NEWS OF SCIENCE AND TECHNOLOGY

WEAK INTERACTIONS

OF E L E M E N T A R Y

PARTICLES

I . V. C h u v i l o Translated from Atomnaya ~nergiya, Vol. 16, No. 9., pp. 160- 163, February, 1964

An international conference on fundamental aspects of weak interactions was held in September,1963 at the Brookhaven National Laboratory (USA), Yu. V. Trebukhovskii and the author of this article attended this conference as delegates from the USSR. As is generally known, the physics of weak interactions of elementary particles attained a particularly broad sweep after the theory of universal four-fermion interaction w as formulated. The physics of weak interactions, having accounted for a large group of phenomena associated with weak interactions in processes involving the participation of nonstrange particles on the basis of this theory (decay of/J - and ~r -mesons, g ' - c a p t u r e , b e t a - d e c a y of the neutron and of nuclei, etc.), has now run up against some urgent unresolved problems involving weak interactions in which strange particles participate. In the first instance, it has been disclosed that an understanding of these phenomena requires a more c o m p l e t e l y f i l l e d - i n e x p e r i m e n t a l picture. And finally, investigations have been started in recent years on the physics of neutrino interactions, results of which will be of fundamental significance in understanding any processes involving weak interactions. These and other reasons account for the intense interest now manifested in resolving the fundamental problems facing the physics of weak interactions. This conference was devoted to a discussion of the latest experimental and theoretical projects in that field. Participants in the conference included about 150 representatives from laboratories in many countries in Europe, America, and Asia. The following problems involving weak interactions were discussed: 1) regenerative effects in beams of K~-mesons, measurement of the magnitude and sign of the mass difference of K~- and K~-mesons; 2) the experimental status at present of the AQ = AS rule; 3) the structure of lepton decays of kaons; 4) leptonic decays of hyperons: 5) nonleptonic decays of strange particles and the fxQ = AS rule; 6) the decay properties o f E -hyperons; 7) leptonic processes with conservation of strangeness; 8) high-energy neutrino interactions; 9) astrophysical aspects of neutrino physics; 10) theoretical aspects of weak interactions physics. Thirty-seven delegates submitted reports as part of the conference program. Below, we detail some of the major findings contained in the papers presented. Neutral K-mesons are one of the most intriguing research objects in view of their specific properties. The i n formation now at hand on the absolute magnitude of the mass difference of K~- and K~ ~ = l mK~ - m 0 I, K2 as obtained in various experiments, has been summarized in a review paper by V. Fitch. 6 is characterized, in K~meson l i f e t i m e units, by the following values:

- ( L '22

1,!t

J- o.2~

I,5 ~ (), 2 6=

1.1;

(),t~5

(), :~;-)

IL5 O, 5

The values obtained for 6 are generally concentrated about some value in the neighborhood of 1.5. But there are still serious deviations from that value. The first three values were the earliest published ones. The fourth was reported by F. Crawford in his experiments with the 72-inch hydrogen bubble chamber, on the basis of observations

186

of K~ from the reaction 7r- + p ~ A~ + K~ The fifth value is the result of experiments on regeneration of K~-mesons in a beam of K~-mesons in a liquid hydrogen bubble chamber, from a report by F. Eisler. The last value was obtained in regeneration experiments in copper plates with K~-mesons detected by means of spark chambers c o m bined with magnetic fields. In this experiments, reported on by D. Cronin, the dependence of the regeneration of K~ into K~-mesons at a distance G between two copper plates of thickness l 1 and l 2 (where 11 = 1 inch and l 2 = 2 inches) was investigated. This relationship was defined by a formula for the amplitude of the Kt~ beyond the regenerator: 12 (;+l 1 ali o (G) =:. 1 (a!611--:; - ~- ) e '-2--ei6(a+ll)

where A is a constant, and G and l i are expressed in units of the radiation lengths of K~ By measuring the intensity of the K~ in back of the regenerator as a function of G, we may secure information on the magnitude of 6. One conspicuous fact is that this arrangement of the experiment is essentially at variance with others in which information is sought on the value of 6, and that in this experiment an appreciably lower 6 value results. But this result as reported is admittedly of a tentative nature, since the experimental materials had not been completely processed. Information was first obtained on the sign of the difference 6 in experiments using the Berkeley (USA) 72-inch liquid hydrogen bubble chamber, discussed earlier by Crawford. For this purpose, experimental material on the generation of K~ of known momentum in the 7r- + p ~ A ~ + K~ reaction with subsequent scattering of the particles on hydrogen: K')

