Upper limit for the branching ratio of Ks0→e+e− decay

11 August94 PHYSICS LETTERS B

Physics Letters B 334 (1994) 234-236

EI~SEVIFR

U p p e r limit for the branching ratio o f K °

e + e - decay

A.M. Blick a V.N. Kolosov ", V.M. Kutjin a, V.I. Shelikhov a, I.E. Chirikov-Zorin b G.A. Chlachidze b, Yu.I. Davydov b, V.B. Flyagin b, V1.V. Glagolev b, A.V. Kolomyichenko b A.S. Kurilin b, I.P. Liba b Yu.F. Lomakin b S.N. Malyukov b I.P. Minashvili b, O.E. Pukhov b, V.I. Romanovsky b, N.A. Russakovich b, N.L. Russakovich b, A.A. Semenov b, A.N. Shalyugin b, A.S. Solov'yov b, G.V. Velev b, A.G. Volodko b a Box 142284, IHEP, Protvmo, Russta b Box 141980, JINR, Dubna, Russta

Recewed 7 September 1993, revised manuscnptreceived 19 May 1994 Editor L Montanet

Abstract

A search for K° ~ e ÷ e - decay was performed by means of a 624-channel lead glass shower hodoscope and a set of proportional chambers. Neutral kaons were obtained in the quasibinary charge exchange reaction. A new upper llmxt of 2.8 × 10 - 6 at 90% C.L. for the branching ratm is obtained.

The K ° ~ e ÷ e - decay is suppressed because of the absence of the flavor changing neutral current, but it can exist at high orders of the weak and weak-electromagnetic interaction as well as beyond the Standard Model frames. A recent value of the branching ratio Br(K°~e+e - ) < 1 × 10 -5 was established in Ref. [ 1 ]. In the frame of the kaonic decay research programme of the " H y p e r o n " facility [ 2 ] measurements were performed to search for the decays K ° ~ e ÷ e - , K ° ~ e ÷ e - zr°, K ° --) e ÷ e - y, K ° ---)~.o~.oy at a more sensitive level. This letter concerns the first results of the experiment. The scheme of the apparatus is given in Fig. 1. K +mesons are identified in the 10.5 G e V / c secondary beam of the IHEP accelerator by means of several Cherenkov threshold counters. The beam size is about 5 cm in the target position region. The total intensity was about 2 × 10 6. The target is a cylindrical piece of copper ElsevmrScienceB.V. SSD10370-2693 (94)00684-Y

92 cm thick. It is followed by the anticoincidence counter Ao to suppress the detection of the charged secondaries in the forward direction. The supposed e ÷ e - pairs are detected with a 2 m m step proportional chambers and a scintillation hodoscope of 32 elements. The electromagnetic calorimeter ( E M C ) , which is a 24 × 24 matrix of 85 × 85 × 350 m m 3 lead glass blocks, is installed at a distance of 5.6 m from the target. The 64 central blocks have a twice smaller granularity: 42.5 X 42.5 × 350 mm 3. So, the total number of cells ts 624 and the total aperture is more than 4 m 2. The anode signals are transported to 10-bit ADCs strobed by a 60 ns pulse. The last dinode signals are used to evaluate the total energy release and to set a minimal threshold of >/3 GeV. Therefore the fast trigger requires neutrals behind the target and showers in the calorimeter: A suppression of the incident kaon beam by a factor of 4 × 10 -4 was obtained. It was found that the majority

A M Bhck et al./Physics Letters B 334 (1994) 234-236

EMC

Xim 1.0

PWC Hx¥

0.5 T

0

G

Ao

"r,¢

-0.5

- 1.0

I

i

I

i

I

I

jZ,

0

1

2

3

4

5

6

m

Fig. l. The scheme of the setup T: target, G and A. anticounters, PWC' proportional chambers, H~y scintillation hodoscope, EMC lead glass wail.

80O > o 0 60O o z 400

2OO

0

8

6

10

E [GeV]

12

235

energy distribution of the zr° pmrs gives sufficient information about the K ° energy spectrum, which is shown in Fig. 2. The decay mode K ° ~ 7r + zr- was also used as a monitor. In this case the hodoscope Hxy was used in a fast trigger instead of the EMC and special mini-exposures were performed periodically. The events were selected if two tracks intersecting in the decay volume were found. The opening angle distribution of the track pairs is shown in Fig. 3. Hence a large contribution of reaction (1) into our monitor decays statistics is confirmed by clear peaks in Figs. 2 and 3 [ 3]. The signals of the neutral mesons (~-°, r/, f ) have been used to calibrate all EMC channels. The corresponding effective mass peaks have been obtained from a pion charge exchange test run. The pions from the K ° ~ ~-°zr° decays have been used for the calibration as well [4,6]. The effective mass distribution of these events must be Gaussian at the K°-meson tabulated mass. The r/-meson peak parameters are a good (conservative) estimate of the resolution, i.e., trm---34 M e V / c 2 [5]. The statistics obtained corresponds to 3.45 × 10 9 incident K+-mesons. To select the e + e - candidates we required the intersection point of two detected tracks to be downstream the target along a 1 m decay base. For each track the distance between the track extrapolaUon and the shower in the EMC plane must not exceed 50 mm. The effective mass spectrum of the separated events, supposing the tracks belong to the e +e - pair, is shown in Fig. 4. 22 events enter the mass region ( 4 0 0 - 6 0 0 ) M e V / c 2, ( ___3o'r,). After a >/8 GeV total energy cut 3 events remain.

