- Email: [email protected]
A Search for the K0L → μe and K0L → ee decays
731c
Nuclear Physics A527 (1991) 73 10734~ North-Holland, Amsterdam
A Search for the ICf + pe and It’: --t ee Decays*
S.S.Yamamoto Department
of Physics,
University
of Tokyo, Bunkyoku,
Tokyo 113, Japan of the Kg --+pp decay
A search for the Kg + tie and I<: + ee decays and a measurement rate were made at the KEK 12 GeV proton - July 1989 (4000 hours of running and December
1989 - May 1990 (1600 hours of running
2.0-2.5~10~~
ppp).
first running
period.
be considered The data
The results
presented
and represent
magnets
an electromagnetic
system,
trigger conditions
identification
resolution
plot between
ing the number
tification
of ?rr events,
assuming
events
is seen above the background
Candidates
by appropriate tracks
and their
angle, B , the angle
were calculated.
and the 0’ resolution
Fig. I shows
of 1000 events.
The mass
around 0.9(mrad)2. was chosen:
From
493MeV/c’
<
8’ < 3(mrad)‘. N,,
, for normalization
purpose
was obtained
to 3.63~10’
from the effective
were pions.
for which a 0’ cut at 3(mrad)* distribution.
by count-
including
the
mass distribution
of
trigger for the 1rx decay mode without that the particles
~TXevents as well as those coming from the most predominant effective mass distribution,
a gas Cerenkov
selected
mass and collinearity
It could also be obtained
by the hodoscope
information,
the
Details of the detector
are given by Inagaki et al.’
of events in the fiducial region, which amounted
factor of 500.
events obtained
and two hodoscopes,
fiducial region for acceptable and
during
of
arm spectrometer
from the two reconstructed
their effective
to be 1.3MeV/c2
effective mass < 502~eV/cz
intensity
the results should
of a double
decays were simultaneously
information
1987 ppp)
best results.
the nl effective mass and o2 for a subsample
was determined
The total number
procedure,
and the vector sum of their momenta,
these values the following
prescaling
with an average proton
consisting
and track reconstruction
info~ation,
between the 1<: direction
our present system
and five drift chambers,
Using the kinematical
December
of 1.0-1.5~10’~
shower counter and muon range counter.
for 1Ci -+ he , ee , pp and 1Cz --+ x+x-
a scatter
intensity
here are based on the data obtained
were taken with a detector
counter,
particle
over two periods:
Since we are still fine tuning the analysis
as preliminary
with two bending
triggers.
synchrotron
with an average proton
any particle
iden-
These events include genuine 111s decays.
has been made.
Dots with error bars represent
Fig. 2 shows such A clear 1ii peak the distribution
“KEK E-137, A KEK - Univ. of Tokyo - Kyoto Univ. - Tohoku Univ. ~olIaboration ( TInagaki, M.Kobayashi, T.Sato, TShinkawa, FSuekane K.Takamatsu, Y.Yoshimura, R.Fukuhisa, T.Kishida, T.K.Komatsubara, M.Kuze, FSai, J.Toyoura, S.S.Yamamoto, Y.Hemmi and T.Akagi ) 0375-9474/91/$03.50 0 1991 - Elscvier Science Publishers B.V. (North-Holland)
of
73%
SS. Yantamoto / I$-
FIGURE 1 Effective mass vs fP scatter plot for I<$ -+ 7r+7r- events
pe and I$-
ee decays
FIGURE 2 TK effective mass distribution of events obtained by the ?tx hodoscope trigger only
the Xrs events generated by a Monte Carlo simulation, normalized to the numbers of events outside the Kg peak. The agreement between the data and the simulation is quite good. Mx?robtained by subtracting the background from this distribution agreed with that obtained by straight counting to within about 2%. From the numbers of the generated Iii -+ xev and Kg 4 ~JN events, N,, and the Ki --P X*K obtained: BR(KE -+ nev)=37%
and BR(Ki
decay rate the foliowing branching ratios were -+ np~)=27%,
which are in good agreement
with the world average values of 38.6% and 27% s, Indicating ’ the imernal consistency of our data. Figs. 3(a), (bf and (c) show ef?ective mass vs @ scatter plots for the pe , ee and y~_ldecay modes.
The fiducial region is superposed in each figure. There are no events in the fiducial
region in the pe and ee plots and 114 events in the top plot. Fig. 3(d) is a projection of the ~JLplot on the effective mass axis with 62 < 3jmrad)‘.
A peak is clearly seen at the Kg mass.
