Report on the τ-mesons

P A I ~ T E S E C O N D A

SEZIONE A: Particelle pesanti instabili.

Report on the ~:Mesons.

E. A~ALDI

Is t i tu to di t r i s iea dell' Univers i td - R o m a

Is t i tu to Naz ionale di Y i s i c a Nuc lea te . Sezione di Ron~a

Introduction.

The v-meson, discovered in 1949 by the Bristol group []] and confirmed shortly af terwards by the London group [2], is probably the best known of all the new unstable particles.

I ts properties have been discussed and reviewed a t many occasions, in !)articular at Bugn~res-de-Bigorre in Ju ly 1953 [3], at Padua in April 1954 [4], and recently, at the l~ocbester Conference in Feb rua ry 1955 [5].

Therefore f here is not much to add to the Mready well known reports published in the proceedings of those Conferences, apar t f rom the fae~ tha t the cxperimelltM da ta available today s tar t to be statistically suffieien~ for definitely establishing" some impor tan t conclusions which previously had been suggested only as being probable.

The repor t presented at the Pisa Conference was based on the data, obtained in cloud chambers and str ipped emulsion work, already collected in the Repor t of the Committee oa z-mesons of the Padua Conference [4] or published since tha t t~me or seat to me before the conference b y the following laboratories to all of which I would like to express m y best thanks: Physikalisches Ins t i tu t tier Universitiit Bern [6], Brookhaven National Labora tory [6], Radiat ion Labora to ry in Berkeley [6], Tara Ins t i tu te of Fundamenta l Research in Bom- bay [7], Imperial College of Science and Technology of London [6], The Phy- sical Laboratories of the Universi ty of 5lanchester [6], Is t i tu to di Fisica del- FUniversit~ d i Padova, Sezione di Padova delFIst i tuto ~N-azionale di Fisica Nucleare [8]~ Is t i tuto di Fisica delFUniversit~ di Roma; Sezione di Roma

180 ]~. AMALD1

d e l l ' I s t i t u t o I~azionale di F is iea Iqucleare [9], I s t i t u t o di F is ica de l l 'Un ivers i th

d i Torino, Sezione di Tor ino de l l ' I s t i t u to ~Nazionale di Yisica lqucleare [10].

More d a t a h a v e been col lected d u r i n g the Conference f rom the Br is to l

g roup [1] and f rom the R a d i a t i o n L a b o r a t o r y in Berke ley whose resul ts have

b e e n r epor t ed b y Dr. Smogg [1]] .

The p re sen t r epo r t follows the same scheme of t he r epor t p r e se n t e d a t t i le

Conference in P i sa and differs f rom i t on ly in t h a t these new d a t a h a v e Mso

been inc luded in ~he discuss ion of the var ious proper t ies of x-mesons in order

to improve the s ta t i s t i ca l s ignif icance of the co r respond ing conclusions.

Tab le I r ep resen t s a revised a n d e x t e n d e d ed i t ion of the Ta b l e p repared

b y the C o m m i t t e on z -mesons [4] of the P a d u a Conference in t he sense t l l a t i t

con ta ins a]] v-mesons observed in s t r ipped emuls ion up to now, accord ing to

t he knowledge of the reviewer.

F o r the reader ' s convenience , I l ist here the code-words i nd i ca t i ng Labor-

atories and Phys ica l I n s t i t a t e s quo ted in this report .

Be = Phys ika l i sehes I n s t i t u t der Un ive r s i t~ t - Bern

Bh -- B r o o k h a v e n l~a t iona l L a b o r a t o r y - Brookhaven (N. ]5.)

Bk ~- -Rad ia t ion L a b o r a t o r y , U n i v e r s i t y of California - Berkeley (Cal.)

Bo = T a t a I n s t i t u t e of F u n d a m e n t a l Research - Bombay

Br = H. H. Wil ls Phys i ca l L a b o r a t o r y - Bristol

B t h - - E x p e r i m e n t a l Biology a n d Medicine I n s t i t u t e of the ~NationM I n s t i t u t e s

of H e a l t h - Bethesda (Maryland)

E p = Labora to i re de P h y s i q u e de l 'Eeole P o l y t e e h n i q u e - .Paris

Go = Max P l a n c k - I n s t i t u t fu r P h y s i k - GSttingen

Mi = I s t i t u t o di Scienze Fis iehe de l l 'Un ive r s i t~ di Milano e Sezione di Milano

de ] l ' I s t i t u to NazionMe di F is iea l~-ueleare - Milano

P d = I s t i t u t o di Fis iea de l l 'Un ive r s i t~ di P a d o v a e Sezione di P a d o v a del-

l ' I s t i t u t o lqazionMe di Fis ica ~Nucleare - Padova

Ro = D e p a r t m e n t of Physics , U n i v e r s i t y of Roches te r - Rochester (N. Y.)

Re = The W e i z m a n n I n s t i t u t e of Science - Rehovoth

]~.o = I s t i t u t o di Fis iea de l l 'Un ive r s i t~ di R o m ~ e Sezione di R o m a de]l ' Is t i -

t u to NazionMe di F is ica ~Nueleare - Roma

To = I s t i t u t o di Fis ica de l l 'Un ive r s i t~ di Tor ino e Sezione di Tor ino del-

l ' I s t i t u t o Naz iona le di F is iea Nuc lea te - Torino

Wi - - D e p a r t m e n t of Physics , U n i v e r s i t y of Wiscons in - Madison (Wise.).

For symbols a n d n o t a t i o n s refer r ing to par t ic les we have followed the sug-

gest ions gdven in the Proceedings of the P a d u a Conference.

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R E P O R T ON T H E ~ - M E S O N 1 9 5

1. -- Q - v a l u e a n d m a s s .

The decay scheme of v-mesons into 3 pions, first suggest.e<t b y its disco- verers as the more p robab le one, and la ter p u t in doub t f rom t ime to t ime b y some par t icu lar observat ion, was definitely verified a t the t ime of the Con- ference of BagnSres-de-Bigorre [12, 3] and confirmed by all the successive exper imenta l results.

The ma jo r i t y of v-mesons observed in nuclear emulsions comes to rest. before decaying, a po in t usually checked by measur ing the scat ter ing and ionizat ion alol~g the t r ack of the p r i m a r y as well as b y veryfy ing the copla.na- ri ty, wi th in lo or 2% of the three emi t ted pious.

I n mos t eases a Q-value has been given us the stun of the kinet ic energies of the three pions. When all the three part icles st.op inside the emulsion, their kinet ic energies are ob ta ined f rom the range-energy relat ion; if one of t hem does not stop, i ts kinet ic energy is de te rmined f rom grain densi ty and scat- ter ing measurements . Somet imes the conservat ion of to ta l m o m e n t u m and angle measuremen t s are used to deduce the kinet ic energy of one or two of the three pious f rom t h a t of one or b o t h of the others.

A t the Padua Conference the Commi t t e on v-mesons t r ied to establish a reasonable es t imate of the Q-value and of the mass.

As a weighted average of the da ta relat ive to 25 ~-mesons observed b y different au thors in s t r ipped emulsions, the Commit tee obta ined

q = (74.7 ~ 0.3) MeV, M = (965.5 ~ 0.7) m e ---- (493.3 ~ 0.~) MeV.

At the same t ime the Commit tee stressed t h a t these values h a v e to be t aken with caution, in par t icu lar 6hat the given s tandard deviat ions, a l though correct f rom a s t r ic t ly algebraic po in t of view, are certainly underes t ima ted and of doubt fu l significance due to the inhomogenei ty of the averaged Q-values.

