Volume 88B, number 3,4 PHYSICS LETTERS 17 December 1979
OBSERVATION OF r/' -~/a+bt - 3" DECAY
R.I. DZHELYADIN, S.V. GOLOVKIN, M.V. GRITZUK 1, V.A. KACHANOV, D.B. KAKAURIDSE, A.S. KONSTANTINOV, V.F. KONSTANTINOV, V.P. KUBAROVSKI, A.V. KULIK, L.G. LANDSBERG, V.M. LEONTIEV, V.A. MUKHIN, V.F. OBRASTZOV, T.I. PETRUNINA, N.S. POKROVSKI, Yu.D. PROKOSHKIN, V.A. VICTOROV and A.M. ZAITSEV Institute for High Energy Physics, Serpukhov, USSR
Received 25 September 1979
The mass spectrum for ~- ' r systems produced in ~r-p interactions at 25 and 33 GeV/c has been studied. To detect muon pairs and photons a wide-aperture magnetic spectrometer and a hodoscope "r-detector were used. In the mass spec- trum a clear peak is seen corresponding to the decay r/' --, tata-'r not previously observed. The branching ratio for this de- cay, 8 X 10 -s, is in agreement with the r~' form-factor model, based on vector dominance.
In the present work, which continues the series of experiments on rare electromagnetic decays of light mesons [1,2], a search was made for the decay
r~' -~/1+/2 - 3'. (1)
The description of the set-up "Lepton-G" used in the experiment, as well as the measurement and data processing procedures was given in earlier pa- pers [1,2]. The main units of the set-up were a wide- aperture magnetic spectrometer used to select muon pairs and a hodoscope multichannel 7-spectrometer.
The charge-exchange reaction
7r-p --> ~7'n (2)
was used as a source for ~7' (958). The experiments were carried out in the negative pion beam at mo- menta 25 and 33 GeV/c. The total flux through the set-up was 3 1011.
The first stage of the experimental data processing involved the reconstruction of charged particle tracks in the spark and proportional chambers and the selec- tion of events with two energetic (E u > 4 GeV) muons produced in the target. It was also required that there should be no additional tracks in the head part of the
1 ITEP, Moscow.
spectrometer emerging from the interaction vertex. Then a search for single showers, that satisfy the selection criteria for showers from 7-quanta with E. r > 1.5 GeV, was made in the 7-spectrometer. To suppress the background caused by hadronic showers it was required that the distance between the shower centre of gravity and the coordinates of each of the two charged tracks at the entrance of the 7-spectrome- ter was larger than 24 ram.
Further, events corresponding to the inclusive pro- cess 7r-p ~ gt+gt - 7n were selected, and the effective mass spectrum of the/a+/a-7 system was studied.
The background conditions were strongly im- proved in the mass range Mu u-'r > M n by increasing the threshold over the 3' energy. Such a cut gave ex- tra suppression for hadron showers, reducing insig- rtificantly, within a certain range of the 3" threshold, the detection efficiency for decay (1).
To reduce the background caused by fast zr 0 me- sons (e.g. from co -> rr0gt+/a - decay ) for which the showers from two photons could not be resolved in the detector, a cut was also imposed on the maximum 7 energy (E'r < 16 GeV).
The f'mal effective mass spectrum for the gt+gt-3' system is presented in fig. 1. In the mass spectrum, along with a peak corresponding to the r/~/l+/a-3' decay , a clear peak is seen in the r/' mass range,
Volume 88B, number 3,4 PHYSICS LETTERS 17 December 1979
V-q, ~51.~ . N , 0 0.5 M g. f fy [GeV/c~
Fig. 1. The mass spectrum for #+/~-7 systems. The shaded histogram corresponds to E 7 > 5 GeV, the open one corre- sponds to E3, > 3 GeV. N is the number of events in a 40 MeV/c 2 mass bin. The arrows point to the tabulated values for the r/and r/' masses. The left peak corresponds to the de- cay r/~ ta+~-7 , the right one corresponds to decay (1).
containing 25 events (E7 > 3 GeV), the nonresonant background being 4 events. The peak mass Mu+u-, r = (0.95 -+ 0.01) GeV/c 2 is in good agreement with the tabulated value for r~' and its width is compatible with our apparatus resolution. Thus, decay (1) of rl' into a muon pair and a photon has experimentally been established in our work.
To define the branching for this decay, BR(~' /~+/2-7) = P0?' -~ ~+/a-7)/P(r/' ~ all) we used for nor- realization the reaction 7r-p ~ p0(co)n, p0(a 0 ~ p+~- observed in the same experiment . The efficien- cies for all the processes were Monte Carlo calculated. The detection efficiency for the reaction zr-p -~ ~)'n,
77' -~/a+/a-7 is practically independent of the form factor in the matrix element of decay (1). The cross sections for reaction (2) for our energies were taken from ref. . As a result the following branching ratio was obtained:
BR0?' ~/a+/~-7) = 8 X 10 -5 , (3)
with a systematic error of 50%. To estimate theoretically the branching ratio for
decay (1) we used the Dalitz formula  with the simplest form factor
2 2 2 22 -1 - _/M;) +(ro/ )21 F (M~+ _)= [(1
chosen in agreement with vector dominance. This estimate provides the following value: BR(~' -~ /~+~-7) = 8.2 10 -5 ,1. Here 15% of all events is expected to occur in the p-mass range (Mu+ u- > 0.7 GeV), what is in qualitative agreement with the fraction of events observed by us in this range, (30 -+ 10)%.
Thus the decay r/' -~/a+/a-7 has been detected in this work. The branching ratio for this decay is in agreement with the r/' form-factor model, based on vector dominance.
,1 If the form factor is not taken into account then BR(rf /a+u--r) = 3.4 10 -s. The form factor plays an important role since the O-pole for decay (1) is in the physical region.
[1 ] Yu.B. Bushnin et al., Yad. Fiz. 28 (1978) 1207; Phys. Lett. 79B (1978) 147.
 R.I. Dzhelyadin et al., Phys. Lett. 84B (1979) 143.  W.D. Ape1 et al., Phys. Lett. 83B (1979) 131.  R. Dalitz, Proc. Phys. Soc. A64 (1951) 667.