Rare decays

Nuclear Physics A434 North-Holland,

(1985)409~

430~

Amsterdam

RARE DECAYS

Hans Kristian WALTER Institut fur Mittelenergiephysik Switzerland

der ETH Zurich, CH-5234 Villigen,

A review is given on the status of presently number violating K- and u-decays.

running or proposed muon

1. INTRODUCTION In this contribution

I would like to discuss some forbidden and rare

kaon-, muon-, and pion-decays.

Table 1 shows examples of such decays, their

present experimental

ratios or upper limits for them as well as the

physical

branching

interest for studying them. From this table we see that the reasons

for a certain decay to be rare are vastly different, interest in most cases concentrates

and that the physical

on studying the suppression

mechanisms.

Many of the decays listed are forbidden within the standard model either because the particles

involved are not foreseen in the model or because the

minimal group structure

does not allow for the interaction

the other hand the standard model is unsatisfactory e.g. to predict masses,angles family replication branching

and coupling constants

problem, these processes

involved. Since on

concerning

its unability

and to deal with the

are most interesting;

ratio for one of these forbidden decays immediately

a finite

indicates

"new"

physics beyond the standard model and opens a new era of low level counting experimental

physics. Double beta decay and proton decay is being treated in separate lectures 8-9 and therefore I will concentrate on muon number violating kaon- and muon-decays. summarized

Some basic mechanisms

in chapter 2, presently

for muon number violation will be

running experiments

and their preliminary

results will be treated in chapter 3, possible improvements future experiments

and limitations

for

will be touched in chapter 4. Always the decay u+3e will be

taken as an example, because because this experiment

I myself am

most familiar with this decay and

has reached the smallest upper limit up to now and

works with the highest stop rate, therefore

allowing best to discuss high rate

and trigger problems.

03~~-9~~41~51~~3.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

electroweak

V+A, S, P, T interaction

-2

l

lo-7

struct. of pion

Electromagn.

Radiative correct,

nO*+e-

(4

V, A structure of pion

Radiative correct.

Ir-+3ev

V, A structure of pion

Radiative correct.

lo-‘O)

* lo-8

-6

?T-=VY

l

Radiative correct.

* lo-5

3.5

p3e2v

< 10

prinesonta

...

baryon number violation

KFqe.

lepton number violation

u, e number violation

New interactions

New interactions

New interactions

-9

N+N'e-e"

5 10

-10

Heavy neutrinos of all flav.

New particles

familon etc.

Axion, majoron,

New particles

10-8

l

Nb of neutrino SUSY inos

New particles

* 10 -7 (lo-")

11

10

7

9

8

5-7

4

3

19, 20

18

18

13, 77, 12

16

15

15

2

14

13

12

New prop. (ref.)

13

CP-violation

(- lo-g)

(5

1

New data (ref.)

expectations.

n+-0 CP-violation

E'/E # 0 ?

CP-violation

* 1o-3 CP-violation

CVC in strange background

io-"1

CVC, Higgs contributions

Small phase space

l

Small phase space

corrections

and

GIM mechanism electroweak

Higher order

10-g)

1o-g

lo-'

* 1o-8

l

l

Physical interest

Reason for rarity

p, e number violation

uj3e, ...

<4

< 1.7

-2

(3

1.02

9

2.6

Branching ratio

New interactions

u*y,

nk2' KR2

K+rrr+Xo

K+W+v;

KtflOe+e-

K!$3n

K&

K'+K'e'v L

+ o+ 7rqev

K+rrr+e+e0 +KL*u u

Decay

TABLE 1 Examples of what is rare and why. Numbers in brackets are theoretical

411c

H. K. Walter / Rare decays 2. THEORETICAL

MOTIVATION

Since there appeared several recent review articles on the subject of muon 21-23 I will omit in the following all references to specific

number violation

models. Figure 1, taken from ref. 24 shows present days elementary

particles,

quarks and leptons coming in at least three families. Strong SU(3)co,or electroweak

SU(2) x U(1) interactions

and

are not unified and their combination,

often called the standard model is a single family theory. Since neutrino masses are zero neutrinos

cannot mix and hence generation

leptonic sector is conserved. the introduction doubly charged

Minjmal extensions

number in the

of this minimal model include

of ad hoc finite neutrino masses, right-handed

currents,

leptons, or more than one Higgs doublet. Depending on the

masses and mixing angles of the particles

introduced branching ratios up to

the present limits can be obtained, which means in fact, that rare decays put interesting

constraints

on these parameters.

standard model include horizontal models and supersymmetric

symmetries,

More substantial technicolor

models and various combinations

extensions

theories,

of the

composite

thereof.

c

SU(3) color FIGURE 1 Todays multiplets of elementary fermions participating weak interactions. Taken from ref. 24.

