Charmed semileptonic B meson decays

N~clearPhysicsB (Proc. Suppl.) 13 (1990)261-269 North-Holland

261

CHARMED SEMILEPTONIC B MESONDECAYS *

Sheldon STONE Wilson Laboratory, Cornell Univ., Ithaca N. Y. 14853 USA Recent 8 meson semileptonic branching r a t i o measurements are summarized and appear to be lower than theoretical expectations. Measurements of the exclusive f i n a l states D°~'u, De°~'~, D*~E"~ and D * * ~ ' ~ a r e reported. The r a t i o of rates D*°/D° and the polarization of the De+ are shown to be consisten¢ with theoretical models. F i n a l l y comparing the D*°~-uand D ~ [ ' ~ rates, the .22O. lifetime r a t i o of neutral and charged B~s is shown to be equal t;o 1.18,0.27+°.a

CLEO

1. INTRODUCTION Theoretical models of heavy quark decay apply to both the O and B systems. Models of semileptonic decays are the simplest. Measurements in D decays of the r e l a t i v e widths for K*£u versus K£u are lower than model predictions and the K* p o l a r i z a t i o n is larger than model predictions. 1 Here I w i l l describe s i m i l a r , but poorer, measurements in B decays. F i r s t , however, I w i l l summarize our knowledge of the average semileptonic branching r a t i o of B°and B" mesons and some recent results from CLEO on the B° semileptonic rate.

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2. INCLUSIVE SEMILEPTONIC DECAYS Spectator model predictions 2 of the

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semileptoni¢ B branching r a t i o BsL=B(B4Xev) are 16S. ~CD corrections enhance the hadronic width and lower t h i s expectation by 25~. 3 An

Momentum D i s t r i b u t i o n of muons and electrons

albernatlve theoretical approach using the 1/N expansion can drive the prediction as low at

from B decay. The contribution from the continuw, below the T(4S) has been subtracted.

12~. 4

The dashed curve is the expected spectrum for B decays to charmed mesons and the s o l i d curve is

Although many 9roMps in the past have

measured BSL, I w i l l summarize only recent ones. 5 The lepton momentum spectrum has two components, one from B*XI~u and the other from the cascade B~DX, D+Y£u. (See f i g u r e 1.)

FIGURE 1

the spectrum from B decays to charmed p a r t i c l e s which subsequently decay into leptons. The histogram is a f i t to the data using the two curves.

* Vork u-ppo,tod by the National 8cfen=e Foundation.

0920-5632/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

S. Stone/Charmed semileptonic 13meson decays

2~2

T o ~ m i n e ~ S L A R ~ U S f i ~ s the lepton s~u~to predictions of theoretical models a~ovep~of 1.4QeV/c (or in so~ecaseel.7 ~Y/©) in order to r ~ o v e ~ s t of the iep~ons f r e d d~ay. CLEO fi~s the entire spat,rum for ~ c ~ p o n ~ s . One theeretical model, A | ~ r e | i i e t a l . , 6 (ACM) is b~sedon free quark decay an~ in~udee one gluon corrections and F~rmi ~ e s t u m o f the b-qu~rk. The other models calcula~ ~ p | i c i ~ final s ~ t ~ s : D~, D * ~ a e d , |node©ass (ZS~), D**~.~.7They calculato~he exclusive ~ i d ~ s in ~er~s of meson f o r ~ f a c t o r s ~icht~eymat~:h with qu~rkmedel wave functions. ZS~Mt~:h atmximummomntum transfer (q2) where the rate is the largest while Vi ~ 8 and KS9 m~tch at qZ=O where the meson I~smaximal momentum. The r e s u l ~ o f the fi~,s ~o the data are given in Table Z. The CLEO results are unpublished numbers ~eriv~d in Ko~l~ski+s~h~s~s 10 and represent the most c(mprehensive analysis of CLEO*s 1985 data sample. Table ~: 8 ~ s o n S ~ i l e p t o n i c Branching Ratio Mo~l AR(IJ~ ~1 CLEB Crys~l Ball 12 AC~

