on decay (2) and a large reduct ion of the ma in background.

Even t s classified in the class m ~ 4 have been re ta ined for fu r ther k inema- t ical analysis. Those which satisfy the k inemat ics of react ions

(5) rr-p ~ r:~

(6) :z-p -~ K~A

or : : - p - ~ ~ (lo), with Z 2 < 12 (99% confidence level), have been discarded. The compat ib i l i ty of the remain ing events with the hypotheses M ~ -~v.~ and M ~ -~ ~]yy has been checked. I f the value of g s is smallest and not larger t h a n 7 (97 % confidence level) when the first hypothes is is re ta ined, events are fu r ther considered as possible candidates for react ion

(7) ~ - p -~ r~~

Then the region of small M ~ t ransverse m o m e n t a PT < 0.3 GeV/c has been excluded in order to reduce the background. At the cost of a 20 % loss of good events (3) (5), this allowed us to suppress the contr ibut ion of react ions like (5), which, going th rough one-pion exchange, are concent ra ted at small PT (11), and the cont r ibut ion of for tui tous coincidences.

Contaminat ion due to reactions like (5) and (6), giving ~o pairs in the final state, is fu r ther suppressed by selecting events in which the mass corresponding to the pair of noncorrela ted y 's in (7) is larger t h a n 180 MeV/e ~ only. Of these

(lO) W. D. APEL, K. H. AUOENSTEIN, E. BERTOLUCCI, S. V. DONSKOV, A. V. INYAKIN, V. A. KACHANOV, R. ABDEL KttALEK, R. : i . KRASNOKUTSKY, ~ . KR~.TGER, U. LEDER, A. A. LEDNEV, I. MANNELLI, YU. V. MIKHAILOV, I-I. ~r G. M. PIERAZZINI, YU. D. PROKOSIIKIN, ~V~. QUAGLIA, H. SCHNEIDER, .A. SCRIBANO, F. SERGIAMPIETRI, ]~. S.

SItUVALOV, G. SIGURDSSON and M. L. u Natl. Phys. B, 193, 269 (1981). (11) V. N. BOLOTOV, V. V. ISAKOV, D. B. KAKAURIDZE, YU. D. PROKOSHKIN, S. A. SAD0VSKI and G. V. KRAUST0V: Yad. Eiz., 24, 106 (1976).

502 ] I I ~ P - I I S N - L A P P C O L L A B O R A T I O N

reactions, the mos t t r i cky source of background is cer ta inly (6) (in the case in which the K ~ decays at a few met res f rom the target) . The detect ion of the decay products of the recoil hyperons by the counters of the guard sys tem, which surrounds the Larget in the set-up, allowed us to reduce the contribu- t ion of react ion (6) to the level of a few percents of the ma in process (3).

The m i n i m u m value of energy threshold Eth ~ . has only been used in the y reconstruct ion p rogram in order to reduce as much as possible the main background which comes f rom decay (2), when any two y 's are being missed. One drawback of such u low threshold is t h a t the p rog ram somet imes finds spurious low-energy y 's genera ted b y the noise of the counters and of the elec- tronics of the spect rometer . At the stage of the k inemat ica l analysis, a quite higher-energy threshold for the y 's , Eta,, has been used. By increasing Eth , the above-ment ioned ins t rumenta l background decreases fast.

4. - M a s s spec tra .

The effective mass spec t rum of the ~oyy system, constructed f rom the events which have passed th rough the selection procedure described above, is shown in fig. 2a). A narrow peak, with a s tat is t ical significance exceeding 7 s tandard deviations, shows up in the region of the ~ mass.

This peak has been fi t ted wi th a Gaussian dis tr ibut ion F ( M - - M , a~), the pa rame te r s _M and ~ and the normal izat ion being left free. The background pa r t of the spec t rum, which is bell-shaped with a m a x i m u m near 500 l~eV, has been f i t ted wi th a polynomial funct ion Fb(M). This procedure provides a s tat is t ical ly significant descript ion of the measured spec t rum as :~2/nDgl (e.g. fig. 2a) and b)).

The evalua ted ~]I value is found to be independent of the threshold energy

E ~ value be tween 0.5 GeV and 3.5 GeV. ~t is equal to (549~=2)MeV/c 2, a value which agrees within 0.2 ~o with the t abu la ted ~) mass.