In order to obtain the upper limits on the Fe and ce decay rates as well as the pp decay rate from the data the particle identi6cation efficiencies were measured by using bptons and pions from ICs events. The average efficiencies are : Epe= 0.66f0.02,
E,, = 0.52f0.02 and E,,= 0.80 3~0.03. The relative acceptances with respect to the Kg -+ r+z- decay were calculated by a Monte Carlo simulation. They are: A-,/A,= = 1.00 i 0.02, A,,/&, = 1.00 f 0.02 and A~~JA~~ = 1.08 i: 0.02. Furthermore, the loss of pions due to nuclear ir~t~ractions in the upstream hodoscope and Cerenkov counter were measured by using the 7rf’s from Ibis events. The momentum corrected percentage loss is 5.0 i 0.2 %. Using these values, N nn and the h’i
-+ ?T+K- decay rate we have obtained the 90%
confidence level upper limits for the Kg -+ ,ue and ee decays with a systematic error of about 6% as follows: BR(Ki
+ pe) < 1.9 x 10-r” and BR(@
Kg ---) ~iy decay rate was measured to be BR(
-+ ee) < 2.5
x
10-lo. The
I<:--+ p;ttf=( 8.2f0.8fstat) i 0.7(sys) )x10-‘.
S.S. Yamamoto / KL-
pe and KL-
ee decays
133c
I
,
8-
I
(b)
-
-60
-
L “0
.
. 2-.
0 480
i
I
I
490
500
510
160f
I
M,, ( MeVicz
,
)
FIGURE 3 Scatter plot of the effective mass vs o2 for the (a) pe and (b) ee events (c) scatter plot of the effective mass vs B* for the pp events and (d) the pp effective mass distribution of the ptp events with 0’ < 3(mrad)’ This pp decay rate is to be compared with the value ( 5.8 rt O.G(stat) i 0.4(sys) )x10-’
based
on 87 pp events obtained by Mathiazhagan et al.3 These two t~izdecay rates barely agree with each other and are consistent with the unitarity lower limit of ( 6.83 i 0.25 ) x10-s based on a formula by Sehgal ’ within the uncertainties. REFERENCES 1. T.fnagaki et al., Phys. Rev. D40, 1712 (1989). 2. Particle Data Group, G.P.Yost et al., Phys. Lett. B204 (1988). 3. C.Mathiazhagan et al., Whys. Rev. Lett. 63, 2185 (1989). 4. L.M.Sehgal, Phys. Rev. 183, 1511 (1969).
Nuclear Physics A527 (1991) 73 10734~ North-Holland, Amsterdam
A Search for the ICf + pe and It’: --t ee Decays*
S.S.Yamamoto Department
of Physics,
University
of Tokyo, Bunkyoku,
Tokyo 113, Japan of the Kg --+pp decay
A search for the Kg + tie and I<: + ee decays and a measurement rate were made at the KEK 12 GeV proton - July 1989 (4000 hours of running and December
1989 - May 1990 (1600 hours of running
2.0-2.5~10~~
ppp).
first running
period.
be considered The data
The results
presented
and represent
magnets
an electromagnetic
system,
trigger conditions
identification
resolution
plot between
ing the number
tification
of ?rr events,
assuming
events
is seen above the background
Candidates
by appropriate tracks
and their
angle, B , the angle
were calculated.
and the 0’ resolution
Fig. I shows
of 1000 events.
The mass
around 0.9(mrad)2. was chosen:
From
493MeV/c’
<
8’ < 3(mrad)‘. N,,
, for normalization
purpose
was obtained
to 3.63~10’
from the effective
were pions.
for which a 0’ cut at 3(mrad)* distribution.
by count-
including
the
mass distribution
of
trigger for the 1rx decay mode without that the particles
~TXevents as well as those coming from the most predominant effective mass distribution,
a gas Cerenkov
selected
mass and collinearity
It could also be obtained
by the hodoscope
information,
the
Details of the detector
are given by Inagaki et al.’
of events in the fiducial region, which amounted
factor of 500.
events obtained
and two hodoscopes,
fiducial region for acceptable and
during
of
arm spectrometer
from the two reconstructed
their effective
to be 1.3MeV/c2
effective mass < 502~eV/cz
intensity
the results should
of a double
decays were simultaneously
information
1987 ppp)
best results.
the nl effective mass and o2 for a subsample
was determined
The total number
procedure,
and the vector sum of their momenta,
these values the following
prescaling
with an average proton
consisting
and track reconstruction
info~ation,
between the 1<: direction
our present system
and five drift chambers,
Using the kinematical
December
of 1.0-1.5~10’~
shower counter and muon range counter.
for 1Ci -+ he , ee , pp and 1Cz --+ x+x-
a scatter
intensity
here are based on the data obtained
were taken with a detector
counter,
particle
over two periods:
Since we are still fine tuning the analysis
as preliminary
with two bending
triggers.
synchrotron
with an average proton
any particle
iden-
These events include genuine 111s decays.
has been made.
