VOLUME 1$, NUMBER 4 P H Y SI CAL RE V I E%' LETTERS 27 JULY 1964
EVIDENCE FOR THE 2rr DECAY OF THE Km MESON*1
J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay~
Princeton University, Princeton, New Jersey
(Received 10 July 1964)
PLAN VIEW
I root
VFEEEPEEEEPz
57 Ft. to =
internal target
Cerenkov
FIG. 1. Plan view of the detector arrangement.
This Letter reports the results of experimental
studies designed to search for the 2m decay of the
K, meson. Several previous experiments have
served"~ to set an upper limit of 1/300 for the
fraction of K2 's which decay into two charged pi-
ons. The present experiment, using spark cham-
ber techniques, proposed to extend this limit.
In this measurement, K,' mesons were pro-
duced at the Brookhaven AGS in an internal Be
target bombarded by 30-BeV protons. A neutral
beam was defined at 30 degrees relative to the
1 1
circulating protons by a 1&-in. x 12-in. x 48-in.
collimator at an average distance of 14.5 ft. from
the internal target. This collimator was followed
by a sweeping magnet of 512 kG-in. at -20 ft. .
and a 6-in. x 6-in. x 48-in. collimator at 55 ft. A
1~-in. thickness of Pb was placed in front of the
first collimator to attenuate the gamma rays in
the beam.
The experimental layout is shown in relation to
the beam in Fig. 1. The detector for the decay
products consisted of two spectrometers each
composed of two spark chambers for track delin-
eation separated by a magnetic field of 178 kG-in.
The axis of each spectrometer was in the hori-
zontal plane and each subtended an average solid
angle of 0.7&& 10 steradians. The squark cham-
bers were triggered on a coincidence between
water Cherenkov and scintillation counters posi-
tioned immediately behind the spectrometers.
When coherent K,' regeneration in solid materials
was being studied, an anticoincidence counter was
placed immediately behind the regenerator. To
minimize interactions K2' decays were observed
from a volume of He gas at nearly STP.
Water
The analysis program computed the vector mo-
mentum of each charged particle observed in the
decay and the invariant mass, m*, assuming
each charged particle had the mass of the
charged pion. In this detector the Ke3 decay
leads to a distribution in m* ranging from 280
MeV to -536 MeV; the K&3, from 280 to -516; and
the K&3, from 280 to 363 MeV. We emphasize
that m* equal to the E' mass is not a preferred
result when the three-body decays are analyzed
in this way. In addition, the vector sum of the
two momenta and the angle, |9, between it and the
direction of the K,' beam were determined. This
angle should be zero for two-body decay and is,
in general, different from zero for three-body
decays.
An important calibration of the apparatus and
data reduction system was afforded by observing
the decays of K,' mesons produced by coherent
regeneration in 43 gm/cm' of tungsten. Since the
K,' mesons produced by coherent regeneration
have the same momentum and direction as the
K,' beam, the K,' decay simulates the direct de-
cay of the K,' into two pions. The regenerator
was successively placed at intervals of 11 in.
along the region of the beam sensed by the detec-
tor to approximate the spatial distribution of the
K,"s. The K,' vector momenta peaked about the
forward direction with a standard deviation of
3.4+0.3 milliradians. The mass distribution of
these events was fitted to a Gaussian with an av-
erage mass 498.1+0.4 MeV and standard devia-
tion of 3.6+ 0.2 MeV. The mean momentum of
the K,o decays was found to be 1100 MeV/c. At
this momentum the beam region sensed by the
detector was 300 K,' decay lengths from the tar-
get.
For the K,' decays in He gas, the experimental
distribution in m is shown in Fig. 2(a). It is
compared in the figure with the results of a
Monte Carlo calculation which takes into account
the nature of the interaction and the form factors
involved in the decay, coupled with the detection
efficiency of the apparatus. The computed curve
shown in Fig. 2(a) is for a vector interaction,
form-factor ratio f /f+= 0.5, and relative abun-
dance 0.47, 0.37, and 0.16 for the Ke3, K&3, and
Eg3 respectively. The scalar interaction has
been computed as well as the vector interaction
138
VOLUME 1),NUMBER 4 PHYSICAL REVIEW LETTERS 27 JULY 1964
QATA: 52ll EVENTS
~ ---- MONTE-CARLO CALCULATION
VECTOR —' -0.5 - 6oo
484&m" & 494 -. IO
- 500
I
0
I
Il~
I
I
I
- 400
- 500
- 200
- IQO 494& m~& 504
30
CA
-- 20
1 LU
LLI
- IO
i iJ
500 550
I I I
400 450 500 550 600 MeV X
p iZ
(b)
504&m" & 5 l4 -- IO
"---"MONTE- CARLO CALCULATION -~ )20
VE CTOR — & 0.5 "- I IOf+
- IOP
90
~,
- 80
ewe $0
r-- ~ sp
r-i ' ~ 5p
y II 40
- 50
. 20
IO
10.998 0,999
cos g
FIG. 2. (a) Experimental distribution in rn~ com-
pared with Monte Carlo calculation. The calculated
distribution is normalized to the total number of ob-
served events. (b) Angular distribution of those events
in the range 490 &m*&510 MeV. The calculated curve
is normalized to the number of events in the complete
sample.
with a form-factor ratio f /f+ =-6.6. The data
are not sensitive to the choice of form factors
but do discriminate against the scalar interac-
tion.
