DECAY AND INTERACTICI OF NEUTRAL STRANGE PARTICLES PRODUCED AT COSMOT1DN UERGIES
by
\T.OF TECHNoL
0JUN 23
195,8
TASM
PALFR^Z-M.Sc., East Punjab University(1949)
SUBMITTED IN PARTIAL FULFILLMD!T OF THE REQUIRU!ENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
(1958)
Signature of Author
Department of Physies, May 19, 1958
Certified
by
Thesis Supervisor
DECAY AND INTERACTION OF NEUTRAL STRANGE PARTICLES PRODUCED AT COS9OTR1 ENERGIES
by Yash Pal
Submitted to the Department of Physics on May 19, 1958 in partial fulfillment of the requirements f or the degree of Doctor of Philosophy.
ABSTRACT
Properties of neutral strange particles produced in iron by n mesons of 1.6 Bev mean energy are studied with a large
multiplate cloud chamber. It is shown that ~w 6% of the 00 par-ticles decay by a neutral mode, which is consistent with being a 2n* decay mode. A particles are shown to decay by a neutral
mode,
probably N + no with a branching ratio of 0.30 + .05.Life times of ,* and Ae are measured to be 1.07 + .17 10- sec and 2.4 + 2 10 sec respectively. Interactions made by the 90 componenE of the long-lived 9 particle are seen and analyzed, as also some cases of decay o? the G* particle. Using particle mixture theory the experimental resuIt on 9 interactions is shown
to indicate that the most likely mass difference between Q and QP2 particles is /-r . Life time of
e*
is determined to e9
+' 1 seconds. Internal consistency of all the neutral K particle phenomena observed in our chamber is discussed in terms of the particle mixture theory.An unusual event is described which could be an example of a K-meson of positive strangeness bound with nuclear matter.
Thesis Supervisor: Bruno Rossi
Professor of Physics
Title:3
TABLE OF CONTENTS Title p1e... . Abstract...----..---.-... 2. Table of Contents..- - --.
---....3.
Chapter I .. .... ..* . .. .. ... * * * * *8.
1. Experimental Details...14,
2. The Cloud Chamber...
...
16.
3. Operation with theCosmotron... 23.
h.
Scanning and Analysis Procedure... 24.ChaptereIIl...r... 27.
A. Neutral Mode of Decay of 0
...
271.
General...
27.2. ERets: .h....
...
28.3. Results for 90-
+
R
0...
36.B. Neutral Mode of Decay Of /\A . ...
44h.
1. General...44.
2. Results: Shower ehd...
47,
00
3.
MissingA\8...,.
50.
C. Discussion of the Neutral Modes of Decay of 9G and A .. 52.
1. Ioic Spin-Selection-Rule--in- and-...D.. a...
52.
TABLE OF CONTENTS (Cont.)
Chapter
IV
--...
57.
1. The Particle Mixture Theory and ; Interactions...
57.
2. K
Interactions...----.-...
66.
3. Quantitative Comparison with Particle Mixture Theory.. 78.
4.
Discussion on the Mass Difference between 90 and 90to1
2*
87*
7
Chapter V.... -...---.--...90.
0 0 1. Life times of
A
and...--... 90.2. Life time of 097
ChapterVI...
1014
The "bound K" event...
104.
Chapter VII - . ...
121.
Summary... - -- -- ... *... 121.
Appendix A. 1. Detection Probabilities of Neutral Modes of Decay of 90 adA0....17
2. Energy Calibration for -y-ray Showers...-.--..-... 133.
Appendix B. 1. Interactions of Neutral K Particles at Large Distances 136. frcm Production... 2. Numerical Values of f and f---... 143.
3. Upper limit on 00, go Mass Difference...
146.
4.
Extraneous Sources of Neutral Induced Strange Particle-Producing Interactions...148.
Appendix C.Maximum Liklihood Procedure for Life time
TABLE OF CONTENTS (Cont.)
Acknowledgments...*...
164*
Figures1. Layout of the Cosmotron floor ...
15.
2. The Cloud Chamber ...
17.
3. Camera set
up...
... 18.4. Effective Camera Position with Respect to the Center
Plate of the Chamber....-... --...
21.
5.
Plate Assembly with Respect to the Effective Camera6. Dynamics of n 0 Decay... 33.
7. Probability of getting y-rays of more than 100 Mev
from the Decay g
-4
2n
,as a Function of GIMomentum..
34.
0
0
8. Associated Production of a
A
and a G
,in which at
least 2n Is Result from the Decay of the 0
...
40.
9.
Probability of getting y-rays of more than 100 Mev
from the Decay of a
A
by the Mode /.0
+n, as
a Function of 0
..
enu...h6.
10.
Plot of Function
against
f or k
0,
1/2,
1 and 3---
.
...
61.
11. Integral Plot of Theoretical and Experimental
Distri-bution of V Interactions in the Chamber... 63.
12. An Interaction of a
;o
Particle in which a
A
is
Produced along with a Charged n-Meson...
65.
13. An Interaction of a io Particle in which a A\ is Pro..
6
TABLE OF CCNTENTS (Cont.)14. An Interaction of a & Particle in which a - is
15. Likelihood Curve for 0 go Mass Difference... 88.
16. Integral Plot
of
\
Decay...
94,
17. Integral Plot of 0 Decay ... ..- --... 96.
18. Associated Production of a
A
0 with a "K-fragment"...105.
19. Sketch of the "K-fragment"event... 106.
20. Sketch Illustrating Time Measurements for Life time
Determination...