Today, following the same procedure, one obtains as weighted average on 54 ~-mesons observed in s t r ipped emulsions in various laboratories,

Q -- (75.0 =~ 0.2) ~[eV, M = (966.1 ~ 0.6) m e ---- (493.7 :): 0.3) 5(eV.

The same remarks made one year ago have to be repea ted here. The first reason for inhomogenei ty of the averaged Q-values is due to the !

fac t t h a t different laboratories still use different range-energ~ relations. In the energy region involved in the de te rmina t ion of the Q-value of z's, the differences.between the more used curves are not very large b u t still sufficient to introduce s is tematic deviat ions be tween the results of various laborat.ories.

F u r t h e r m o r e only few laboratories have adopted the procedure of measuring the Stopping power of the used emulsions, b y determining, in a convenient n u m b e r of cases, t.he range of ~-mesons produced in the decay of pious a t rest.

196 ~. AMALDI

B u t Mso w h e n such ~, p r o c e d u r e is app l i ed , t h e a g r e e m e n t b e t w e e n t h e

res|~lts of v a r i o u s l a b o r a t o r i e s is n o t s a t i s f a c t o r y .

T h e d i s c r e p a n c y , a l t h o u g h n o t de f in i t e ly o u t s i d e of t h e e s t i m a t e d e r ro r s (*),

i n d i c a t e s t h a t t h e r e ~re s t i l l a p p r e c i a b l e d i f ferences in t h e p r a c t i c a l det;Mls

of t h e cMib r~ t i on o r p e r h a p s t h a t t h e p r o c e d u r e i t se l f is n o t suff ic ient to gua-

r a n t e e ,~ g o o d u n i f o r m i t y of r e su l t s .

B u t also if t h e v a r i o u s l a b o r a t o r i e s w o u l d m a k e s a t i s f a c t o r y e M i b r a t i o n s

of t h e s t o p p i n g p o w e r of t h e e m u l s i o n a n d t h e y wouJd use t h e s a m e range -

e n e r g y r e l a t i o n , t h i s l a s t w o u l d stil l i n t r o d u c e an a p p r e c i a b l e u n c e r t a i n t y in

t h e Q~-value of q -mesons (see t h e d i scuss ion of t h e r a n g e - e n e r g y r e l a t i o n con-

r a i n e d in t h e r e p o r t of the B ~ r e a n of S tanda , rds) .

One can c o n c l u d e t h a t ~or t h e t i m e b e i n g one can no t c lMm to k n o w ~he

Q-va lue b e t t e r t h a n w i t h i n 2 ~ a n d the re fo re , i n s t e a d of t h e va lues g iven

~,bove i t is b e t t e r to use t h e fo l lowing ones :

(1)

(2)

Q = (75.0 _~_: ]..5) M e V ,

M-~ (966 ~ 4) m e = (494 ~ 2) M e u

Some effor ts to i m p r o v e th i s s i t u a t i o n m u s t b e done a lso in c o n s i d e r a t i o n of

t h e f~c t t h a t t h e mass of t h e z - m e s o n can be used as a s t a n d a r d fo r com-

p a r i s o n wi~h t h e masse s of o t h e r K-p~r t i c l e s .

Be fo re c los ing th is s ec t ion on t h e Q-value , one shou ld m e n t i o n an ~ b n o r m M

z - meson d e c a y (Bo~) o b s e r v e d b y DANIEL a n d PAL a t B o m b a y [13]. T h e sum

of t h e k i n e t i c e n e r g y cf t h e t h r e e p i o u s g8 d e t e r m i n e d f rom t h e i r r anges , a m o u n t s

(*) The error given usually by the various authors represents the result of Uncer- ta int ies in range determinations due to straggling, distorsion of the emulsions and variat ions of the thickness of the various pellicles of a stack. I t does not t ake into account the uncertainties due to the used range-energy curve and in par t icu lar the uncer ta in ty due to the water content of the emulsions at the moment of exposure. In order to reduce this last source of uncer ta in ty the stopping power of the emulsion is measured, but as mentioned in the text , there are still discrepancies as is s by the following examples

Range of i~-mesons Labora to ry ' f r om = ~ ~ decay

(~m)

Bombay . . . . . . . . . 579.9 i 3 .5% '(+) Rome . . . . . . . . . . 595 ~= 3.5% Turin . . . . . . . . . . 590 , ~ 3.5%

613 ~= 3.5%

Number of tz-mesons used

10 7

! 3

1

Average Q-value (MeV)

76.3 ~= 0.3 73.9 =~ 0.4 74.5 ~ 1.3 7 0 =t= 3.2

(+) The indicated experimental'errors corrospoil4 to the theorical range straggling.

REPORT ON THE ff-MNSON ~97"

to 46 M e V i n s t e a d of 75 MeV, a n d t i l e r e s u l t a n t of t h e i r momen ta , to 32 3{eV/c.

T h e r e f o r e t h e co r r ec t b a l a n c e of energy a n d m o m e n t a can b e o b t a i n e d b y

a s s u m i n g t h a t , fo r i n s t ance , a s ingle p h o t o n of hv == 32 5{eV has b e e n e m i t t e d

a c c o r d i n g to t h e s c h e m e

(3) ~+ -+ 7:+ + ~+ + ~ - -t- hv + Q' .

I)ALITZ [14] has r e c e n t l y c a l c u l a t e d t h e p r o b a b i l i t y of emis s ion of a y - r a y in

t h e 3~: d e c a y p roce s s of h e a v y mesons a n d h a s s h o w n t h a t fo r a pseudoscM~r"

v - m e s o n (see s e c t i o n 7) t h e p r e v a i l i n g p rocess is t h e emiss ion of b r e m s s t r a h l u n g

due to t h e s u d d e n a c c e l e r a t i o n of t h e e l ec t r i c charge .

H e e s t i m a t e s t h a t t h e p r o b a b i l i t y of emiss ion of a p h o t o n of ene rgy l a r g e r

t h a n 30 MeV is of t h e o r d e r of 10 -~ a n d ~herefore conc ludes t h a t in a b o u t

100 v -mesons obse rved u n t i l now, one h a s a p r o b a b i l i t y of t h e o r d e r of one.

p e r c e n t to obse rve such a ease.

T h e p r o b a b i l i t y of emis s ion of p h o t o n s of lower energy is c o n s i d e r a b l y

h i g h e r b e c a u s e t h e y show a b r e m s t r a h l u n g s p e c t r u m . As a c o n s e q u e n c e o n ~

has to e x p e c t t h a t t h e d i s t r i b u t i o n funci~ion of f i le o b s e r v e d Q-vMues has a~

t a i l t o w a r d s t h e low energies due to th i s effect .

One has ~o e x p e c t to obse rve also a t a i I t o w a r d s t h e h igh ene rgy d u e t o

v -mesons d e c a y i n g in f l ight .

B o t h cases cart u s u a l l y b e r e c o g n i z e d f r o m an a c c u r a t e check of t h e mo-

m e n t u m ba l ance .

2 . - E l e c t r i c c h a r g e .

T a b l e I I shows t h e s ign of t h e e lec t r i c c h a r g e of x -mesons o b s e r v e d t(~

d e c a y in fligh~ in c loud c h a m b e r s .

TABLn I I . - Sign of the charge o] v-mesons observed in cloud chambers.