The region of the mass spec t rum around i GeV/c 2 has been studied to check the l ineari ty of the spec t rometer and the accuracy of its calibration. Here too a peak appears in the mass spec t rum of the 7:~ system. I t is due to the rare decay ~ ' -+o)$-+~:~ (BR ~ 2.10 -3) (2) of ~'(958) mesons produced in react ion (4). The f inal-state topology is the same as th,~t of decay (]). The posit ion of the ~' peak agrees to 0.4 ~o with the va lue given in the tables for the ~' mass. The measured value of the Ks ~ mass in react ion (6) is also in good agreement (0.3 ~o) with the tables, which demonst ra tes the accuracy of the

mass scale of GAMS. The width of the ~ peak shows also no dependence on the y energy

threshold Eth. The value aM--~ (7.2=[:1.5)MeV/e 2 obta ined in the fit ugrees wi th the intrinsic resolution of the appara tus (8.5 h[eV/e~).

OBSERVATION OF THE D~CAY ~--~r@yT 503

30

f/

20

b)

510 i

55O 590

40

N

20

a)

0 I i &00 500 600 70O

M~ol (M eV/c z)

Fig. 2. - a) Mass spectrum of the n0yy system measured in reaction (7). The arrow points to the ~] mass. The dashed curve is a polynomial function ~b which describes the background par t of the spectrum, b) Mass spectrum in the region of the ~-meson obtained from a) after subtraction of the background distr ibution ~b (dashed curve). The dot ted curve is the Gaussian distr ibution 17,(M--_~r, a~) obtained with the best est imates of the parameters , after fitt ing 2~ = 549MeV/e ~, a M = 7.2 MeV/c% The measured spectrum has been fi t ted by a superposition of / 7 and /~b with z~/n, : 0.7.

5. - S i m u l a t i o n p r o c e d u r e .

A s i m u l a t i o n p r o c e d u r e b a s e d on a b a n k of m e a s u r e d showers has been

d e v e l o p e d to d e t e r m i n e t h e r e g i s t r a t i o n eff ic iency for d e c a y (1) a n d t h e v a l u e

of BR(~ 1 _ . ~ o y y ) : F (~ --* ~~ --* all) .

The 7's f rom t h e d e c a y B --* 27, wh ich a re d e t e c t e d in t h e s p e c t r o m e t e r

t o g e t h e r w i th t hose p r o d u c e d in d e c a y ( ] ) , a re wel l s e p a r a t e d s p a t i a l l y . T h e y

504 II-IEP- IISN-LAPP COLLABORATIOI~

have been used as a source of real individual showers. The set of ampl i tudes A; in the (~ fired ~) cells of showers produced b y 150000 y ' s of var ious energies and co-ordinates have been stored in a spec t rometer da ta bank.

I n a first stage, events produced in react ion (3) have been genera ted b y the me thod of Monte Carlo by using the known t-dependence of the differential cross-section (5). The phase-space dis tr ibut ion has been assumed to be uniform. Fur the r , the informat ion stored in the bank of showers has been used for each y enter ing GA~S . After , the genera ted events have been analysed with the help of the ve ry same set of p rograms which has been used for the analysis of the exper imenta l data . The detect ion efficiency s of the spec t rometer de te rmined in this way takes na tura l ly into account the acceptance of the set-up, the efficiency of the programs ensuring the reconstruct ion and the k inemat ica l analysis and the selection cri teria for the events.

I n order to check the accuracy of this procedure, s imulated values of s have been de te rmined for bo th decays ~ -+ 2~ and ~ -* 37:0. They have been compared with the num ber of events de tec ted in the spec t rometer . The efficiency s decreases quickly as the mul t ip l ic i ty m increases, essentially following the relat ionship ( 1 - - m E t J E ) TM. For the considered decays, s amounts to 0.8 (m = 2) and to 0.23 (m ~ 6) wi th Err = 0.5 GeV. Notwi ths tand ing such a large difference, the rat io of decay widths F(~--* 2y)/F( ~ --~ 37:o) obta ined f rom the n u m b e r of events found exper imenta l ly and the calculated efficiencies agrees within 10 % with the value quoted in the table (2).

A smaller error is expected for the detect ion efficiency in the case of decay (l), which has a much simpler topology (m : 4) t han decay (2).

6. - Partial width of the decay ~ - ~ ~0yy.

The number N of decays ~--~ ~~ , observed exper imenta l ly for several p rede te rmined values of Eth , the threshold energy for Y detect ion in GAMS, is shown in fig. 3. With increasing threshold energy, iY decreases rapidly, in qual i ta t ive agreement wi th the factor (1 - - Eth (GeV)/7.5) 4 (see above).