10.1~0.7~ 8.7~0.5~

~0.1~0.5~ g.3~0.5~

12.0"0.5~ 10.8~0.~

s ~ i l e p t o n i c branching ratio in these decays. This is about20Z since the v i r t u a l W canner materialize a s ~ s or ~ . Thus there can be an additional 1.5~ which could add ~ the IO.SSto raise BsL to 11.5S s t i l l a l i t t l e below the theoretical expectation. On the other hand, all AC production in B decay could proceed via mechsnim which don't have semileptonic decays so t h i s correction is somewhere between 0-1.5S. 3. THE B° SE)GLEPTONZCB,~ICHING RATZO The T(4S) resonance decays to B-B+ or B°i °. The fraction of decays to the charged and neutral B's modes has not been measured d i r e c t l y . D i f f e r e n t numbers would arise from d i f f e r e n t amounts of phase space caused by d i f f e r e n t B mosses. Previous CLEO measurement gave 2.0~1.1MeVmoremassto the B°than the 13".14 Using t h i s central value and p-wave phase space, CLEO found 431 neutral B's and 57~ charged B's. A new, preliminary, CLEO mass difference measurement, based on the data shown in figure 2, is -0.2t0.6 MeV. I f the mass difference is zero there can't be any difference between the charged and neutral B fractlons. Zn the rest of t h i s paper, 1 w i l l assume t h i s is the case and that the fraction of charged or neutral B's on the T(4S) is equal to 1/2.

There i~good agreement between CLEOand ARQUS whlc~ measure both e's and ~'s using

By looking at the relative numbers of single lepton events and dilepton events, CLEO had

magnetic ~omentum analysis. BSL appears to be approxi~tely 10~. Crystal Ball'~ electron

previously constrained the r a t i o of B~L/B;L to be between 0.+4 and 2.05, I now discuss recent direct measurements of B~l.

~sur~n~ is somewhat higher. BSL appears to be lower than any expec~tion. One possiblesource of leptons that hasn:t been considered are those coming from the reaction B"'+AC+~..,."~.These lepton have a soft ~ n t u m spectrum and would not be included by the f i t s to the da~ in the above quoted experi~nts. An estima~ of the magnitude of t h i s e f f ~ t uses the CLEO measurement13 of Br(B+ACX)=7.5~ multiplied by the expected

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The f i r s t method uses a sample of "tagged" B°'s to investigate the decay of the other B° meson in the event. Samples of B° mesons are found by I p a r t l a l l y " reconstructing the decays i%D*+~"- and B°+D*+~'~. Procedures for selecting the l a t t e r channel w i l l be discussed later. Here m+J throughout t h i s paper, charge conjugate r=~ctions are also used. D*+~" are found by using t h e ~ from D*++~+D° decay

S. Stoae / ChaJrmc.i semileptoaic B meson dec~ys

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e f f e c t i v e l y maxi=izes the p o s s i b | e u s s v a l e for a given track p s i r . This B mass d i s t r i b u t i o n is shown in f i g u r e 3 along with the mss d i s t r i b u t i o n found f o r l i k e ¢ ~ r ~ pion pairs. The F o x - t ~ l f r m s ~ : p e v a r ~ | e 15 R2 is required to be l e s s ~ n O . S t o r,c~ceCP~ conbinuum contribubien. The fitt,~trough the

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FTcURE 2 Fully reconstructed B meson decays from CLEO. a) is f o r charged B's and b) f o r neutral B's.

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FI(~IRE 3 T h e m s s spectrum of candist~s for i%D*@~r" using a p a r t i a l reconstruction technique. The histogram is D*÷tr~ candidates. Although the signal to background r a t i o is poor, i t turns out t h a t 80~of the background @ -

without f i n d i n g the D°. Events with a f a s t w= and slow r + with the cosine of the angle between the two pions (-0.8 are searched f o r . A

from lepton-poor continuum • e a n n i h i l a t i o n s and can be d i r e c t l y s u b t r a c t . The remaining background from the T(4S) is evaluated using the continumsubtractad event~

B mass is then calculated by a t~:hnique which

which r m a i n s t masses j u s t below the B mass.

