Download - 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.
406 G. Sanders/Rare decays
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
408 G. Sanders/Rare decays
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.
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