Horizontal Discrete,

symmetries

in strong and electro-

have been proposed to confront the family problem.

gauge and unified groups from U(1) up to SO(l8) have been considered.

CP violation,

parity violation and flavor changing neutral currents

are easily

41%

H. K. Walter 1 Rare decays

incorporated

and must be suppressed

by high enough masses

small mixing angles) for the corresponding diagram for the decay u+3e mediated Technicolor unappealing

new generation

gauge bosons. Figure 2% shows the

by such a horizontal

models are mainly motivated

elementary

gauge boson,

by the possibility

Higgs scalars. Unfortunately

of techniparticles

(or unnaturally

to avoid the

they bring about a whole

and gauge bosons, which should be experimen-

tally found. Figure 2b shows a loop diagram for u+3e involving technifermions and Extended Technicolor Counting

gauge bosons.

the particles

of the standard model we find 45 fermions,

bosons and a number of Higgs particles.

Economists

12 gauge

feel uncomfortable

these numbers reaching almost the number of elements,

with

and propose new elemen-

tary objects mostly called preons, from which quarks and leptons and/or gauge and/or Higgs bosons should be composed. could be some kind of excitations

In these models the tau and the muon

of the electron and p+ey, u+eyy, u+3e could

proceed by radiative ,transitions between the two states as shown in Figure 2c. The nonexistence

of these decays above the present limits gives severe con-

straints on transition

moments and form factors.

Finally supersymmetric

models should be mentioned.

to solve the gauge hierarchy fermions,

unfortunately

too heavy particles,

Potentially

being able

problem these theories try to unify bosons with

again introducing

a whole new generation

of partly not

offset from the normal particles by a spin l/2. Figure 2d

shows the decay p+3e through a box diagram

involvjng superleptons

and the

partners of the W, the socalled Winos. Summarizing

we find that in all models extending

the minimal grand unified theories)

the standard model

there exists an intermediate

(except

mass scale of

the order of lO1'2 TeV, well within the grand plateau predicted by the minimal GUTS, which can be tested indirectly

by sensitive

searches for rare K- and JJ-

decays. Since not only the different branching

ratios but also their ratios

depend on the specific model, the experimental

search for each of these decays

must be done independently. at e'e- colliders is mediated

Here the question might be asked whether

can be used for testing muon number violation.

by a horizontal

reactions

Assume u+3e

gauge boson as shown in Figure 2a. Then the ratio 21 versus the process p+3e is given by

of count rates for the process e+e-+u+eR=s

. L . G;/N

where s = c.m. energy squared and L = luminosity

of the collider,

constant and N = stop rate for a decay experiment.

GF the Fermi

Setting s = lo4 GeV*,

413c

H. K. Walter / Rare decays

L = 1o32

cm-*

sensitivity

s-l,

it follows R = 5

N = IO7 s.-’

l

lo*', again showing the

of decay experiments.

+

+

a

P

t

+

+

B0

1 I AG -0

+ e

P

e-

e

+

e-

p+m

(b)

(a)

El

e

Ml

+

e-

El

e

+

FIGURE 2 n+3e decay mediated by (a) flavor changing gauge boson (b) technicolor particles (c) radiative transitions in composite models (d) supersymmetric particles

3. PRESENT UPPER

LIMITSFOR MUON NUMBER VIOLATING DECAYS AND CURRENT EXPERI-

MENTS AND ACCEPTED

PROPOSALS

Table 2 shows upper limits for muon number violating decays. Tau decays have not been listed since the upper limits for their branching

ratios are bet-

ween 4 * 10m4 and 2 * low3 25 and therefore not yet competitive.