The detect ion efficiency ~ for decay (1) is shown in the same figure (full curve). I t has been normal ized to the n u m b e r of measured events with E~h --~ 2 GeV (at this point z - 0.13, tak ing into account all selection criteria). The calculated curve reproduces well the exper imenta l ly observed N(Et~). This confirms the correctness of tile me thod followed to separa te events corresponding to decay (1) f rom the background.

The par t ia l width of the decay ~ -+ ~~ has been deduced f rom the n u m b e r of events detected wi th Eth ~ 2 GeV (the error is m i n i m u m at this threshold value) normal ized to the n u m b e r of ~ -+ 2 7 decays measured simultaneously. The result ing rat io of decay widths is Fi~ -~ ~~ --~ 2y) --~ (2.5 =L0.6). 10 -3.

OBSERVATION OF THE D]ECAY ~-~r~~ 505

150

N

100

50

1 I I I I I 1 I ]

] 2 3 E~h (5 e V)

Fig. 3. - Number of events observed for the decay ~)--*=oyy for various values of the threshold energy Eth. The curve is the detection efficiency for this decay mode nor- realized to the point Eth = 2 GeV.

From this and from the known branching ratio for the decay ~ --~ 2y (2), it follows tha t

(8) BR(~ --~ ~oTT ) = (9.5 •

The largest contribution to the error is statistical (19 %). The error also includes a systematic contr ibution (14%) (including the uncer ta in ty linked

with the background subtract ion procedure). The measured ratio of decay widths and the previously measured value

of F(~ -~ 2y) (12) give F(~ -+ ~oyy) _~ (0.81 4-0.22) eV.

7. - Background analysis. The contr ibution of various processes to the background par t of the spectrum

(dashed curve in fig. 2) has been evaluated with the same simulation method, based on a bank of real showers. "The simulated events have been submit ted

to the same t rea tment and selection procedures as those which were used to

construct the experimental mass spectra.

(12) A. BROWMAN, J. DEWII~]~, B. GITT]~LMAN, K. M. HANSON, R. LEWIS and E. LOll: Phys. Rev. Lett., 32, 1067 (1974).

5[}6 IIIEP- IISN-LAPP COLLABORATION

The curve 2~: o in fig. 4 shows the mass s p e c t r u m of c a n d i d a t e ~ y n

even t s which could have b e e n confused w i t h ~~176 events . The ana logous

c o n t r i b u t i o n f rom Ks~ is l abe l led K ~ These spec t ra have been no rma l i zed

to t he n u m b e r of r eg i s t e red even t s of each type . The n o r m a l i z a t i o n is in good

a g r e e m e n t w i th the k n o w n cross-sect ions for these reac t ions . The c o n t r i b u t i o n

of the process ~:-p --~ 3r~~ has been d e t e r m i n e d in the same m a n n e r . ] t ap-

pears to be one order of m a g n i t u d e lower t h a n in the case of ~ - p - - ~ n~

~ 2m ~ 10 0

I I I

~oo 5oo 6oo 200 H ( M eV/c :~ )

Fig. 4. - Simulated background using a bank of real showers. N is the number of events candidate for the reaction =-p--~7:~ per AM = 5 MeV/c 2 mass interval of the 7:~ system. 27: ~ and K ~ are the contributions of processes (5) and (6), respectively. (37} is the background linked to events ~( pumped )> from the class m = 3 into the class m ~ 4. ~1 and ~ are the contributions to the background of the decay ~ --~ 3~ ~ (see text). ~ is the sum of the above-mentioned contributions. /7 b is a polynomial function which describes the background part of the spectrum in fig. 2. This spectrum corresponds to E t h ~ 2 GeV. Analogous agreement between ~ and 2' b is obtained for other threshold values E t h , ranging from 0.5 to 3.5 GeV too.

The c o n t r i b u t i o n to t he b a c k g r o u n d due to the smal l n u m b e r of even t s

be long ing to the topological class m = 3 which have b e e n (( p u m p e d >) i n to

t h e class m - - 4 in the presence of a spur ious shower is r e p r e se n t e d in fig. 4 b y

t he curve (37). I t s e v a l u a t i o n is based on the obse rva t ion of s imi la r misclas-

s i f icat ions of ~ -~ 27 which are wrong ly p u t in the class m ~-- 3.

The c o n t r i b u t i o n s of all these reac t ions to t he measu red b a c k g r o u n d spec-

t r u m F b are no t v e r y large. The i r sum is p rac t i ca l ly c o n s t a n t over the con-

s idered mass i n t e r v a l a n d does no t exceed 10 % of the ~ peak.

