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Volume 84B, number 1 PHYSICS LETTERS 4 June 1979

OBSERVATION OF to ~ t 0 F + l t - DECAY

R.I. DZHELYADIN, S.V. GOLOVKIN, M.V. GRITSUK 1, D.B. KAKAURIDZE, V.A. KACHANOV, A.K. KONOPLYANNIKOV, A.S. KONSTANTINOV, V.F. KONSTANTINOV, V.P. KUBAROVSKI, A.V. KULIK, L.G. LANDSBERG, V.M. LEONTIEV, V.A. MUKHIN, V.F. OBRAZTSOV, T.I. PETRUNINA, N.S. POKROVSK1, Yu.D. PROKOSHKIN, V.A. VICTOROV and A.M. ZAITSEV Institute for High Energy Physics, Serpukhov, USSR

Received 9 April 1979

The results of studying the mass spectrum of ~+~-rr systems produced in n-p collisions at 25 and 33 GeV/c axe pres- ented. The wide-aperture magnetic spectrometer with a hodoscope 3,-detector was used to identify muon pairs and photons. There is a clear peak in the mass spectrum which corresponds to the decay to ~ rr0#+~ - previously not observed. The branching ratio for the decay is estimated to be 9 × 10 - s with a systematic error of 50%.

In the present experiment a search was made for a rare electromagnetic decay o f the co meson

co -~ 7r0p +/J- . (1 )

The b ina ry reac t ion

zr-p ~ con, (2)

was chosen as co source. This reaction provides favourable background condit ions for the measurements. The experiment was carried out at the 70 GeV IHEP accelerator.

The measurements were performed with the experimental set-up " L e p t o n - G ' , which allowed to de- tect efficiently processes with simultaneous produc- tion o f muon pairs and particles decaying further into photons (see our earlier work [1] reporting the observation of the decay r / ~ t~+ta-3'). The set-up consisted of incident particle detectors, a target with guard counters, wire proport ional chambers and drift chambers to measure the coordinates o f charged particles downstream the target. A hodoscope spectrometer with 64 lead-glass counters was used to measure the coordinates and energies of the photons. A wide aperture magnetic spectrometer selected processes with

1 ITEp, Moscow.

muon pairs in the final state. The set-up was triggered by a system of scintillation counters and hodoscopes, that selected events with two penetrating particles produced in the target.

10

0

~ 3 0 0 - -

> -

~ " 2 0 0

N

5

1 0 0

I " • ° •

• ° ~ .

° •

. , ° .

• - - - - ° - - - - _

• " "1 ° ' : : ' : I ° . • .

° ° * , • °

I " 1' •

r "'1 I " P

M~W.-y Y, MeVlc 2 N

Fig. 1. Two-dimensional distribution of events of the type - - + -

~r p ~ ta la "y'rn over the effective masses M30, and Mla~r r. The dashed histograms for the Mrr (Mt~trr) distributions correspond to the selection of events over Mt~,r. r (Mr,r) shown by a broken line.

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Volume 84B, number 1 PHYSICS LETTERS 4 June 1979

The experiments were carried out in the negative pion beam at 25 and 33 GeV/c momenta. The beam intensity was 3 × 106 per cycle. The total 7r- flux that passed through the set-up target during the experiment was 3 × 1011.

At the first stage of the data processing events were selected with two high-energy muons (E u > 4 GeV) originating from the target. Then we selected events corresponding to the reaction r r -p ~ u+u-v'yn in which the muon pairs were produced in association with two photons (E v > 2 GeV). Fig. 1 shows a two- dimensional distribution for events of this type over the effective masses M~r. r and Muu ~ . In this distribution one sees an evident concentrat ion of events in the mass region 214_ ~ M o and M. . _ ~ M. . .

"FY /r P4.~,'F Y t,~

Fig. 2 presents the 7r0/J+/J - mass distribution for events with Mvv ~ Mr0. The tabulated value for the lr 0 mass was used to calculate the masses MTr0u+ - . In the obtained spectrum there is a clear peak corresponding to the co meson. It contains 30 events, after 20% background subtraction. The peak width ("~ 60 MeV) is defined by the resolution of the apparatus.

Thus in the present work we have experimentally established the existence of co decay (1 ) i n to 7r 0 and a muon pair.

To determine the branching ratio of this decay,

BR(co --* rr0t~+~ - ) = P(co ~ r r0~+p-) /P(co -+ all) ,

we normalized for the number of events of the reac-

tion

r r - p - * o ( c o ) n , p(co)-~u+U - ,

observed in the same experiment. The cross sections for p and co product ion were taken from refs. [2,3] . The set-up efficiencies for all reactions were Monte- Carlo calculated. For decay (1) we used a standard matrix element with a form factor F(q 2) = const. The state of co was described by a density matrix with

the elements P00 = PI0 = 0, P l l = P l - 1 = 1/2. Similarly to ref. [1] we used the value BR[p(co)

~+tJ-] = 4.6(7.6) X 10 -5 [4]. The branching ratio for decay (1) is found to be equal to

N

15

500 1000

M r t , p + ~ , MeVlc 2

~ 1~°~ ÷ ~ -

Fig. 2. Mass spectrum of the ~r°~t~t system. The peak corresponds to decay (1). The arrow points to the tabulated value for the mass. N is the number of events per interval of 40 MeV/c 2 .

with a systematic error of 50%. This value is in agree- ment with the available theoretical estimates.

The present work showed that the background con- ditions for the detection of decay (1 ) in the described experimental approach are favourable for its further quantitative investigation. It opens a possibility to study the electromagnetic form factor, corresponding to the transition co ~ 7r 0 .

References

[1 ] Yu.B. Bushnin et al., Yad. Fiz. 28 (1978) 1507; Phys. Lett. 79B (1978) 147.

[2] O.I. Dahl et al., Phys. Rev. Lett. 38 (1977) 54. [3] CERN/HERA Compilation, 72-1 (1972);

Estabrooks et al., Nucl. Phys. B81 (1974) 70. [4] Particle Data Group, Review of Particle Properties, Phys.

Lett. 75B (1978) 1.

BR(co --* lr0u+u - ) = 9 X 10 -5 , (3)

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