The limit for

the decay KF-+ue has been taken from ref. 25 because the authors Clark et a1.26 0 in their paper give an upper limit for K,_+l.~u, which later was found 27 to be low by a factor of five. Ref. 26 quotes B < 1.6 * lo-' and thus 10m8 seems to be lie limit,for this decay. The two K-decays are byproducts of experi-

a reasonable

ments primarily

designed

for measuring

the decays K:-HJ~ and K++&te+e-

respect-

ively. They both have to be studied since Koi,,e only involves axial vector (or pseudoscalar)

currents, whereas

interactions.

Concerning

K+m+pe

their relative

proceeds through vector (or scalar) relevance theory 37,38 generally

the former mainly from phase space arguments.

Experimentally

favors

KF+ne is disfavored

H.K.

414c

Published

Decay

Wulrer /Rare decays

TABLE 2 upper limits for muon number violating decays

Branching

ratio (90%) limit

Reference

KF*c*e+

<

6

-

10-6

25

K+q+u+e-

<

48

-

lo-"

28

<

700 *

W-To

29

<

1.9

.

lo-lo

30

P-WY

<

84

*

lo-lo

31

p+3e

<

1.6.

10-l'

32

U-k-N

<

0.7

.

lo-lo

33

u-N*+N'

< 3

.

,o-‘0

34

t +lJ-te vevF(

<

0.02

35

p+e-q-e+

<

0.04

36

since a 38% e- singles rate from KL*+e-v energy for the two neutrinos,

can fake the signature.

lution is necessary

and the detection

The main background

for the decay K+q+pe

the IT- misidentified

Excellent momentum

comes from Ktrrrtntr- with v'*~*Y and particle identification is needed. 29 of the K'q+ue experiment.

comes from a reanalysis

therefore,

(and *'+e+e" and possibly ~'-6)

searching

for K+w+ue

can also look for sOqe

with similar sensitivity

using the tagged TT'

from Kr2 decay. Let me now turn to the specific experiments i)

reso-

of kinks helps reducing this background.

as e-. Here excellent

The limit for eoye Future experiments,

together with .'-~I+v and small

or proposals.

KF*ue

A proposal to search for this decay and the decays KFe'e-, of 10-T' has been accepted at Brookhaven

u+u- at a level

(AGS E780)17. The experimental

is shown in Figure 3. 2 * lo7 Kt/p (plus 6

l

lo8 neutrons/p)

setup

at a proton

current of 1012 p/p are purified by sweeping magnets and collimators

to enter

the decay zone, where 1.2% of the KF decay. Use of a single magnet together

415c

H. K. Walter / Rare decays

Pb-*I.*. H_ Ccrankov

c.,orlmerer

/

-

Apparatus

FIGURE 3 used for the search for the decay KF-+pe, taken from ref. 17.

with high resolution mass resolution hodoscope

of 1.5 MeV. Segmented

discriminate

crimination

electrons

Hp Cerenkov counters and a Pb glass

by a factor of 100 against 71 + p. n/u dis-

is done at - 10 -' in a

total acceptance

* 'OS3 X0) mini drift chambers lead to a

(ZOO u) thin (2

concrete-steel-scintillator

is lo%, the experiment

filter. The

should start to take data in spring

1985. An even more ambitious

proposal to look for the same decay at a level of

lo-'2 also has been approved at Brookhaven difference

compared

(AGS E791)13 recently. The main

to E780 is the introduction

of an evacuated

pipe for the

straight beam, which again consists mainly of neutrons and the use of two magnets

per arm, which facilitates

kink-detection

The addition of a '40 t Al muon polarimeter

and trigger logic (Figure 4).

will allow polarization

measure-

ment of the u+ with a precision of 14% and the addition of Pb glass hodoscopes 0 will allow to study also the decays KFq'e+e- , II ve, uuy, eey. The experiment needs 1013 p/p, a large fraction of the total proton current of the AGS.

416c

H. K. Walter /Rare decays

2

Drcft Chombel

I

o-

1

Scintillatkfi Counters I

u

Apparatus

I

IO

5

I

I

I

I

I

15

20

25

30

35

I

40

FIGURE 4 used for the search for the decay KF+ue, taken from ref. 13.

Also this experiment

is under way at Brookhaven

is shown in Figure 5. An unseparated 6 . lo8 n/p plus 1.8 MWPC spectro~ter

l

(AGS E777)".