... 157.21. Plot of S() against -r for Maximum Likelihood Deter-mination of 9 0 Life time... 161.
Tables I. Details of Cloud Chamber Operation... 19.
II. Detection Probabilities for y-rays frcan 2n 0 Decay of go. 35. III. Result of Shower Scan for Determining Neutral Modes of 37. Decay of 00and
A?
... * .. 10 IV. Detection Probabilities for y-rays fram N + n0 Decay of A 0 ... V. List ofe
Int erac-
......---.
69.VI. Expected Number of go Interactions for Different Values
of M
90M o ,
9 1 2*''''''''''''''''''''''''''''''''''' .''''9 VII. Expected Number of Apparent Two-hyperon Production Vvents... 83.7
TABLE OF CONTENTS (Cont.)IX.
Likelihood of Apparent Two-hyperon Production Result for
Different Values of M0 ... 0 087.
1
2
X.
Ionizations of Tracks in "bound K event "...
..108.
XI.
Survey of
Neutron Stars in Second and Third Plates of
the Chamber... 149
XII. Survey of Stars made by
2.7 Bev Protons in Iron Plates..
149.
8
CHAPTER I
1. Introduction
Discovery of all the elementary particles known so far, other than the proton, the neutron, the electron and, of course, the neutrino, was made in cosmic rays. In 1947 when Rochester and Butler1 saw the first examples of neutral and charged V par-ticles in a cloud chamber, one knew about the existence of only the -rmeson and the position besides the above-mentioned four fermions. This was also about the time when nuclear emulsion technique was being perfected in Bristol and the discovery of the n-meson was made. Then came a period of intense activity and improved techniques which resulted in the discovery and establishment of a large number of masses and decay modes. Even before the Brookhaven Cosmotron went into operation most of the new particles, which have been subsequently seen in working with accelerators, had been identified in cosmic rays. One knew, for example, that the K particles have a mass close to 1000 me, that they decay into 2 n-mesons, 3 n-mesons and also into a p-meson and a neutrino. Three body decays of K particles involving p-meson and electron had also been identified. Besides the
1. Rochester, G. D., and Butler, C. C., 1947, Nature, Lond.,
160, 855
2
Brookhaven diffusion chamber results, it had also been suggested
in early cosmic ray work that the heavy unstable particles tend
to be produced in pairs. Negative K mesons had been seen and
shown to be less abundant in cosmic rays than positive K mesons.
Cloud chamber pictures also gave evidence that the absorption of
negative K particles leads to A
0
particles.
Amongst the neutral
particles, one had identified
go decay into 2 n-mesons with a Q
value of about 2164 Mev and the most abundant neutral particle, the
A
0whose Q value for decay into a proton and a
ni
meson had been
determined both by cloud chamber and emulsion workers to be about
37
Mev .
Besides these two body decays, one had a few anamolous
neutral decays in cloud chambers, which have recently been shown
to be, predominantly, the decays of the long-lived 90
meson by a
2
6
three body decay
mode.
Amongst the charged hyperons one had seen
the positive 7 hyperon decaying into a proton and perhaps a
n
0, a
negative hyperon decaying into a neutron and a n and Q values for
2. Fowler, W. B., Shutt, R. P., Thorndike, A. M., and Whittemore,
W. L., 1953, Phys. Rev., 91, 1287; 1954, Ibid.,
93,
8613.
3.
Lal, D., Yash Pal, Peters, B., 1953, Proc. Ind. Acad. Sci.,
XXXVIII, A, 398.
h.
Thompson, R. W., Buskirk, A. V., Etter, L. R., Karzmarck, C.
J., and Rediker, R. H., 1953, Phys. Rev., 90, 1122.
5.
Armenteros, R., Barker, K. H., Butler, C. C., and Cachon, A.,
1951, Phil. Mag., 42, 1113. Bridge, H. S., Peyrou, C., Rossi,
B., and Safford, R., Phys. Rev., 91, 362. Lal, D., Yash Pal,
and Peters, B., 1953, Phys. Rev., 92, 438.
6. Lande, K., Booth, E. T., Impeduglia, J., Lederman, L. M., and
Chinowsky, W., 1956, Phys. Rev., 103, 1901.
10
7
these processes had been determined The "Cascade" particle had been seen in cloud chambers and later in emulsions, and its decay mode had been established, together with the fact that only nega-tive cascade particles had been encountered.
Inspite of all this information the situation was quite confused. One was far fram aware at the time that perhaps the bulk of the particles had already been discovered. One had no way of organizing all the available information into a scheme. One was puzzled, for example, by the fact that negative K particles
were much rarer than positive K particles and that the positive K particles, though nuclear interacting, were usually not lost in
nuclear collisions. One did not know that all the K mesons had the same life time and very closely the same masses. Dalitz's
9
analysis to determine the spin and parity of the r meson was in-conclusive because of smallness of the sample available for investigation. There was no information on the spins of K par-ticles or of the hyperons. Though one had a plausible explana-tion for the metastability of these particles in the form of Pais'1 0 associated production hypothesis, it had not been established
that associated production was the only mode of production of these
7. See H. S. Bridge in Progress in Cosmic Ray Physics;
1956,
Vol.
III, North-Holland Publishing Company, Amsterdam 8. Cowan, E. W.,
1954,
Phys. Rev.,94, 161.
9. Dalitz, R. H., 1953, Phil. Mag., 4h, 1068; 1954, Phys. Rev.,
94,
1046.
lI
particles. This essentially was the situation in early
1954:
The number of heavy unstable particles available was too small for an accurate quantitative study of their properties. Then the accelerators entered the field in full force; first the Brookhaven Cosmotron and then the Berkeley Bevatron. Soon high-energy proton n-meson and even K particle beams were available from the accelerators.
