kI.SKVIKR

UCLEAR PHYSICS

Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392

PROCEEDINGS SUPPLEMENTS

R e c e n t C L E O B R e s u l t s

William Ross ~

~Department of Physics and Astronomy University of Oklahoma Norman, OK, 73019

Using a large data sample accumulated at the T(4S) resonance with the CLEO II detector at the Cornell e+e - storage ring CESR, we have fully reconstructed decays of both/~0 and B - mesons into final states containing either D**, D*, D, ¢ or ¢ ' mesons. We present recent measurements of many hadronic and semileptonic branching fractions. Using these results and predictions from Heavy Quark Effective Theory, we perform tests of the tkctorization hypothesis and extract the CKM matrix element V~b. We also present results on a search for color suppressed decays of B mesons.

1. I n t r o d u c t i o n

We present recent results of several analyses using either all or a subset of the 2.0 fb -1 of data taken at the T(4S) resonance. These in- clude measurements of various two-body hadronic branching fractions, allowing tests of Heavy Quark Effective Theory and of the factorization hypothesis. We also present several semileptonic branching fractions, which permit the extraction of the CKM matrix element Vcb.

2. C E S R a n d t h e C L E O I I D e t e c t o r

The data used for all the analyses reviewed in this paper were obtained using the CLEO II detector at the Cornell Electron Storage Ring (CESR), a symmetric e+e - collider operating on the T(4S) resonance at a center-of-mass energy of 10.58 GeV. The T(4S) is the lowest lying bb state able to hadronize to BB mesons. At present, we have taken 2.0 fb -1 of data on-resonance. In order to study events from the continuum un- der the T(4S) peak, data is taken 60 MeV be- low resonance. The maximum CESR luminosity has recently been recorded as Z;in~ = 2.9 x 1032 cm-2/sec.

The CLEO II apparatus [1] is a general pur- pose 4~r detector, designed to detect both charged and neutral particles with excellent resolution. Charged particle tracking is achieved by with

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three concentric tracking volumes, yielding a combined momentum resolution of

(@/p)2 = (0.0015p)2 + (0.005)2 (1)

in a 1.5 T field. Neutrals are detected with a 7800 CsI(T1) crystal electromagnetic calorimeter, which provides an energy resolution of

~E/E = 0.35/E °75 + 1.9 - 0.1E EinGeV (2)

Particle identification is provided by time-of- flight array, having a time resolution of ~ ,~ 160 ps, and by means of dE/dx in the tracking vol- umes. Muons are identified using iron-drift tube muon chambers located on the outside of the su- perconducting coil.

3. H a d r o n i c B D e c a y s

Bottom decays proceed primarily by the exter- nal W-emission, or spectator, process. In this in- stance, the charm quark hadronizes with the spec- tator quark to form either a D, D* or D** meson, with the W decaying to a 5d pair, leading to final states of the form D(n)Tr. The B may also de- cay via the color-suppressed internal W-emission process. In this case, with the W decaying into a cs pair, the daughter quark hadronize into vari- ous charmonia, leading to final states containing either a ~, ~ or Xcl and a strange meson. Feyn- man diagrams for these processes are shown in Figure 1

382 VY Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381 392

D * - D ° mass difference be within 2.5 cr of our mea- sured value.

~b-rnesons are reconstructed by searching for pairs of identified leptons (e+e - or /~+/~-). We require tha t the reconstructed invariant mass sat- i s fy -150 < m ( e + e - ) --m(~b) < 45 MeV/c 2 or [m(#+l~ - ) - r n ( , b ) l < 45 MeV/c 2. ~b' mesons are selected by reconstruct ing e+e - , >+I t - or rr+Tr-¢ combinations.

d b c -.. w .. w . . . . <

b o/" c

(a) (b)

Figure 1. B meson decay diagrams: (a) External W-emission (spectator) , (b) Internal W-emission (color-suppressed).

We present a search for b --+ c decays into vari- ous exclusive final s tates based on a da ta sample of 0.923 f b -1

3.1. S e l e c t i o n c r i t e r i a a n d r e c o n s t r u c t i o n In order to be considered as a possible B can-

didate, we demand than an event pass s tandard C L E O analysis cuts for a hadronic event. Photon candidates are selected f rom showers in the barrel region of our CsI calorimeter, passing m in imum energy and topo logy requirements, re ° candidates are reconstructed f rom photon pairs whose invari- ant mass is within 2.5 cr of the known neutral pion mass. r/-mesons are reconstructed using the 77 mode, where the two photons are kinematical ly constrained to the known 7/ mass. Candidate ~' mesons are obta ined th rough the r/rr-rr + and 77 modes. Candida te co mesons are reconstructed in the rr+rr-rr ° mode.