Labora to ry To~al number of observed -c

Manchester (Jungfraujoeh) . �9 i 7 London - Imperial College (Pie i,

du Midi) . . . . . . . . . i 1 Paris - l~eole Politeehnique (Pie i

du Midi) . . . . . . . . . . ~< 9 5f.I .T . . . . . . . . . . . . I Princeton . . . . . . . . . . . 1 Indian~ . . . . . . . . . . . I CM Teeh . . . . . . . . . . . 4

rl'o~al . . . . . . . 24

t I Number : Number !Number of x ell

of ,~+ i of ~- uncertain sign , i

i i

1 5 1 i

5 < 4

1

3 1

16 ~< 6 .

1

19,q JS. AMALDI

As one can see the positive ones are at least twice as many as those of ,opposite sign. I t should be noted tha t t h e definitive proof t h a t an 'event

.observed in a cloud chamber is due to a -r-meson is in general not easy. The <tat.a presented to the Conference by the I~eole Polytechnique group ho-

wever, show definitily the existence in at least 2 cases of negative z-me- ~ons, whose mass is very close to tha t of the positive ones [15].

The observed positive excess represents correctly the si tuation at the instant

o f production provided the mean lives of bo th z+ and z- are long with respect

to the potential t ime of observation of the cloud chamber, a condition which

seems to be usually satisfied.

Wi th the exception of one or two eases, all v-mesons observed in nuclear .emulsions come to rest before decaying. This is obviously a Consequence of

their long mean life which has been measured~ as we shall see in Sect. 4~ in the case of z +. I f the z- is the charge conjugate of the z +, it mus t have the

.same mean life, and this is sufl3ciently long to allow it being brought to: rest, captured into an a tom of the emulsion and by subsequent Auger processes and

.optical transitions, to go in a low-lying Bohr orbit and finally absorbed by the nucleus. As we shall see in Sect. 3, the nuclear absorption is expected to prevail on the spontaneous decay even in the case of weak interaetion~

between z-meson and nucleons. An experimental argument in favour of such a conclusion can be based

on the fact tha t in all cases observed until now the three pious emitted by w-mesons at rest are eoplanar within the experimental error (Sect. 1), while,

if at the time of decay the z- would be in a low lying Bohr orbit, it would have ~ momentum high enough to upset the coplanari ty by ~ 3 ~ (*).

Table I I I shows the evidence on the sign of the charge of z 's observed in

stripped emulsions. One can add the following remarks:

a) a single uncertain case has been observed in which the charge of the x could be negative (Bo,); on the cont rary one has observed: 49 (~-~- - - )~- ~- 10 ( ~ ~-) = 59 certainly positive cases ;

b) if all the z-mesons decaying in the emulsions are positive and there

is no experimentM bias for the observation of the unlike pion (~:-) with respect

to the two equal ones (=+), one has to expect to observe twice as many events in class ( + - - ) than in class (J , -+) and twice aS many in class ( + ) than in

class (--). A non-negligible percentage of z- would produce a n increase of the first

ratio and a decrease of the second o n e . I f the statistics were not so poor one

(*) The momentum of the "r- in a orbit of principal quantum number n is of the ~)rder of (Z/137n) ~'L:c.

REPORT ON THE T-MESON 199

could use the numbers of column 2 of Table I I I to estimate the experimental

bias affecting the efficiency of observation of the dee~y pions of different sign.

TABI.E I I I . - Evidence on the sign o/ the charge o] z-mesons observed in stripped emulsions.

Class

(+ + - ) (+ - ) (+ +) (+) ( - ) 0 ( - - + )

Number of T-mesons

49 29 l0 10 4 3

~o5 + 1 (?)

Number of :v-mesons

~+

98 29 20 10

157

49 29

4

82

Probably, for the t ime being, a very rough estimate of this effect can be ob-

tMned by comparing the total numbers of ~+ and ~- tha t have been observed to be produced in the decay of T-mesons; they are 157 ~nd 82. Therefore one

can tentat ively say that, on the ~verage, the efficiency of observation of e~ch

.one of the two like pions is between 0.9 and 1 times tha t of the mflike one.

3 . - I n t e r a c t i o n o f z - m e s o n s w i t h m a t t e r .

One can now t ry to derive from the da.ta of Table I I I the existing evidence

for the assumption, frequently done, tha~ all (or almost all) the T's decaying ~t rest in emulsion in 3 charged pions are positive.

The weak point in a consideration of this type is due to the fact tha t the

observation or non observat ion of one of the 3 pi,,:ms emitted in a -:-decay is no t independent of the observation of the other two.

I f one neglects this type of correl~tion one can develop a quant i ta t ive

statistical consideration of the following type [16]. Let us consider the uni-

verse of all z-mesons tha t stop in the emulsion and decay in 3 charged pions and let us call p the probabil i ty for one of ~hem to be positive. We c~n ask,

for u given value of p, what is the probabil i ty P of the two following independent experimentM observations:

a) out of

M = ] ( + + - - ) + 1 ( + 4 ) + t ( - - - - + ) = ~9 + ] . o + 1 = 6o

~ - , g u p p l e m e n t o a l :'v;~lovo Cin~en~o.

200 ~. AMALDI

~-mesons for which we know the sign of the charge, to have at least

positive z-mesons;

b) out of

Y ' = l ( + + - - ) + t ( + + ) = 59

n = ? ( + ) + i ( - - ) = l o + 4 = ~

cases in which we know the charge of a single decay pion, to have at least;

m - - ] ( + ) = 10

positive pions. By taking the efficiencies of detection of ~+ and ~- in the

ratio 0.9:1 as suggested above, one can calculate P as a funct ion of p and

one finds tha t in order to have P ~ 8% we must have p ~ 95 %. Considering tha t at the ins tant of product ion the number of z+ is between 2

und 3 times larger thnn tha t of z-, one can conclude tha t the probabi l i ty for

a ~- to escape nuclear capture when brought to rest in nuclear emulsion, is

most probably not larger than about 10-15~ . Unfortunately, emulsions are such a complicated medium tha t no simple

conclusion can be drawn from this data.

A very low value of the probabil i ty of escape of nuclear absorption by a z- when captured in an atomic K-orbi t is expected according to DALI~z. This

author shows tha t even if one assumes tha t the only direct interaction of the=

z-mesons with nucleons is tha t due to the decay scheme of the z, namely

/ ~ - § (3) z- + p ---> =+ + = - + ~ - -]-p --+ [~+ -7 =- + n ,

z - captured in a K-orbit of a heavy element (Br, Ag) present in the emlflsion

will certainly undergo unclear absorption. About 30% of the z- will be captured by light elements (C, N, O). These will reach the K-orbi t before decaying

and the nuclear capture will strongly compete with the decay process even

ii reaction (3) would be the only process responsible of the nuclear absorption (*). Experimental evidence for interact ion of fast d-mesons with mat te r seems

to be provided by two events, observed by the Bombay group [17], whose

interpretat ion however is not unique.

(*) This is mainly due to the fact that the kinetic energy released in the ~bsorp- tion process is much larger than that released in the decay process a circumstance, which reflects on the corresponding volumes of the space of phases.

R ~ P O R T ON T H E T - ~ E S O ~ 201

I n these events a fas t K-par t ic le (]48 and 246 }IeV kinet ic energy respect- ively) produces a star, f rom which a z-meson emerges showing the typica l 3 s -decay when b rough t to rest. The energy loss in the s tar is respect ively 101 ~leV and 202 MeV, and the angle of deflection between the direction of mot ions of the t w o K-pgrt ic les producing the s tar and emerging f rom it are respect ively 78 ~ and 38 ~ .

A similar event in which the emerging K-par t ic le is not a z b u t a K~-meson has also been observed b y the same group.

These events a l though ve ry interest ing are still too few to allow the establishing of any quan t i t a t ive dat~ on the nuclear in teract ion of z-mesons with mat te r , apa r t f rom the fac t t h a t in thei r in te rpre ta t ion the question, remains open whether the K-par t ic le producing the s tar has the same na tu re as t h a t emerging f rom it or not.