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S. SCone/Chimed semfleptonic B meson dec&ys

iept~ns of mmntum grem~er than 1.4 QeV/c ar~ c o u n t . The c o n t r i b u t i o n from semi leptonic

D*~ and L" and then c a l c u l a t i n g the missi:,g mass s q n r e d given by

O ¢iecays i s re~mv~ and the r e s u l t i n g B~L= 9.~2.~ ( p r ~ | i ~ i ~ r y ) . The number from the O * ÷ ~ e v ~ t s i s 9.1,3.4~ ( p r e l i m i n a r y ) which ~ a v e r e ~ gives g.4~2.0~, censiston~ with

where EB i s known beam energy, S0 and PD refer t o the m s u r e d D*~ candidate energy and

• ver~3~ ~SL" ~o~,her intor~sting~ but no~ decisive r e s u i ~ c~ms from ~ust tagging s O~ . Here CLEO looks f o r an ~.~ t o insure t h a t the lepton comes from the other B. They f i n d 8 ( I ~ L ~ ) / B ( ~ D * ~ X ) : 7.1"1.9~. He,ever, there ~mst be a correction f o r ~ mixing because when a ~ mixes i t wi I| no longer decay i n t o an ~..~ but into an #.-. This mixing c o r r e c t i o n can be done esactly i f i~ is t r u e t h a t D*÷ are only predu=ed by B°:s. Then the corrected r e s u l t is B~L = 9.2-2.2~.

momentum, EL and P~. r e f e r to the lepton and ~B i s the unknown B momentum. A c t u a l l y is known t o be 320 MeV but the d i r e c t i o n is unknown. The best guess i s made by ~.4king ~B--O. D i f f e r e n t processes f o r producing D*÷ and P."

IPBI

lead t o d i f f e r e n t shapes in MU2 as shown in f i g u r e 4. This technique was pioneered by ARGUS.16 There are new, pre!iminary, ¢LEO r e s u l t s which i w;[! de.~crlbe. The gU2 d i s t r i b u t i o n s from CLEO are shown in f i g u r e 5 f o r D~'÷ P.- " r i g h t s i g n " events and D~÷ JL÷ Wwrong-sign" events, f o r 2.4OeY/¢ ) pp. ) 1.4 OeV/c. D*+ are found by 1 7 7 0 6 8 g - OI I

4. THE REACTION~%D*÷e.'~ Candidate events are i d e n t i f i e d by f i n d i n g 1?70689-018

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FIC~RE 4 The W 2 spec~r~ f o r three d i f f e r e n t processes r e s u . l t i , g in a D** and an ~.-.

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S. Stone/Charmed semileptonic B meson decays

2~5

t h e i r decay into ~÷D° where the D° subsequently

The angular d i s t r i b u t i o n of tF.~ D° in the D~÷

decays into K-~ + or K-~+~+~- . The D*+

rest frame with respect to the D*~ direction in

background was estimated by using O*÷

the laboratory is then investiga~=d and f i t t e d to the form (Z/N)dN/dcose = t • = cos20. (2~

sidebands. The other curves are determined by a fi~tothe

data l e t t i n g the normalizations of

each component vary. CLEO expresses t h e i r

= relates the amount of longitudinal O~+ to

results in terms r a t i o ' s . The denominator is

amount of transverse, a = ~i'L/rT-1. They found the polarization parameter = = 0.7~O.g.

taken as BSL. They f i n d Br(B°~D*~&'~)/Br(B÷X£~) = 0.46~0.05~0.07 where the l a s t error is due to the measured errors on the D*÷ and DO branching

CLEO uses events with pp.. ) t . 4 ¢=eV/c, since

or D*~ component,

they are worried about contamination fron leptona from D decays. They select events with W 2 consistent with zero (wi~hin *:lQeY2) and

Br(B%D**~E-~)/Br(B*X~) = 0.18-0.11~0.03.

fit

r a t i o s . CLEO also finds weak evidence for a D**

for the number of D~+'s. This removes the 177o5~J--oo7

The amount of D** is rather large (and has large e r r o r s ) ,

ARGUS

but i t is agreemnt with

prediction Of 13~ for D== in the IS~Wmodel. 7 These rates are based on the assumption that the production of neutral B~s and charged B~s at the 7(4S) are equal as implied by the recent