We reconstruct D-mesons by requiring tha t the measured invariant mass of a candidate be within 2.5 a of the accepted value, using the modes D o --~ K - r r + , D ° ~ K-r r+ r r °, D o -+ K- r r+ r r -~ r + and D + -+ K-Tr+rr +.

We find D* candidates th rough the decays D *+ ---, D°rr + and D *° --+ D°Tr °, using recon- s t ructed D o candidates and demanding tha t the

3.2. S e a r c h f o r B --+ D, D*, D**(n)rr d e c a y s Having found charmed meson candidates, we

combine these with other hadrons to form B can- didates. All B-meson candidates are required to have a measured energy within 2.5 cr of the beam energy ( A E ) and satisfy I cos(0b~ . . . . . diaat~)] < 0.95. In order to suppress con t inuum back- ground, we also demand Icos(0,ph~ric,y) I < 0.8 (0.7) for decays involving n=1 ,2 (n=2,3) pions. Furthermore, the reconstructed mass of s t rong resonances, such as the p, is required to fall within one full width of the known value.

To measure all signal yields, we calculate the beam constrained mass, defined as

MB = ~/E2~a,,~ - ( E p ~ ) 2 (3)

The resolution of this quant i ty is about 2.7 MeV/c 2, roughly an order of magn i tude less than the resolution in invariant mass.

We first consider final s tates containing a D meson and one or more pions. The result- ing branching fractions are given in Table 1. We use the CLEO absolute branching [2] ra- tios B ( D ° --+ K - r r +) = 3.91 4-0 .08 4 -0 .17% and B(D + --+ K- r r+ r r +) = 9.1 + 1.4% and the Particle Da ta Group values [3] for the ra- tios B ( D ° ---+ K- r r+rr° ) /13 (D ° ----+ K - r r +) and B ( D ° --+ K - r r + r r - r r + ) / B ( D ° --+ K - r r + ) . The first sys temat ic error quoted in the branching ra- tio results includes contr ibutions f rom the back- ground shape(--~ 5%), Monte Carlo Statist ics (2-4 %) and uncertainties in tracking and rr ° detection efliciencies. The second sys temat ic error is due to errors on the D branching ratios.

We now consider final states containing a D* and one, two or three pions. Beam-const ra ined mass distr ibutions for the channels B - --, D * ° r r - , B ° --+ D * - r r + , B - -+ D*°p - and B ° --+

W. Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381 392 383

> rD

2~ cN

C {D >

l~J

. . . . I . . . . 40 . . . . I . . . .

(o) (b)

20

2O

o 5 ~ 0 5 ~ 5 5 ~ 0 5 ~ 0 5 ~ 5 5 ~ 0

32 . . . . [ . . . . ~ . . . . [ . . . .

(c)

16 |6

o 5 ~ 0 5 ~ 5 5 ~ 0 5 ~ 0 5 ~ 5 5 ~ 0

M, (GeV)

Figure 2. The beam constrained mass distr ibutions for: (a) B - --+ D*%r-, (b) B - ---+ D * ° p - , (c) /3 0 --+ D*-~r + , and (d) B ° --+ D * - p + .

D * - p + are shown in Figure 2. The signal is fit with a Gaussian having a fixed c~ of 2.7 MeV/c 2. The background is parameter ized by a straight line with a parabol ic kinematical cutoff. This shape is shown to accurately model the back- ground f rom A E sideband studies. Branching ratio results are shown in Table 1. We use the following C L E O D* branching ratios [4] in our calculation

B(D *+ -+ D%r +) = 68.1 :k 1.6%

B ( D *° -+ D%r °) = 63.6 -4- 4.0%

In addit ion, we have studied the possibility of D** product ion in B decays. The two states D**(2460) ( J P = 2 + ) and D**(2420) ( J P = 1 + ) have been searched for in the modes B - --+ D*+rr rr and B - -+ D*+rr-rr-Tr °, where the we required the D*+rr - invariant mass to fall within one full width of the nominal mass of ei- ther excited state. We have also searched for the D**(2460) resonance in the channels /~0 __+ D°rr-Tr + and B - + D+Tr-rr - . Upper limits for these processes are shown in Table 2.

3.3. S e a r c h f o r B --+ ec d e c a y s If the B meson decays by internal W-emission,

the resultant final state mesons consist of a char-

mon ium particle and either a s trange meson (if the W decays to a 6s pair) or a light unfla- vored meson (via the Cabibbo- suppressed de- cay W --+ ~d). We have searched for decays of the former type, where the B decays to the fi- nal states ~b(/)K - , ¢(,)1(0, ~b(,)t(,0, and ~b(')K *- . In addition, we present results for final s tates of the form X~IK(*). Beam-const ra ined mass dis- t r ibut ions for the modes B --* CK(*) are shown in Figure 3. The fitt ing a lgor i thm is analogous to tha t described previously. Branching fract ions and upper limits for all B --+ C h a r m o n i u m modes are listed in Table 3.