11
12
In
1953
Gell-Mann and Nakano and Nishijima had made a successful classification of all the known particles by making suitable isotopic spin assignments for these particles. Associated production was introduced in their scheme by giving each particle a new property called the "strangeness" number, which could be empirically derived by the rule:where Q is the charge + 1 or 0, T3 is the third component of the
isotopic spin, v is the nucleon number and S is the strangeness quantum number. Further, it was stated that all strong inter-actions must conserve "strangeness" and the weak interinter-actions occur through a violation of strangeness. Thus when the machines were ready to produce heavy unstable particles capiously under
controlled canditions, there already existed a frame-work of semi-empirical postulates which seemed to be in agreement with the
11. Gell-Mann,
M.,1953, Phys. Rev., 92, 833.
12.
Nakano, T.,
add Nishijima,
K., 1953,
Progr. Theor. Phys.,
Japan, 10, 581.
experimental information available till
that time and which
pro-vided motivation for some of the experiments which were done
later.
The present experiment was undertaken in late
1954.
A
large multiplate chamber had been constructed for the study of
cosmic ray produced heavy unstable particles, but it was decided
to operate the chamber with the Cosmotron at the Brookhaven
Na-tional Laboratory, to study the artificial production of strange
particles and their decay and interaction properties. It was
ex-pected that a large amount of material in the chamber and its big
sensitive volume would prove very useftil for the investigation of
a series of interesting problems, even though the nature of
ul-timate analysis for which the pictures were taken over a period
of a year and a half was not completely visualized at the
incep-tion of the experiment.
The strange particles included in the
present study were obtained by putting negative n-mesons of about
1.6 Bev kinetic energy through the chamber.
In the following
report we will discuss our results on:
1) Neutral mode of decay of go
2) Neutral mode of decay
of
A
0
3) Interactions of neutral K particles at large distances
from their production and detailed verification of
the particle
mixture theory, including the effect of
0 0
13
4) Life times
of
A
0, go, and 9
2
5)
A possible "bound K" event.
The results on production cross-sections for different
kinds of particles, and asymmetries of production and decay
proc-esses will be discussed elsewhere
1 3.
The body of this thesis has been divided into seven
chapters. In Chapter II we discuss the experimental details of
this investigation. Chapter III will be concerned with the
dis-cussion of the neutral modes of decay of
90
and
A0
particles.
The detection efficiencies for y-ray showers will be discussed in
Appendix A. In Chapter IV we will discuss the interactions
pro-duced by neutral K particles at large distances from their
produc-tion, as also a method of determining the mass difference between
0 and
2
particles. Details of the calculations involved, as also
the discussion of the spurious causes of neutral induced
inter-actions is relegated to Appendix B.
Chapter V
is concerned with
the life time measurements on A
090
and
00.
Details of the
max-1
1
imum likelihood procedure for determining life times will be
dis-cussed in Appendix C. In Chapter VI we will talk about a peculiar
event and finally in Chapter VII we will present a review of our
results.
13. Boldt,
E., Bridge, H. S., Caldwell, D. 0., and Yash Pal
Proceedings of the Venice-Padua Conference 1957,
-
to be
published.
'4
CHAPTER II
1. Experimental Details
The cloud chamber used for the experiment was designed and built by Dr. H. S. Bridge. Several members of the M.I.T. cosmic ray group participated in the operation of the chamber from time to time and the author very gratefully acknowledges their basic contribution to this experiment.
General Set-up
General lay out of the Cosmotron floor with the posi-tion of the chamber is shown in Fig. 1. The chamber was located in a shielded, temperature-controlled rom approximately 50 feet from the machine. The particles from the machine were allowed to emerge through a narrow (8" wide) channel in the shielding. A steering magnet outside the channel was used to sweep out the low energy secondaries made in the channel walls, and deflect the beam by about 80; so that the chamber did not look directly at the tar-get. During most of the operation a n meson beam was used. This was obtained by ramming an Al target in the acceleration chamber and collapsing the proton beam on to it. The negative n-mesons emitted in the forward direction after being momentum analyzed by the Cosmotron magnetic field emerged from a channel in the shield-ing wall, the momentum of the accepted particles dependshield-ing upon
0 6 12 GLOUW CHAMBER STEERING -MA---.T -BEAM EMERGES UNDEFLECTED ---BEAM BENG TARGET SHIELDN EE
16
the position and orientation of the channel with respect to the target. Some of the pictures were taken with the proton beam, which consisted of the so-called "blow up" beam of the Cosmotron. The channel f or this mode of operation was, of course, approxi-mately tangential to the acceleration chamber of the Cosmotron, and the beam as before, was bent into the cloud chamber by a steering magnet located outside the shielding wall. In the present inves-tigation we will discuss, mainly, the results obtained using a n beam.
2. The Cloud Chamber
The cloud chamber has been described elsewhere . It had a well-illuminated volume of about 54" X 48" X 20" and it contained 17 plates of 1/2" thick steel coated with 20 mil sheet-ing aluminum for high reflectivity. The chamber was set horizon-tally so that the beam entered it on one of the 60" X 21" sides approximately normal to the iron plates. Pictures were taken through the top glass window of the chamber by three cameras sit-ting on the floor and looking through a front-surface mirror
fixed on top of the chamber. Figures 2 and 3 show the positioning of the chamber and the camera set up.
