TO THE QUESTION OF ELECTRON ANGULAR DISTRIBUTION

AT It-MESON DECAY

M. FIClML, A. MAZUR

Faculty of Technical and Nuclear Phyxice, Pra~e

The paper describes the measurement of the angular distribution of electrons at the decay of/t-mesons in a longitudinal magnetic field. The results of measurement indicate that electrons are emitted with preference not only in the direction parallel to the spin orientation of fz-meson but also in the antiparallel direction in disagreement with the angular distribution predicted by theory.

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INTRODUCTION

In the theory of a two-component neutrino, in the same way as in the theory of a four-component neutrino, the angular distribution of the electrons emitted a t the decay of polar ized it-mesons is der ived in the fo rm

(1) N(O) d O - - K(1 + a cos O) sin O dO

where K is a cons tant , a the a s y m m e t r y coefficient and O the angle be tween ~he direct ion of the i t-meson spin and the electron m o m e n t u m [1], [2].

I n p a p e r [3] we compared the expe r imen ta l values ob ta ined by us wi th those of an analogical p a p e r [4]. W e ar r ived a t the conclusion t h a t agreement be tween the values of the a s y m m e t r y coefficients of b o t h measu remen t s can be reached if the a s y m m e t r y coefficient is calculated in the same way as in [5]. This m e t h o d of de te rmin ing the a s y m m e t r y coefficient, however , assumes t h a t the cases of i t-meson decay wi th an electron emi t ted in the direct ion of i t-meson emission and in the ant i -para l le l direction have g rea te r s ta t is t ical weight t han o ther cases. However , this does not agree wi th the d is t r ibut ion g iven b y Eq. (1). I n order to de te rmine whe the r devia t ions f rom this dis tr ibu- t ion real ly exist, we measured the angu la r d is t r ibut ions of e lectrons a t i t-meson decay in a longi tudinal magne t i c field, which or ien ta tes the i t-meson spins in to cer ta in precessional angles.

EXPERIMENTAL ARRANGEMENT

The experiment was performed according to our suggestion in the Joint Institute of Nuclear Research in Dubna. The experimental arrangement is shown schematically in Fig. 1.

5 5 4 qex. ~ t 3 . H~. B 11 (1961)

To the Question oJ Electron Angular Distr ibut ion. . .

A beam of ~--mesons having an energy of 80 MeV was led from the syn- ehrocyclotron and from it the ~*-mesons were separated by a graphite and aluminium filter (d, e). The most suitable filter thickness was found by means of the coincidences of three scintillation counters, two of which (a, b) were placed in front of the filter and one (c) behind it. The initial thickness of the filter was 19"8 g/em ~ of graphite. The optimum thickness of the additional filter was determined from the absorption curve (measurement of coincidences a, b and a, c) at 7.8 g/era 2 of alumi- nium. Graphite and aluminium were used on account of the small depo- larization of the ~-mesons in ~ight materials. The intensity of the ~- -meson beam was approximately 650 particles per 1 cm 2 per minute. Nuclear emulsions NIKFI -R (/), 10 • 10 cm ~ and 400 ~ thick, which were placed in the longitudinal mag- netic field of the solenoid (g), were

Fig . 1. D i a g r a m of e x p e r i m e n t w i t h ~ - m e s o n

d e c a y in l o n g i t u d i n a l m a g n e t i c f ie ld: a, b,

c - - s c i n t i l l a t i o n c o u n t e r s ; d, e - - f i l t e r (d

g r a p h i t e , e a l u m i n i u m ) ; f - - n u c l e a r e m u l s - ions ; g so leno id .

used to detect the ~-meson decays. One part of the emulsion (stack I; 82 layers) was irradiated in a magnetic field anti-parallel to the direction of motion of the particles and the other part (stack II; 84 layers) in a magnetic field parallel to the motion of the particles. In both cases the intensity of the magnetic field was 6300 Oe. The emulsions were placed so that one edge of each emulsion was parallel to the direction of the magnetic field and to the direction of motion of the beam of particles. The thickness of each emulsion was measured before developing them.

RESULTS OF MEASUREMENT

Of all the decays of positive ~-mesons we chose for measurement those cases which satisfied the following conditions:

a) the ~-meson track formed an angle smaller than 5 ~ with the direction of the magnetic field;

b) the projection of the electron track on the plane of the emulsion was at least 20 ~x in one emulsion;

e) the electron track formed an angle with t he plane of the emulsion of less than 68 ~ .

Measurement consisted in determining the direction of electron emission in the point of ~-meson decay. The cosine of the angle between the direction of electron emission and the direction of the magnetic field was calculated for each measurable case (i.e. the case satisfying the above conditions). In this way 1030 cases were elaborated for stack I and 1002 for stack II. Correction

Czech. J. Phys . B 11 (1961) 5 5 5

M. Friml, A. Mazur

for the finite thickness of the emulsions used was made for the measured angular distributions. The shrinkage factor was found for each emulsion separately. The histograms of the angular distributions are shown in Fig. 2.

If we turn the angular distribution given by Eq. (1) about a precessional angle a with respect to the direction of the magnetic field we get a distribution

(2) N(v ~) dv ~ -- K(1 -~ a cos ~ cos v ~) sin v~ dye,

i.e. again a linear function of the space angle (v~ is the angle between the direc- tion of the magnetic field and the direction of electron emission). Figure 2 shows this distribution as lines Pl (histogram I , i.e. measurement on stack I) and PH (histogram II, i.e. measurement on stack II).