-12-

" K~

IJ

and decay: ts was

:t +

:t-

processed. Because of the difference in the interactions between K~ and K~

2e

- Y~ ~ "~ t :" - {( a 2 _ . ' 2 )

c~JsS l - i - 2 I / n q * a

with protons, the formula

silt ~:},

where iz is the decay probability ofa neutral_K-m eson of the i-th type, and a and ~ are functions taking into account the difference in the interactions of K~ and K~ with protons, was obtained for the intensity of the K~-component in such an ensemble of neutral K-mesons scattered on protons. This formula contains a term with sin ;~t, which is an odd function of the mass difference 6 of K~ and Kg-mesons, and leads to the result that the t i m e dependence of the K~ in this ensemble of neutral K-mesons will have a different shape depending on the sign of 6. As a result of processing information on 18 events of this type, it was shown that when [ 6 ] has a value of about 1.5, 6 = 1.4. This means that the Kg-meson is heavier than the g~ The statistical treatment of the results of the measurements bear witness to the fact that the probability of a reversed sign to the value of 6 is of the order of 1/1000. One should bear in mind, however, that our knowledge of the value of 6 is still too inadequate to rely upon and this inference is consequently inconclusive. At the conclusion of this section, it is well to mention some recent measurements of the l i f e t i m e of the K~ meson, carried out at Brookhaven National Laboratory by spark chamber methods. These experiments were described in a report by Yovnovich. The following values were reported for the lifetime of the K~ rK0 = (5.83 • 0.68) x 10"8 sec. Information has appeared in recent years implying that the &Q = AS rule for strongly interacting particles in leptonic decays of strange particles is not accurate. More experimental material relating to that question is being accumulated at present. Data on the ~ - d e c a y and /l -decay of Z +-hyperons reported by Plano and Snow still yielded no new information in support of the existence of AQ = - A S transitions, and the now-old Barkas event is viewed with strong suspicion, inasmuch as it can be given another interpretation which would e l i m i n a t e it from the list of evidence in support of this rule.

187

At the Radiation Laboratory at Berkeley (USA), the Ke+4-decay is being investigated. Fifty cases of this decay mode have been uncovered out of a total of 3 9106 conventional K+-meson decay processes; 49 of these follow the scheme K ~ e - Ir +Tr'e+u in obedience to the AQ = AS rule, One such event might conceivably correspond to a K+ -+ ~r ~r + v decay channel where AQ = - AS holds, but it is subject to the more plausible interpretation of a r - d e c a y of the K+-meson with charge transfer involving a rr--meson and a neutron, when g a m m a photons from a ~r~ yield, upon conversion, one apparent electron, i.e., according to the scheme /s

~-.

--I-I-~+

~-L

->.'i-p-

..q%-

,2yl~-

,eyl~.

Finally, the__result obtained by D. Steinberg in experiments on the time dependence of leptonic decays of an ensemble of K~ and K ~ -mesons generated in the annihilation of stopped antiprotons, fails to correspond to expectations based on the results of earlier experiments, and more likely constitutes evidence of the nonexistence of &Q -- - AS transition in K~ decays. The findings of the new experiments on this problem thus fail to provide any confirmation of the AQ _- - AS rule. If the contribution from that channel to leptonic E +-decays actually occurs, then available data indicate that the ratio of the amplitudes of AQ = - A S and &Q = AS transitions is not greater than 0.06. The structure of leptonic decays of K-mesons is being studied with the object of shedding light on the variant of weak interaction describing Ke3" and K/~r-decay events, as well as obtaining information on the form factors of strongly interacting particles at matrix elements describing this decay mode. Experimental data had been obtained earlier on the spectra of K+~ _ and K~ _-decays. Information is now available on the Kes-decays of K~-mesons. A combined analysis of Ke3-deca~as of K+- ~and K~-mesons was furnished in a review paper by D. Wills, indicating that the vector variant of the interaction with constant form factor in the matrix element of the Ke3-decay is the best way to describe the experimental data. Data on K/.~-decay are inconclusive, even though they too apparently support the vector variant of the theory. The definitive solution to the problem awaits more accurate measurements of the spectrum of g -mesons in the low energy region. Another possible approach would be to measure the polarization of/J -mesons in K~3-decay, and preparations for such measurements are now under way. Extremely laborious measurements are called for when information is sought on leptonic decays of hyperons. Th~ "world statistics" now contains 430-103 A~ ~ p Tr - - d e c a y s , 520.103 Z ' - d e c a y s , 255" 103 Z+-decays, and 969 - -decays. Available data on the probabilities of leptonic decays of hyperons, cited in a review paper delivered by A. Rousset, are the following: A0 _