Dots with error bars represent
Fig. 2 shows such A clear 1ii peak the distribution
“KEK E-137, A KEK - Univ. of Tokyo - Kyoto Univ. - Tohoku Univ. ~olIaboration ( TInagaki, M.Kobayashi, T.Sato, TShinkawa, FSuekane K.Takamatsu, Y.Yoshimura, R.Fukuhisa, T.Kishida, T.K.Komatsubara, M.Kuze, FSai, J.Toyoura, S.S.Yamamoto, Y.Hemmi and T.Akagi ) 0375-9474/91/$03.50 0 1991 - Elscvier Science Publishers B.V. (North-Holland)
of
73%
SS. Yantamoto / I$-
FIGURE 1 Effective mass vs fP scatter plot for I<$ -+ 7r+7r- events
pe and I$-
ee decays
FIGURE 2 TK effective mass distribution of events obtained by the ?tx hodoscope trigger only
the Xrs events generated by a Monte Carlo simulation, normalized to the numbers of events outside the Kg peak. The agreement between the data and the simulation is quite good. Mx?robtained by subtracting the background from this distribution agreed with that obtained by straight counting to within about 2%. From the numbers of the generated Iii -+ xev and Kg 4 ~JN events, N,, and the Ki --P X*K obtained: BR(KE -+ nev)=37%
and BR(Ki
decay rate the foliowing branching ratios were -+ np~)=27%,
which are in good agreement
with the world average values of 38.6% and 27% s, Indicating ’ the imernal consistency of our data. Figs. 3(a), (bf and (c) show ef?ective mass vs @ scatter plots for the pe , ee and y~_ldecay modes.
The fiducial region is superposed in each figure. There are no events in the fiducial
region in the pe and ee plots and 114 events in the top plot. Fig. 3(d) is a projection of the ~JLplot on the effective mass axis with 62 < 3jmrad)‘.
A peak is clearly seen at the Kg mass.
In order to obtain the upper limits on the Fe and ce decay rates as well as the pp decay rate from the data the particle identi6cation efficiencies were measured by using bptons and pions from ICs events. The average efficiencies are : Epe= 0.66f0.02,
E,, = 0.52f0.02 and E,,= 0.80 3~0.03. The relative acceptances with respect to the Kg -+ r+z- decay were calculated by a Monte Carlo simulation. They are: A-,/A,= = 1.00 i 0.02, A,,/&, = 1.00 f 0.02 and A~~JA~~ = 1.08 i: 0.02. Furthermore, the loss of pions due to nuclear ir~t~ractions in the upstream hodoscope and Cerenkov counter were measured by using the 7rf’s from Ibis events. The momentum corrected percentage loss is 5.0 i 0.2 %. Using these values, N nn and the h’i
-+ ?T+K- decay rate we have obtained the 90%
confidence level upper limits for the Kg -+ ,ue and ee decays with a systematic error of about 6% as follows: BR(Ki
+ pe) < 1.9 x 10-r” and BR(@
Kg ---) ~iy decay rate was measured to be BR(
-+ ee) < 2.5
x
10-lo. The
I<:--+ p;ttf=( 8.2f0.8fstat) i 0.7(sys) )x10-‘.
S.S. Yamamoto / KL-
pe and KL-
ee decays
133c
I
,
8-
I
(b)
-
-60
-
L “0
.
. 2-.
0 480
i
I
I
490
500
510
160f
I
M,, ( MeVicz
,
)
FIGURE 3 Scatter plot of the effective mass vs o2 for the (a) pe and (b) ee events (c) scatter plot of the effective mass vs B* for the pp events and (d) the pp effective mass distribution of the ptp events with 0’ < 3(mrad)’ This pp decay rate is to be compared with the value ( 5.8 rt O.G(stat) i 0.4(sys) )x10-’
based
on 87 pp events obtained by Mathiazhagan et al.3 These two t~izdecay rates barely agree with each other and are consistent with the unitarity lower limit of ( 6.83 i 0.25 ) x10-s based on a formula by Sehgal ’ within the uncertainties. REFERENCES 1. T.fnagaki et al., Phys. Rev. D40, 1712 (1989). 2. Particle Data Group, G.P.Yost et al., Phys. Lett. B204 (1988). 3. C.Mathiazhagan et al., Whys. Rev. Lett. 63, 2185 (1989). 4. L.M.Sehgal, Phys. Rev. 183, 1511 (1969).