Figure 2(b) shows the distribution in cos8 for
those events which fall in the mass range from
490 to 510 MeV together with the corresponding
result from the Monte Carlo calculation. Those
events within a restricted angular range (cos8
&0.9995) were remeasured on a somewhat more
precise measuring machine and recomputed using
an independent computer program. The results of
these two analyses are the same within the re-
spective resolutions. Figure 3 shows the re-
0
0.9996 0.9997 0.9998 0.9999 I.OOOO
cos 8
FIG. 3. Angular distribution in three mass ranges
for events with cos0 & 0.9995.
suits from the more accurate measuring machine.
The angular distribution from three mass ranges
are shown; one above, one below, and one encom-
passing the mass of the neutral K meson.
The average of the distribution of masses of
those events in Fig. 3 with cos8 &0.99999 is
found to be 499.1 + 0.8 MeV. A corresponding
calculation has been made for the tungsten data
resulting in a mean mass of 498.1 + 0.4. The dif-
ference is 1.0+0.9 MeV. Alternately we may
take the mass of the E' to be known and compute
the mass of the secondaries for two-body decay.
Again restricting our attention to those events
with cos0&0.99999 and assuming one of the sec-
ondaries to be a pion, the mass of the other par-
ticle is determined to be 137.4+ 1.8. Fitted to a
Gaussian shape the forward peak in Fig. 3 has a
standard deviation of 4.0 + 0.7 milliradians to be
compared with 3.4+ 0.3 milliradians for the tung-
sten. The events from the He gas appear identi-
cal with those from the coherent regeneration in
tungsten in both mass and angular spread.
The relative efficiency for detection of the
three-body E, decays compared to that for decay
to two pions is 0.23. %e obtain 45+ 9 events in
139
VOLUME 1$, NUMBER 4 PHYSICAL REVIEW LETTERS 27 JuLY 1964
the forward peak after subtraction of background
out of a total corrected sample of 22 700 K,' de-
cays.
Data taken with a hydrogen target in the beam
also show evidence of a forward peak in the cos0
distribution. After subtraction of background,
45+ 10 events are observed in the forward peak
at the K' mass. We estimate that -10 events can
be expected from coherent regeneration. The
number of events remaining (35) is entirely con-
sistent with the decay data when the relative tar-
get volumes and integrated beam intensities are
taken into account. This number is substantially
smaller (by more than a factor of 15) than one
would expect on the basis of the data of Adair
et al. '
We have examined many possibilities which
might lead to a pronounced forward peak in the
angular distribution at the K' mass. These in-
clude the following:
(i) K,' coherent regeneration. In the He gas it
is computed to be too small by a factor of -10' to
account for the effect observed, assuming reason
able scattering amplitudes. Anomalously large
scattering amplitudes would presumably lead to
exaggerated effects in liquid H, which are not
observed. The walls of the He bag are outside
the sensitive volume of the detector. The spatial
distribution of the forward events is the same as
that for the regular K,' decays which eliminates
the possibility of regeneration having occurred
in the collimator.
(ii) K&3 or Ke3 decay. A spectrum can be
constructed to reproduce the observed data. It
requires the preferential emission of the neutrino
within a narrow band of energy, +4 MeV, cen-
tered at 17+ 2 MeV (K&3) or 39+ 2 MeV (Ke3).
This must be coupled with an appropriate angular
correlation to produce the forward peak. There
appears to be no reasonable mechanism which
can produce such a spectrum.
(iii) Decay into w+7t y. To produce the highly
singular behavior shown in Fig. 3 it would be
necessary for the y ray to have an average ener-
gy of less than 1 MeV with the available energy
ext nding to 209 MeV. We know of no physical
process which would accomplish this.
We would conclude therefore that K2 decays to
two pions with a branching ratio R = (K2- w++ w )/
(K,'- all charged modes) = (2.0+ 0.4) && 10 where
the error is the standard deviation. As empha-
sized above, any alternate explanation of the ef-
fect requires highly nonphysical behavior of the
three-body decays of the K,'. The presence of a
two-pion decay mode implies that the K,' meson
is not a pure eigenstate of CI'. Expressed as
K,0=2 "'[(K,-KO)+e(KO+KJ] then I&I'= R&T—IT2
where 7, and T, are the K, and K,' mean lives
and RZ is the branching ratio including decay to
two r'. Using RT = &R and the branching ratio
quoted above, l et =—2.3x 10
We are grateful for the full cooperation of the
staff of the Brookhaven National Laboratory. We
wish to thank Alan Clark for one of the computer
analysis programs. R. Turlay wishes to thank
the Elementary Particles Laboratory at Prince-
ton University for its hospitality.
*Work supported by the U. S. Office of Naval Re-
search.
This work made use of computer facilities sup-
ported in part by National Science Foundation grant.
~A. P. Sloan Foundation Fellow.
~On leave from Laboratoire de Physique Corpusculaire
h Haute Energie, Centre d'Etudes Nucldaires, Saclay,
France.
M. Bardon, K. Lande, L. M. Lederman, and
W. Chinowsky, Ann. Phys. (N. Y. ) 5, 156 (1958).
D. Neagu, E. O. Okonov, N. I. Petrov, A. M.
Rosanova, and V. A. Rusakov, Phys. Rev. Letters
6, 552 (1961).
3D. Luers, I. S. Mittra, W. J. Willis, and S. S.
Yamamoto, Phys. Rev. 133, B1276 (1964).
R. Adair, W. Chinowsky, R. Crittenden, L. Leipun-
er, B. Musgrave, and F. Shively, Phys. Rev. 132,
2285 (1963).
140

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