9 l,-v

? k r~ _

- l,!t-v

x'--

< ).s2-}-- ,),l;;).

__

\ ",'--

_

.\'c-v - .\%+v

.V'c-v

l,)

::."

I. II)-). 9

~ . I(r:I

(i~(i -r2_0,:;). i~j-4, II.25. It)-I

2,:;. I~)-:L

There is unfortunately still no way to compare this information with any theoretical predictions. Data on the spectrum of A~ ~ pc- u -decays are not in contradiction to the V - A-variant of the theory, but to date it has not been possible to draw any more explicit inferences on this variant of the theory, on the basis of an analysis of 92 events. Information on nonleptonic decays of strange particles has been analyzed by Dalitz from the point of view of satisfying the empirical rule IAII = 1/2 followed in variation of the magnitude of the isotopic spin. Experimental data allow in principle a contribution from the amplitude ]ZxIt = 3/2. The amplitude ratio of the 12~II = 3h and ILxI! = 1,/,a processes has been estimated ~n different experiments to be of the order of 0.05 to 0.I. One notable exception is the result obtained from a comparison of the probabilities of decays of the hyperfragments AH4 and AHe4, where the

188

Theoretical and Experimental Data on the Rates of Various Processes in Absorption of/1 "-Mesons by.Nuclei Process

Experimental data, sec "1

Total probability of absorption in liquid hydrogen Total probability of pvp

p-+ Hes ~

Hs + v

/.t" + C I~- --~ B19- + .v

Theoretical data, sec -I

445 ~ 43 464 ~ 4 2 1520 ~ 50

reaction

585 570 1530 ~ 150

1440 ~ 90

+300 6750-750

reaction

6 9 0 0 + 1500

Total probability of absorption

in He4 g - + 0 Is ~

368 ~ 47 N 16.

270

reaction

(0"/1")

0.38 ~ 0.07

Radiative capture in C a40, /~" + C a 4 ~ 7 " + C a4~ ~ any other reactions if- + Ca 4~ n reaction, asymmetry

0.55

(3.05 :~ 0.35). 10 -4

2.3. I0 "4

+ 0.15

-0.2

-I.0

[AI{ = I/2 rule predicts that the ratio will be F(AH4)/2F(AHe4) ~- 2, but experimental evidence indicated a value of 0.2. Here, however, these is an uncertainty in the analysis, associated with the inexact information available on the nuclear matrix elements describing these processes. Measurements of the probability ratio of K~ decay in decay to three ~r~ to the probability of all channels of K~ decay involving the emission of charged particles have been carried out at Dubna. The value of 0.24 ~ 0.08 reported to the conference is in excellent accord with predictions based on the ~ I ] = 1/2 rule. Information has been made available recently on the properties of E -hyperons and their decay properties. The new data on this topic were presented in a review paper by H. Ticho. The mass of the E -hyperon is characterized by the quantity mE--= (1321,2 i 0,3) MeV. The g01-hyperon islighterthan the obtained w as

El

The information on the lifetime of the periments are the following:

-hypemn by (5.6 ~: 1.4) MeV. For the lifetime of the E ~-hyperon, the value ~-=

(1,77 • 0,05). 10-! 0 s e e .