Figures
h
and 5 give the chamber and camera lay out14. Annual Progress Report, pp 51, June 1, 1955 to May 31, 1956, Laboratory for Nuclear Science.
r
At-Il
I1
17
-ft . --;I18
ANN
LLN.
firV
Ifrom the optical point of view. Other details of the chamber operation are listed below.
TABLE I
Type of operation
Illuminated volume
Operating temperature
Compressed pressure
Gas
Vapour
Expansion ratio
Delay between expansion
and arrival of beam
Recycling time
Plate assembly
Number of plates
Size
Material
DensityVolume controlled f or about
5000
pictures. Pressure controlled for about 5000pictures.
54"
x 48"
X
20"t
21.5
1
95
cm
Hg.
Argon
(Argon with 10% helium for part of the run)
70/30 alcohol water
mix-ture in some runs,
95%
alcohol in others1.07 to 1.12 depending on
the mixture used
a 40 milliseconds
1
1/2
- 3 minutes17
21" X 60" X 1/2"
Iron coated with 20 mil polished aluminum
TABLE I (Cont.) 10.05
g
cm-
2, 0.7
radia-tion lengths
Photography
Number of cameras3
Focal length Film Photographic background Light delay IlluminationKodak Linograph Ortho
70 mm
Black velvet cemented on the piston surface
Z
150 milliseconds
Two 60" xenon filled flash
lamps with "Alzak"
cylin-drical reflectors on either
side of the chamber
Thickness
Lens
Fiducial Marks
(5 cm Separation)
174.411 I211"
EFFECTIVE POSITIONS OF CAMERAS WITH RESPECT
TO CENTER PLATE OF THE CHAMBER
)
66"
10.5
66"
E
/-/,v
60
-J
-4
.03
474
____ ___34"
"
---
1
Line of
Camera Lenses
Plates
:
60" x 21 x
-L
Iron
2
PLATE ASSEMBLY AND EFFECTIVE
CAMERA
POSITIONS
j
k
21"
"1
3.
Operation with the Cosmotron
Cosmotron "programming" was used to control the time sequence. For a particular n run the following time relation-ships were obtained for the various events mentioned, the time being measured with respect to the start of the acceleration cycle of the Cosmotron.
Time of expansion 945 milliseconds Time of R. F. turn off 980 milliseconds
Time of arrival of the
beam at the chamber 990 milliseconds
Time of light flash 1100 milliseconds
For a typical run with protons, the time sequences were:
Time of expansion 940 milliseconds
Time of R. F. turn off 1000 milliseconds Time of arrival of the
beam 985 milliseconds
Time of light flash 1090 milliseconds
Channels for n and proton exposures were laid out with the help of the mamentum orbits calculated by R. H. Sternheimer of the B.N.L. Cosmotron group. The most probable erergy of n mesons was 1.6 Bev, the spread extending from 1.2 Bev to 1.9 Bev. The mean beam energy for the proton runs was 2.7 Bev. The Cosmotron programming control adjusted the particular requirement for the beam pulse immediately succeeding the expansion of the chamber
with regard to energy of the circulating beam, target, intensity, time of arrival and the time width of the beam. The optimum in-tensity aimed for was about one to two dozen tracks per picture. In practice it was difficult to maintain a constant level of in-tensity, particularly for proton rms, because the necessity of operating at a low circulating beam renders the "noise injection" mechanism of intensity control ineffective. For some runs
scin-tillation counter telescopes were used to monitor the intensity of the beam pulses.
d4.
Scanning
and AnalEs ProcedureThe film was scanned by projecting the three views on three projectors layed side by side. The projected image for scan-ning was about 1/2 the life size. In scanning, the main emphasis was to look for charged or neutral Vs decaying in flight. S par-ticles were also picked up but with a small efficiency. However, after one strange particle had been found in one picture, search was made to find another one in the same picture either caning obviously from the same origin as the first, or otherwise. Origins were assigned to the events whenever they were unambiguous.
Most of the film was scanned only once, but enough was scanned twice in order to determine a scanning efficiency. This was found to be -:s 80% for 1 scan. Detection of an associated
event is easier than detection of a single event. It is believed that the scanning efficiency for associated events was close to
26'
100%, if both decays were in flight. After the scan each event was examined carefully and traced on translucent sheets of paper. The tracks required for measurement were labelled and the observed ionizations were specified for all relevant tracks. Using proper criteria as discussed in section on neutral decay of 90, a sur-vey was made for electron showers associated with the events.
The tracks, drawn up and labelled were measured by a group of scanners, directly on the projectors using an accurate drafting machine and proper scales. The measurements required for analysing a track segment are two spatial coordinates of a point on the segment and its angle with respect to vertical in each of the three projected views.
The measurements were then transcribed in a proper form for key punching and analysis with a type 650 I.B.M. ccmputer. Besides the measurements made by the scanners, one had to include other information like the sense of motion of the particles, the weighting factors for various tracks depending on picture quality,
and correction factors (called stretch factors) to allow for ex-tremely small distortion of the angles due to shrinkage of the film, curvature of the photographing mirror or non-uniformity in the particular projector used for the measurement.
