II. - INTERACTIONS FAIBLESSOME IMPRESSIONS OF EXPERIMENTAL PROGRESS IN WEAK INTERACTIONS

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II. - INTERACTIONS FAIBLESSOME IMPRESSIONS OF EXPERIMENTAL PROGRESS IN WEAK

INTERACTIONS

James Cronin

To cite this version:

James Cronin. II. - INTERACTIONS FAIBLESSOME IMPRESSIONS OF EXPERIMENTAL

PROGRESS IN WEAK INTERACTIONS. Journal de Physique Colloques, 1971, 32 (C3), pp.C3-

13-C3-16. �10.1051/jphyscol:1971302�. �jpa-00214583�

SOME IMPRESSIONS OF EXPERIMENTAL PROGRESS IN WEAK INTERACTIONS

James W. CRONIN

Princeton University and National Accelerator Laboratory

RhumB. -

On fait une revue du progres significatif qui s'est produit dans le domaine des interac- tions faibles durant les sept dernieres ann6es. On souligne aussi quelques problemes actuellement importants et l'on indique les progres qui peuvent Etre esp6r6s dans un proche avenir.

Abstract. - A

review of some of the significant progress in weak interactions in the past seven years is presented. Also some of the outstanding problems are pointed out and the prospects for progress in new areas are indicated.

Dr Tran has asked me to give an introductory talk to start this meeting. The weather is so beautiful and the skiing so pleasant I am sure that many of you would prefer to remain out of doors rather than hear another review of weak interactions. In fact, I do not intend to give a review but rather will talk about some random impressions concerning the experimental progress in some weak interaction problems with which I have been associated during the past few years.

As a general conclusion, experimental progress is considerably less rapid nowadays despite the vast improvement in techniques of detection and the quality of performance of accelerators. The reason seems to be scale and complexity of the experiments. You may recall that the time between the discovery of parity violation in 1957 and significant progress was very short, a few years at most. The analogue of parity violation in 1964 was the failure of another presumed space time symmetry, CP symmetry. Now after seven years we finally have a number of consistent experi- ments which measure the phenomological parameters, but as yet no real understanding of why the violation occurs.

Last summer at the Kiev Conference, six years after the announcement of CP violation, a set of experiments giving precise and consistent values for the KL

-

Ks mass difference and y +

-

phase were presented by three independent experimental groups. These are given in table I.

Recent Measurements of

K:

Parameters

I+-

Refe-

Mass difference

(s-1)

(degrees) Group rence

-

-

-

-

(0.542 f 0.006) x 10'0 44.5 f 5

Chicago

11

1

(0.542 & 0.006)

x

1010 48.7

+

¹²

^CERN

^i-21

(0.535 31 0.007) x 1010 37

+

¹⁰

^Princeton

^[3]

After considerable fluctuation the value of q +

-

has settled to a n average of 42.5

^_f

30. This is in excellent agreement with the phase of 43.2

$.

0.40 which would be expected if the entire violation were contained in the mass matrix. Such a result is predicted by the

cc

super- weak

⁾⁾

theory [4] and the term

cc

superweak >> has come to describe all those theories which would have the effect only in the mass matrix to at least the precision presently available experimentally.

As a generalization it seems that the early experi- ments gave a qualitative understanding of the experi- mental parameters, but only after a number of years of hard work do the detailed numbers actually emerge.

In support of this generalization, I remind you of the beautiful experiment done by Fitch and collabora- tors

[ 5 ] .

This experiment demonstrated constructive interference between Ks decays to n f n- induced by a diffuse beryllium regenerator and CP violation K L decays to the same state. If one assumed the regenera- tion amplitude was essentially imaginary, the total constructive interference would have been the result of

q + -

-- 450 which Fitch suggested at the time, and which after five additional years of work has been shown to be indeed the case.

Recently, as Dr. Repellin will review, a great deal of progress has been made in the measurement of 1 yo, 1.

The most recent result of the CERN group [6]

1 yoo Ill ^y+- I

⁼

1.00 + 0.06 agrees well with the weighted average of the other results. The net conclu- sion is that the magnitude of I yo, I is very close to 1 ^y

⁺ -

1. Further, since q +

-

is very close to the

cc

superweak

^)>

phase we do not expect the phase q,, to be greatly different from q + -.