3.4. T e s t s o f t h e F a c t o r i z a t i o n H y p o t h e s i s The Factorizat ion hypothesis is the basis of

most theoretical t rea tments of hadronic B decays. It postulates that , for two-body hadronic decays proceeding via external W emission, in analogy to semileptonic B decays, the ampl i tude of the process can be expressed as the produc t of two in- dependent hadronic currents, one describing the format ion of a charmed meson f rom b o t t o m de- cay, and the other representing the hadroniza t ion of the decay products of the virtual W

A = Cr/vSV bV:d(D+h-I(-eb)( u)l, o) (4)

384 W. Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392

Table 1 Branching Ra t ios (%) for B -+ D(* ) (nr r )

B Mode B(%) B - --+ L ) ° 7 r -

B - --+ D ° p - B o -+ D - r r +

B ° -.-+ D - p +

t~o --+ D%r-Tr +

B - .-+ D+rr rr

B - ---+ D*° rr -

B - --~ D*°p - B o _..+ D*-Tr +

B o ___+ D * - p +

B - --+ D*° rr+ Tr 7r

B ° --+ D*-rr+r~ rr

0.55:t:0.04-t-0.05-t-0.02 1.35+0.124-0.144-0.04 0.294-0.044-0.034-0.05 0.81:t=0.114-0.12+0.13

< 0.16 < 0.14

0.524-0.07-t-0.06-4-0.04 1.684-0.214-0.254-0.12 0.264-0.034-0.044-0.01 0.744-0.104-0.144-0.03

0.94 4- 0.20 4- 0.16 4- 0.06 0.63 4- 0.10 -4- 0.11 4- 0.02

Table 2 Branching Ratios (%) for B ~ D**(nrr)

B Mode B - --+ O**°(2420)rr -

B - ---.+ D**°(2460)rr - {D**°(2460) ~ D*+rr-)

t3- ---+ 0"*°(2460)rr - (D**°(2460) --+ D+rr - )

BO --+ D**+(2460)rr - (D**+(2460) -+ D°rr +)

B - --+ D**°(2420)p - B - --+ D**°(2460)p -

B(%) 0.11 -4- 0.05 4- 0.02 -4- 0.01

< 0.28

< 0.13

< 0.22

< 0.14 < 0.5

= G F / v / 2 V c b V * d ( D + ] ( ~ b ) l t 3 ° ) ( h - l ( - d u ) ] O } (5)

If Fac to r i z a t i on is val id, decay ra tes for hadron ic processes can be re la ted to the cor- r e spond ing semi lep ton ic decays in the fol lowing m a n n e r [5]:

R~xp

r(B 0 =,6 21V dl2fglcll dr l B D*l~)]q~=,~

/~t heo

(6)

where q2 is the f o u r - m o m e n t u m t ransfer f rom the B to t he D*, V~a arid f~ are the well- measu red C K M m a t r i x e lement and pion decay cons tan t . T h e c~ t e rm accounts for ha rd gluon

corrections. Thus Fac to r i za t ion can be tes ted ex- p e r i m e n t a l l y by de t e rmin ing if the above re la t ion holds. A compar i son of Reap and Rth¢o is shown in Table 4 for var ious values of q2, where we have

used c 1 : 1.12 i 0.1 [6] and IVy, a] = 0.975. As can be seen f rom these results , Fac to r i za t i on

seems to hold up to the al mass scale. Another , more subt le tes t of Fa c to r i z a t i on can

be pe r fo rmed by s tudy ing the po l a r i za t i ons i n / 3 decays to two vector mesons. For B ---+ V V de- cays, the ra t io of l ong i tud ina l to t ransverse po la r - iza t ions between hadron ic and the (cor respond- ing) semi lep ton ic decays should be equal [9], eval- ua ted at a p p r o p r i a t e q2

W. Ross~Nuclear Physics B (Proc. Suppl.) 39t?, C (1995) 381-392 385

Table 3 Branching Rat ios for B -+ c6 Decays

B Mode ~(%) B - --+ ~bI(- 0.110 4- 0.015 4- 0.009 B ° -+ ~bI( ° 0.075 4- 0.024 4- 0.008 B ° --+ ¢ I ( *° 0.169 4- 0.031 4- 0.018 B - ---* CK*- 0.178 4- 0.051 =t= 0.023

B - ---+ ~b'/(- 0.061 -}- 0.023 =t= 0.009 B ° --+ ~O'K ° < 0.08 B ° --+ ~b'K *° < 0.19