6 GeV/c beam of 3

l

lo8 p/p) at a proton current of 5

with 5% acceptance.

lo7 K+/p (plus

- 10"

filter and electrons

suppressed

at the 5 * 10 -"

are selected by H2 Cerenkov counters

< 10m5) and a Pb glass array (rejection

from K'+?n+n-

with IT+-~J+~and IT- misidentified

level so that the experiment

in a 1000 hr run. According

from

by a Fe-PWC

to low5 by C02+N2 Cerenkov counters and Pb

side electrons

(heavy particle misidentification The main background

p/p enters a

Charges are separated magnetically

the intense beam. On the positive side st. and pt are identified

glass. On the negative

The apparatus

lo*).

as e- enters

should reach a 10-l' sensitivity

to the authors the proton beam has to be limited

to 5% of what it could be because of the large singles rate of lo6 ufs*m' due to the n halo from T- and K-decays.

Interesting

byproducts will be a sample

of +. 10'000 Kt+stete- events and studies of the tagged 71' decays into e+e(- 1000 events) and pe.

H. K. Walter 1 Rare decays

APPARATUS-PLAN

417c

VIEW

LL IDENTIFIER ,

‘-

t

P4

Apparatus

iii)

FIGURE 5 used for the search for the decay K+rrr+p+e-, taken from ref. 12.

u+ey, p-teyy, p+3e

(Crystal box, LAMPF)

A search for these three decays is under way at LAMPF5. The apparatus is the crystal box shown in Figure 6. It consists of 396 NaI crystals rounding a 8 plane stereo drift chamber. A surface muon beam of 3 with a duty factor of 6.8% is stopped in a 52 mg/cm2 polystyrene photon energy resolution

used

sur-

- 10 5 s-1 target. The

of the NaI is 6.5% at 130 MeV, its time resolution

1.1 ns. The single particle acceptance

is 45%, the acceptances

events are 12%, 40%, and 14% respectively.

is

for 3e, ey, eyy

The ey mode reaches accidental

back-

ground already after one day of running, the two other modes are background 11 free down to the lo-l1 level. Preliminary data from 2.2 - 10 stopped muons with a total efficiency

of 8.5% yield a limit for the branching B < 1.3 u+3e

* 10 -lo

(90% C.L.)

ratio for u+3e5

418c

H. K.

Walter

/ Rare

decays

DEEP ACROSS

L

36

HODOSCOPE

COUNTERS

FIGURE 6 Crystal box detector as used for studying the decays u+ey, u-q-y and p+3e at LAWPF, taken from ref. 5.

Also 11 events of the type irc3eZv have been seen in agreement with the expectations

from standard electroweak

theory. Analysis of the u-fey and p+eyy

data is in progress and more data will be taken this summer. The expected -11 is around 10 for all three decays.

final

sensitivity iv)

p-N-+e-N

(TPC, TRIUMF)

The anomalous muon conversion the help of a time projection with 73 Metl/c momentum

chamber

3

l

resolution

(TPC), shown in Figure 7. Cloud muons -1 lo5 s in a 2 g/cm2 thick

are stopped at a- rate of 5

target. The 100 MeV/c electrons momentum

in Ti is being searched for at TRIUMF6 with

l

can be detected with 20% acceptance

in a magnetic

lOl* muons were stopped. With a total efficiency

branching

of 3.5% a limit for the

ratio is givenG: B c2 !J+e

and 4%

field of 0.9 T. During a first running period

-10

-11

(90% C.L.)

H. K. Walter J Rare decqvs

419c

FIGURE 7 Perspective view of the TPC used for the search of anomalous muon conversion at TRIUMF, taken from ref. 6.

The experiment

is continuing

and a final sensitivity

of 5

* 10 -12 is ex-

pected. v)

p+3e

(SINDRUM, SIN)

At SIN a new experiment7 has been done to search for the decay p+3e with a -12 of a few 10 . Part of the data have been published32. The mag-

sensitivity

netic spectrometer momentum

SINDRUM is shown in Figure 8. A surface p+ beam of 28 MeV/c 7 -1 s was focussed on the entrance of a magnetic lens,

and a rate of 10

which transported

it to a hollow double-cone

target (58 mm 6 x 220 mm). The

target was made of low density foam with a wall thickness corresponding

to a thickness

coil produces a homogenous

in the beam direction (AB/B < 2%) magnetic

of 1 mm (11 mg/cm2),

of - 90 mg/cm2. A solenoid

field of up to 0.6 T parallel

to the beam axis in a volume of 110 cm length and 75 cm diameter. A hodoscope of 64 scintillation

counters and five cylindrical

multiwire

proportional

chambers concentric with the beam axis are used to measure particle tracks. The chambers consist of two concentric density foam and aluminized (0.9 or 1.7

cylinders

(half-gap 2 or 4 mm) made of low

Kapton having a thickness of 30 or 60 mg/cm2

- 10m3 radiation length). The inner and outer cathodes of chambers

1, 3, and 5 are divided into -I 45' helical strips (0.1 urn Aluminum).