E o -hyperon is still imprecise. Measured values obtained in different ex-

( {" 2 49 @0' 3 "~. 10-10 see, \

Tzo=

I

' - -0,2

-',-o.o~ \ ~ , o --o17.) / o =

2 "t0-1~

see,

L ( 3 , 9 -@ ' 0 ) " t0--10 sec. - t0,7

The most probable value of the spin of the E -hyperon is 1/9.. If we assign the value ~z to the spin of the .E: -hyperon, then its decay, obeying the schema E ~ A 0 + ~''; may occur in the S-state and P-state of the TrA~ The following values of the asymmetry factors of that decay were obtained, under that assumption, from an analysis of the angular distribution of A~ and the polarization of A~ in E -decay: a E _ = --0,55 + 0,07;

13a-= --; 0,32 + 0,17;

y E _ = + 0 , 8 0 4- 0,08.

189

The phase difference of

~rA~

in the S- and P-states at an energy of the order of 70 MeV is ~ 20 ~

The results of experiments on ~l'-capture by various nuclei was reported on, in a generalization, in papers by C. Rubbia and L. Wolfenstein, and were found to be basically in e x c e l l e n t agreement with the predictions of the theory of universal four-fermion interaction (see the table on p. 189). The reduced value of the probability of the /~" + He3 ~

H3 + v reaction is (1440 i 90). s e c ' t as obtained at

the Carnegie T e c h n o l o g i c a l Institute (USA) in work using a gaseous scintillation counter. This reaction had been studied earlier with a He 3-filled diffusion chamber, by B. Pontecorvo and colleagues at the Dubna Joint Institute for Nuclear Research. In a new experiment performed by R. Edelstein and associates, a He 3-filled gas scintillation counter in which ~l--mesons were stopped to produce the above reaction, amongothers, was employed in combination with a gaseous xenon counter. T h e new procedure made possible a significant enhancement of the statistical accuracy in measuring the reaction rate. It becomes clear from a comparison of the tabulated data that the agreement between the experimentally obtained and theoretically predicted values is entirely satisfactory within 25-30%. We find an exception to this in two experiments on /a "-capture in Ca 4~ The CERN experiments on radiative capture g - - m e s o n s in Ca 4~ yield a result leading to a value of the pseudoscalar constant such that gp = 12 gA instead of the gp = 8 gA predicted by theory. An even more contrasting result is that reported from Dubna experiments on the asymmetry of neutrons in g ' - c a p t u r e in Ca 4~ gp ~ 30 gA" The reasons for this deviation are obscure. Another bizarre result was obtained in CERN investigations of positron polarization in the decay process /l + e + u ~" , where the polarization was found to be zero. Several more experiments based on the use of the effects of the interaction between positrons and magnetized iron were set up to check this finding. Still another attempt was undertaken to detect the /a+-meson decay following the schema /i + ~ e+y. It is general acknowledged that the absence of this mode of decay served B. M. Pontecorvo as a basis for the hypothesis of two modes of decay, one neutrino-electron and the other muonic, a view which met with brilliant confirmation in high-energy neutrino experiments. A new attempt by H. Anderson to find this mode of decay again yielded a negative result. It was established that the rate of this process would be 0.8 910 -8, with 50~ confidence, and 2 . 2 . 1 0 "8, with 10% confidence. In a recent paper by Yu. Prokoshkin of the Joint Institute for Nuclear Research, information was provided on the beta decay of the ~r+-meson. The rate of the Tr+ ~ ~r~ + u reaction proves to conform to the value flowing from the hypothesis of conserved vector current. The value

was found for the ratio of the constants of this decay and /a-decay. found to correspond to the resolved spectrum of beta decay.

