rare decays

Nuclear Physics B (Proc. Suppl.) 19 (1991) 39940 North-Holland

RARE DECAYS

Gary SANDERS Los Alamos National Laboratory, Los Alamos, New Mexico 87543 USA

Hare decay experiments have made sign&ant advances in the last several violating decays of kaons have been tightened. Large sampks of st decays have been collected. For several substantially narrowed. Opportunities for a new background to one decay mode and

1. INTRODUCTION

In structuring this review, I have chosen to respect

the brief time permitted by concentrating on those areas

in which significant progress has been made in the last

year or two in areas that may signal new physics. I shall

give highest priority to new results, which in one case

includes the observation of a new background, and to

promises for the near future made by mature and, there-

fore, more likely reliable experimental teams. I shall

devote only scant attention to promises for future ex-

periments in the far future, or for new accelerators. My

report will, given these limitations, concentrate on rare

decay research with kaons. This is the area in which the

greatest progress has been made recently. I will men-

tion one pion decay, and since no new work is available

in muon or beta decays, I shall exclude these except to

say that at Neutrino 92 we may expect to hear new rare

muon decay results from the meson factories.

1.1 The Best Rare Decay Experiment Ever Done

Having started with all of these limitations, I will

promptly ignore them for a few moments and describe

to you what is likely the best rare decay experiment

ever done. Two notable facts bear mentioning. This

experiment was done nearly 100 years ago, and more

important, it appears that the result was due to an ac-

cident!

As you may recall, Becquerel had a problem with

film being exposed when placed in proximity to a piece

querel, the Curiq andRut

beta rays, respan&ble f<a expkng the f&n_

to violate the -on of

to consider abandoning this

generation after Becquenzl’s orig+l

followed by Fermi, rescued the

the very primitive notion of

emission of a light, neutral part&k which cwried the

missing energy and balanced the books.

We all know the FEZTIC theory of beta decay_

recall that in the generation after that theory was pro-

posed, it was extended to include the picture of a weak

current carried by a charged bosom With the advent of

the quark picture, this charged weak current coupled to

the d and u quarks in the decaying nucleon. The boson

carrying the neutral current was added to the picture,

and this current was finally observed after yet another

generation.

It was not, however, until 1983, almost a century

after Becquerel’s “unfortunate” film exposure, that the

weak bosons were directly observed at CERN. Mare-

over, they were observed at a mass scale of 1O’l eV,

while Becquerel was working on a scale of 104 eV. The

spread between the observation of this “rare” decay and

the direct observation of the underlying mass scale is

seven orders of magnitude!

0920~5632/91/%3.60 @ Eleevier Science Publishers B.V. (North-Holland)

400 G. Sanders/ Rare decays

The lesson for experimentalists is to pay careful

attention to small effects from beyond the current state

of knowledge. As in the case I have cited, they can be

a vestige of physics at an inaccessible mass or energy

scale.

1.2 A hk&rn Analogy: The Dynamical Mm

Scale of the Bkunily Interaction

Modern rare decay experiments often have a sim-

ilar motivation. Consider the family problem. In the

minimal standard model, the quarks and leptons are or-

ganized into the we&known families, motivated by our

observations of quark mixing, the hierarchy of masses,

and some more technical considerations.1 This orgh-

zation is put in, but no rigorous or fundamental mo-

tivation exists. If the &&lies reflected a true gauge

symmetry, as in QCD or QED, there would be an exact

conservation law. This law would correspond to a new

force in nature and the theory would have to incorpo-

rate a massless vector boson, analogous to the gluon or

photon.

Such a scheme does not reflect observations. Fam-

ilies are mixed in the quark sector. The suppression

of family mixing in the lepton sector may be a man-

ifestation of a broken family symmetry. This would

be the result of a massive neutral family vector boson.

Such a model has been discussed by Cahn and Harari2

In Figure 1, the diagrams for the conventional decay

K+ --) p+v,, and the lepton family violating process

KE * pe are shown. The known process is mediated

by the exchange of a W+. The rare decay is postu-

lated as resulting from the exchange of a neutral family

vector boson. The couplings are indicated. By dimen-

sional analogy between the two diagrams, and equating

the analogous couplings, an estimate can be made of

the mass of the family boson in terms of the rate or

branching ratio for the mocess.