4 . - M e a n l i f e .

Cloud chamber observat ions have not given unti l bow much informatiol~ a b o u t t h e mean life of z-mesons, main ly because the space dis t r ibut ion of the decay points is: roughly uni form through the useful region of cloud chambers of usual dimensions. The only conclusion t h a t can be drawn f rom ~hese observat ions seems to be t h a t the mean life is not short.

This agrees w i t h conclusions reached f rom the inyes t igat ion of emulsions exposed to cosmic rays. The to ta l t ime of flight spent b y 53 T-mesons emi t ted f rom observed stars, plus 9 whose origin is not observed, amoun t s to 1 16.10 -s s. O a t of these 62 T-mesons only one (Pdg) decays in flight; ano ther case was repor ted by GO~TSTE[~ a t the Pisa Conference in which an appreciable deviat ion f rom the m o m e n t u m balance indicated t h a t the decay did not take

place ~t rest. The observat ions are obviously too scarce to allow the ~pplicat ion of the

methods used for o ther part icles [18, :[9]. One can however t en ta t ive ly say t h a t the mean life of T-mesons is no~

much shorter than 10 -8 s, p rovided one can exclude t h a t the lack of observa t ion of m a n y T-mesons decaying in flight migh t be due to exper imenta l bias. The es t imate of the influence of such a bias is r a the r uncertMn, nnd while some authors t hough t t h a t i t is no t too serious as a consequence of the typ ica l ' a spec t of the z -mode of decay and of the r a the r high ionizat ion shown b y these par t - icles along the ma jo r pa r t of their observed t rack [20], others t hough t necessary the in t roduc t ion of a large correct ion fac tor which takes down tile mean life o f the z-meson to a round 10 -s s [21].

This poin t seems however to be set t led in" fgvour of the longer value of the mean life, b y the resul ts obta ined a t Berkeley b y the CItUPP-CTOLDHA~ER~

202 E. AMALDI

group and repor ted b y Dr. STOI~K [23], who obta in

v = (0.95 __~ 0.14).10 -s S ,

and by those of ALV~REZ and S. G0~DmtBER [24] who compare the mtmber of x-mesons and pro tons b o t h of m o m e n t u m around 350 MeV/e observed in the emulsions exposed a t various distances f rom the t a rge t of the beva t ron : these authors find

+0,7 (4) -r = (1.0 o.8 ) . !0-~ s .

5. - F r e q u e n c y of o b s e r v a t i o n a n d a s s o c i a t e d p r o d u c t i o n .

Very few and uncer ta in da ta are known abou t the f requency of product ion of v-mesons.

Usually one tries to compare the to ta l m tmbe r of Kr-par t ic les wi th the nnmber of x-mesons observed under given conditions of product ion: such a eornparison however , is ve ry uncer ta in because ~he probabi l i ty of observa t ion in emulsions is s t rongly different for K~ decaying in a single L-meson and v-mesons. F r o m this po in t of view the s i tuat ion is certainly much worse in the ease of emulsions exposed ~o cosmic rays, t han when they are e~posed to machines. Keeping in mind this difficulty I will recall t h a t a t the P a d u a

Conference, the Commit tee on v-mesons gave, as an average of the results obta ined b y various laboratories in stacks of emulsions exposed to cosmie

rays a t high a l t i tude

NKr __ --~-~25 5 , N~ 5 052 ~& 5 N ~ 9

--~ 5 6 0 .

Yiore recently HILl , SALANT and WIDGOFF [25], working wi th the eosmotron, find

N~L 10

by summing together the results of exposure done a t two different angles (900 and 45 ~ in the l abora to ry sys tem) with respect to the direct ion of the beam of p ro tons whose energy was also var ied (2.2 and 3.0 GeV).

According" to the results of the Berkeley group repor ted to the Conference by Dr. STORK, one has

2V~r __ 2 . (5) - - = ~ 4

REPORT ON Tnl~ Z-5[ESON 203

The higher value of this rat io m a y be due in p a r t to the different condi- t ions of exposure, b u t in p a r t also to a difference in the p robabi l i ty of observa t ion of the var ious events, which p re sumab ly affects more the cosmic ray data.

F inal ly one has also to men t ion t h a t two cases have been repor ted (Bo) [26]

(Go~) [27] in which the z -meson is produced in a s tar toge ther wi th ano ther heavy, uns table particle, nmnely a charged hyperon which decays in flight into a light meson, and a th i rd case (Ton) in which f rom the same s tar a recta- stable f r agmen t (~H) is emi t ted .

The problem, a l though ex t remely interesting, does not appear, a t least for the moment , to bear ~ specific interest for r -meson. The associated product ion is a p rob lem of wider in teres t which has to be discussed in general for all the heavy uns table part icles and not for z-mesons alone.

6. - A l t e r n a t i V e m o d e of d e c a y .

One can enquire whether v-mesons undergo o ther modes of decay besides t h a t considered unti l now in 3 charged pions. The exper imenta l problem of establishing what, among all possible processes, are those actual ly h a p p e n i n g in nature, and of measur ing the corresponding branching rat ios are obviously ex t remely useful in order to p rov ide in format ion abou t the s t ruc ture of K-pa r t - icles in general and in par t icu lar of z-mesons.

A few events have been observed in which a K-par t ic le decays in a single pion whose kinet ic energy is compat ib le (p ~< 125 ~,IeV/c) wi th the assumpt ion

of the existence of the process

I t is very difficult of course to s ta te if all events satisfying the above ment ioned condition do real ly correspond to this mode of decay. B u t wi th this assump- t ion one eau t ry to compare the f requency of observat ions of this class of events wi th the class of z-part icles (decaying in 3 charged pious). Such an es t imate is ra ther uncer ta in because in generM, var ious exper imenta l biases will affect in a s trongly di'fferent way the probabi l i ty of observat ion of these

two classes of events. A t the Padua Conference t h e Commi t t ee t en ta t ive ly suggested t ha t the

exper imenta l value of the branching rat io w~s be tweeu 1/3 and 1/5. The Berkeley group working wi th the Beva t ron seems to find 3]20. :[ do

not know if this resul t is a definitive one or not. I t is, however, eertMn t h a t the machine exposures seem to be much more convenient t han cosmic ray work in order to establish the correct vMue of this i m p o r t a n t r~tio.

Here I like only to recall the results of some theoret ical work done on this

20~ :E. AMALDI

prob lem b y DALI~Z [28] some t ime ago. I f one assumes t h a t the concept of isotopic spin can be ex tended to z-mesons and with a definite assumpt ion ~bout i ts value, one can establish l imits for the branching ratio be tween the two considered modes of decay, p rovided the main interact ions responsible for the decay of z-mesons are cha rge independent (i.e. invar ian t for ro ta t ions in the isotopic spin spaces). DALI~Z has shown ti~at for T---- ] (so t h a t in na ture there will exist z +, -r ~ z-) these l imits are

1 < iO(-~• --> r : : t: J-,- 2~o)

Other values of course are found for these limits for o ther assumpt ions abou t the value of T, as repor ted a t the Pisa Conference by Dr. EISENBEP~G.

7. - Sp in a n d par i ty .

The angular dis t r ibut ion and energy spec t rum of the ~:-mesons emi t ted in the decay of the z-mesons have been inves t iga ted b y DALITZ [29].

Wi th the use of s t r ipped emulsion technique it becgme possible to establish the sign of the emi t t ed pions in a v e r y large number of cases and conse- quent ly i t was felt necessary to reconsider the p rob l em in more detail.