The ampli-

tudes of the signals induced on these strips are measured to determine coordinate

the

along the cylinder axis. The solid angle defined by the hodoscope

H. K. Walter / Rare decays

420~

H

Hodoscope

C

Chambers

SINDRUM

S

Solenoid

P

Photomultipliers

A

Preamplifiers

L

Light guides

M

Magnet

T

Target

B

p beam0

The magnetic

Coil J /

spectrometer

FIGURE 8 SINDRUM used for the study of the decay u+3e at SIN.

is 73% of 4a. With the magnetic

field of 0.33 T used in this experiment

trons and positrons with a transverse momentum hodoscope.

The geometrical

acceptance

elec-

of > 16 MeV/c trigger the

for u+3e events is 24%.

The selection of events to be readout by the computer was done in four successive

stages. The time signals from both ends of the hodoscope elements

were combined in 64 mean-timers which three hodoscope quired

and fed into a majority

coincidence

unit in

clusters occuring within a period of r 6 ns were re-

(rate 15000 s-l). In the second step a search for tracks of negative

curvature was made by comparing the hit patterns of the hodoscope three outermost

and the

chambers with 3200 preloaded masks (decision time 180 ns, rate

- 1000 s-l). In the third step a search for tracks of both polarities was performed using the information

of the hodoscope

and all wire chambers

(mean

decision time - 2.5 us, rate - 40 s-l). Both trigger stages used growps of 3 to 16 wires as input. In the fourth step e+e+e- combinations

having a total

42lc

H. K. Walter / Rare decays

transverse

momentum

below 25 MeV/c were sought (mean decision time .- 5 ~JS, 39 and is housed in one

rate - 4 s-l). The trigger system was built by Struck FASTBUS crate; a detailed description

can be found in ref. 40.

Since the allowed decay p++e+ete-veiu

can be used as a check of the normal-

ization, the events read out by the online PDP 11-44 computer were scanned for both decays p+3e and u+3eZv in a software filter before going to tape (decision time 30 ms, rate - 0.6 s-l). After the offline analysis in the plane perpendicular

to the magnetic

field

137000 events remained. A clear prompt peak of 11000 events over a flat background could be seen.

In the next step the z-coordinates from the strip infor-

mation were used. For all e+e+e- combinations culated

(o = 1 mm), and a correlated

a common vertex in space was cal-

time-vertex

cut was applied.

shows the distribution

of total energy E versus missing momentum

remaining 8838 events.

Indicated is the cut E t IC&i

Figure 9a I&l

for the

= 112 MeV below which

u+3e2v events should be confined. The events from Figure 9a are projected a E t /Cccl axis and their distribution

gram). A number of (7833 + 90) p+3eZv events is found after subtraction 4.7% accidental is dominated

background

(dotted histogram).

Since the uncertainty

by the errors in IpI a separate coplanarity

uses only the three unit vectors F/IpI. of the three emission

onto

is plotted in Figure 10 (solid histoof a

of I&

cut was applied, which

Events were rejected for which the sum

angles relative to any plane exceeds 5'. The resulting

899 events are plotted in Figure 9b, together with contours,

in which 90% and

68% of the p+3e events should occur. No event was found within these boundaries. The acceptance

of the detector and the efficiencies

mined by Monte-Carlo

simulations.

Standard electroweak

of all cuts were deterinteractions

were assumed

for p+3e2v and a constant matrix element for p+3e. Table 3 summarizes efficiencies.

the

The total number of muons stopped in the target was evaluated

using the hodoscope

counts corrected

side the target (31%). A total of 7.5 during the experiment. overall efficiency

for photon background

(6%) and decays out-

- 1012 pt were stopped in the target

Having not observed any p+3e candidate

and taking the

of (13.7 f 1.5)% we obtain an upper limit for the branching

ratio B 2.4 p+3e ' With an overall efficiency tain a branching

of 3.3

* lo-l2

(90% C.L.)