Fig. 1. Diagrams for the ordinary decay K+ + pup

and the lepton-number violating decay I<L ---) ccc. The

first reaction is facilitated by the W+, the second by a

possible neutral family-changing boson.

The estimate is shown as

MF = 24 TeV C

1 x 1o-8 1 f

B(KL-,P~) '

which involves a i power dependence of the mass on the

sensitivity of experimental searches. I have expressed

the formula in terms of the approximate limit on this

process when the current round of experiments began.

They will be described later in this talk. More revealing

is a plot of this formula. Figure 2 shows how progres-

sively more sensitive experiments in this area probe ever

higher mass scales, though only for specific signatures.

At lo-“, the mass scale is 75 TeV. This is an order

of magnitude beyond the highest scale accessed at the

SSC! I will report limits in this range and promise more

to come very soon.

G. Sanders/Rare decays 401

Fig. 2. The relation between the rate for the rare decay

KL + Fe and the mass of a family-changing boson,

estimated using a simple dimensiomiI anaIogy appkd

to the process shown in Fig. 1.

These arguments have been made for most of the

interesting rare decays of the muon and kaor~,~ even

though very difkrent physical processes are thought to

be responsible. The analogy to the Becquerel obser-

vation is obvious. In the rare kaon decay mentioned

above, the search is carried out at a scale of lOlo eV,

but the underlying family boson mass scale is approxi-

mately 1014 eV. This is not quite the “reach” that Bec-

querel had in 1896. That is why his experiment is the

best rare decay search ever done.

Having just established the motivation for rare de-

cay research, in general, and for searches for the de-

cay KL + pet in particular, I will describe the recent

progress on the three leptonic decays of the KL together.

While the physics gods are difbent, these

carried out together in the same

ofaleptquark- Pi3

*-p t c

z-e

Fig.3. ThedecayKL+pmia

aSpX?diCtedby

decay KL -+ JL~? This

the higherorder induced

currents involving

duced Z*, or by

current round of rare decay sear&es, 27

tuted the world ~ample,~ with a

(9.1 f 1.9) x 10-9, sIightly above the

by unitarity considerations and the measured branch-

ing fraction for the decay KL + 3”1.5v6 More acaxmk

402 G. Sanders/Rare decays

measurements of the pp decay rate might stimulate the

theoretical community to produce more precise calcula-

tions which could then be used to constrain the quark

mixing angles, the top-quark mass and new interactions.

The really new physics, however, might come from the

observation of muon polarization in this decay. I shall

discuss this later.

target with a set of magnets and defines the remaining

neutral component with a series of precision collimators.

This system has been the bane of many experiments,

with poor shielding, improperly aligned collimators and

fen>cious dominance of neutrons over neutral kaons.

7 d

27 w d

cilkwJP s I c

Fig. 4. The leading standard model diagrams for the

d-Y PL --) PP-

The process KL + ee is closely related to the pre-

vious topic. It is helicity suppressed with respect to

the pp decay and comparisons of the absorptive part

of the two photon diagrams for the two decays leads

to branching ratio estimates in the neighborhood of 2-

5 ~10~‘~. This strong suppression opens a big window

for new physics observations.

These three processes are generally sought simulta-

neously in the same apparatus. Figure 5 shows a generic

“kit” for studying neutral kaon decays. The kit is di-

vided into three functional sections. The first, a neutral

beam, separates charged secondaries from a production

Fig. 5. A generic “kit” of detectors for a neutral kaon

decay study.

The second functional section consists of an evac-

uated decay volume for the candidate kaon decays that

are viewed by a precision magnetic spectrometer. The

spectrometer must track the decay daughters and pro-

vide a precise measurement of the track angle and mo-

mentum to facilitate reconstructing the parent invariant

mass. The key features here are magnetic field strength

for momentum analysis, and precise spatial resolution

for the spectrometer detectors. The ability to handle

high rates and the requirement of low detector mass are

also paramount. Remember too, that the neutral beam

survives along the spectrometer, and it must not be al-

lowed to confuse the instrument.