This has been nlade independent ly by F.4]~I [30] and DALITZ [31] whor each similar conclusions and to whose work I will refer in the following.

Fig. 1. - Variables used in the description of the final s~ate of ~hc decay process

of z.mesons.

W h e n a z-meson decays, the final s ta te of the th ree pions flying away m u s t h a v e the same pa r i t y and to ta l angular m o m e n t u m of the ~ before decay.

To represent the final s ta te it is convenient to use re la t ive coordinates, namely, apa r t f rom some numerical factor, the dis tance r be tween the t w o like pious (7:+) and the dis tance r ' of the unlike one wi th respect to the center of mass of the other two. The conjugate m o m e n t a are indicated by p and p ' and the corresponding angular moment~ (in units ~) by I and l ' . Due to the conservat ion of energy and m o m e n t u m , the final s ta te of a decaying x-meson is complete ly de- t e rmined (apar t f rom non-essentiM ro~ations and t ranslat ions of the sys tem as a whole) b y giving the m o m e n t u m p ' of ~he unlike meson and the

angle 0 (Fig . 1).

R~PORT ON THE T-MESON 2 0 5

l and l ' obviously compound to give the spin yz of the

(8) J~ = l + 1'

~nd a.re re la ted to the pa r i ty of the final s ta te b y the equat ion

(9) /~x ----(--1)3(-- 1)~'+' = ( - - 1 ) ~'+~,

because 1 is a lways even as a consequence of the fac t t h a t the wave funct ion m u s t remain unchanged b y exchanging the two like pious (equal bosons).

.With the assumpt ion t h a t the dimensions of the z meson are small wi th respect to the wave length of the emi t ted pions, one can show that , for a given initial state, i.e. for given J , and p a r i ~ P~, only s ta tes wi th the m i n i m u m possible l+l' are impor t an t , because all o ther t e rms are apprec iab ly smaller [32].

Now DALI~Z [29] has shown t h a t in 5 cases there is a single pair of vMues of ~ and l' which satisfies such a condi t ion while in all the o ther eases there ~re ~t leas~ two.

I n these 5 simple cases the energy dis t r ibut ion of the unlike pion (~-) and the d is t r ibut ion funct ion of the values

of cos v~ are complete ly determined, while in the o ther e~ses they are given b y the combinat ion of a t least two known funct ions: the coefficients, ho- ~vever, a.re unknown been.use de te rmined b y the details of the s t ruc ture of the

~-D~ e s o n .

Fig. 2 shows the spectra Gn, in non-

relat ivist ic approx imat ion , for various pairs of values of (l, l') as a funct ion of the kinet ic energy of the negat ive pion, divided by its m a x i m u m value. While the 5 cases [0--], [ I+ ] , [ 1 - - ] [ 2 + ] ~ud [ 3 - - ] ~re simple in the sense ex-

plained above, the eases [ 2 - - ] , [3#-],

0.5

0.5 �88

~

""" ...... G22 ......... "..

0 ~ ~.~ ~ o , Fig. 2. - Theoretical spectra G w of the negative pion emitted in the decay of z-mesons, as a function of its energy divided by the corresponding maximum value: corresponding values of spin and parity indicated in square brackets.

[ 4 - - ] are complex i.e. due to the superposi t ion of two or three simple spect ra ; the do t t ed curves correspond to the special ease in which the par t ia l spect ra are combined wi th equM coefficients. I n par t icu lar one notices tha t , for even spin-odd pa r i ty as in the case [2---], [4---], one of the componen t

spec t ra corresponds a lways to l '----0.

203 ~. a~*aLDI

The exper imentM da ta collected in Table I are still r a the r scarce; one cait however t r y to compare t h e m with the theoret ical results in the 5 simple cases ment ioned above.

A general view of the avai lable exper imenta l da ta can be obtained by p lo t - t ing them in the t r iangular d iagram

c of Fig. 3. Fig. 4, similar to Fig'. 3, refers to 16 z-mesons observed in cloud chambers ; i t was pre- pared and kindly sent to me b y Dr. DALITZ shor t ly before the Conference. I n such a representation the height of the tr iangle is t aken equal to the Q-vMue {75 MeV) of the z-meson, so t h a t the th ree distances of a po in t P internal to the t r iangle (whose sum is always. equal to i ts height) correspond to the kinet ic energies of the 3 emitCeA pious [29]. Momen tum conservat ion imposes a fu r the r res t r ic t ion to the points represent ing z-meson*decays: in the non-relat ivis t ic app rox ima- t ion they can fall only inside the

o ~ ~ inscribed circle whose d iameter is

Fig. 3. - Representation of z-meson dee~ys (2/3) (-2 (505~cV). The relat ivist ic observed in emulsions, corrections res t r ic t somewhat more

the pe rmi t t ed region whose bounda ry remains tangent ia l to the sides, while the distance f rom the center is reduced b y 8 % in correspondence to its intersections with the heights of t he tr iangle [30]. In such a representa t ion the stat is t ical factor, relat ive to a given e lementary energy in te rva l of the

4

emi t ted pions (i.e. to a given e l e m e n t of sur- face inside the pe rmi t t ed o~;^ = r~ . , ,n ) , is cons tan t [31] and the relat ivist ic corrections never exceed ' 3 2 % [30]. I n Fig. 3 the kinet ic energy E_ of o

the negat ive pion is represented b y the dis tance ~ c ~ ~ f rom the side A B , while the dis tance f rom the

side C B corresponds to the kinet ic energy E"+ o MANCSeSTe~ �9 CALTECH

t �9 & INDIAN~

& 8EPKE[EY

[3 PARIS

Fig. 4. - tl, epresenta, tion of v-meson dee~ys observed in elottd chamber (DALI~Z, private communication).

REPORT ON THE ";-MESON 2 0 T

of the posi t ive pion of lower energy (E+ < E+). To m a k e clear the use of this graph (in non-relat ivist ic approximat ion) I will refer t9 an example : t h e dis tance PE is propor t iona l to p'~, the dis tance PH to p~ and cos ~ is given by the rat io PG/FG, the line OC corresponding to cos ~ = 0 and �9 the se-

micircle ODI to cos z9 = 1. l~rom Fig. 3 one sees t h a t the exper imenta l points are roughly un i fo rmly

dis t r ibuted in the pe rmi t t ed area. One notices fu r the rmore t h a t there are m a n y points in the region near ~

the po in t D. T h a t means t h a t there are m a n y eases in which a posi t ive p ion is emi t ted with re1T low energy. Also in the region near O there are a few points which indicates, a l though With less exper imentM evidence, t h a t some- t imes also the negat ive pion is emi t ted wi th low energy.

These two remarks [5] appea r still more evident f rom the inspect ion of Table IV, which contains the spec t ra of the three emi t t ed pious.

The fac t t ha t one of the ~+ is often emi t ted with ve ry small energy can be used in the following a rgument : let us consider a 0 ~ which undergoes t h e decay into two pions: 0~ ~+ + ~-. The angular m o m e n t u m of these two. particles (in the f rame of reference of the center of mass of the h e a v y meson). will be equal to the spin Jo of the 0 ~ whose pa r i t y turns out to be obviously

Re - (--:[)% I f the particle which undergoes the 3 pious deeay~ i.e. the ~+-meson, had

the same spin and pa r i t y of the 0 ~ the third pion i=+) would be always emi t ted with an odd angular m o m e n t u m (in the same f rame of reference). B u t such a conclusion contradicts the exper imenta l observat ion t ha t quite f r equen t ly one of the positiYe pious is emi t ted with negligible kinet ic energy, a circumstance which can happen only if i t is in a S-state .