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mass measurements. I f we take BSL as 10£~ The quoted value from the o r i g i n a l ARGUS measurement is 7.0,1.2,1.9~. However the difference in the CLEO and ARQUS numbers is merely the r e s u l t of using d i f f e r e n t m u l t i p l i c a t i v e factors rather than a difference ~ actual measurements. ARGUS assumed a charged ~

neutral B r a t i o of 55/45, Br(DJ%~+D °) of

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CLEO uses the

model of ISGWto account f o r the lost leptons. ARG~I~ u~es a value of 0.75~0.08 as the f r a c t i o n above 1.0 ~eV/c while the IS~W model gives 0.88. Putting these three factors together

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The p o l a r i z a t i o n of the D*~ can be used to d i s t i n g u i s h among models of semileptonic heavy quark decay. This has been done for D~K*E~ decay by E6911 and was f i r s t

~one in B decay by

FIGURE 6 The angular d i s t r i b u t i o n s of t;~e D° in the D~ rest frame with respect to the l)~

ARGUS.15 They selected D * ~ " events with M~2

flight

close to zero and required t ~ m t p E )

ARCUS.

1.0 ~eV/c.

line of

in the laboratory for both CLEO and

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S. Stone~Charmedsemi!eptoaicB meson decays

pro~|e~ of how to account for fake ~*÷=s. They fi~¢i ~ = 0.7~0.7 The number of events in d i f f e r e ~ ¢os8 bi~s from both groups is shown in f l g a r e 6. The reeul~s are compared with predictlons of three theoretical models in figare 7. The KS and IGS~ models are in

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FI~E ? ¢o~parison of some of the mode~s with the data. The p r e d i c ~ ~. i s integrated from the lower E~. cu~=of~ ~o 2.4 GeV/c.

5. THE REACTIOHSB'*D°E'~AND B%D*°E-~ Let us f i r s t consider the inclusive rates of charm meson production for PE > 1.4 GeV/c. The CLEO i n v a r i a n t m s s spectra for candidate D°~K" ~ a n d D%K'~÷~÷ events when a n ~ " is present are shown in figures 8a and 8b respectively. After correcting for effects of semi|~ptonlc D decays, and BSmixing the r e s u l t i n g rates are: 8 r ( e ÷ D ° x ~ - v ) / e r ( e ÷ x ' e ~ ) = o.Ts.o.og and er(S÷~÷Y~=u)/er(S÷X'~v) = o . s 2 , o . o 8 . The error bars contain s t a t i s t i c a l and s~st~matic errors but do not include the u n c e r ~ i n t l e e in the D branching r a t i o s (which are 13~). The sum is 1.05*0.12,consistent with other data showing that charm dominates the semile~to~ic r ~ e s . 17 Zt ~|so implies that the centre| values of the O branchln~ r a t i o s are more or Oess correct.

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The charmed meson candidate mass d i s t r i b u t i o n in events containing a E-. (a) :s f o r K'w÷mass comblnatlons and (b) is for K'w÷w~ combinations. The points are for data taken on the T~S) while the histograms are for data taken at center-of-mass energies j u s t below and have been scaled to account for the difference in Iominosit¥.

S. Stone/Charmed semileptonic B

meson

decays

26?

CLEO has taken the concept of using the missing mass squared d i s t r i b u t i o n one step f u r t h e r . They calculate

MM2

80

from events with a

D° candidate andS- when the D°÷K-f +. The idea

m

60

here is to be able to e x t r a c t branching r a t i o ' s f o r B'+D°E'~ and B-+D*°E-~. The MM2 distribution

is shown in f i g u r e 9~ fo

po > 1.4

OeV/c. There is a clear peak near MM2=0 ~with a great deal of background. The data have been

W

2O

continuum subtracted. Also shown is the d i s t r i b u t i o n of the continuum subtracted D° sldebands. (In p l o t t i n g t h i s l a t t e r

-I0.0

d i s t r i b u t i o n the K and • 3-moments are constrained so t h a t the K~ invariant mass is that of the Do.) After sideband subtraction the MM2 d i s t r i b u t i o n appears as shown in f i g u r e ~0. To e x t r a c t branching ratios certain components are