Given these facts, and the assumption of CPT, we can consider the difference between y

⁺ -

and yo, to be a small vector. We then can ask questions about the precision required of given experiments to establish

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971302

C3-14 JAMES W. CRONIN

whether this vector is zero or not. We use the standard

relationships,

y + - = E

+

^8'

y o 0 = E

-

^{2 8 ' .}

Then we resolve these vectors into components parallel and perpendicular to the direction

(P,, =

43.20, the

⁽⁽

superweak

^>)

direction. From consideration of unitarity [7] the value of

^E~

cannot be larger than - ^.07

^E,,.

Figure 1 shows the construction of

y + -

REAL

FIG. 1.

and

yo,

interms of the components. One observes first that the ratio I

^{y o o}

111

^{y + -}

1 is a measure of

[l

- 3

E/,/I

^E

11. The measurement of

(pa,

-

(P+-)

is

a measure of 3

^E

I and a measurement of

(cp,

- -

qsw)

is a measure of

( E ~

+ ^&;)/I

^E

^{I. Assuming}

AQ =

- A S transitions do not occur, the charge asymmetry measures 2 Re

E.

The ratio

Re &/I

7

+ - I

COS (Psw

measures [l -

(el

+ &iI)/I

^E

I]. Table I1 shows the required precision in the measured quantity to achieve a given precision in the derived quantity. For this table we assume that 1

y + -

1 and

qsW

are measured with negligible error.

Desired precision in derived quantity + ^.05 + ^.Ol

Derived Required precision Measured quantity quantity of measured quantity

-

-

-

-

l l o o

l2

"jI/I&

I

5 3 0 % i 6 %

l o o - I+-

&;/I& I

^{4 go i 1.5O}

9+-

+ &i)/I

8

I

& 3' i 0 . 6 O Charge asymmetry

+

^eil)/

¹

^e

I

^{& 5} % & 1 %

What is known about the

nn

phase shifts would indicate that is most likely larger than

^E;

so that the most promising measurement to continue is I

^yo,

1'.

It might be possible to measure to a precision of 0.01 with respect to 1

^E

1 with a new measurement of I

^{y o o}

12. At present the most accurate experiment limits

Eil/l E

I to

$-

0.02. The measurement of perpen- dicular components requires very accurate measure- ments of phases or charge asymmetry.

One must bear in mind that all the analysis above assumes the original phenomological analysis of WU and Yang [8] is correct. If CPT is violated or in some way the whole analysis is incorrect, then one might find inconsistencies between the various possible measure- ments. It does not seem likely, however, that the charge asymmetry will contribute significantly to the measure- ment of the phenomological parameters. It also is clear that the precision of I

^{y + -}

I should be reexamined. Its measurement requires the subtraction of leptonic backgrounds which might involve some systematic errors which up to now have not required serious consideration.

We are now in a regime where the search for diffe- rences of I

yo,

1 and I

y + -

I are at the few percent level.

One can stop and cry,

⁽⁽

superweak

^>)

However, when viewed objectively experiments can be done to push

E'

to the order of 1 %. The experiments are difficult but probably no more so than many of the negative expe- riments which have searched for CP or T violation in other systems. Despite how fatigued we have become with the question of CP, it does seem experimentally feasible to continue the study of the neutral K system.

It is a challenge, at least, to the next generation of physicists.

What has been the progress in finding CP violation or T violation in other systems

?

Until the present there has been no real evidence of such violations but there have been hints. One source of possible C viola- tion rests with the electromagnetic interactions. There is an asymmetry of (1.5 + ^{0.5) % in the}

^{y -, ?I+ n- no}

decay 191, If the asymmetry is to be believed, the interfering state must be I

=

2 so the violating electro- magnetic current could have either a I

=

1 or I

=

2 character.

I t is curious that experiments recently done at UCLA which measure

n-

+ p

+

n +

y

find some evidence for a failure of reciprocity with the inverse reaction [lo].

In studying the details of the reaction they also find some evidence for an isotensor component to the elec- tromagnetic current. Thus there is the intreguing possibility that a bizzare electromagnetic current carries also a CP violation.

