IL NUOVO CIMENT0 VoL. u N. 3 1 o Settembre 1957

The 7~-~-e Asymmetry.

D. H. W~LKTNSO~

Cavendish Laboratory - Cambridge

(rieevuto il 9 Maggio 1957.)

S u m m a r y . - - By comparing the apparent asymmetry of the ~-e decay observed for 9-mesons stopping in Ilford G5 nuclear emulsion in cyclotron experiments with tha t for <~ non-quenching ~ substances such as graphi te i t is possible to measure y~5, the probabi l i ty tha t the ~-meson forgets i ts orientation on stopping in nuclear emulsion. This determinat ion is independent of the degree of polarization of the ~-meson beam. This result may then be combined with the apparent asymmetry 1-t-A cos 0 d i rec t ly observed for 7:-~-e events in nuclear emulsion to find the t rue asymmetry l + a cos 0 for the fundamental ,~-i• process for all electrons observed without energy discrimination. A combination of results from the Columbia and Chicago cyclotron groups gives y~5=0.55 ~ 0 . 0 3 . A combination of results from eight groups of emulsion workers gives A ~ - -0 .129 • 0.015. These yield c~= --0 .287 =~ 0.039 which corresponds to the value of the parameter of the two-component neutrino theory

= 0.87 :~ 0.12.

1 . - I n t r o d u c t i o n .

I t is now v e r y well e s t a b l i s h e d b y m a n y e x p e r i m e n t s t h a t p a r i t y conser -

v a t i o n a n d cha rge c o n j u g a t i o n b o t h b r e a k d o w n in b o t h legs of t h e : : -~-e c h a i n

of decays . This was f i rs t d e m o n s t r a t e d u n e q u i v o c a l l y in t h e e x p e r i m e n t s of

GARWIN~ LEDER~cIAN a n d W E I ~ m H (1) w i t h c o n t e m p o r a n e o u s ev idence c o m i n g

f r o m FRIEDNIAN a n d TELEGDI (2). I t is n o w of i m p o r t a n c e to m a k e th i s q u a n -

t i t a t i v e . I d e a l l y we shou ld m e a s u r e t h e a n g u l a r c o r r e l a t i o n 1~-c~ cos 0, w h e r e 0

(1) R. L. GARWIN, L. M. LEDERMAN and M. WEINamH: Phys. Rev., 105, 1415 (1957). (~) J. FRIEDMAN and V. L. TELEGDI: Phys. Rev., 105, 1681 (1957) and pr iva te

communication.

THE 7~-~-6 ASYMMETRY 517"

is the angle between the directions of emission of the ~-meson and the decay electron as a function of the energy of the decay electrons. Such measurements can and have been made (8) using modera ted beams of ~-mesons derived from the decay in flight of 7:-mesons made by cyclotrons. Such measurements de- pend on the assumption tha t the modera ted ~-mesons retain their initial spin orientat ion unti l they decay viz. t ha t there is no (( quenching ~ of the initial polarizat ion by the moderat ing medium. I t is assumed tha t such quenching would take place chiefly af ter the moderat ion (4). There exist many sub- s t ances~graph i t e and several meta l s - -which exhibit just the same appa ren t a symmet ry in the cyclotron experiments a n d so it is ve ry reasonable to sup- pose tha t for them the quenching is very small. However these measurements also depend on the assumption tha t the degree of polarization of the ~-meson beam from the cyclotron is whatever is given, by the breakdown of par i ty and charge conjugation in the ~-~ decay. This is equivalent to saying t h a t