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FI~JRE lO The sideband subtracted p/~2 spectrum. The histogram is the f i t to the various components as indicated on the figure. fixed. The r a t i o of the D=* te 9" is fixed from

i 7T06~9-009

from D*+E- events. The dot dashed curve is

Dc Even',s r~ D° Sidebond Events

I00

the previously discussed analysis of C h e w 2 fixed te the calculat~KI amount of mixing and fake leptens. The data are then f i t ~ ¢ l f o r the magnitude of the 9°~"u component (solid curve}

80

and the magnitude of the sum of D*°~'u ÷ D*+~"~ (dashed curve). The dotted curve is the D**

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60

component. The D°~'u part comes d i r e c t l y out of

+tt

F3

4O

the f i t ,

while the D * ° ~

is derived by

subtracting the previously determined D~÷ part from the f i t of the sum. One immediate result of the f i t s

is the

r a t i o of branching ratios Br (B-+O*°~'~)/mr (B-+DOC-~) = 1.6.0.8_0 . + °.S' ; The s t a t i s t i c a l error includes the correlation

20 +

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between t h s D ° and D*° rates from the f i t .

-8

-6

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MM2(GeV2) FIGURE 9 The MM2 spectrum from K'f+~ - . The data have been continuum subtracted. The data points are those f o r which the K'~ + are close to the 0e mass while the histmgram is f o r D° sidebands.

The

syst4makic error is given by varying the D*e/D*~ rate. The rate has been correct~sd by the fraction of leptons expected above 1.4 QeV/¢ in each mode by usins the ICSWmodeU. To proceed further I assume that the charged and neutral B f r a c t i o n s o n the T(4S) are equal. Again~ the results are expressed in terms of ratios to the average BSL above a lepton

S. Stone/Chimed seraileptonic B meson decays

268

a factor of 1.36.) The s i m i l a r p l o t in the case

~ n c L , , of 1.4 GsV/c. The results are: Sr (8-.~)o~-;)/Br (B÷X~.--~) = 0 . 2 4 - 0 . 0 8 ÷0 " 0~ o.s

of D decay gives JD/JDO equal to about 0.6.

o.08_0

6. ANOTHERMEASURE OFT HE B°-B - LIFETIME RATIO

~r(B'*D**°t.'u)/Br(B~X~"u) = 0.16,0.10,0.03,

The semileptonic widths of B° and B- into

and the sum = 0.79,0.15-0.11 The amount missing 0.21~0.19 is consistsnt

~heir respective DR channels are expected to be equal. Since BsL--rSL/rto t and Fro t is inversely proportional to the l i f e t i m e j we have t h a t Br(~%D*÷~-~)/Br(B%D*O~-~) = ~o/~-. We can

with zero. In the model of Wirbel and Bauer 18 (WB) there is a free parameter which they call JB/JBo. This paramoter represents t h e i r i n a b i l i t y to calculate some of the c o e f f i c i e n t s

compare the measured values of the two ratios

of the form factors. In f i g u r e 11 the CLEO

• D*°L'u)/Br(B~X~-~) = 0.39*0.08 to obtain ~o/~- = 1.18,0.27÷~ .22

polarization moasure~ent and vector to

8r(~°~D~÷L'~)/Br(B4X~-u)--0.46*O.05 and Br(B-

-

.30"

The systematic e r r o r results from varying

pseudoscalar r a t i o in semi leptonic decay are compared with the ~B predictions for p~. > 1.~

the D~ component in the f i t s

¢~eV/c and r e s t r i c t 0.62 (JB/JBo( 1.08. (In t h i s

inaccuracy in the D~÷ t o r s o ° branching r a t i o .

~ntum

This r e s u l t represents the most accurate

region, the V/P ra~io is scaled up by 1770689-

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determination by CLEO of t h i s quantity. I t is

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~o is less than or equal to ~-, t h i s would push

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~ ~ -~ IUpper limil oni

7. CONCLUSIONS The average B semileptonic branching r a t i o

~°°°

..... -"

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!s lower than theoretical expectations and

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c e r t a i n l y in the range from 9.5-11~. Asid8 from

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Upper h'mi~on G (CLEO} ! 0.0 -J ~ I , 0.50

0.70

t h i s , t h e o r e t i c a l models seem to be able to

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match the semileptonic B decay data.