N o one really believes all of this. The inversereaction

y

+ ⁿ

^-t

^n- + p requires the unraveling of the neu-

tron target from deuterium. An I

=

2 electromagnetic

current is not a welcome concept. Fortunately

one expects greatly improved measurements of

v l + n + n- no.

TWO experiments each with 400,000

events have been run respectively at the Rutherford

laboratory and at the PPA. One hopes to have these

results within one year. Also, it is expected that the

photo production and inverse reactions will be impro- ved. Such an explanation of CP violation seems far fetched, but all possibilities should be vigorously pur- sued. Of course, the fact that the limit on the neutron electric dipole moment is now < ^{5 x e.cm}

makes the electromagnetic explanation of CP rather implausible.

Let me now turn briefly to another field of research, the three body K decays and particularly the K:

decays. Here a pioneering experiment was done some ten years ago by Willis and collaborators [ll]. He showed that the interaction was essentially vector, and that 2' in K> decay was positive. He measured the branching ratios, and give a crude slope for the no spectrum in K:, decay. In a very real sense we have not learned very much more. The present experimental situation is very confused as I will indicate by a few examples below.

In the past year there have been three high statistics experiments on K:

+

nev, npv, and nf n- no. In table I11 the results of some of these experiments are presented. One can see that there are significant disagreements between many of these measurements which are far outside statistical fluctuations. These seem to be particular to the KO experiments ; the K + experiments seem to be more consistent with one another. Thus there remains considerable work to do before one will have confidence in the results. We should point out that the

^KO

experiments are in a much better position to explore the entire Dalitz plot. The background from two body decays of the K' make it difficult to explore the entire kinematical region.

Although there may be some interesting physics results inherent in these decays, it is probably an

<(

exercise for the student

>>

to show that the KO decays

give the same results as K + . If the K + results for A+

and ((0) are correct, there does seem to be some trouble with the Callan-Treiman [I21 predictions which

uses current algebra and soft pion extrapolation techniques. One thing that does seem rather certain about measurements of non leptonic K decays is the violation of the AI

=

3 rule. In four separate cases ((1) K f

^-t

n f no, (2) KS

^-+

nn, (3) K

+

3 n rate, and (4) K

-,

3 n slopes) there is now a convincing indication of a A1

=

9 amplitude of strength 0.05 with respect to the dominant A1

=

$ amplitude. Exactly what the origin of this AI

=

3 amplitude is not clear.

A genuinely intreguing result is the upper limit for KL

+

pi p- measured at Berkeley [13]. They find the branching ratio KL

+ p f

p-/KL

^-+

all < 1.8 x lo-' with 90 % confidence. The predicted lower limit using the measured KL

+ y y

rate is 6.1 x lo-'. At present it is difficult to understand this discrepancy since in this case it is very difficult to find fault with the relevant experiments. The resolution of this problem may well lead to a new discovery, or better understanding of present experimental facts.

If we turn away from the present and the pas1 and look towards the future there seem to be new areas in the weak interactions which are opening up. Already at Brookhaven and CERN hyperon beams are a reality.

These will permit greatly improved experiments on hyperon beta decay so that stringent tests of the Cabbibo theory can be made.

Also, with the advent of large detectors such as Gargamelle and the new accelerator at NAL, a new stage of neutrino physics will become accessible. This is potentially a very rich field in which the weak interac- tion structure can be explored at high momentum transfer. Also, hopefully, the elastic form factors can be carefully measured. Despite all the effort a severe critic would say that the only solid result obtained in neutrino physics has been confirmation to a certain precision of the two neutrino hypothesis.

There are many other areas of weak interactions that

Recent Measurements of K: Parameters

2'(Ke3) If ( 5 3 ) t(o) (K,3) Slope n

O

in K,, Reference

-

- - -

0.022 f 0.003

(")

- - - CERN

0.053 + 0.011 (b) 0.078 + 0.012 (b) - 2.5

^{( b )} -

0.290 + ^0.006

^(')

^Daresbury

0.257 + ^0.005

^(S)

^SLAC

0.036

_+

0.006 0.045 + ^0.012 - 0.85 & 0.20

(') -

K + Average

(")

Reported by C. Rubbia at Meeting of american Physical Society at Austin, Texas November 1970.