- 10m5 (1 + 0.16) for the decay p+3e2v we ob-

ratio of B = (3.2 f 0.5) . lO-5 p+3eZv

422c

H. K. Walter / Rare decays

SINDRUM 1984

.

.

: : .:‘.:’

.:... ‘..

..i..

.

‘.,

- 40-

. . . ... : . : * :. . . . : . . *. ...f. . .::: . *.*ye *.*A’ :.:.:.. .;:: * . . . . : :*::Lf:“~:,+iz*. ..* . . . * **... *:.zz.:.::~:: ..: . 2 :.:..?i:‘..:‘..: - 20*I. ‘..i:iiiriftiiiffiii~~~.:.. : . *. : .*:*: . .::i:::..::.: ................... ...:’...... . . . . . *.**.:::::* .. . :. ...::..:.:‘:.:” :.I .::“:.. . “Z.:: :i’?.i*”.. .*. .. ,_,._____, . . “::*:. . :y.__J./ r----Y : 7. I. I. 8. I. I. I, I100 110 120 050 60 70 80 90 100 110 120 E (MeV) E (MeV) :’

50

60

70

80

90

FIGURE 9a Distribution of missing momentum versus total energy of 8838 eTe+e- events, remaining after a correlated vertex;time cut. Indicated is the line E t IZpcl = 112 MeV, below which v-+3e2v events should be confined. The encircled event is the one shown in Figure 11.

.

FIGURE 9b 899 events from Figure 9a with an additional coplanarity requirement (see text). The contours contain 90% and 68% of u+3e events.

TABLE 3 Results for the 3e search with SINDRUM.

p+3e Hodoscope

acceptance

24%

p+3e2v 2.6

l

10-4

Trigger efficiency

85%

26%

Chamber efficiency

92%

92%

Other efficiencies

81%

53%

90% contour Total efficiency

90% (13.7 f 1.5)%

Events observed Number of stops Branching Theory

ratio

0 7.5 < 2.4

3.3

- lO-5 (lkO.16)

7833 f 90

- 1012

7.5

. 10 -12

3.2 . lO-5 3.5

* 1012 * 10-5

423~

17.K. Waber / Rare decays

70

80

90

100

110 120 130 E + ICjijcl (MeV)

FIGURE 10 Distribution of E + /&I for prompt events (solid histogram) events (dotted histogram).

The calculated

value is 3.5 * 10m5.

Figure 11 shows the p+3eZv event encircled to the p-+3e contour.

p-+3&V events.

in Figure 9a, which lies nearest

It does not survive the n+3e coplanarity

shows the projections r.ight

and accidental

perpendicular

to the magnetic

cut. Figure 12

field of the 12 most down-

Only three of them have additional

hits or old tracks

showing that a higher stop rate could be used to further improve the result.

4. POSSIBLE FUTURE IMPROVEMENTS For the K-decay experiments generation

of experiments

shown unexpected

problems.

considered

it is difficult

to think about a new

before the present one has started data taking and One problem at least, the large a'/k

ratios in the incoming beam, could be solved soon by trading quality for new secondary

beam lines. For stopped k

lar to the ones used at col7iding

and n/K:

intensity versus

a 4~r detector very simi-

beam machines has been approved at 8rookhaven

(E787)15, mainly to search for the decay K'YII+ + neutrals.

It also can be used

and Ke3 and tagged T' decays. Stopping beams have the advantage of for K 113 smaller F/K contamination, better mass resolution, better n/u separation, and better veto efficiency

for photons.

424~

H. K. Walter / Rare decays

FIGURE 11 Projections perpendicular (left) and parallel (right) to the magnetic the p+3eZv event, marked with a circle in Figure 9a.