OBSERVATION OF THE DECAY ~_._~,~0~,y 5 0 7

I n order to simulate the background due to B's decaying into 3~ o, two processes have been considered. The first is l inked with such events t ha t have been misdirected in the class m = 4 by the reconstruct ion p rogram (overlapping showers in the spect rometer , ~,'s of energy lower t h a n the threshold value Ev~i~, etc.) though all six 7 's h~ve reached the spect rometer . This kind of background is represented in fig. 4 by the curve ~ .

The second source of ~ --* 3r. ~ background is due to events in which one or two T's have fallen outside the l imits of GAMS ~nd have not been detec ted by the guard sys tem counters (7) due to some inefficiency of these for low- energy T's. This background is represented in fig. 4 b y the curve ~ . I t s normal iza t ion depends on the num ber of ~ --~ 3Too decays and also on the efficiency of the aper ture-def ining counters. I t h~s been de te rmined by com- par ing measured and s imulated 57 spectra in the neighbourhood of the ~ mass.

The spec t rum obta ined a f te r summat ion of all contr ibut ions to the background is represented in fig. 4 by the curve ~ , T h e m a i n source of background observed in the mass spec t rum of the v:~ events (fig. 2) is clearly the decay ~ -* 3~ ~ Though only 0.5 % of these decays af ter se]ection are re- sponsible for the main pa r t of the background, it is fair ly accttr~tely described (compare curves F b and ~ in fig. 4) b y the simulation procedure '

8 . - D i s c u s s i o n .

Star t ing with the pape r of Okubo and Saki ta (13) a series of works in which various theoret ical models have been applied (3,4,14-~o) has been devoted to the calculation of dee~y ~ --* ~oyy. The width of this decay has been found to be very sensit ive to the choice of the model. Values of BR(~ --* ~:~ ) v a r y by five orders of magni tude , f rom 6"10 -7 up to 6.10 -5.

The exper imenta l value (8) considerably reduces the va r i e ty of theoret ical models which might claim to describe the radia t ive meson decays.

The nearest theoret ical es t imates (BR ,~ 10-~) have been obta ined with models considering process (1) as media ted by the exchange of vector mesons (vector-meson dominance model and similar models).

(13) S. OKUBO and B. SAKITA: Phys. Rev. Lett., l l , 50 (1963). (it) W. ALLIES, A. BARACCA and A. TIEMB~,O RA~OS: NUOVO Cimento A, 45, 272 (1966). (15) G. J. GOUNARIS: Phys. Rev. D, 2, 2734 (1970). (16) N. D. H. DASS: Phys. Rev. D, 7, 1458 (1973). (17) ~r I). GOKHALE, S. H. ])ATIL and S. D. RINDANI: Phys. Rev. D, 14, 2250 (1976). (is) M. K. VOLKOV and D. EBERT: Tad. ~iz., 30, 1420 (1979). (19) S. CHAKI~ABARTY, A. l~. •ITRA and I. SANTHANAM: Phys. Lett. B, 84, 119 (1979). (20) G. V. EFIMOV, M. A. IVAXOV and E. A. NOGOVlTSYN: Tad. 2'iz., 34, 264 {1981).


We would l ike to t h a n k A. T. FILIPPOV, S. S. GERSTEIN a n d L. D. SOLOVIEV

for e n l i g h t e n i n g discussions conce rn ing the resul ts . We t h a n k the Di rec to ra te s

of CERh7 a n d IHiEP for t he i r suppo r t to th is e x p e r i m e n t .

�9 R I A S S U N T O (*)

Si ~ osservato e studiato in maniera efficace il raro decadimento radiativo ~--~7:~ all'acceleratore di 70 GeV I H E P con lo spettrometro dell'odoscopio GAMS-2000, Si trovato un rapporto di ramificazione per questo decadimento pari a BR(~--*~~ = (9.5 4- 2.3). 10-'.

(*) Traduzione a cura della Redazione.

Ha6Juo~eHHe pacnana ~ _ ~ O y y .

Pe3ioMe (*). - - BbnI 3(~CKTHBHO 3aperiicTpHpoaarI n HCCne~OBaH pe/IrUfi pa~InalLqOHI~l~ pacna~ ~ -> ~~ Ha ycropnTcnc I H E P 70 FaB c nOMOm~m cneKTpoMeTpa GAMS-2000. HonyqcHo, qTO OTHOmCnnC BCTBCI~ ~YI~I 3TOrO pacna~a COCTaBH~CT Ba(~-->~~ = (9.5 :k 2.3)" 10 -4.

(*) l-lepeeedeno peOar~ue~.