Output of the first program gave us:
1) Spatial angles for all segments analysed
Z6
3)
All space angles between any three tracks and their
departure from coplanarity.
h)
Root mean square deviation from the weighted mean
of the three determinations for range and space
angles between tracks
5)
Depths in the chamber for the points analysed
6)
Ionization corrections for
charged tracks and life
time corrections wherever required
A second and a third programe were written up later,
which used the output of the first programme with same additional
infarmation to calculate angles relevant for detecting possible
asymmetries in the production md decay processes. The present
report does not include a discussion of these asymmetries or their
interpretation
CHAPTER III
A. Neutral Mode of Decay of 0
1. General
Two pion decay is the predominant mode of decay of the neutral K particle:
K-
1r1U*+ lr~
In the following we will refer to the neutral K par-ticle, decaying into 2 n-mesons with a life time of the order of.
of 1010 sec, as Q particle.
Since the final state in this decay consists of two bosons, the spin of 00 must be integral. Consider now a possible
1
decay mode for
:
01'--> TT
rTr
If the spin of 0 is j, then
j
= 1, where 1 is the rela-tive angular momentum of the two pions. Since the final state consists of two identical particles, their interchange should leave the state unaffected. However, an interchange of the two particles is equivalent to subjecting the final state to a space reflection, which would introduce a factor (- 1) in front of the wave junction so we have:t
I-
= (0=(-i.e. j must be even. Therefore in order that decay of 0 0 into 2 1
2. Experimental
In order to detect the 2 n0 decay mode or any other y-ray producing decay mode for G0, we decided to look for electron showers associated with events where the presence of a G0 could be indirectly inferred. Our cloud chamber has seventeen 112"
thick iron plates and a large illuminated volume, which makes it good y-ray detector. To achieve satisfactory criteria for shower survey we assume that associated production is established and that a hyperon production event indicates the simultaneous production of a K particle (as distinguished from a K particle). With n mesons on iron, the elementary reactions will be
TT~ -t- N -- >E+ K* TT ~ + P - + K t
+ Ka A0 + K0
Thus to a first approximation a neutral hyperon must always be accompanied by a Ko and therefore the presence of a
A
could be used as a "9 0 Indicator". However, in ccming out of the
nucleus, both
t
and Ko have a fair chance of interacting and the fact that a A\ is seen to decay could imply either the productiono +
of a K or of a K . Therefore we introduce the following two criteria to pick up a "10 Indicator". (These events are also referred to as "pure"
A'S
in the following discussion.)1) All
A
0 producing interactions made by n mesons on iron, in which no charged prong is seen to emerge out of the iron plate, or2) Tose
A
0 producing interactions which have all the charged secondaries stopping in the chamber, withnone of these secondaries being a K meson.
These two criteria will lead to an overestimate on the number of "0 Indicators" because some of the K+ particles would decay in the first plate while others may decay via a three body mode in the succeeding plates (thus remaining undetected) and the
corresponding events will be wrongly included amongst " 0 Indicators".
This error may not be as trivial as it may seem, because the K par-ticlesinvolved are predominantly produced along with E hyperons and the experimental evidence of the Columbia bubble chamber group16
shows that they are preferentially emitted towards 900 in the labor-atory system with low momenta and therefore have a good probability of coming to rest in the plate of production. We will keep the above fact in mind while discussing the results of shower scan. Selection of Showers
In order to restrict the background of yb-ray showers to a reasonable value, as also for a good efficiency for detection, one needs to set a lower limit on the size of showers one should look for. We decided to include only those showers which have three or more electron segments associated with them. As discussed in Appendix A.2, there is a direct relation between the number of electron segments in a completely developed y-ray shower and its energy. On the basis of considerations mentioned there, a three electron shower at normal penetration in the plates of the chamber corresponds to a y-ray of about 80 Mev. But if we survey for
30
showers of a minimum of three electron segments associated with a
"10 Indicator" we are looking for showers produced by y-rays whose
energy will vary slightly depending on the angle of penetration of the shower. This fact hi's been adequately taken into account in obtaining a detection efficiency for showers in Appendix A. In order further to minimize the inclusion of showers which are definitely due to causes other than the decay of the 90 into a -ray pi-oducing neutral mode, we adopt the following criteria:
1) We make a shower survey only for those "Indicators"
where the picture is of good quality and the region around the origin is not completely cluttered with other beam tracks and their interactions. This enables us to have a high efficiency of finding the showers.
2) We accept only those showers which originate with-in
4
plates of the AU origin. Since each chamber plate is -.0.7 radiation length at normal penetra-tion, the y-ray will almost surely be converted in this much distance.3) When a shower is found we draw a "best line of
flight" of the ray through it and see if the y-ray could have obviously originated in another inter-action in the chamber. Since the interinter-actions of 7! mesons in iron plates can be quite well located in space, a y-ray originating (from n 0 decay for
example) in one of them can be easily recognized as such. Actually quite a large number of showers seen are of this category and are not counted. If, howe-ever, there is a reasonable doubt about an assign-ment to one of these extraneous origins (as for
ex-ample is the case when the direction of the shower is not well defined), the shower is accepted in a
"yes minus" category, if it fulfills the other crite-ria.
h)
An appreciable fraction of the 90 Is decay right in the first plate, quite close to the origin, there-fore quite a few y-rays resulting from a possible 27
31
appear to come from the origin of the interaction.
For this reason we cannot discard the v-rays which
seem to originate directly in the nR interaction
of the "o Indicator". However for ease and
effi-ciency of
scanning
we require that the impact
param-ater of the shower direction with respect to the
n-interaction be less than or equal to 6
cm.
In
Appen-dix A.1 we have studied this problem to estimate
what fraction of y-rays due to 2 nO decay mode will
be lost by introducing this criterion.