all ~-mesons in the beam must have originated from ]orward emission in the rest system of the ~-meson, viz. in the direction of the beam. At the m o m e n t the polarization of the cyclotron ~-meson beams has not been measured di- rect ly and the hope tha t it is high derives solely from ideas about the 7c-meson product ion at the ta rge t and the kinematics of the decay process. This means tha t even if the delicate questions of the energy sensitivity of the electron detector and the (~folding-out ~) of its (consider,~ble) angular acceptance are satisfactorily resolved the asymmetries observed with the cyclotron beams still cannot be immediate ly interpreted. The problem would be solved if the 7:-~-e process could be wholly observed inside one of the substances for which we can infer tha t the qnenching is sma]l. This i s not possible, The only substances within which it is at the moment practicuble to observe the whole 7:-~-e chain are nuclear emulsions and fluids which operate in bubble chambers. In such detectors the geometry is precisely defined and all electrons are de- tec ted without regard for their energy. Bu t the observed correlation 1 + A cos 0 is not the desired fundamenta l correlation because it is known from the eyelo: t ron experiments tha t nuclear emulsion and the bubble chamber hydrocarbons show considerable quenching of the ~-meson spin. However as was pointed out earlier (~) one may make use of the cyclotron experiments accurately to measure the quenching in a chosen substance irrespective of the unknown

degree of polarization of the cyclotron ~-meson beam and of poor geometry and unknown energy response so long as the set-up is identical for the chosen

substance and for the unquenching comparator such as graphite.

(s) L. M. LEDERMAN: Report to the 1957 Rochester Con]erence. (4) L. WOLFENSTEIN: :,Phys. Rev., 75, 1664 (1949). (5) G. B. CHADWICK, S. A. DURRANI, L. IV[.' EISBEaG, P. B. JONES, J. W. G-

WIGNALL and D. H. WILKINSON: Phil. Mag., 2, 684 {1957).

5 1 8 D. H. W I L K I N S O N

So far no ve ry accurate measurements of the correlat ion have been re-

por ted f rom bubble chambers . I n any ease it would not be ve ry simple to

use liquid hydrocarbons a t high t e m p e r a t u r e and pressure in the cyclotron e x p e r i m e n t to measure the quenching. Such quenching measuremen t s are howeve r easy to make with nuclear emulsion. Quite good values of A are also ava i lab le f rom several laboratories for the ~-~-e a s y m m e t r y observed in emu!- sion. I t is the object of this pape r to gather together the re levant da ta and

use t hem to find the best present value of the t rue fundamen ta l correlat ion 1-!-~ cos O.

2 . - The q u e n c h i n g in nuclear emuls ion .

I t seems tha t the quenching of I l ford G5 emulsion is a r a the r good constant . At least 16 different batches of such emulsion have beeu tes ted a t the Co- ] umb ia (1.8) and Chicago (~) cyclotrons a t t empera tu res f rom tha t of liquid air to room t empera tu r e and no depar tu re f rom uniform behaviour has been

found. I t is also established t h a t the quenching is rapid and does not seem to be a funct ion of t ime af ter the arres t of the ~-meson. We therefore seek

YG5 as before (5) as the f ract ion of all polarised ~-mesons s topping in ordinary

~G5 emulsion which lose their m e m o r y of their initial spin direction.

F rom the published Columbia work (1) and f rom fur ther da ta supplied b y Professor IJEDERMAN we find the Columbia value using graphi te as the eom-

p a r a t o r Ya5 ~ 0.48 =[= 0.07. The corresponding figure derived f rom the Chicago da ta (e) using graphi te and certain metals as compara tors is YG5 -= 0.56 =k 0.03. We combine these da ta and use yG5----0.55 • 0.03.

3 . - The a s y m m e t r y in nuclear emuls ion .

Many groups of workers, mos t of t h e m s t imula ted by repor ts of the en-

,couraging results being obta ined by FIL1EDMAN and TELEGDI (2) have measured

the a s y m m e t r y coefficient A found for the N-~-e correlat ion in nuclear emulsion.

Their results are shown in Table I.

The to ta l num be r of events represented b y this table is more t h a n 16 000.

A word is necessary abou t the two Cambridge results. Cambridge I is t h a t measured in ordinary G5 emulsion. Cambridge I I is the result ob ta ined

(6) N. P. CAMPBELL, E. L. GARWlN, J. C. SENS, R. flA. SWANSON, V. L. TELEGDI, S. C. W~IGHT and D. D. YOVANOVITCH: Report to the 195,7 Rochester Con/erenee. Also :private communication from Dr. W~IGHT and Professor TELEGDI.

�9 7:-~-e ASYmMeTRY 519

i n twice d i lu t ed emu l s ion which has been here correc ted to its vu lue in ordi-

n u r y emul s ion for purposes of inc lus ion in th is t~ble (*).