A possible by a AGS-II

field of

future limit of lo-l3 for the decay K+m*pe has been discussed 41 . Such experiments certainly need higher pro-

Task Force Subgroup

ton currents. As in Brookhaven tories are under discussion.

also at LAMPF, TRIUMF and SIN socalled

These are high intensity

K-fac-

(- 700 VA) proton acce-

lerators with an energy of 20-40 GeV, which can deliver DC- or pulsed beams for the study of kaons, antiprotons, these machines normalized

pions, muons and neutrinos. A comparison

scheme of the SIN II machine is shown in Figure 13. As already discussed, careful design of the secondary is very important Concerning

of

to the present AGS is listed in Table 4, a possible a

beam lines with respect to small contaminations

(see e.g. refs. 42-44).

improvements

of muon decay experiments

posal is a third generation u-q

experiment

the only approved pro16 . By reconfiguering and at LAMPF

doubling the 400 NaI crystals of the crystalbox each can be built, between which a magnetic

spectrometer

measures electron momenta with < 1% resolution NaI walls. The branching

two walls of 8% solid angle

ratio limit envisaged

is situated, which

and confines them to shield the -12 . is 10

425~

H. K. Walter / Rare decays

f

FIGURE 12 Projections perpendicular to the magnetic field of 12 u+3e2v events, which are located nearest to the down-right corner of Figure ga. The events marked c survive the coplanarity cut.

A 4~r detector

for rare muon decays CRYSP has been presented by G. Sanders 45 . Proportional chambers for e' momentum deter-

at a recent LAMPF II workshop mination

(< 1% resolution)

- 4000 crystals)

are surrounded

inside a magnetic

by a BGO detector

(- 400 litres,

field of 1.5 T, which is high enough to

prevent 70 MeV/c e' from entering the BGO calorimeter.

With a stop rate of

lo8 ~.r+/sand a 100% duty cycle beam available at a LAMPF II facility it is -15 for the decay I_r+eycould be claimed, that a branching ratio of > 5 - 10 measured

in principle.

426~

H. K. Walter / Rare decays

FIGURE 13 A possible layout of a future High Intensity Proton Synchrotron (HIPS) at SIN. The machine would have a 80 JJA, 20 GeV pulsed or DC-beam.

ACCUMULATOR Uw.Ll.nl, . SYNCHROTRON

.

At these high stop rates the most critical background cidences.

Since this background

is proportional

.

are accidental

to N-T-Ax-AY’A~~,

where

coinN is

the stop rate, T the time resolution,

and Ax, Ay, A0 are the electron energy,

photon energy and angular resolutions

respectively,

angular resolution

are most important.

good photon energy and

At SIN we therefore

TABLE 4 Normalized yields of several possible kaon Factories

already in 198146

(Cu production

target).

v +e 1-I

K- (1 GeV/c)

K- (5 GeV/c)

KEK

0.3

0.4

0.07

0.02

AGS

1.0

1.0

1.0

1.0

47.0

47.0

170.0

119.0

81.0

128.0

319.0

334.0

363.0

92.0

257.0

312.0

481 .O

AGS II

19.0

SIN II

68.0

TRIUMF

II

LAMPF II

p (5 GeV/c)

47.0

427c

H. K. Walter 1 Rare decays

proposed to build a 4%

pair spectrometer surrounding a 4~r electron spectro-1 meter. Assuming a stop rate of lo8 s , a time resolution of 400 ps, electron energy resolution resolution branching

of 1X, photon energy resolution

of 30 mr a background ratio for u*ey of 7

-

of 3% and angular

free measurement of a,1 upper limit of the -14 is possible. It is not clear to me how 10

this limit can be improved by use of a BGO calorimeter. As noted earlier

the p-f3e experiment

Also the v+e conversion Here only upgrading

experiment

of the existing

could be done with higher stop rate.

at TRIUMF uses the full available 1-i"flux. proton beam and/or the design of high

luminosity muon beams will lead to improvements moderately hardware

better momentum

resolution

is needed for v+3e. For p*

and better momentum Summarizing

resolution

of the sensitivities.

but fast readout and filter soft- and conversion

smaller beam ~--contamination

is necessary.

I like to stress, that any effort is justified

detect these muon number violating

processes.

to eventually

No firm prediction

any theory but on the other hand almost no theory is invalidated ratios near the present

Only

is made by if branching

limits would be found. Even if only upper limits are

improved for those decays, either the experiment the same detector yields very interesting

itself or an experiment

using

results on allowed rare processes

K'+e+e- , p+1_1-,nOe+e- , K+-tTl+e+e-,r'-te+e-, TI*VY, Ir-t3ev,u+3e2v). 'L The two decays ~-tevy and Ir+3ev for example are the subject of proposals 18,47 coming up at all three meson factories . (e.g

I would like to thank all my colleagues many discussions.

of the SINDRUM collaboration

All authors, who contributed

unpublished material

for

in written

or oral form are warmly acknowledged.