G. Sanders/Rare decays

The third functional system consists of particle

identification detectors. In order to identify the decay

final states, clean identification of the pions, muons and

electrons is required. Even more important is the elim-

ination, to a very high sensitivity, of any misidenti&

cation of particle type. This is of key importance in

eliminating backgrounds to these sear&es.

The principal progress made in the last few years

has come from two experiments. Each made careful use

of aJl of the tools in the “kit.”

The first experiment, a UCLA-LANEIrvine-St

Temple-Texas-William & Mary team (Brookhaven AGS

experiment 791) used the apparatus shown schemati-

cally in Figure 6. The neutral beam section worked es-

pecially well, with halo-free collimation, effective shield-

ing of the charged particle dump and problem-free sup-

pression of leaking neutrons.

0

s

8

f 3

i!3

%

Fe. Filter or HODO.

P Range-Fihder

nleammment.

Alloftheparticle

electrons identi6ed in the

andintheleadglass

iron6lterandhodoscopeandintbe

rangefinderwhoserange

thespectrometermomentum

ingredients are lquired for sens%+q

measurements.

The second group, KEK

apparatus shown in Figure 7. This group

many of the same innovations as the

The principal difference was the

Fig. 6. Schematic layout of BNL E791 detector.

404 G. Sanders/ Rare decays

bends that facilitated a parallelism trigger and drove combined 90% confidence level limits from the two ex-

down the physical size of the downstream apparatus. periments of about 3 x 10-l’ for the pe process. Of

course, the data analysis could show that the decay has

been observed!

Experiment

BNL E780,1987

BNL E791,1987

BNL E780,1988

KEK E137, 1988

BNL E791,1988

BNL E791,1989

KEK E137, 1990

BNL E791,1990 Fig. 7. Schematic layout of KEK El37 detector.

Table 1 summarizes the progress these two exper-

iments have made in the last several years, along with

a less sensitive earlier effort by another Brookhaven ex-

periment. The table uses the published limits on the

KL + pe search for data collected in the indicated year.

The last column also benchmarks these experiments by

the number of KL + pp decays collected. Varying ef-

ficiencies for these two decays among the experiments

accounts for the imperfect tracking of the pe limits and

the observed pp sample sizes. The 6naI three entries ac-

count roughly for the ,UP samples potentially collected

in 1989 and 1990 by these teams. These should not be

taken as more than indications as the final two entries

are based upon data coIlection that was completed by

both teams only two weeks ago. Only a smah portion

of the data samples have been subject to preliminary

analysis. The 1989 experiment 791 data analysis, how-

ever, is nearly complete. What is remarkable about this

table is the great progress it signifies! We may expect

Table 1.

B(Kw.4

< 6.7 x 1O-g

< 1.1 x 1o-8

< 1.9 x 1o-g

< 4.3 x lo-lo

< 2.2 x lo-lo

N PP

2

2

8

54

87

> 250

N 200

N 600

Figure 8 shows a preliminary plot of the 1989 data

collected by BNL 791 for the decay KL --) pp. The

plot illustrates the clean separation of signal from back-

ground. These data will lead to a more precise branch-

ing ratio and they will be surpassed, in turn, by the 1990

data.

Fig. 8. Invariant mass distribution for KL * pp cancu-

dates from data in 1988 and 1989.

G. Sanders/Bare decays

Figure 9 shows the scatter plot of ICL + pe candidates

displayed versus invariant mass and the ‘kink” angle re- weremindyouofthebest

ferred to as colinearity angle which is used to separate

out events from decays of pions to leptons. tie Fe can-

didates should fall near the kaon mass and at colinearity

angle zero. The plot shows no candidates. takenatKEK

HUE FT COL-2 VS MASS

l....l....l....l.... 0.405 1 o.soo OJM 0.510 moss

P

Fig. 9. Scatter plot of KL + Fe candidates showing

invariant mass vs. the square of the colinearity angle.