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Results on exclusive semileptonic decay

, 1.30

JB/a~

rates are becoming available from CLEO and A R m . The data much of i t new and preliminary indicate t h a t ~o and B" have s i m i l a r decay

FIGURE 11 The curves lab;ed R and e refer to the

rates in contrast with O° and D÷. The l i f e t i m e r a t i o of charged and neutral

predictions of the ~ m o d e l as func~ioL of the

B's, ~ o / ~ -

free parameter JB/JBO f o r p~ ) 1.4 GeVjc. The range is limited on the low side by th~ CLEO

exclusive branching ratios Br(B°*D*÷L'~) and

.~asurement of a and on the high side by the CLEO measurement of R=F(B%D*°~"u)/r (B%D°~'~). The 90~confidence level upper l i m i t s on • and R were used.

is measured by CLEO as

1.18"0.27~ "22 by using measurements of the .30 Br(B%D~°~'u). I t

is apparent t h a t the B° and

B- do not have the large l i f e t i m e differences that the D° and D÷ have. The Br(B÷D*÷~'~) is measured as 0.46*0.05*0.07 f o r an average B

S. Stone/Charmed semileptonic B meson decays semileptonic branching r a t i o of 108 the to~al semileptonic, while the D**÷ID *+ r a t i o is found 1;o be about 38£. Here D** can include c o n t r i b u t i o n s from non-resonant D*7 combinations. The r a t i o of vector to pseudoscaler, D*°E'~/D°E-v = 1 . 6 ~ 0 . 8 _ 0+0 ~ 5 7 , is consistent with model expectations. The D*~ p o l a r i z a t i o n coupled with the above measured r a t i o places r e s t r i c t i o n s on the models. With ARGUS taking data and CLEO I I about to

2Gg

12. K. Wachs e~ =1., Z. Phys. C4___22(19e9) 33. 13. M. S. Alam et a l . , Phys. Rev. Left. 599

(lo87) 2 2

14. C. Babek e t a | . , 1289.

Phys. Rev. D36 (1987)

15. Q.C. Fox and S. Wo|frama Physo Rev. ~ t . 41, (1978) 15811 we use R2==H2/~. 16. H. Albrecht et a l . , Phys. Le~t. 197B (1987) 452; ibid ~ l g ( 1 ~ 9 ) 121. =='-

s t a r t up, we can expect s i g n i f i c a n t improvements in these measurements in the next

17. S. Behrends et s | . , Phys. Bey. Le~t. 59 ( t g u ) 407.

few years.

18. M. Bauer and M. Wlrbeio "Form F = c ~

Effects in Exclusive D arvJ 5 Decays", HDTHEP 88-22 (1988).

REFERE~ICES 1. For K-e+v there are two measurements: E691J. C. Anjos et a l . , Phys. Rev. L e t t . _~X(1989) 15371 Mark I I 1 J. Adler et a l . , s~ Rev. Lett. 62 (1989) 1821. For KV°e~vE891J. C."Anjos e~ a l . , Phys. Rev. Le~t. 82 (1939) 722. 2. B. Gittelman and S. Stone, "B Meson Decay", in High Energy_Electron-Positron ,.Ed. by A. Ali and P. Soding, l e n t i f i c ~ Singapore (1%80) 275. 3. J. P. L e v e i l l e , "B-Decays" Univ. of Michigan preprintUMHE81-18 (1981). 4. Private communication from I . 1. B i l l . 6. P a r t i c l e Data Group, G. P. Yost et a l . , Phys. Let. 204B (1933) 1. 6. O. ~ l t a r e l l i (1982) 365.

e~ a i . , Nucl. Ph$s. B208

7. N. Zsgur et a l . , Phys. Rev. D3...~9(1989) 799. 8. W. Wirbel eb a l . , Z. Phys. C2..99(1985) 637. g. J. Q. K o r ~ r and Go A. Schuler Z. Phys. C38 (1988) a n . Lo. R. V. Kowalewski, "Semileptonic B Meson DecayS, Ph.D. Thesis Cornell Univ. unpublishedUMZ 88-21246-m (microfiche) (1988). 1~. H. Albrecht et a l . , paper 0811B =ubmitted to Munich Conf. (1988).