Volume 97B, number 3,4 PHYSICS LETTERS 15 December 1980

STUDY OF q ~ Ix+Ix - DECAY

R.I. DZHELYADIN, S.V. GOLOVKIN, V.A. KACHANOV, A.S. KONSTANTINOV, V.F. KONSTANTINOV, V.P. KUBAROVSKY, A.V. KULIK, L.G. LANDSBERG, V.M. LEONTIEV, V.A. MUKHIN, V.F. OBRAZTSOV, Yu.D. PROKOSHKIN, V.A. VIKTOROV and A.M. ZAITSEV

Institute for High Energy Physics, Serpukhov, USSR

Received 3 October 1980

The muon pair mass spectrum in the reaction ~r-p -+ tt÷p-n has been investigated at 25 and 33 GeV/c. A peak corre- sponding to the rare r/+ p+/a- decay is observed. The measured branching ratio for this decay is (6.5 +- 2.1) X 10 -6 in agreement with theoretical estimates.

The rare decay

77 -+ p + p - (1)

is a 4th order electromagnetic process. It is tightly connected with the main 2nd order decay r~ -+ ~'3' (fig. 1). Their widths are related by (see refs. [1,2]):

P(r/-+ p+p-)/P(rl ---> 3'7) = 20e2~ 2 (1 -- 4~2) -1/2

X {ln{[1 + (1 - 4~2)1/2]/2~}}2(1 + X 2 / y 2 ) . (2)

Here ~ = mu/m ~ is the ratio of the muon and r/masses, Y is the absorptive part of the amplitude, which may be found from the unitarity relation

y2 = (1 - 4~2) -1 {ln{[1 + (1 - 4~2)1/2]/2~}} 2 ,

and X is the dispersive part of the amplitude. For X 2 / y 2 ~ 1 the branching ratio, BR(r7 ~ p + p - ) = r(r~ -+ p+p-)/lV(r/-+ all), is close to its unitarity limit, BR(r~ -+ P+P-)unit = BR(r~ -+ •"y) • 1.1 X 10 -5 = 4.0 X 10 -6 .

Up to now, decay (1) had been seen in a single ex- periment [3], its branching ratio being observed to be

~ T bt+ ]f

~q

/ F(q~,q,z) - - g-

Fig. 1. Feynman diagram for n -+ #+#- decay.

BR(~ -+ p + # - ) = (2.2 -+ 0.8) X 10 -5 . Such a value is much higher than the unitarity limit quoted above. This may be evidence for the dominating contribution from the dispersive amplitude, X 2 / y 2 >> 1.

The quantity X for a point r/3'7 vertex diverges logarithmically. When calculating it, one has to intro- duce a cut-off parameter determined by the vertex structure. Theoretical estimates of X, which were made using various model assumptions [1,2,4,5], show that its contribution is small. For instance, in the vector meson dominance model (VDM) X 2 ~ y2 and BR(r/-+ p+/a-) = 4.1 X 10 -6 . As was shown in ref. [2] it is rather difficult from a theoretical point of view to obtain a value higher than 10 -5 , so the interpretation of the experimental results [3] meets with problems.

These conclusions are even stronger since the mod- els with a dominating contribution from X (and con- sequently with large cut-off masses) result in a very small slope of the r/form factor in the 77 -+ P+P-7 decay (see e.g. ref. [5]). This is in direct contradiction with the results of our measurements of the ~7 form factor [6].

We have studied decay (1) in the course of experi- ments on rare electromagnetic decays of light mesons [6 -10 ] . The description of the "Lepton-G" set-up used in these experiments as well as the measurement and data processing procedures have been given else- where [6,9,10]. The principal units of the set-up were

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Volume 97B, number 3,4 PHYSICS LETTERS 15 December 1980

a wide aperture magnetic spectrometer used to select muon pairs and a hodoscope photon spectrometer.

The binary reaction

rr-p ~ nn ( 3 )

has been used as a source of r/mesons. The measure- ments were carried out at the IHEP accelerator with negative pions o f 25 and 33 GeV/c. The total number of etas produced in reaction (3) on the set-up target amounted to 2 X 107.

Simultaneously with the study of decay (1) the events of the more intense decay [6,7]

7? ~/~+/z- 7 (4)

have been detected as well as those of the reaction

rr-p -+ p (co)n

~.+u- • ( 5 )

This has provided a detailed investigation of the back- gr6und for the detection of decay (1) and the required normalization of the branching ratio for this decay.

During data processing we have selected the events with two muons (E u > 4.5 GeV) produced in the set- up target without any other additional track coming from the interaction vertex. Also the absence of pho- ton or hadron showers in the hodoscope spectrometer has been required. So the 7-spectrometer has been used in an "anticoincidence" mode with low energy threshold (0.5 GeV for photons).

For the final analysis the events whose kinematics corresponded to the exclusive reaction 7r-p ~/a+/a- n have been selected and the effective mass spectrum of muon pairs for this reaction has been studied. As is seen from this mass spectrum (fig. 2) the events of the (p, co) ~/~+/~- decays, corresponding to reaction (5), are dominating. In the 7/range there is a peak in the spectrum, which corresponds to the r/-+/u+/a - decay in reaction (3).

The width of this peak is defined by the resolution of the apparatus. The mass, equal to (551 -+ 4) MeV/c 2, coincides with the tabulated value for the 7? meson. The peak contains 27 + 8 events of decay (1), exceed- flag the background by more than 5 standard deviations.

The background under the r/peak is of physical origin and it may be described by the VDM as direct muon pair production. The contributions from the r/-+ ;t+/a-7 decay as well as from decays in flight are negligibly small.

:E60 o

~z

z,O

20

I i I

i _ 1 ~ III

0/~ 0.6 0.8

÷

1.0

Fig. 2. The/~+p- pair mass spectrum. The arrow points to the tabulated value of the r/mass. The curve fits the background.

To define the branching ratio for decay (1) we have normalized to the number of simultaneously detected events of decay (4) and to the value BR0? -+/~+/~-7) = (3.1 + 0.4) × 10 -4 measured in ref. [6]. The set-up efficiency for decays (1) and (4) was calculated with the Monte Carlo method.

The resulting value for the branching ratio of decay (1) turned out to be

BR(rt -+ #+/a-) = (6.5 + 2.1) × 10 -6 . (6)

This is three times smaller than the one obtained ear- lier [3]. It agrees well with the theoretical estimates quoted above.

References

[1] D.A. Geffen and B.L. Yong, Phys. Lett. 15 (1965) 316. [2] C. Quigg and J.D. Jackson, UCRL-18487 (1968). [3] B.D. Hyams et al., Phys. Lett. 29B (1969) 128. [4] A.I. Ivanov and V.M. Shekhter, Yad. Fiz. 31 (1980) 530. [5 ] M. Pratap and J. Smith, Phys. Rev. D5 (1972) 2020. [6] V.A. Viktorov et al., preprint IHEP 80-57 (1980). [7] Yu.B. Bushnin et al., Yad. Fiz. 28 (1979) 1507; Phys.

Lett. 79B (1978) 147. [8] V.A. Viktorov et al., Yad. Fiz. 29 (1979) 1513; Phys.

Lett. 84B (1979) 143. [9] V.A. Viktorov et al., Pis'ma Zh. Eksp. Teor. Fiz. 30

(1979) 387; Phys. Lett. 88B (1979) 379. [10] Yu.B. Bushnin et al., preprint IHEP 80-57 (1980).

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