Background and Scanning Efficiency
Because of a large number of n. interactions in plates
of the
chamber, there is an apprecia ble
amount of background of
showers
in the chamber. Though most of the background showers are
eliminated by
inuroduction of the selection criteria mentioned above,
some of them might
be included. By background here we mean all
showers which are due to causes other than the possible neutral
decay mode of the 9
0(for example, it will definitely contain
con-tributions due to the decay mode 1->
A
+
y).
This can be
estimated very well by looking for showers, which satisfy all the
selection criteria, but are found instead in association with
visible "associated production" events. This has been done.
Since we restrict ourselves to a well-defined region in
the chamber f
ar
a shower scan, we believe that the
chanx
e of
mis-sing a shower of three or more electron segments is very small.
One systematically examines all electron tracks in this region to
see if they can define
an acceptable shower.
Having found a
shower one tries to see if it could come from another origin. If
not,
it
is accepted as a
tsignal".
Having found an acceptable
3z
shower, its elimination requires a more stringent criterion (namely, to point directly to another interaction) than its in-clusion as a signal (that it have an impact parameter of (. 6
cm with the "Indicator" origin). Therefore, if anything, one will tend to overestimate the "signal". Hence the signal has been divided into two kinds of showers: "yes" showers and "yes minus" showers. A "yes minus" shower is defined as a shower, which satisfies all criteria with respect to the "Indicator"
origin, as also with respect to another n interaction in the chamber, except that it may not point directly to this other n interaction. Including all the "yes minus" showers as signal would be a slight overestimate, while excluding them all would clearly be an underestimate on the true number of showers.
A "yes" shower, of course, is one which does not have any other origin in the chamber except the "Indicator" origin, with an impact parameter of 4 6 cm.
Detection Probabilit0 for 00. n0
"Detection" of a y-ray in the cloud chamber requires that it have an energy greater than that required to give a three segment shower, and that it go through enough material in the illu-minated region so as to convert itself into an electron shower.
The question of detection probability has been discussed at some length in Appendix A.l. We have made an analytical calculation to find out the probability, as a function of go momentum, of
1.00
// Pf (100) Pf (200)'- - - -.90 -I-10 Pf (150) Pf (250)//--.70 0--/ A A00,.60
/.000.---o.50
k1z
/
/
40 -Oleo',~ /a I000,/000
.30
00.I0,.20
10"/
10
0
4
.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
5.2
5.6
6.0
FIG.
6
Luu
0
600
TOO
Soo
900
1000
1100
pe
(Mem)
FIG.
7
-P:-I.C
P.
7
1
.6
.5
.4
.3
.2
Pif (I00,Pe)
PI2
f f (O0,
Pe)
.9-
.8--1(oP)I
Plf b(IOOP)
P12bb(looPe)=
qffbb(looPe)
100
200
300
400
500
1
I
1
35'
TABLE II Event No.6838
7209
7218
7622A
7622G
7827
7860
7870
8008
8208
8608
8500
7240
7364
7946
7151
7222
8252
7678
7524
Plr %
93
75
0
0
84
96
66
84
18
11
0
95
82
0
73
0
87
h
0
0
Plb46
0
6
0
27
38
27
18
15
22
0
71
53
0
0
0
23
10
0
0
2f
0
96
0
90
90
0
95
0
0
0
8h
20
94
9,4
87
95
92
95
ho
0
2b
%
0
35
0
37
38
0
50
0
0
0
0
16
57
61
63
46
54
3h
0
0
Probability of seeing a shower of 3 or more electron
0 0 0 0
segments if the 0 had decayed according to the mode 90 .-
+ RtThe probability is given separately for each of the four y-rays
involved.36
getting one or more y-rays of greater than 100 Mev through the process 0 - 2n -
4-Y.
The results of this calculation are given in Fig. 7. These are meant only as indications of how the probabilities go. The actual detection probabilities would involve the angular distribution, the momentum distribution and places of production in the chamber for the I0s concerned.These have been obtained by an analogue method which calculates the probabilities of getting showers of more than
three electron segments for a random sample of observed
e
0 decays, if these decays had occurred by the 2n mode instead of thecharged mode. Details of this method are discussed in Appendix
A.1 and the results are given in Table II. We find that the average probability of seeing at least one y-ray from the 2R0 decay mode of a 90 is 78% while the probability for seeing at least two y-rays is close to
40%.
3. Result for 0 -4 n0 + R
Table III gives the results of the shower scan. A
TABLE III Associated with
A*
A
0A
0A
0A
0A
0A
0A
0A
0 00
0 @00o
04 0* 0* + 0 + 0 Type of shower yes -yes yes yes yes -yes -yes -yes -yes yesyes
yes-yes
yes
yes-yes
yes
yes
yes
yes
yes
yes
Number of electron segments3
3
2-33
3
17
8
3
17
10
.4-7
5
7
3
3-4
5-6
3
14
5
3-4
3
Energy of shower Mev86
100
55-80
156
78
470
280
102156
h
42
260
115-200
180
240
95
117-156
156-187
95
110
23480-104
125
37
Event No.8553
9085
6785
6406
6929
441
43028
Impact Parametercm
5.1 2.30
0
h(0)
22.5
3-4(0)
1
1
8608
7913
7870
7827
8783
7364
6320
10176
9492
7151
10335
A
0A4
I
TABLE III (cont.) Associated with
A
+A
0 +A
0+
Type of Number of shower electron segments yes -yes -yes-13
3
Energy of shower Mev4o1
62-156
132List of
showers found in a shower scan on 130 pure A*
t s
47 pure Gots, and 27 pure A
1'A
+
0 events.