B - ~ ~b'K*- < 0.30

B - ---* X< 1( - 0.097 4- 0.040 J: 0.009 B° --~ X¢1 K ° < 0.27 1913 ---+ Xal I( *0 < 0.21

B-- --+ Xcl l ( * - < 0.21

Table 4

Compar i son of [~e~p and t~theo

B ° Mode f (MeV) R~.~p l~theo D*+~r - f~ = 131.7 -t- 0.2 1.1 + 0.1 4- 0.2 1.2 4- 0.2 D*+p - fp = 215 4- 4[7] 3.0 4- 0.4 -t- 0.6 3.3 4- 0.5

D*+al(1260) fal = 205 4- 1618] 4.0 -4- 0.6 4- 0.5 3.0 4- 0.5

PL(Bo --+ D._p+ ) = PL (B ---+ D*lE/)[q~=.~ (7) Pr P7

At q~ = rap2 = 0.60 GeV/c ~, the Factor izat ion predict ion [10] is F L / F = 88%. This agrees well with the exper imenta l measurement of VL/F = 93 -{- 5 4- 5% for B ° --+ D * - p+ decays.

3.5. T e s t s o f H Q E T S p i n S y m m e t r y In the infini te quark mass l imit (rob, mc --+ oo),

Heavy Quark Effective Theory (HQET) predicts that , to lowest order, the ampl i tude for a process no longer depends on the heavy quark magnet ic m o m e n t [11]. Therefore, assuming the val idi ty of the Factor iza t ion hypothesis, certain two-body hadronic B modes can be used to test H Q E T spin symmet ry . Specifically, H Q E T spin symme- t ry implies t ha t

r ( B ° ~ D - F +) = F(B ° --+ D * - ~ + ) (8)

P(B ° --, D - p + ) = V(B ° --, D * - p +) (9)

After Corrections for non- inf in i te quark masses, one expects [12] B ( B ° --+ D - r r +) = 1.03 B ( B ° ~ D*-:¢ +) and B ( B ° ~ D - p +) = 0.89 B ( B ° --, D*-p+) . Blok & Shifman [13] predict B ( B ° -+ D - r r +) = 1.2 B ( B ° ---+ D*-rr +) using a QCD sum rule approach, the difference be ing due to non-factor izable contr ibut ions . From our

data, we measure

B ( B ° - - + D - r + ) = 1.12-1-0.194-0.24 B ( B ° --* D*-~r+)

B(BO ~ D - P + ) = 1 .10- t -0 .144-0 .28 13( B o --+ D*- p+ )

As can be seen, both ratios suppor t the H Q E T prediction, as well as tha t of Blok & Shifman.

3.6. S e a r c h fo r C o l o r - S u p p r e s s e d d e c a y s

We present a search for various color- suppressed decays which occur via in terna l W- emission, with the W decaying to a ~d pair. Due to the need for a color match between the quarks produced from W decay and the c and specta tor

386 W. Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392

4 0

2O

o 5.20

> 10 , ,

(c)

5

o ~, 5 . 2 0

Figure 3.

(a)

5 . 2 5

10

O)

0 , i , i i 5 . 3 0 5 . 2 0 5 . 2 5 5 . 3 0

i i

5 . 2 5 5 . 3 0 5 . 2 0 M B ( G e V )

(d)

i

5 . 2 5 5 . 3 0

The beam-const ra ined mass distributions for: (a) B - --+ e K - , (b) B ° --+ ~bK °,, (c) B - e K * - , and (d) B ° -+ e h ".0.

quarks, one expects, in the absence of gluons, a suppression factor of ~ 1 for decays involving 7r0, p0 and w mesons. In real heavy quark decay's, the effects of gluons cannot be neglected. QCD based calculations [14] predict a suppression fac- tor on the order of ±

5 0 "

The relevant modes, along with upper limits on branching ratios and comparisons with various theoretical models, are presented in Table 3.6.

3.7. D e t e r m i n a t i o n o f a 1 a n d a2 In the BSW model [15], the emission of sob glu-

ons is accounted for phenomenological ly by the inclusion of two undetermined factors a l (p ) and a2(#), such tha t the effective t tamil tonian for bot- torn decay reduces to

GF He f] :. ~-~Vcb{al(~)[(cIu) 4- (gc)](cb) (10)

+a2(tt)[(cu)(db) 4- (~c)(sb)]}

in the 1/N~ ~ 0 limit. The term proport ional to al describes decays proceeding via external W- emission (such as B ---* D r ) , while the term mul- t iplying a2 describes (color-suppressed) modes which occur by internal W emission (B ~ e K ) . From our results, we obtain [16]

lal l = 1.15 4- 0.04 4- 0.05 4- 0.09

[a2] = 0.26 4- 0.01 4- 0.01 + 0.02

Modes where the first and second terms inter- fere provide informat ion as to the sign of a2/al . The BSW model [15] predicts