(b) Reported by Manchester group (P. G. Murphy) at K. Meson Study Weekend Daresbury, January 1971.

(')

Reported by SLAC group (Drickey et al.) at Kiev Conference.

(d) CHIEN (C. Y). et a1. Phys. Letters, 1970, 33B, 627.

(")

ALBROW (M. G.) et al. Phys. Letters, 1970, 33B, 516.

(f) BUCHANAN (C. D.) et al. Phys. Letters, 1970, 33B, 623.

(')

CHOUNET (L. M.)

<<

Review of K,, Form Factors

^)>

CERN Report 70-14, 1970.

C3-16 JAMES W. CRONIN

one might discuss. I hope all of these will be brought experimental physics is often difficult and in particular out in our discussion. I have tried t o show by historical the passage from a rough survey of a situation such as example that our progress while slow is steady. I hope K,, decay to precise answers t o all the questions they that our theoretical colleagues will understand that may pose is a time consuming process.

References

[I] AARONSON (S. H.), EHRLICH (R. D.), HOFER (H.), JENSEN (D. A.), SWANSON (R. A.), TELEGDI (V. L.), GOLDBERG (H.), SOLOMON (J.) and FRYBERGER (D.)

Phys. Rev. Letters, 1970,25, 1057 ;

JENSEN (D. A.), AARONSON (S. H.), EHRLICH (R. D.), FRYBERGER (D.), NISSIM-SABAT (C.) and TELEGDI (V. L.),

Phys. Rev. Letters,

1969, 23, 615.

[2] CULLEN (M.), DARRIULAT (P.), DEUTSCH (J.), FOETH (H.), G ~ o s s o (C.), HOLDER (M.), KLEMKNECHT (K.), RADERMACHER (E.), RUBBIA (C.), SHAM-

BROOM

(D.), SCI& (M.), STAUDE (A.) and TITTEL (K.), Phys. Letters, 1970, 32B, 523

;

BOHM (A.), DARRIULAT (P.), GROSSO (C.), KAFTANOV

(V.),

KLEINKNECHT (K.), LYNCH (H. L.), RUBBIA (C.), TICHO (H.) and TITTEL (K.), Nuclear Physics,

1969, B

9 ,

605.

[3]

CARNEGIE (R.), CESTER (R.), FITCH (V.), STROVINK (M.) and SULAK (L.), reported at International Confe- rence on High Energy Physics, Kiev, 1970.

[4] WOLFENSTEIN (L.), NUOVO

Cimento,

1966,

42A,

17.

[5] FITCH (V.), ROTH (R.), RUSS

(J.)

and VERNON (W.),

Phys. Rev. Letters,

1965, 15, 73.

[6] CERN-Aachen-Torino Collaboration, private com- munication.

[7] See for example BELL (J. S.) and STEINBERGER

(J.),

Proc. Int. Conf. on Elementary Particles, Oxford, 1965 (Rutherford High Energy Lab., Chilton, 1966), 193.

[8] Wu (T. T.) and YANG (C. N.), Phys. Rev. Letters, 1964,13, 380.

[9] GORMLEY (M.), HYMAN (E.), LEE (W.), NASH (T.), PEOPLES (J.), SCHULTZ (C.) and STEIN (S.), Phys.

Rev. Letters,

1968,21,402.

[lo] BERADO (P. A.), HADDOCK (R. P.), HELLAND (J.), NEFKENS (B. M. K.), VERHEY (L. J.), ZELLER (M.

E.), PARSONS (A. S. L.) and TRUOEL

(P.), Phys. Rev.

Letters,

1971, 26, 205.

[ I l l LUERS (D.), MITTNA (I. S.), WILLIS (W. J.) and YAMA-

MOTO (S.

S.), Physical Review, 1971, 133B, 1276.

[12] CALLAN

(C. G.) and TREIMAN

(S. B.), Phys. Rev.

Letters,

1966, 16, 153.

[13] CLARK (A. R.), ELIOFF (T.), FIELD (R. C.), FRISCH

(H. J.), JOHNSON (R. P.), KERTH (L. T.) and

WENZEL (W. A.), reported at International Confe-

rence on High Energy Physics, Kiev, 1970

;

also

private communication.