The b e t a - d e c a y spectrum of the ~r+-meson was

An extended experimental treatment of high-energy neutrino experiments carried out recently at CERN was submitted to the conference. The experiments were conducted with a large bubble chamber filled with freon to a density of 1.5 g / c m 3, t . 1 5 - m e t e r chamber diameter and total weight 750 kS. The chamber was placed in a 27-kG magnetic field. In addition, a network of spark chambers of total material weight ~ 40 tons was employed. To data 250 events in a neutrino beam passed through the bubble chamber and about 2000 events obtained by means of the spark chambers in both neutrino and antineutrino (tentatively in the latter case) beams were obtained. The analysis of this material is not yet completed. The major inferences flowing from the preliminary analysis are the following: 1) the muon neutrino differs perceptibly from the electron neutrino; 2) the number of elastic events in the u + Z -- /a- § Z' schema is equal to the number of inelastic events in the u + Z --* /l - + mr + Z' schema; 3) events with a total energy yield greater than 4 BeV are consistently of an inelastic nature; 4) the ratio of the number of observed inelastic events to the elastic cross section calculated theoretically by Lee and Yang increases sharply with neutrino energy; 5) one event with a total energy release of 11 BeV was found; 6) the effect of generation of 3/2, 3/2-isobars apparently makes a large contribution to inelastic events; 7) if there exists an intermediate boson, then its mass must be greater than 1.5 BeV; 8) inasmuch as no events of type u + e ~ u + e have been discovered, 2 . 1 0 -s /a B must be the upper l i m it to the neutrino magnetic moment (experiments involving pile antineutrinos yield the value 1 . 1 0 -9 /aB) ; 9) for processes featuring an energy release greater than 10 MeV, the cross section over an averaged nucleus is found to be 2 . 1 0 -87 cm 2,

190

accurate to a factor of 3 with respect to the known neutrino flux intensity; 10) it is possible that elastic scattering of type u + p ~ u + p was found in one case only; 11) generation of the following strange particles was observed in the bubble chamber: one A~ one K~ and one K+-meson (possibly, still another K+-mes0n). Clearly, even the preliminary treatment of the experimental data resulted in a rich yield of important findings. Neutrino experimental physics has not attained an advanced stage of development and has become one of the o u t standing subdivisions of the physics of elementary particles. The following prominent achievements and problems in neutrino physics were formulated in a report by Yang: 1) the nonidentity of the muon neutrino and electron neutrino must be accepted as a rigorously established fact; 2) the law of lepton conservation must also be viewed as confirmed; 3) clarity must be brought into the problem of the local character of lepton currents; 4)the question of neutron lepton currents remains obscure; 5) further study is r e quired on the Iml = 1 rule in AS = 0 transitions; 6) investigations are required on the differential cross sections of inelastic processes; 7) the existence of an intermediate boson, its mass, and its magnetic moment must be clarified; 8) the validity of the hypothesis o f conserved vector current remains to be established; 9) the momentum dependence of the a x i a l - v e c t o r form factor gA must be studied further. Several theoretical and experimental aspects of the problems enumerated above were revealed in Yang's report. Summing up the achievements of the conference as a whole, we may state that considerable progress has been registered during the past year in the physics of weak interactions, from both theoretical and experimental standpoints, in the understanding of processes involving weak interactions. The reports made it c l e a r that many experiments of a c o m p l e x nature are in planning or now in execution. The delegates to the conference were afforded the opportunity to become familiar with the experimental research in progress at Brookhaven National Laboratory. Two experimental arrangements consisting of l a r g e - s i z e spark chambers, Cerenkov and scintillation counters are either already operational or getting their finishing touches, and will be useful tools in the following planned research projects: 1) investigation of the leptonic decays of K~- and K~

with the object of :

a) measuring the K ~ --* r + e ' u / K ~ --* r - e + u ratio in order to verify PC invariance in weak interactions; b) measuring the t i m e dependence of the K1~ --* Ireu/K~ .-* rreu ratio in order to shed light on the existence of AQ = - AS transitions in the leptonic decay of K~ 2) investigation of B-decays of polarized A~ with the aim of solving the problem of the variant of weak interaction describing this decay. Also figuring in the Laboratory's plans are experimental studies of the p o l a r i z a tion o f / l +-mesons in K ~ s - d e c a y . This brief rundown alone is entirely adequate, however, to indicate the general trend of the research being planned. From an analysis of the materials of the conference, visits to the laboratories, and talks with leading ~merican and European physicists, we must draw the conclusion that we may anticipate in the very near future some new e x p e r i m e n t a l findings which will constitute significant progress in our knowledge of the properties of processes involving weak interactions.

191