R1 = B ( B - --+ D % r - ) = (1 + 1.2302) 2 (11) B ( B ° ---+ D-rr+ ) al

B(B- --+ D°p - ) = (1 + 0 .66a2) 2 (12) R2 ~- B(Bo --+ D_p+ ) al

R3 = B ( B - --~ D*°rr - ) = (1 + 1.29a2) 2 (13) B(B ° --+ D*- r r+) al

B ( B - --+ D*°p - ) = (1 + 0 .75au) 2 (14) R4 = 13(B o -~ D * - p + ) a I

Table 5 shows a compamson between our ex- perimental results and one of the two allowed so- lutions in the BSW model, as well as predictions f rom the Reader-Isgur model[8].

4. S e m i l e p t o n i c B d e c a y s

We present new measurements of the B semileptonic branching fractions, as well as the ratio of charged to neutral B lifetimes.

W. Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392 387

B Mode B(%) B ° --+ D°Tr ° < 0.048 17 ° --+ D°p ° < 0.055 /~0 .._+ D0r/ < 0.068 /~0 _+ D0r/, < 0.086 /~0 __+ D0¢o < 0.063

/~0 __+ D*07r0 < 0.097 13° -+ D*°p ° < 0.117 B ° --+ D*°r/ < 0.069 t7 0 ---+ D*°71 ' < 0.27 17 0 --+ D*°w < 0.21

] 'able 5 Ratios of branching fractions f rom CLEO data to determine sign of a u / a l

Ratio a2/al = 0.24 CLEO II [17] Reader-Isgur model

/{1 1.68 1.89 + 0.26 -+- 0.32 1.20-1.28 R2 1.34 1.67 4- 0.27 4- 0.30 1.09-1.12 R3 1.72 2.00 4- 0.37 4- 0.28 1.19-1.27 R4 1.85 2.27 A- 0.41 4- 0.41 1.10-1.36

4.1. T h e B s e m i l e p t o n i c b r a n c h i n g f r ac - t i o n s a n d l i f e t i m e r a t i o

In contrast to hadronic decays, bo t t om semileptonic decays proceed uniquely via exter- nal l/V-emission, with the b quark decaying to ei- ther a c or u quark, and the the W - decaying to a g - ~ pair. In this picture, the isospin partners B - and /~0 have equal semileptonic widths. In analogy, hadronic decays proceeding via the spec- ta tor process should have equal hadronic widths. In this case, however, nonspec ta tor effects break isospin symmet ry , leading to a deviation f rom uui ty of the rat io of lifetimes on the order of 10- 15% [18]. Assuming equal semileptonic widths, this is equivalent to unequal semileptonic branch- ing fractions:

p,o,(g0) r,o,(B0) L, r ( B °) - F,o~(B-) Psi - ' P , o , ( B - ) - (15)

B(B- ~ X g - u ) _ b+

B( t7 ° --+ X g - v ) - bo

Previous measurements of the B semileptonic branching fract ions have been sensitive to uncer- tainties in the product ion ratio of charged to neu- tral B mesons (it is assumed tha t f + / f o = 1). Moreover, the average B senlileptonic branching

fraction [19] from T(4S) decay is significantly lower than than theoretical predictions. Possible causes include n o n - B B decays of the T(4S) , as well as unknown hadronic modes. The problems associated with these uncertainties are reduced when tagged measurements are performed, as the number of B/ ) pairs is then directly counted. Here, tagging refers to reconstruct ion of exclusive modes.

These measurements were carried out on a da ta sample of 1.35 f b -1 of on-resonance data. B de- cays were reconstructed using three methods:

• Full reconstruct ion of various hadronic modes

• Part ial semileptonic reconstruct ion

• Part ial hadronic reconstruct ion

For full hadronic reconstruction, B mesons were obtained using the following channels: D~r-, D*rr- , D p - , D ' p - , D a l , D ' a T , , b K and ~bK*. All events must have a topology consistent with tha t of a hadronic event. Note tha t this is the only method of measuring b+. Part ial recon- st, ruction of/30 __+ D . + g - ~ decays is achieved by searching for the slow pion from D* decay and an

388 Ig Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381 392

appropriate lepton. For partial hadronic recon- struction, we use the mode ~0 __+ D*+Tr-, D*+ __+ D%r +. This decay chain produces a hard rr- and a soft rr +, which are used to identify this mode without explicitly detecting the D o . For all three types of tag, we search for a high momen tum lep- ton consistent with being a pr imary decay prod- uct from the other B meson in the event. The pr imary lepton m om en t um is required to satisfy 1.4 < IPel < 2.4 GeV/c. In order to extrapolate to pe = O, we use the model of Isgur et al. [20]. From our Monte Carlo simulation, we find that the fraction of the spectrum above 1.4 GeV/c is 48.1% (51.4%) for electrons (muons). The results are:

13(B- --+ X t - u ) = 10.1 + 1.8 4. 1.4%

B ( B ° -+ Xg+L ,) = 10.9-t-0.74. 1.1%

Assuming equal semileptonic widths, this yields a ratio of lifetimes of

b+ _ 0.93 4- 0.18 4- 0.12 b0

The B semileptonic branching fraction can also be obtained, albeit in a more model dependent fashion, from the single lepton momen tum spec- t rum, if one assumes equality of the B lifetimes. By fitting the shape of the spectrum to various theoretical descriptions, we have extracted both a value for B(B ~ X~u), as well as the CKM matr ix element V~b, based on 0.9a5 f b -1 of on- resonance data.

Leptons are required to be stringently identi- fied, and satisfy 0.5 < [pc[ < 3.5 GeV/c and 1.3 < [p,[ < 3.5 GeV/c. Major sources of back- ground come from secondary leptons, as well as leptons produced from @') decay, ~r ° Dalitz de- cays, B -~ m , X , r -+ g Y decays, and photon conversions. Once all extraneous leptons are re- moved, the spectra are fit with shapes predicted by the inclusive model of Altarelli et al. (AC- CMM) [21], as well as those obtained from the ISGW model[20].

The fit with the ACCMM model has five un- determined parameters: B(b -+ c g - ~ ) , B(b --+

u£-~) , B(D --~ ~ -~ x g - p ) , rue and PF (the

mass and Fermi momen tum of the final state charmed quark, respectively). These were found to be 10.48 + 0.07%, 0.17 4. 0.04%, 9.58 -t- 0.25%, 1700 MeV/c 2 and 230 MeV/c respectively for a combined fit to both the electron and muon mo- mentum spectra. Using this model, the over- all semileptonic branching fraction was measured to be B(B --~ X £ - ~ ) = 10.65 4- 0.05 -t- 0.33%, where the systematic error quote contains contri- butions due to uncertainties in particle identifica- tion, tracking and ~r ° detection, as well as errors arising from subtraction of leptons from the con- t inuum and from ¢(') decay.

A fit using the predictions of the ISGW model is simpler, as there are no free parameters to de- termine. The shapes and relative proportions of B --+ Dg-£, , B --+ D * g - ~ and B ---* D**g-£, are predicted. We have made a simple modification to this model, however, by allowing the D**g-~ branching fraction to be a free parameter (the modified model is referred to as ISGW**). A fit to our data yields a relative branching fraction of 21 + 2 4. 8%, higher than the 13% of the ISGW model. Using this, we extract a B semileptonic branching fraction of B ( B --+ X g - £ ' ) = 10.98 4. 0.10 4. 0.33%. These two values of the semilep- tonic branching fraction are consistent with each other, but are still significantly below the theo- retical prediction of 12.5% [22].

Using an average B lifetime of rB = (1.49 + 0.03) x 10 -12 s [23], we obtain the value of [Vcb[ = 0.042 4- 0.001 4. 0.004 for both the ACCMM and ISGW** models.

4.2. M e a s u r e m e n t o f t h e B e l e c t r o n i c b r a n c h i n g f r a c t i o n

We present a measurement of the B electronic branching fraction using lepton tags, based on 2.07 f b -1 of on-resonance data, In this instance, a decaying B meson is identified by the presence of a hard lepton (l .4 < IPel). We then search for a pr imary electron in the rest of the event, requir- ing that 0.6 < IPel. Events were required to have topologies consistent with hadronic decays. All leptons were required to satisfy stringent particle identification criteria.

Since leptons from the two B-mesons in an event should be uncorrelated, we eliminate elec-

lg Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392 3 8 9

t.rons and tag leptons from the same B by im- posing kinematical constraints on the eg pair. Furthermore, we obtain good separation between primary and secondary electrons by demanding the correct relative sign between the tag lepton and the electron (unlike-sign for primary elec- trons, like-sign for secondaries). Background contributions to the electron spectrum are the same as those described in the previous section. The mesured electron momentum spectrum after background subtraction is shown in Figure 4. In order to extrapolate to zero electron momentum, an average of the ACCMM and ISGW models is used. From our Monte Carlo simulation, we find that the fraction of electrons from B decay having IP~I < 0.6 GeV/c to be 5.8-t-0.5%. We measure the electronic branching fraction to be B(B ---+ Xef,) = 10.56 4- 0.17 4- 0.35%. The con- tributions to the systematic error are the same as those discussed in the last section. By integrating <he Pe spectrum, we obtain an upper limit of 0.04 at the 95% CL on the non-B J9 fraction of T(4S) decays.