Fig. 2 The energy spectrum of the ~.o~o events

of events recorded with such a trigger belonged to the following quasibinary charge exchange reaction on the target nuclei: K + +A~K

° +A',

(1)

followed by K ° ~ 27r ° ~ 4 7 decay. We decided to use these events as a monitor in searching for K ° ~ e +e -. The reconstruction of monitor decays was based on the absence of charged tracks in the PWC and the presence of 4 showers m the EMC. The effective mass distribution of these events was very close to the K ° mass value but slightly wide and shifted because of the unknown decay vertex position. Nevertheless the

800

~) 6 0 0 o d z '~ 6 0 0

200

0 0.05

~

I 0.09

0.13

e [rGd]

Fig. 3. The opening angle spectrumof the 7r+~r- events

236

A.M Bhck et al ~Physics Letters B 334 (1994) 234-236

Table 1 1200

I[ 0 z

8O0

400

200

400

600

M [MeV/c~]

Fig 4. Two track events mass dlstnbunon assunungthat the tracks form an e +e - parr The main background in this experiment as well as in our previous measurements [7] is the K ° ---,7r ÷ 7rdecay followed by hadron showers in the lead glass. Other sources of background (pion Dalitz decay, external conversion, K 0L~ ~-ev decays) were estimated using the Monte Carlo method and found to be negligthle in this experiment. The background suppression is based on the difference in the decay kinematics: the opening angle spectra of the pion and electron pairs are not overlaying. The m a x i m u m angle is 0(~- + ~-- ) = 88 mrad and the m i n i m u m one is 0(e ÷e - ) --95 mrad in reaction (1) with accounts for the momentum transfer and resolution. Returning to our 3 events one can see that one of them must be excluded because of the opening angle cut 0 > 9 5 mrad. In the remaining events the "trackshower" distance in the EMC plane is more than 3Crx, at least in one of the projections, where O'x is the space resolution according to the shower energy and location in the detector cell [8]. Since our cell size is 85 mm, such a situanon occurs either in the case of a hadronic shower or for showers overlaying. One could see quite strong neighbours at the opposite sides of the main

Decay

Nevents

Aaccep

Br(PDG)

Br(e +e - ) [90% C.L ]

•r+Tr~'%r° e+e -

6.40× 105 2 05 × 105

0 794 0.545 0.697

0 6861 0 3139 < 1× 10-5

<2 81 X 10-6 <2.75 X 10-6

detecting glasses in these events. All such cuts were taken into account when monitoring was carried out. The upper limit of the decay branching ratio can be obtained on the basis of the absence of the events in the mass region considered. The results obtained are presented in Table 1. The K--* ~-°rr° events were observed directly in the experiment, while the K--* rr + ~- - events were obtained from the test run data. The acceptance was Monte Carlo calculated, the branching ratios of the monitor decays were taken from Ref. [9]. The upper limits obtained by means of the different decay modes are in very good agreement. So, we obtain an upper limit for the branching ratio for the K ° ~ e ÷ e - decay of 2 . 8 × 10 - 6

with 90% C.L.

The authors express their gratitude to Yu.A. Budagov, Yu.D. Prokoshkin and V.P. Dzhelepov for their support of the work. The research described in this publication was made possible in part by a grant from the International Science Foundation.

References [ 1] V V Barminet al, Yad. Phys 44 (1986) 965 [2] A G Asmolovet al, JINRprepnntR13-92-526 (Dubna, 1992) [3] N D Gonna and YuD Prokoshkin, IHEP prepnnt 71-94 (Serpukhov, 1971). [4] S.A Aklmenkoetal.,IHEPprepnnt82-149 (Serpukhov, 1982), Pnb. Tekh Eksp 1 (1984) 86 [5] S A Aklmenkoet al, IHEPptepnnt 86-67 (Serpukhov, 1986), Pnb Tekh Eksp I (1988)45 [6] A M. Bhck and I P. Llba, IHEP preprlnt 93-86 (Serpukhov, 1986). [7] G S. Bltsadzeet aL, JINR preprintE1-85-610 (Dubna, 1985), Phys Lett B 167 (1986) 138. [8] S.A Akimenkoetal ,IHEPprepnnt84-194(Serpukhov, 1984) [9] Particle Data Group, K. Hlkasa et al., Review of particle properties, Phys Rev D 45 (1992) 7