Volume 84B, number 1 PHYSICS LETTERS 4 June 1979
OBSERVATION OF to ~t0F+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 of the co meson
co -~ 7r0p +/J- . (1)
The b inary react ion
zr-p ~ con, (2)
was chosen as co source. This reaction provides favour- able background conditions for the measurements. The experiment was carried out at the 70 GeV IHEP accelerator.
The measurements were performed with the ex- perimental set-up "Lepton-G' , which allowed to de- tect efficiently processes with simultaneous produc- tion of muon pairs and particles decaying further into photons (see our earlier work  reporting the obser- vation of the decay r /~ t~+ta-3'). The set-up consisted of incident particle detectors, a target with guard counters, wire proportional chambers and drift cham- bers to measure the coordinates of 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 aper- ture 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.
~ 300 - - >-
. , .
- - - - - - - - _
" "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.
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 experi- ment 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 rr-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 distribu- tion one sees an evident concentration 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 corre- sponding 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) into 7r 0 and a muon pair.
To determine the branching ratio of this decay,
BR(co --* rr0t~+~ - ) = P(co ~ rr0~+p-)/P(co -+ all),
we normalized for the number of events of the reac- tion
r r -p -*o (co)n , p(co)-~u+U - ,
observed in the same experiment. The cross sections for p and co production 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.  we used the value BR[p(co)
~+tJ-] = 4.6(7.6) X 10 -5 . The branching ratio for decay (1) is found to be equal to
M r t ,p+~ , MeVlc 2
~ 1~~ ~-
Fig. 2. Mass spectrum of the ~r~t~t system. The peak corre- sponds 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 .
[1 ] Yu.B. Bushnin et al., Yad. Fiz. 28 (1978) 1507; Phys. Lett. 79B (1978) 147.
 O.I. Dahl et al., Phys. Rev. Lett. 38 (1977) 54.  CERN/HERA Compilation, 72-1 (1972);
Estabrooks et al., Nucl. Phys. B81 (1974) 70.  Particle Data Group, Review of Particle Properties, Phys.
Lett. 75B (1978) 1.
BR(co --* lr0u+u - ) = 9 X 10 -5 , (3)