0 . 1 5

> 0.10

.e 0.05

0.00

' ' ' ' I ' ' ' ' r ' ' ' '

./.",~

1 2 p~ (OeWc)

Figure 4. Momentum spectrum of primary elec- trons (solid circles), and that of secondary (open circles). Solid lines are fits to the Altarelli model.

The relation between the electronic branching fraction and the CKM matrix elements G,b and V~b is straightforward, but model-dependent. The semileptonie width of the B meson is given by

where the 7i must, be obtained from theory. Using our 13(t? ---* Xeu) result and the average charged and neutral B lifetime [24], we measure IGbl = 0.042 + 0.002 + 0.002.

4.3. D e t e r m i n a t i o n o f B(B ~ D*gu) and V~b f r o m H Q E T

In order to measure the exclusive branching fractions B(/~ ° --+ D*+g.P) and 13(B- ----+ D*°gf]), we use the the relatively low-background decay chains D *+ ~ D%r + and D *° --+ D°rr °, where D O --+ K-~r +. To insure a pure sample of D* mesons, we demand that the D * ° ( D * + ) - D ° mass difference be within 3(4) MeV of the accepted value. Furthermore, the momentum of the D* must be consistent with that expected from B decay.

Leptons are identified, and required to have 0.8 < [p~[ < 2.4 GeV/c. We can then calculate the invariant mass of the neutrino:

= , ( z B - zD. ) 2 - 2 - Ivs. L2 + ( 1 7 ) Y

M M ~

l rr25.+!cos(ot C

Setting P~ = 0 defines a kinematically allowed M M ~ - C region. A source of background in this region are B ~ D**gu decays, where D** refers to any charmed state decaying to D*Tr (cor- related D* - g pairs). Random KTr and slow 7r combinations provide the main source of combi- natoric background. There are also random (un- correlated) D* - g combinations which pass all kinematical cuts. In order to obtain a signal yield after these backgrounds have been subtracted, we fit the D o mass peak with a Gaussian of fixed width for the signal. The background is fit with a straight line. Using the CLEO II results for the D* branching fractions [25]

B(D *+ ~ D%r +) = 68.1 4- 1.0 -t- 1.3%

B(D *° ~ D%r °) = 63.6 ~: 2.3 4- 3.3%

as well as the CLEO II result [25] for the D O branching fraction:

B(D ° --* K-Tr +) = 3.9 4- 0.08 4- 0.07 4- 0.16%

390 W Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381 392

we measure, assuming equal charge and neutral B product ion fractions (f0 = f+) ,

B( f i 0 --+ D*+g - ~) = 4.49 4- 0.32 + 0.39%

B ( B - --+ D * ° g - u ) = 5 . 1 3 + 0 . 5 4 4 - 0 . 6 4 %

where the sys temat ic error quote contains con- t r ibut ions f rom the uncer ta in ty on the D O and D* branching fractions. If we do not assume equal product ion fractions, but instead F ( B - --+ D*°g-u) = F ( B ° -+ D*+g-v) and f0 + f + = 1.0, we measure:

F(B --+ D ' e - u ) = (2.99+0.19+0.274-0.20)x 10*°s -*

which is independent of f+/ fo . The decay" ampl i tude of /) --~ D ' g > is com-

mon ly expressed in terms of three form factors: V(q2), A,(q2) , A2(q 2) [26]:

(D*+(p ', e)ley"(1 - 7 5 ) b [ / ~ ° ( p ) } = (18) 2iem'~ * I T r [ 2X

%P~PZviq ) + rUB + roD*

(roB + rnD*)e*VAl(q 2) +

e* .p (p + V,)UA2(q2) m B + f r l D *

In the H Q E T limit of infinitely heavy quark masses, all three form factors become expressible in terms of a single form factor ~(y) (known as the Isgur-Wise Function). Here, y = v .v ' , is the inner p roduc t of the four-velocities of the heavy quarks involved in the decay. The variable y is related 1o the more conventional variable q2 (mass of the vir tual W) by

Y = ru~ + m 2. - q2 (19) 2reBinD*

At the y = 1 point, corresponding to a maxi- m u m in q2, the Isgur-Wise funct ion is normalized to unity:

e(1) = 1

and the relation between the form factors simpli- ties to the following

q ~ , 2 ] - ' ( 2 0 ) V(q2) = A2(q2) = A1(q2)[1 (rnB ~- D*)