Figure 10 summarizes the measurements made in

the last 20 years of the pp decay branching ratio.‘,*

The solid line indicates the unitarity limit on this rate

imposed by the absorptive part of the two photon pro-

cess. The 1988 KEK and BNL results fall just above

the limit and 1.5 standard deviations below it, respec-

tively. Any distress caused by the low point will likely

be clarified by the new data taken by both groups.

Fig. 10. MV tsofthebmnchingl&io

decayKLqq&omrekrences?and8.

For the fixture, the KEK group9 having

its data collection, has emba&ed upon the

hassubmittedanewletterofintenttotheKEK~-

agement. They propose to return with a signScantIy

improved beam, a better duty f&or, and larger spec-

trometer acceptance, enabling a f&ctor of 20 w

ment in sensitivity. TheBrookhavengroupisatan

earlier stage in its future plans, but they too plan to

return. Possibilities are also being considered at a main

ring injector upgrade for Fermilab and at the Canadian

initiative for a kaon factory.


The KEK and Brookhaven experiments would

achieve a sensitivity that permits observation of the

& + ee decay at the Standard Model rate, thus com-

pletely closing the new physics window. They would

also make possible the search for polarization of the

muons in the final state of the KJ + pp decay, a signa-

ture for unobserved GP-violation.

SEARCH FOR POLARIZATION OF THE

MUONS FROM Kr, + P/J

I have already pointed out the leading Standard

(or electroweah) di s for this decay (Fig-

ure 4). Though others wrote about this at au earlier

time?O Herczeg set the stage for modern treatments of

muon polarization in this decay. Figure 11 illustrates

the kind of “naneiectroweak” processes considered by

Herczeg. All are outside the Standard Model.

Fig. 11. Nonelectroweak diagrams for the decay Kt -I

crcr*

He shows that the small admixture of Kl in the KI,

leads to a small polarization of the final state muons

that he calculates is

This is the Standard Model background. Any po-

larization larger than this must come from new physics

processes that involve GP-violation of a new, direct va-

riety. His estimates of the possible polarization from

the processes displayed in Figure 12 range all the way

up to 100%. These estimates are model dependent.

Many others have followed Herczeg in considering

the possible polarization. I gave a more extensive review

of this subject at Moriond in 1989.‘l The range of mod-

els and predictions is extensive. The important point is

that no experimental search has been carried out in this

area and the search is what I have called a “window”

experiment in which the Standard Model suppression

of the signal permits new physics to be observed. Most

important, the sensitivity achieved by the Brookhaven

and KEK experiments approaches that needed to carry

out a search for this phenomenon. The two experiments

have collected a combined total of about 800 events.

Both the Brookhaven and KEK groups have pro-

posed to carry out the measurement with a seg-

mented marble or aluminum polarimeter. The tech-

nique has been proven by the Brookhaven group12 and

a 300-ton polarimeter is partially incorporated into

the Brookhaven experiment. Table 2 illustrates the

estimated sensitivity of these past proposals to the

polarization. l3 The required sample of raw events is

somewhere between 1000 and 10000. The recent let-

ter of intent from the KEK group identifies a goal of

10% with 4000 raw events.

G. Sanders/Itan decays

Table 2. Past Experimental Possibilities.

Et01 (ISM) KEY (TLRIYF tbbshkq 1282)

This is an exciting prospect. A novel means of

searching for CP-violation now appears to be feasible.

4. OBSERVATION OF A NEW BACKGROUND

PROCESS TO THE DECAY KI, + a’ee

For some time, the interest in sear&ing for the

decay KL + vr”ee has been motivated by the poten-

tial to observe direct CP-violation. This process is ex-

pected to occur through a CP-conserving twophoton

exchange. Branching ratios for such processes lie in

the range 2 x lo-l1 - 3 x 1O-1o.14 It is also possible

through CP-violating mechanisms. Estimates of these

yield branching ratios between 4 x lo-l2 and 2 x lo-l1 .15

Since these rates are suppressed, this a also a “window”

experiment or an opportunity to observe the predicted

rates and to attempt to unfold the contributing pro-

cesses.