1
.11
38
Event No.4494
9355
Impact
Parameter cm4.7
0
0
I I39
two criteria given on page 2' . In the n. pictures we had a total of 166 pure A
's,
52 pure 91 's, and 31 cases of pure events where both aA
0 and 00 were seen to decay. Out ofa 0 0
these 130 pure A's, 47 pure Q,'s and 27 pure A +9 events
were scanned for showers. From Table III we see that 9
A
0's
00and
.5A
0'+e'events were associated with showers. Showers
asso-ciated with the visible A+9,events cannot obviously be due to a neutral mode of decay of 01, therefore they are expected to give the number of background showers. These showers may be due to direct n0 production, due to 0 decay into a y-ray and
A
or due to other causes. Fram this number we conclude that the number of background showers expected for 130 pure A''s will be of the order of 6 130 5 t Ii while the total number of showers is 9 + 3. In order to put an upper limit on the number of 90's
which decay by a y-ray producing neutral mode, we assume that the back-ground is too high by 1-1/2 standard deviations and the number of showers with pureAI's
is too low by 1-1/2 standard deviations. Probability for this is only about 25. Thus this would give us an upper limit for the number of 0 Is decaying by the neutral mode~~t~e
00
)
~
+
(~i4. -7)i-65)=
This number corrected for detection efficiency of 78% is 6.5. This will be an upper limit also because some showers may conceivably
come from events in which the invisible particle is other than a
0 +
decay-0
4434C
a
a
/ b l/1e
f7-_Xl
. I - .'II
FiG.
8
4o
3
4
5
6
7
8
9
I
rooe
I I I Iing by the mode K- n +n0 ). In the same sample we have (27 X 1.62 = 43.2) decays of 9 into two charged n mesons. (1.62 is
1
the average inverse of the probability for a 9 to decay in the visible region of the chamber.) Thus we have
[
(
00) -
-
5-
Z 0
i
s"-.,y.
Therefore we conclude that on the basis of our data not more than 13% of the 00 particles can decay by a y-ray producing neutral mode. This upper limit will not hold if the neutral decay
occurs predominantly into a mode involving only 1 y-ray or only
1 R , because one will have a lower detection efficiency f or y-rays in these two cases.
Inspite of the fact that the ratio of neutral to charged mode of decay for 90 is consistent with zero on the basis of the above data, we have other evidence to show that the neutral mode does exist. During the shower scan we have found one event, in which we observe two showers associated with a "IQ Indicator". A
sketch of this event is reproduced in Fig. 8.
a is the incoming negative n meson. b is the line of flight of the A which has a momentum of 570 + 20 Mev/c. There is no charged secondary coming out of the interaction. The two showers are S and S 2 i and h represent the lines of flight of
-k
the y-rays prodicing S and S 2 i and h intersect in space. If
S and S2 arise from the decay of a neutral particle produced in this interaction, then g would represcnt the line of flight of this neutral particle. The spatial angles and other relevant in-formation is given below.
Angles ab = 17.050 + 2.93 ag = 8.270 + 1.82
cd
=
38.66
+0.14
ih = 57.85* + 1.2 gi =)0.76
+ .3h gh - 19.43 + 1.1 Un-coplanarities 6(bed)
=
h.
6 (ghi) =4.5
0 6 (abg) = 6.350 0 Momentum ofA
570 + 20 Mev/C Energies of y-raysNumber of track Average penetration Energy
segments (in plate thickness) (Mev)
s 8 1.33 280 + 90
S 17
1.06
470
+ 120
2-Errors on energies are probable errors due to statistics.
r
43
in S2 which would introduce an additional uncertainty of about
50
Mev in its energy.
If the showers are due to 2 y-decay of a particle, then the mass of the particle is given by
M
4
E
1E2,
S &,, 57. F=340 t
8o
lev Thus we conclude that1) The two -reys do not arite from the decay of a single n
2) It is unlikely that they arise from 2 y-decay of a 90 (whose mass is known to be about
h95
Mev)The small angle of uncoplanarity 6 (ghi) = !1.50 suggests
that if the decay is in 2n0 Is, then the two y-rays carry most of the energy of the two n'Is. Further we find that 2n 's arising from the decay of a 90 at a relative angle of 57.80 and with rela-tive momenta in the ratio sin
40.76
will have total energiessin 19.43
of 295 +
10
Mev and 600 + 35 Mev respectively. These energies are in approximate agreement with the observed y-ray energies. There-fore the present event strongly suggests the decay modeGO -- > iT* + - TT a
It is not possible, of course, to rule out the possibility that we have here a case of 3i" decay, except on the grounds that 2n decay mode fits rather well, coplanarity and dynamics both, and that
short-lived 3n decay modes of neutral K particle have not been
observed.
If
2n0 mode is accepted, the
9
0lives for
0.75
X
10'10
44
sec before decay.
As shown in Appendix A.1, the probability of seeing two or more y-rays in 2n0 decay of a 0 is equal to about
40%.
Thus our having seen one case of two y-rays gives a ration
e(G.o)
.o 4 foM(.e..)
+
n
te,..)
This may be considered in the nature of the most probable value for the fraction of 90 particles which decay by the 2n0 decay mode.