One can conclude tha t since the 0-mode of decay belongs to the class. even-spin even-par i ty or odd spin-odd parity~ the x-mode of decay mus t necessarily belong to one of the classes even spin-odd parity~ or odd spin-even~ pari ty , i.e. z and 0 mesons mus t be different particles.

DALITZ develops also a seceded a rgumen t similar to the previous one by considering the fac t t ha t also the negat ive pion is emit ted, a t least sometimes~ with low energy. I n order to compare, the spec t rum of the =- with tha t of the ~+ one has to inspect Table V which is obta ined f rom Table IV co~!sidel~ing only the events of the following classes ( + + - - ) ( + - - ) ( + + ) and not the events of the classes ( + ) and (--) ; these last b a r e been excluded because they can not be used in ull the three pa r t s of Table IV.

The a rgument of DALITZ consists s imply in noticing tha t the angular m o - m e n t u m l' of the negat ive piou mus t be zero because it is emitted~ at least sometimes, wi th zero energy and therefore in a S-state. Therefore one con- eludes f rom Eqs. (8) and (9) t ha t its spin mus t be even and its pa r i t y P~ = - - 1 .

The same author gave a thh 'd a rgument of the same type bu t more genera l

~ 2 0 8 ~ . A M A L D I

0

)

I l

+

I I

r

v

c~

-F

+

? § §

F~

, ? - o

i I I I I ! li~ ~

I

I ~ ~ [ § i i ! 1 I

[ I

§

! ) I I I ! i I I I I

�9 ~ * ~

~ o

~ ~ . ~

~, ~i ~ m ~

o ~ N

~ ~

~i ~

]R30.'POtgT O N T I - I E Z - _ ' 3 I E S O N 209

based on the consideration of the fact t h a t in the d iagram of Fig. 3 there is :an appreciable num ber of points which fM1 ve ry close to the bounda ry cos ~ = 1 of the allowed region. A si tuat ion like this can be m e t only if the :spin and pur i ty of the z are different f rom those of the 0 because in this last case the densi ty of the points mus t go to zero a t the boundary cos ~9 = 1.

~fA~L~ V . - Comparison of the experimental data on the energy spectra o] re- and ,=+ emitted in z-decay.

0-4

4-8

8-12

12-16

16-20

20-24

24-28

28-32

32-36

36-40

40-44

44-48

1 4

4 9

10

13

9

5

10

8

11

3

87

4.5

5

4.5

9 13.5

7

7.5 8.5

12

5.5

6

4

87

In this table one makes lxse only of the events of the following classes: ( + -t---) (+ - - ) ( -~ +).

The preceding considerations do not allow us to express the conclusions ~bol~t the spin of z-mesons in a quan t i t a t ive way.

I n order to do t h a t the s implest ~ a y consists in tes t ing the spec t rum of t h e negat ive pious by means of the F A ~ I rule which seems to be the more ~ppropr ia te in the ease of r a the r poor stat is t ics ~nd ra the r large exper imenta l

,errol~s,

F _ ~ I gives for each of the 5 values of spin ~nd par i ty , the frequencies ~vith which the energy E_ of the unlike p ion falls in 3 regions defined as follows:

region a E~ ~ E'+ , E+

! H )) b E+ > E_ > E+

i // ~) c E+, E > E_ § �9

Table VI shows the resul t of a Z ~ tes t applied to the 5 simple cases. In the :first p a r t the tes t has been m a d e b y considering only the 91 ~ mesons for which

2]0 :E. AMALDI

we k n o w t h e ene rgy of t h e r:~; in t h e second and t h i r d p a r t t h e t es t has b e e n

done b y a d d i n g to t h e 91 wel l k n o w n cases, t h e 10 be long ing to class ( + ) , a n d

a s suming once t h a t t h e y b e l o n g all to t h e h ighes t poss ib le region, and t h e n

secondly to t h e lowes t poss ib le region. I t is c lear t h a t w i t h such a p r o c e d u r e

we b r a c k e t t t h e t r u e s i t u a t i o n in t h e sense t h a t we o v e r c o m e t h e in f luence

of any e x p e r i m e n t a l bias. F r o m t h e i n spec t i on of th is t ab l e we can e x c l u d e

w i t h c e r t a i n t y [ 1 + ] , [ 1 - - ] and [ 2 + ] b u t n o t [ 3 - - ] . W e can, h o w e v e r , do

an i n d e p e n d e n t t e s t w i t h r e spec t to t h e o t h e r i n d e p e n d e n t v a r i a b l e co.s 8

(see Fig . 1).

TABLE VI. - z=-test of the energy distribution o] =- (Fabri rule).

Number of -~-ille~ons

[0 --] [ 1 + ] 0 --~ [2 + ] [3 --]

91 = 32 + 31 + 28 a b c

Z 2 P

0.3 1 - - e 22 1.7" 10 -5 22 1.7.10 -a 20 4.5-10 -~

2.5 0.29

101 = 42 + 31 + 28 a b e

Z 2 P

3.3 0.20 15 5.5- I0 -4 35 10 -6 39 10 -~

8.3 1.8.10 -2

101 = 32 q- 40 + 29 a b c

Z = /)

1.9 0.4 23 10 -~ 14 9.10 -~ 19 7.5"10 -~

0.3 1 - - ~

Out os 106 v collected in table I, only 5 can not be used: 3 because of class (0), Bo~ because i anomalous and 13o, because uncertain [possibly of class (----+)]. The first pa~t o~ the table i. is made with 91 certain events [(+ +--) (+ +) (+--) (--)]. The 10 events of class (+) have I always two possibilities; therefore they have been placed once in the highest possible region i (second part of the table), and once in the lowest one (third part of the table). [

One ea~t t a k e a d v a n t a g e of t h e f ac t t h a t whi le t h e angu l a r d i s t r i bu t ion is

i so t rop ic in t h e eases [ 0 - - ] and [ 1 + ] , i t d epends on ~ in al l o t h e r cases: in

p a r t i c u l a r in all o t h e r cases i t con t a in s a power of sin ~ wh ich br ings to zero

t h e dens i ty of pa r t i c l e s on t h e semieere le O D I of Fig . 3 [30] ; in t h e case [3 --]:

one has

15 5-6 (5 + 3x~)(1- x~) wi th x = c o ~ .

The i n t e r v a l 0 ~ x ~ t can be d iv ided in t h r ee equa l pa r t s (called A, B', C:

A - ~ 0 ~ < x ~ . � 8 9 and t h e e v e n t s of Tab l e I are classif ied wi th r e spec t to

t h e m : 95 e v e n t s h a v e a ce r t a in des igna t ion , 3 h a v e two poss ib le d e s i g n a t i o n s

whi le t h e r e m a f n i n g 8 can n o t be used. T h e r e f o r e one can cons ider t h r e e pos -

sible cases (Table V I I ) : in t h e first one, I m a k e use of t h e 95 ce r t a in n's, wh i l e

in t h e second a n d t h i r d cases t he 3 a m b i g u o u s x 's h a v e been p l a c e d r e s p e c t -

i ve ly in t h e lowes t and in t h e h i g h e s t poss ib le i n t e r v a l of x = cos ~..

I~PORT 02q T H E T - M E S O N

TABL~ V I I . - z2-test of the angular distribution /or the [3--] case.