REFERENCES 1) A. Zylbertstejn,

this conference,

contr. N26.

2) R. Handler et al., "A measurement of the CP violation parameter n+_oll, Fermilab exp. 621, G.B. Thomson, Rutgers, Univ. of New Jersey, priv. co~un., July 1984. 3) T. Ishikawa et al., this conf., contr. K4. 4) T. Numao et al., this conf. contr. Dl. C. Amsler et al., this conf. contr. D2. 5) 6. Hogan et al., this conf., contr. N12, and session K. 6) D.A. Bryman et al., this conf., session K.

H. K. Walter / Rare decays

428~

7) W. Bert1 et al., this conf., contr. K5. 8) E. Takasugi,

this conf., session D.

9) M. Koshiba, this conf., invited talk. 10) A. Bay et al., this conf., contr. Kl. 11) N.W. Tanner et al., this conf., contr. K3. 12) AGS exp. E777, M. Zeller, spokesman. 13) AGS exp. E791, S.G. Wojcicki, 14) SIN proposal R-82-04.1, 15) AGS exp. E787, I.-H.

spokesman.

B. Hahn, spokesman.

Chiang et al.

16) LAMPF proposal 444, J.D.

Bowman and R. Hofstadter,

spokesmen.

17) AGS exp. E780, R.C. Larsen et al. 18) TRIUMF proposal 284, M. Blecher, spokesman. 19) TRIUMF proposal 277, C.E. Waltham,

spokesman.

20) L. Bergstrom, Talk presented at "Workshop on the Physics Program at CELSIUS", Uppsala, Nov. 7-9, 1983. 21) C.M. Hoffman, LAMPF preprint LA-UR-84-1327, to be published in Proc. 4th Course of the Int. School of Physics, Erice, Italy, March 31 - April 6, 1984. 22) G.L. Kane and R.E. Shrock, TSIMESS, AIP Conf. Proc. No. 102, Part. and Fields No. 31, Santa Cruz, 1983. 23) 0. Shanker,

Nucl. Phys. B206 (1982) 253.

24) J. Ellis, CERN preprint TH 2942 (September

1980).

25) Part. Data group, Review of Particle properties,

Phys. Lett. 1llB (1982) 1.

26) A.R. Clark et al., Phys. Rev. Lett. 26 (1971) 1667. 27) R.L. Kelly et al., Rev. Mod. Phys. 52 (1980) 51. 28) A.M. Diamant-Berger

et al., Phys. Lett. 628 (1976) 485.

29) D. Bryman, Phys. Rev. D26 (1982) 2538. 30) J. Bowman et al., Phys. Rev. Lett. 42 (1979) 556. 31) G. Azuelos et al., Phys. Rev. Lett. 51 (1983) 164. 32) SINDRUM collaboration;

W. Bert1 et al., Phys. Lett. 140B (1984) 299.

429~

H. K. Walter / Rare decays

33)

A. Badertscher

34)

R. Abela et al., Phys. Lett. 1058 (1981)

35) J. Frank, XXII

et al., Lett. Nuovo Cim.

28 (198@)

401.

Conf. on High Energy Physics, Leipzig, July 1%25(1984).

Int.

36) G.M. Marshall et al., Phys. Rev. 025 (1982) 1174. 37) P. Herczeg, Proc. Workshop on Nucl. and Part. Phys. at Energies up to 31 GeV, Los Alamos, Jan. 1981. 38) R. Cahn and H. Harari, Nucl. Phys. 6176 (1980) 135. 39) Manufactured

by Dr. 8. Struck, LIP-Tangstedt, Hamburg, Germany.

40) W. Bert1 et al., Nucl. Instr. and Meth. 217 (1983) 367. 41) Report of the AGS II'Task Force, Febr. 1984. 42) J. Doornbos, TRIUMF Report TRI-ON-83-48 43) Proc. 3rd LAMPF II

Workshop,

44) Report of the AGS II 45) G.H. Sanders, 46)

SIN proposal

(Dec. 1983).

Los Alamos, July 1983,

Task Force, Febr. 1984.

Proc. 3rd LAMPF II R-80-06.1,

Workshop,

Los Alamos, July

W. Bert1 et al., (August 1981).

47) G. Hogan, LAMPF, private communication,

August 1984.

1983,

p.

691.