TABLE I.

Group A

Bristol (7) . . . . . . . . . Cambridge I (5) . . . . . . Cambridge I I (5) . . . . . . Chicago (~-) . . . . . . . . Copenhagen (s) . . . . . . . GSttingen (9) . . . . . . . Minnesota (15) . . . . . . . Rochester (n) . . . . . . . Rome (~) . . . . . . . . .

- - 0.08 ! 0.05 - - 0.149 _4:0.033

- - 0.142 ~ 0.035 - - 0.174 :J: 0.038 - - 0.17 :J: 0.07 - - 0.095 • 0.044 - - 0.030 • 0.038

- - 0.19 ~- 0.06 - - 0.222 -V 0.067

Before c o m b i n i n g these resul ts we m u s t usk if t h e y fo rm u consisten~ set.

T h i s we do in the u s u a l w~y f ind ing ~r 13.5 which, w i th 8 degrees of f reedom,

cor responds to P = 0.10. Since this is b e t t e r tha.n the usuu l 5 % level of

(*) To reduce the results found in twice diluted emulsion ( A ~ - 0.190 =[= 0.033) to ordinary G5 emulsion we must know the relative numbers of ~-mesons stopping in the halide and in the gelatine and the relative quenching powers of the two com- ponents. Earlier (5) it was assumed that the numbers of positive ~-mesons stopping in heavy and light elements stpod in same ratio as for negative particles namely about 7:3. This is probably wrong as can be seen by examining the measured proton range-energy relations for protons at the lowest energies. The correct ratio is probably nearer 1:1 which we use here. (A discussion of this point with Dr. J. 0REAR was very useful). The correction is very insensitive to this assumed ratio however. As to the quenching we may take it to be complete in the halide or about 90% as may be indi- .cared by cyclotron asymmetry measurements on silver bromide (3). Again this cor- rection is insensitive to the choice. Extreme var ia t ions in the assumptions lead to correction factors differing by only about 10%. We have quoted the value of A reduced from the twice diluted to ordinary emulsion using the stopping ratio 1:1 and comp]ete quenching in the halide. The standard deviation of this corrected result would be • 0.025; this has been increased to • 0.035 which amply covers the un- Certainty in evaluating the correction.

(7) B. BHOWMIK, D. EVANS and D. J. P~ows~,: .Yuovo Cimento, 5, 1663 (1957). K. BOGGILD, K. HANSEN and M. SCHARFF: Report to 1957 Rochester Con- (s) J.

Jere~bce. (~ N. (10) p.

t~eport to

N. B~swAs, M. C~CCAR]~LLI and J. Cl~VSSAR]): NUOVO Cimento, 5, 756 (1957). H. FOWLER, P. S. FI~EIER, C. M. G. LATT~S, E. P. N~Y and S. J. ST. LORANT : the 1957 Rochester Con]erence.

(11) D. DAVIS, A. ENGLER, C. GOEBEL, T. 1~, HOANG, M. F. KAPLON and J. KLAR- ~ANN: Report ~ to the 1957 Rochester Con]erence.

(lz) C. CASTAG~OLI, C. FJaANZINETTI and A. MA~REDINI: ~UOVO Cimento, 5, 684 (1957).

5 2 0 D. I t . WILKINSON

significance we are not able to conclude tha~ these results may not be com-

bined. We may note in passing, however, tha t more than half this value of g 2

is contr ibuted by the low Minnesota result (*); had this been excluded we

should h~ve found P - - 0.6. The value of P = 0.1 for all the results does no t

just ify exclusion of the Minnesota point so we retain it and find the best mean

value A = - - 0.129 • 0.015.

4. - The t rue a s y m m e t r y .

The above values for ~'~5 and A are now combined to yield the t rue asym-

me t ry coefficient for the fundamenta l process for all electrons without energy

discrimination

-- - - 0.287 :J:: 0.039 �9

The best values for the a symmet ry coefficient observed directly with the

cyclotron beams, corrected for the angular resolution and extrapolated to zero

energy of the decay electrons, seem to run around A ~ 0.26 (3.6.~8). This, taken

in conjunction with the present value for g seems to indicate a polarization

for the cyclotron 9-meson beams of about 90 % although complete polarization

is obviously not excluded.