2 1 1

A B C Total Z 2 P o + 1

31

3 3

31

32 33 33

2 9

2 9

31

95 98 98

1 1 . 5

1 0 . 2 5

1 4 . 0

3 ' 1 0 - 3

0 . 6 7 �9 1 0 - 3

9 " 1 0 ~

O u t of t h e 106 e v e n t s c o l l e c t e d i n t a b l e I , h e r e one does n o t u s e o n l y t h e f o l l o w i n g e v e n t s : i .hc 3 e v e n t s of c l a s s (0); t h e e v e n t Be9 b e c a u s e u n c e r t a i n ; t h e e v e n t Bo~ b e c a u s e a n o m n l o u s ; t h e 2 e v e n t s Rc~ ~ n d ~ r i , of c l a s s t - - ) a n d Br~ of c l a s s ( + ) h c e a u s e n o i n f o r m a t i o n is a v a i l a b l e ~ p a v t f r o m t h e e n e r g y of a s i n g l e p i o n ; Le. all t o g e t h e r 8 e v e n t s .

6 of t h e r e m a i n i n g 9 e v e n t s of c lass ( + ) f a l l i n a wel t d e f i n e d r e g i o n of cos "a, w h i l e t h e r e m a i n i n g " 3 (Bk~, B h , , Bo~) h a v e t w o p o s s i b l e a s s i g l m ~ e n t s .

I n t h e f i r s t r o w one m a k e s u s e o n l y of t h e 95 c e r t a i n v - m e s o n s , i n t h e s e c o n d t h e 3 a m b i g u o u s c a s e s h a v e b e e n p l a c e d i n t h e l o w e s t p o s s i b l e i n t e r v a l , w h i l e in t h e t h i r d r o w t h e y a r e p l a c e d i n t h e h i g h e s t p o s s i b l e i n t e r v a l of x = cos ~.

From the da ta of Table VI I one recognizes tha t the angular distr ibution is in all cases quite isotropic so tha t while the Z~-test with respect to the [ 0 - - ] case gives a Pearson probabi l i ty P very close to 1, the [ 3 - - ] assigne- merit appears to be ra ther improbable.

If this Z2-test with respect to the angular distr ibution were made with the relativistic expressions of cos ~ the Pearson probabi l i ty of the [3 - - ] ease would g u m out still smaller.

In conclusion i t appears tha t the exper imental material available today permits the exclusion of [1 + ] [1 ---] [2 + ] and makes improbable the ease [3 --J.

Therefore the most probable in terpreta t ion is tha t the v-meson is a pseudo- scalar i.e. of the t ype [0 --] . One can not exclude with arguments of this type any even spin-odd par i ty ( [2--] , [~ - - ] , ...) because the corresponding spectra and angular distributions are combinations of various terms, one of which corresponds always to l ' - - 0 (see Fig. 2).

Fur thermore , independent ly of this uncer ta in ty a~bout the value of the spin we have seen tha t according to the DaLITz argaments reported ~bov% i t is very unlikely~ tha t the v and 0 have the same spin and par i ty .

8 . - S p i n a n d p o l a r i z a t i o n .

I t has been recent ly noticed by TEUCttER, T m ~ I ~ G and WINZELEI~ [33] tha t if the v-mesons have a spin > 0, they can be produced polarized in the process of emission, and tha t the slowing down does not remove this pola- r izat ion unless the v has a high anomalous magnetic molnent [34]. Therefore

212 E. AMALDI

ev~ts perA cosOn. ~

-~ BERKELEY

0 O.S 1

they have suggested to look for such ~ polariz-

at ion by collecting data on the angular distri- bnt ions of the following angles:

0~ = angle between the direction of emis-

sion of the x and the norton,1 to the ~o~O,~, plane of decay;

0~- =:~ngle bet.ween the direction of emis: Pig. 5. sion of the z and the direction of

emission of the ~+, o r bet ter ;

angle between the direction of emission of the ~. and the direction:

of the vector p ' shown in Fig. 1.

the Bern group are shown ~~ '~ ...... tven~s

in Fig. 5 (*), while ill Fig. 6 ~ I are given two graphs pre- ] pared by DALITZ and kindly /

- 1

sent to me before the Con . . . . s~0

ference, relative to events

observed in cloud chgmber.

§

cos #lrp

Fig. 6. - Angular distribution with respect to the a.ngle 0~ between the dlrectio~ of emission of -:-meson and the nor. ms,1 n to the plane of decay.

0 0'.2 o.* o:s o:B ,.o cosOnl:

From all these data as well us f rom similar results obtained in Berkeley and reported by Dr. S~0BK, one c~n conclude that , ~ least for the moment ,

no indication is found in favour of marked deviutions from isotropy.

I like go express my thanks to Prof. HOISTEIC~A~S and the Bern group

for providing me with the data of Fig. 5 (*), to Dr. DALI~Z for sending me the graphs of Fig. 4 and 6 and for making available various information ubout,

his work on ~.-mesons before publication, to Dr. E. ~FABRI for various discus- sions during the preparat ion of this report, and finally to Dr. G. BA~O~I for

he lp in the collection, ordering and elaboration of the experimental duty.

(*) H. ~VI_~Z~LEB: A~tggular Correlations in~x-Meson Decay, in this issue, pug. 2~16[.

R E ~ O R T ON THE V-MESON 21~

R E F E R E N C E S

[1] R. BROWN, U. CA)I]~RINI, P. H. FOWLER, H. ]YIUIRHEAD, C. F. FOWELL and D. M. RITSON: Nature, 163, 82 (1949); P. H. FOWLER, M. G. K. MENON, C. F. POW~LL and O. ROCHAT: Phil. Mag., 42, 1040 (1951).

[2] J. B. HARDING: Phil. Mag., 41, 405 (1950); P. E. HODOSON: Phil. Mag., 32,. 1060 (1951); A. J . HERZ, P. E. HODGSON and R. M. TENNENT: Phil. Mag., 44, 85 (1953).

[3] Proceedings o] the Con/erence of Bagn~res de Bigorre (July 1953). [4] E. AMALDI, E. FABRL J. F. HOANO, W. 0. LOOK, L. SOARSI, B. TOUSCHEK and

B. VITALE: Suppl. Nuovo Cimento, 12, 419 (1954). [5] H. DALITZ: Repo~t on z-mesons in Proceedings o/ the /i/th annual ~ochester Con-

/erenee on high energy nuclear physics. [6] I am indebted to the following persons for providing the data: for Bern,

Prof. HOUTERM~NS and Dr. WINZELER; for Brookhaven, Prof. SALANT; for Berkeley, Dr. DALITZ; for London Imperial College, Dr. BUTLER; for Man- chester, Dr. NEWTE and COO~ER.

[7] P. S. G OEL and K. A. N~ELAKAXTAN: Proc. Ind. Acad. of Science, in press. [8] M. BALD0-CEOLIN, presented by MERLI~ at the Pisa Conference; in this issue,

pag. 221. I thank Prof. N. I)ALLAPORTA ~or sending the d~ta of the Padua- group.

[9] G. BARONI: Nuovo Cimento, 2, 169 (1955). [10] A. I)E BENEDETTI, C. M. G~ELLI, G. LOVERA, L. TALLONE and M. VIGORS:

Nuovo Cimento, i l , 420 (1954); A. D~ BENEDETTI, C. M. GARELLI, L. TALLONE. and M. VIGON~: ~Vuovo Cimento, 12, 466 (1954). I am indebted to these authors for sending more details about their measurements.

[11] I thank Dr. FRIEDLANDER for providing the data of the Bristol group, and Dr. S~oRK for the data of Berkeley.

[12] E. AMALm, G. BARO~'I, C. CASTAONOLI, G. COR~INI and A. MANFR~DINI: NUOVO~ Cimento, 10, 937 (1953).