The work of several groups has improved the sen-

sitivity of the searches for this decay by several orders

of magnitude, with the most recent result of 5.5 x lo-’

Fig. 12. Event scatter plot of the square

reconstruction angle vs. z%+e- effective mass_

Howwer,inaveryrecentpreprix&theB

grouphasrepoltedt~o~of~

closely mimicked the signaL” These are

ure 13. The only observable diEkren=

events and the flee decays is the

the two photons hawe the so invariant znass. ln a

companion paper:* one member of the group has pre-

sented a scholarly and elegant calculation of the rate


for the process KL + Tee accompanied by a hard in-

ternal bremsstrahlung, which yields the additional pho-

ton. This preprint demonstrates that the observed rate

should be about five events corresponding to the mea-

sured branching ratio of (5.0 f 2.3) x 10B7. This process

was not previously anticipated by anyone as a back-

ground.

m,,, (MeV)

Fig. 13. Display similar to Fig. 12 for the Tree events.

These authors also demonstrate that this back-

ground is not e&y sepuable and presents a substantial

obstacle to any planned background-free search. Thus,

the prospects for a search for direct CP-violation via

this route appear to be substantially diminished.

I find these papers convincing. Nevertheless, a

KEK group (KEK experiment 162) will commence a

sensitive search for the decay KL + r”ee later this year.

We will then have more information. Plans for such re-

search at FNAL may also be tiected. The possibility

for a search for KL + n”pp should be considered. This

rate is lower by a factor of five. The background is

at least two orders of magnitude lower, however. This

should be investigated as a potential avenue for observ-

ing CP-violation.

5. SEARCH FOR THE DECAY K+ -+ n+uc

Theorists tell us that this process is the only known

second order weak process that can be calculated un-

ambiguously. The principal input to these calculations

is the Cabbibo-Kobayasbi-Mkawa matrix with three

generations constrained by B meson decay observables

such as I&. The calculation produces the branching

ratio as a function of the top quark mass. A recent

calculation of this rate is displayed in Figure 14.1g

I I

100 150

m t (Gev/c 2 )

Fig. 14. Upper and lower bounds on the allowed branch-

ing ratio of K+ + n+z& as a function of ml from refer-

ence 19.

Any rate observed above this range signals new

physics. Precise measurement of the rate constrains the

mixing angles and top quark mass. Thus, this is a re-

action of great interest.

An experiment at the Brookhaven AGS (AGS ex-

periment 787) is in the early stages of this search. They

G. Sanders/Rare deays

have already substantially improved over the previous

best search2’ which reached 10B7. In the most recent

publication, 21 they reported a limit (90% confidence

level) greater than 3.4 x low8 for data collected in 1988.

The team has had another data run in 1989 in which

they expect to report a result more than one order of

magnitude more sensitive. This year, in the run just

completed, they may reach the upper range of the ex-

pected Standard Model rate. With a proposed beam

improvement and several detector upgrades, they antic-

ipate reaching the lower range of the Standard Model

predictions.

This search has many side benefits, as well, such as

the search for K+ -+ R+H + a+/~p for Higgs masses

in the range 220 to 320 MeV/c2 where the group has

already reported a limit greater than 1.5 x 10m7. In the

light of the new ALEPH results limiting light Higgs, this

may seem less interesting but science is the business of

independent confirmation.

Another side benefit of this experiment is related

to the next topic.

6. SEARCH FOR THE DECAY 7r” + vc

This decay is forbidden with normal currents by

helicity considerations. An observation of the decay is

a signal of new physics. Limits based upon BEBC and

CHARM data for the decays into e, ~1, and r neutrinos

have been set in the 10B6 range.22

The rare K+ decay experiment just described has

reported a new limit on the inclusive decay to any neu-

trino type (or, indeed, any light neutral objects) of

greater than 10 -6. This is a preliminary number. A

final publication is being prepared.

ALAMPFneutri~~asci&ti~

645)isreportingapr&minaqlimiton

and p neutrinos at the! 5-g x lo-’

pectmoreprogressinthisarea~

years fkom the Brookhawen

LAMPF beam stop.