Thus on the basis of our work we find that '
VI
(
)
+
(e....)
and that it is definitely greater than zero. We may also express
our result as .06 + .05. Columbia bubble chamber people find
.14 + .07 for the same ratio. Combining the two we get -11 :t .05
B. Neutral Mode of Decay of A 0
1. General
A
0 particle is known to decay according to the mode /* -.. -t- T7There is no known restriction against the decay of
A
0into a neutron and a n 0. This decay mode may also be detected by
observing showers made by y-rays from the no's. For this purpose
16. Plano, R., Samios, N., Schwartz, M., Eisler, F., and Stein-berger, J., Nevis Cyclotron Laboratory Report 46, May 1957, Columbia University and private communication.
we make a shower scan for pure @'s, where ttpurity", as before, is defined by the two criteria,
1) Either tgere be no charged prongs from the origin of the 9 , or
2) if there are any charged prongs,they should all stop in the chamber and none of them be a charged V or an S particle.
By using these criteria, we will select all events
0 + +
which have an accompanying
A
, T~j, K or a 9 . (all invisible, of course)Any showers associated with these events, which are in
excess of the background expected on the basis of shower scan on
associated events, will be due mainly to the neutral decay of the
0 +
A
. Contribution of . decaying in the plate of production by+ 0
the mode 1...p + n is expected to be negligible because
a) A T+ can be produced in association with a 00 only
by a second order process in a n~ nucleus collision, and therefore will happen very rarely;
b) less than 1/2 T.+'s produced are likely to decay in
the same plate in which they are produced, and
c) only 8ne-half of those which do decay will decay by p + n mode.
Contribution from K decaying in first plate will be negligible be-cause 2K production is very small and hardly any K - particle is expected to stop in the first plate. Showers due to 0 decay into 2n Is will not be appreciable firstly because of the small cross-section for 29 production, and secondly, as snown above, because
P
(100, P)
P
b ,P )b(bOO,
()
-
b (
A .200
300
400
500
600
P
700
800
900
Mev
(-cFIG. 9
1.0
.9
.;
.5
P
4
.3
.2
.1
0
100
1000
1100
oil 'I Apqmp47
of the small branching ratio for decay of a 0 by a neutral mode. Even though all the contributors to showers associated with pure
@ s are unimportant, except perhaps the neutral mode of decay of the
A
, the ratio (of neutral to charged mode ofA
) we get this way will perhaps tend to be an overestimate.Selection criteria for the showers is exactly the same as discussed for the case of the neutral mode of decay of G0. It
turns out, as expected, that the detection efficiency is much
0 0
smaller than it was for the case of 2n0 decay of Q,. This is clear from Fig. 9 where we have plotted the probability of getting a y-ray of more than 100 Mev from the decay of A0 in a neutron and
n0 , as a. function of the
A
momentum. The actual value of the detection efficiency has been found, as for 2n0 decay of Qe', by ananalogue method discussed in Appendix A. In Table IV we show the detection probabilities for the two y-rays due to n0's for a randomly
chosen sample of A 's. The weighted mean of the probabilities
for seeing one or more y-ray showers of three or more electron
seg-ments is about 36%.
2. Resultsj Shower Method
The result of shower scan is given in Table III. Though strictly speaking the background should be estimated only by using the associated events, we may improve the statistical accuracy of the background determination by taking the showers associated with all events involving A0 or A0+ Go. This can be done because,
10514
0
0
10213 0 010173
0
0
10211 0 010335
0
0
10432 0 0 101415 90 1210391
52
0
10290 93 lit 1022244
010265
74
0
10285 92 010140
55
0
Probability of seeing a shower of three or more electron segments for a few A 0's if they were to decay by the mode
4 q
as already seen, the contribution of showers due to
2n0 decay of
90 is
extremely small, and should be covered by the statistical
error of the genuine background. As in the case of 2n0 decay of
@,
we give full weight to all showers, whether yes or yes minus.
This is because a yes minus shower is more likely to be connected
with the event than not. From Table III we see that there are
(5
+ 9
=
14)
"background"
showers associated with (130
+27
=
157)
cases of pure
A
and pure
A +
events.
Number of pure 9,
events for which a shower scan was made is 47 and 9 of these are
associated with a shower. The background amongst these will be
equal to
( lh
X 47
=+
1). Therefore the "signal" is equal to
l!77
(9
+
3)
-(4
+ 1)
5
+3. Average detection probability for
see-ing at least one y-ray from
N+
n decay mode of
A
has been
found to be 0.36
in Appendix A.l.
Therefore our sample has
5 +.
l
+
8
cases of neutral
decay
of
A
00.*36
i.e.
YV,( Aao
141+9
0
But, taking the average inverse probability of A's into account,
the number of charged decays of
^p:
K (A-)
7
Y,1-9 Y1 10IIo)
-17 1and
=
- 22 -13h, (
A)
gives the fraction of A Is which decay by a y-ray producing neutral mode.
3. Missing A0 s
Frequency of the neutral mode of decay of A 0 can be
determined in another way without using a shower scan. Our sample consists of 52 pure go's and 31 pure (
A
+ GO) events. A pure 61 may be associated, as mentioned already, with another 0 0 or a which decays in the plate of production, or a A, We can estimate the fraction of pure 0, events which wmuld be accompaniedo
+ 0 0by a 9,,or a . by comparing the relative number of 6 + A0, 6 + I
+ a 0
- and 0, + 6, events observed in the sample from which this data was talien. We find that the observed numbers are
0, + * 1* : : =+ * 31 : 13 : 2
+
Inverse probabilities of
A*,
S - and Q, are of the same order, therefore a go will be accompanied by aA
0, , and a e with the above relative probabilities. Only half the 's decay in the plate of production and out of these nearly half may have secondaries which will tend to go out of illumination and render the event impure. Further f or @, + e , one must also ta'M' intoaccount the fact that half the 4's decay by the long-lived 6* mode
2
and hence are not observed. Therefore for pure ;E's we will have for the accompanying particle+