We may also note tha t the above result for ~ is consistent with ~he two

component neutrino theory (~4.~5) which allows (x4) the extreme value ~ = : - �89

The above result corresponds in the notat ion of LEE and YA~a (~4) to

= ( I f , i ~ + I b i~ ) - ' ( / g~ + I,F~) = 0.87 • 0.~2.

These agree well with earlier (5) less accurate values.

I t therefore seems quite possible tha t the extreme asymmet ry al lowed by

the two component theory is not realized in this process al though we mus t

remark tha t if our assumption tha t there is no quenching in graphite and some

metals is incorrect then ~ must be raised.

(*) A sample of the same emulsion as was used by the Minnesota group h~s been tested at the Chicago cyclotron (6) and found to show the same asymmetry as other samples.

(1,) j . M. CASSELS, T. W. O'KE]~E, M. RIGBY, A. M. WETHEEELI, and J. R. WORMALD: Proc. Phis. Soc. (Londml), A 70, 543 (1957).

(1~) T. D. L~E and C. N. YANG: Phys. Rev., /05, 1671 (1957). (1~) L. LANDAU: Nuclear Physics, 3, 127 (1957); A. SALAM: Nuovo Cimento, 5,

299 (1957); B. F. TOUSCHEK: Nuovo Cimento, 5, 754 (1957).

THE 7~-~-e ASYMMETRY 521

This p a p e r d r a w s f r o m t h e w o r k of a b o u t f i f ty peop le . M a n y peop le h a v e

b e e n v e r y gene rous in a l lowing t h e use of u n p u b l i s h e d w o r k w h i c h t h e y h a d

r e p o r t e d a t t h e 1957 R o c h e s t e r Confe rence : Dr . SC~[ARFF t h a t of t h e Co-

p e n h a g e n g r o u p ; Dr . P. I t . FOWLEIr t h a t of t h e M i n n e s o t a g r o u p ; P r o f e s s o r

) [ . F . KAPLON t h a t of t h e R o c h e s t e r g roup . I h a v e also been v e r y g r a t e f u l

for m u c h i n f o r m a t i o n f r o m P r o f e s s o r L. M. L E D E ~ A ~ a b o u t t h e C o l u m b i a

.cyc lo t ron w o r k a n d f r o m P r o f e s s o r V. L. TELEG])I a n d Dr . S. C. ~RIGI tT a b o u t

t h e Chicago c y c l o t r o n work . W i t h o u t t h e he lp of a l l t he se peop l e th is r e p o r t

,could n o t h a v e b e e n p u t t o g e t h e r a t th i s t ime .

R I A S S U N T O (*)

Confrontando l ' apparente asimmetr ia del decadimento ~-e osservata nei mesoni :arrestati in emulsioni nucleari Ilford G-5 negli esperimenti al ciclotrone con quella .data da sostanze prive di smorzamento come la grafite, ~ possibile misurare ~ 5 , la probabili ts che il mesone ~ dimentichi la sua orientazione arrestandosi nelle emulsioni nucleari. Tale determinazione ~ indipendente dal grado di polarizzazione del fascio ,di mesoni ~. Si pub allora combinare questo r isultato con l 'as immetr ia apparente I + A cos 0 di re t tamente osservata negli eventi ~-iz-e in emulsioni nucleari per t rovare l ' as immetr ia vera l + a cos 0 per il processo ~-~-e fondamentale per t u t t i gli elettroni .osservati senza discriminazione d'energia. La combinazione dei r isul tat i dei gruppi dei ciclotroni di Columbia e di Chicago d~ ~o~ = 0.55 • 0.03. La combinazione dei risul- t a t i di ot~to gruppi di sperimentatori con lastre ds A = - -0 .129 • 0.015. Ne risulta

= - - 0.287 • 0.039, che corrisponde al valore del parametro della teoria deI neutrino .a due componenti $ = 0.087 • 0.12.

(*) Tradttztorte a c~tra della Redazion~.