7. SUMMARY

Therehasbee.nsubstantiat

eralyearsinraredecayresear&B

K~+j4ewarchesappeartohave

range of sensitivity. Thest2 same

crezwzdthewox3dsampleaW.w

about 800, enabling new research into

thefinalstatemuons. Thismaybean

search for direct CP-violation The

violating process KL +3r%eappeaIsto

cult than a3kipated due to the

Standard Model prediction for the second order

decay K+ + &ZIG. New limits have been set cm the

helieity suppressed decay Ire + UG by at

Brookhaven and LAMPF.

REFERENCES

1.

2.

T. Goldman and Michael Martin Nietq =The Fam- ily Probkm,“Los Alamos Science 11,1984, pp. 114- 124.

T. Goldman and Michael Martin Nieto, “The Fam- ily Problem,” Particle Physics: A Los Albrws Primer, N. Cooper and G. West eds., Cambridge University Press, Cambridge, England, 1988, ISBN- O-521-34542-1, pp. 114124.

R N. Cab and H. Harari, &cl. Phys. Bl74135 (1980).


3. B_ D. &u&x+, %mma,ry of Dimensional Argu- mmts Regarding Rare Processes,” internal memo- m&m KL-153, Department of Physics, UCLA.

4. P. Herczeg, Phys. Rev. lM7,1512 (1983).

5. Particle Data Group, M. Aguilar-Benit=, et uL, Phys. Mt. IrOB, 14 (1986).

6. H. Burl&a&, et uZ., P&3. kit. 199B, 139 (1987).

7. W. C. &&hers, et a& Phys. Rev. Lett. SO, 1336 (1913); W. C. &&hers, et d, Phys. Rev. Leit. 31,1025 (1973); Y. Fbkushba, et al-, Phys. Rev. Lett. 36, 348(19?6); M. J. Shochet, eb al., Phys. Rev. D 19,1965 (1979).

R~QJ. D 40, 1712 (1989); Phys. Rev. Left. 63,2185

et &, Phys. Rev. Lett. 63,218l

10. E. de R&t& Pbya. Rev. 157,1486 (1966); A. Pais and S. &iman, Phys. Rev. 176, 1974 (1968); G. V. Dass and L. W-kin, Phys. Lett. 38B, 435 (1972).

11.

tiq ISBN 2-86332-965-3, p. 383.

12. C. J. Kenney, et OL, IEEE 1988 Nuclear Science Symposium, November 1988, Orlando, Florida.

CP-Violation Workshop, TRIUMF,

14. G. Eckx, A. Pith and E. de Rafael, Nucl. Phys. B 291 (1987) 692; B303 (1988) 665; L. M. Seh- gai, Phgs. Rev. D 38 (1988) 808; T. MO~OZUXIG and H. Iwasaki, KEK preprint 88-4, TH-206 (April 1988); J. M. Flynn and L. Randall, Phys. Lett. Bgl6 (1989) 221.

15. M. K. Gaill~d and B. W. Lee, Phys. Rev. D10, (1974) 897; J. Ellis, M. K. Gaillard and D. V.

Nanopoulos, Nucl. Phys. B 109 (1976) 213; J. F. Donoghue, B. R Holstein and G. Vale&a, Phys. Rev. D 35 (1987) 2769; C. 0. Dib, I. Dunietz, F. J. Gihnan, SLAGPUB-4762, Nov. 1988; F. J. Gilman and M. Wise, Phys. Rev. D 80 (1979) 2392; 21 (1980) 3150; J. M. Flynn and L. Randall, LBL- $631 O/RAL-88-1 OS, Nov. 1988.

16. K. E. Ohl, eb al, YALE preprint YAUG-A-90/2, submitted to Phys. Rev. Left., March 12, 1990.

17. W. M. Morse, et aL, YALE preprint YAUG-A-90/4, submitted to Phys. Rev. D, May 17,199O.

18. H. B. Greenlee, YALE preprint YAUGA-90/3, submitted to Phys. Rev. D, May 17,199O.

19. Q. C. Gang and J. N. Ng, Pirys. Rev. (to be published).

20. Y. Asauo, et al., Phys. Leti. MB, 195 (1982).

21. M. Atiya, eb al, Phys. Rev. Left. 64,21 (1990).

22. C. M. Hoffman, et eZ., Phys. Lett. B 208, 149 (1988).

23. Ali Fazely, private communication.

rare decays

Documents