125

100

75

50

125

N

75

IO 0.5 0 -0.5 -1.0 5 0 - -1.0 cos L~ COS

50

25

tO

Fig. 2. Histograms of angular distributions of electrons from ~-meson decay in longitudinal magnetic field. Histogram I is for case of anti-parallel orientation of magnetic field with respect to direction of motion of particles, histogram II for parallel orientation of magnetic field. Lines P1 and PII represent distribution predicted by theory (i.e. distribution given

by Eq. (2)).

The results of our measurements can be summed up as follows:

1. The experimental values of the two angular distributions (I and II) do not agree with the lines PI and PI1 which represent the distribution given by Eq. (2).

2. Distribution I (anti-parallel orientation of magnetic field with respect to direction of motion of beam of particles) and distribution I I (parallel orient- ation) differ quali tat ively from one another. Histogram I has the greatest deviations from the theoretical line around the angle v ~ = ~:; the experimental

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To the Question o/Electron Angular Distributior, . . . .

distribution I I has two pronounced maxima. This is an unexpected and sur- prising result.

3. I t is obvious that in case I the asymmetry of the angular distribution is greater than in case II . This means that in case I (anti-parallel orientation of magnetic field) the depolarization is smaller. I t can be inferred from this tha t the polarization of ~+-mesons with respect to the direction of their emission at ::*-meson decay is negative. This agrees well with the results of paper [6].

DISCUSSION AND CONCLUSION

It is seen from Fig. 2 that angular distribution II, in particular, does not agree with Eq. (2). It can be shown that for the above-mentioned transform- ation (transition from expression (1) to expression (2)) the two maxima cannot be given by a linear function of the space angle (i.e. a linear function of the cosine of the angle) but only by a non-monotonous distribution with respec~ to the space angle with two sharp maxima in distance of an angle 7:, i.e. in both direetion-wises of a given direction.

When measuring angular distribution in a longitudinal magnetic field it is usually assumed that the angular distribution is described very well by Eq. (1). All cases when the ~-meson trajectory made a relatively large angle (30 ~ and more) with the magnetic field were therefore chosen for the measurement [5], [7]. With such a choice apparent deviations from the distribution given by Eq. (1) could not be determined. I t can be assumed of the angular distributions measured without using a magnetic field tha t for the most part they are the result of the depolarization of ~-mesons. This hypothesis is supported by the fact that the asymmetry coefficients, measured in different laboratories under conditions which differed relatively little, are very different.

The dependence of the form of the angular distribution on the mutual orientation of the magnetic field and the polarization of ~-mesons can be inferred from the qualitative difference in distributions I and II. I t points to the asymmetry of the electromagnetic interaction of the ~-meson with the magnetic field. This can be understood, for example, to mean tha t the precessional motions of ~=mesons for parallel and anti-parallel spin orient- ation of the particle with respect to the direction of the magnetic field are quali tat ivelydifferent .

The conclusions reached from our experiment can be summed up in three points:

I. The angular distribution of electrons at ~-meson decay in a longitudinal magnetic field is not a linear function of the space angle, i.e. as it is described by Eq. (1) or (2).

II. The angular distribution of electrons depends on the orientation of the magnetic field with respect to the polarization of the ~-mesons.

Czech: J. P h y s . B I1 (1961) 5 5 7

M. ~riml, A. Mazur: To the Question o] Electron Angular Distr ibut ion. . .

I I I . The polarization of ~+-mesons at ::+-meson decay is negative with respect to the direction of emission of the ~+-mesons.

I t should be remarked that the results of our measurements can be inter- preted on the assumption that electrons are emitted with preference not only in the positive direction-wise but also in the negative wise of the spin orient- ation; the correctness of this assumption is, however, not sufficiently con- firmed. Either experiments with greatly polarized beams of ~-mesons and with more suitable detecting equipment than nuclear emulsions (in which large depolarization occurs due to the presence Of heavy nuclei) would have to be made or the angular dependence of the form of the electron energy spectrum would have to be investigated in order to give a convincing proof (or denial) of the correctness of the above-mentioned assumption. We have decided to continue in the work of investigating energy spectra measured for different intervals of the electron emission angle.

The authors thank 1N. V. R a b i n for irradiating the nuclear emulsions, Prof. Dr. V. P e t r ~ i l k a and M. S u k for mediating relations with the Joint Inst i tute of INuclear Research in Dubna and Dr. J. P e r n e g r for valuable discussions and remarks. Thanks also go to H. K o u t o v s and H. S l&movs for carefully scanning the nuclear emulsions.

Received 15. 2. 1961.

Reference8

[1] L e e T. D., Y a n g C. N.: Phys . Rev . 105 (1957), 1671. [2] Landau L.: Nucl. Phys. 3 (1957), 127. [3] Friml M.: Czech. J. Phys. 9 (1059), 552. E4] Castagnoli C., F r a n z i n e t t i C., Manfredini A.: ~Tuovo Cimento 5 (1057), 684. [5] Gurev i~ I . I . , K u t a k o v a V . M., Miw P., Nikolskij V.A., Surkova

L. V.: ZETF 34 (1958), 280. [6] Goldhaber M.: Report at Eighth Rochester Conference, 1958. [7] Weissenberg A. O., Smirni tski j V. A.: ~ETF 39 (1960), 242.

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