[13] R. R. DANIEL and Y. PAL: Proc. Ind. Acad. Science, 40, 114 (1954). [14] H. DALITZ: Phys. t~ev., 99, 915 (1955). [15] R. AR~ENTEROS, B. GREGORY:, H. ttENDEL, A. LAGARRIGVE, L. L]~emNc]~-Rm-

GUET, F. MULLER, CH. PEYROU, R. RAU, presented by A. HENDEL: this issue,. pag. 217; m~+ = (964• m z - = (990•

[16] E. AMALDI, G. BARONI, G. CORTINI, C. FRANZINETTI and A. MANFREDINI: Suppl. N~ovo Cimento, i2, 181 (1954).

[17] R. R. DANIEL and D. LAL: Proc. Ind. Acad. Science, 41, 1, ]5 (1955). [18] C. CASTAGNOLI, G. CORTI~I and C. FRANZI~E~I: Suppl. ~uovo Cimento, 12,

297 (1954); E. A~[ALDL C. CAS~AGNOLI, G. CORTINI an4 C. FRA~[ZI~-ETTI: ~UOVO ~ Cimento, 12, 668 (1954).

[19] C. DAHANAYAKE, P. F. FRANCOIS, Y. FUJII~OTO, P. IREDALE, C. J. WADDINGTON and M. YASI~: ~u Cimento, 1, 888 (1955).

[20] E. AMALI)I: Suppl. Nuovo Cimento, in press. [21] M. BALDO, G. BELLIBONI, M. CECCAI~ELLI, M. GRILLI, B. SECHI, B. VITALE and

G. T. ZORN: Nuovo Cimento, 1, 1180 (1955). [23] W. W. CtluPP, G. GOLDHABER, S. GOLDHABER, E. L. ILOFF, J. E. LANNU~TI,~

A. PEVSNER and D. RI~SON, presented by Dr. S~oRK: Otis issue, png. 361.

214 E. &MALDI

[24] L. W. ALVA~I~Z and S. GOLDI~AB]~R: NUOVO Cimento 2, 344 (1955). [25] R. D. H~LL, E. O. SALAXT and 3I. WIDGO~T: Phys. Rev., 94, 1794 (1954); 95,

1699 (1954) 99, 229 (1955). [26] D. I~AL, Y. PAL and B. P]~TJ~RS: Prec. Ind. Aead. o/ Science, 38, 398 (1953). [27} K. GOTTST]~IN: NUOVO Cimento, 1, 284 (1955). [28~ R. H. ~)ALITZ: Prec . Phys. See., 64, 710 (1953). [29] R. H. D~LITZ: Phil. Mag., 44, 1068 (1953). [30] E. F:~BRI: Nuovo Cimento, l l , 479 0954). [3I] R. H. DALITZ: Phys. Rev., 94, 1046 (1954). ~32] E. FaBRI has examined this point in some detail and has shown that assuming

for the dimensions of v-mesons the corresponding Compton wavelength

/i 273 1.4I - = 1 . 4 1 " 1 0 - l a - - 1 0 - a a ~ 4 " 1 0 -14 cm ,

M~c 966 3.54

and considering that the wavelength corresponding to p~ is always larger than

14 h h m_c 139 ~'m~-- ~ -- ~ -- 0 " 8 1 6 " l ' 4 1 " 1 0 - 1 a - - c m = 12-8"10-~aem,

p .... ' 3 Pamax m;zc Pa .... 125

the neglected terms are of the order of one percent of the principal one. [33] 3t. TEVOI~R, W. THIRnI>~G and H. WI~ZEL]~R: Nuovo Cimento, 1, 733 (1955).

[34.] G. W~xa:znL: Helv. Phys. Acta, 22, 101 (1949); E. H~IBnnG, U. KRvsn, J . MAn- SHALL, L. M.anSHaLL and F. SOL)~ITZ: Phys. Rev., 97, 250 (1955).

I N T E R V E N T I E DISCUSSIONI

- - V. L. TELEC, DI:

In discussing the branching ratios

[27~• 4- a + Z~

l 2= ~ 4- ~ +

according to isotopic spin conservation (as done by DALI~Z) it has to be borne in mind ~hat according to the theoretical views now favoured (G~LL-MA>Z~) the decays occur violating spin conservation. Therefore it would be interesting to check the branching ratios with the purpose of confirming this violation.

TE~EGDI has just pointed out that in current theories the decay of the v-meson and other strange particles depends on the violation of charge independence. He then correctly objects to taking seriously the supposition that the v-meson has isotopic spin 1 and decays into three n-mesons with a total isotopic spin of 1. However, even in a theory in which the v-meson has an isotopic spin of say ~, it may be that the decay mechanism is such that the final pions are in a state of definite isotopic spin, say 1.

Thus the ratio of v' to v obtained for a fixed isotopic spin value may still have significance, but. the isotopic spin involved is that of the final three pions and not necessarily that of the initiM z-meson.

R E P O R T O N T H E T - M E S O N 215

- - Y. EISENBERG:

f have the following two remarks about the branching ratio of v-mesons (9'/v):

1) The experimentally observed ratio is probably only a lower limit, since many z'-mesons must be misinterpreted, whereas one seldom looses a v-meson.

2) The branching ratio is not very sensitive to the isotopic spin of the -r-meson, being 1 for T = 2 and } for T = 3. Therefore, on the basis of the experimental data existing so far one can not conclude anything.

- - ~ . SCHEI~:

1) Negative v-mesons can be possibly found by looking for negative ~-mesons emerging from the alternate mode of decay of the x-meson: v - - > ~ : - + ::0+ ~0+ Q; the secondary re--meson could be identified by its property of being captured by the nuclei of the emulsion.

2) The question whether or not the T-meson has a spin higher than 0 can possibly be decided by measurements of the angular correlation between the line of flight of the primary directly producing the z-meson and the decay plane, in hydrogen. Experiments in heavier nuclei can yield results considerably distorted since T-mesons of low energy can frequently be produced by secondary collisions of v-mesons or nucleons, created inside of the same nucleus, having, different directions from the visible pr imary producing the nuclear disintegration from which the v-meson emerges.

- - Cm PnYRoe:

The polarization which was checked, by the Bern group was a correlation between the spin of the I: (if i t exists) ~nd the line of fight of the -~ when it enlerges from ~he nucleus. Therefore one at tempts to find out if the x-meson is polarized after some nuclear interaction, either the production or the last scattering in the nucleus. We have not tried to find a correlation depending from the direction of the primary of the nuclear interaction i n which the v is produced. The remarks of Prof. ScI~I~ would certainly apply to this kind of correlation.

- - K. GOTTS'rEI~q :

I t might be mentioned *hat momentum bManee is a more sensitive test for a <( pure ,> three-body decay than is coplanarity with the comparatively large experimental errors involved in its measurement. We have, e.g., observed a z-meson decay in which the three secondary tracks are eoplanar within experimental errors but in which there is a momentum excess in one direction of 4 MeV/o (outside the error limits) suggesting that the z was still moving at t, hc moment of decay. Such an effect can introduce

errors of a few electron masses in the determination of the , -mass , particular]y if one ~-track leaves the emulsion and the v-mass is derived from momentum and energy balance considerations. Thus, if it comes to precision measurements of the z-meson mass precise momentum balance checks - - not just rough ones - - ought to be made apart from the application of a correct range-energy relation and due consideration of the moisture content of the emulsion.

- - E. AMALDI:

We have always made such checks of momentum balance and never observed appreciable irregularities.

15 - Su, pplemento a~ Nuovo Cimento.

Documents

Report on the τ-mesons