Given tha t H Q E T gives the relationships be- tween # and A1, A2 and V only to leading order, we absorb these, as well as the corrections for non-infinite quark masses in the the funct ion 5 c. The decay rate is given by [27]

dr_ @ m .(mB--mv.)=lV bl2.r (Y) x (21) dy 48rr a

- l [4y(y + 1)1 - 2yr + r 2 (1 - r) 2 + (y + 1)2]

We can re-express dF/dy in term of physically measurable quantit ies in the following way':

d r = ~(y)iV~b 120~=(y) (22) dy

where all the known quantit ies are folded into ~. In the labora tory frame, y = ED* ~roD* in the B rest frame. Since the produc t [V~blbv(~) is to be determined from the data, we expand .P about y = l

5r(y) = 5r(1)[1 - a2(u - 1)] (23)

After taking into account smearing due to de- tector resolution, a m a x i m u m likelihood fit to the da ta yields

IV~bl:r(1) = 0.0351 + 0.0019 -4- 0.0018 + 0.0008

a 2 = 0.84 4- 0.12 + 0.08

Using 5 r = 0.97 + 0.04 [27], we measure

]V~bl = 0.0362 + 0.0019 -4- 0.0020 -4- 0.0014

4.4. M e a s u r e m e n t o f IVtd/Vt~] The observation of the radiat ive penguin pro-

cess B -+ K*(892)7 has led to a search for the rarer flavor changing neutral current B decays B - --+ P-V, B° --'+ w7 and B ° ~ p° 7. One ex- pects, f rom isospin symmetry , tha t

B ( B - --+ p-7) = (24)

8 ( B 0 --+ K*07)

where #, the ratio of form factors for the decays, accounts for Su(3) s y m m e t r y breaking, and Q

Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392 391

represents the r a t io of phase space factors . In order to ex t r ac t a value for I V ~ / v . I, we must rely on var ious theore t i ca l p red ic t ions for bo th ( and w. The ra t io of phase space factors is given by:

( - 4 -

- - 1 . 0 4 ± 0 . 0 1 ( 2 5 )

Various p red ic t ions for ( are given in Table 6. We ob t a in the fol lowing upper l imi ts on 3 radia- t ive penguin decays, based on 1.56 f b -1 of on- resonance da t a :

B(B- ---, P - 7 ) < 1.84 x 10 -5

B(B- ~ a:7) < 1.38 x 10 - s

B(B- --+ p°7) < 3.1 × 10 -5

The upper l imi t s a t the 90% CL for IVtd/Vt~[ as a funct ion of C are shown in Table 7, where we have used a value of B(B --+ K*7) = (4.1 =~ 2.0 ± 0.8) x 10 -5 .

5. C o n c l u s i o n s

We have presented new m e a s u r e m e n t s of m a n y hadron ic B b ranch ing ra t ios . W i t h these results, we have carr ied out an extensive series of tes ts of the f ac to r i za t ion hypothes is , as well as tes ts of H Q E T spin s y m m e t r y . The fac to r iza t ion hy- pothes is is consis tent wi th the d a t a up to the al I I l a S 8 .

The B S W p a r a m e t e r s a l and a2 have been mea- sured to be

l a l l = 1.15 4- 0.04 ± 0.05 ± 0.09

la21 = 0 . 2 6 ± 0.01 -t- 0.01 ± 0.02

with a2/a 1 posi t ive . The semi lep ton ic b ranch ing f rac t ion B(B --+

X g P ) has been measu red to be

b_ = B ( B - ---, X g u ) = 10.1 4- 1.8 ± 1.4%

bo = B( t~ ° ~ X g u ) = 10.9 + 0.7 ± 1.1%

giving a charged to neu t ra l B-meson l i fe t ime ra t io of

T+/ro = 0.93 ± 0.18 ± 0.12

We have also measu red the B ~ D*gu branch ing f ract ion to be

B ( B ° ---, D * + g - ; ') = 4.49 + 0.32 + 0.39%

13(B- ~ D * ° g - u ) = 5 . 1 3 ± 0 . 5 4 - t - 0 . 6 4 %

al lowing us to ex t rac t a pa r t i a l wid th of

v(B ~ D * g - . ) = ( 2 . 9 9 ± 0 . 1 9 ± 0 . 2 7 + 0 . 2 0 ) x lO~°s -1

Using the fact tha t , f rom HQET, ~(1) = 1, we have measured

[E=bl = 0.0362 -t- 0.0019 4- 0.0020 ± 0.0014

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392 W. Ross~Nuclear Physics B (Proc. Suppl.) 39B, C (1995) 381-392

Table 6 Theoretical predictions for ratio of form factors (

Model BSW[28]

Narisoi, (QCD Sum Rules)[29] Ali (QCD Sum Rules on Light Cone)[30]

¢ 0.90 0.88 0.75

Table 7 Upper l imi t s for the ra t io of CKM m a t r i x e lements ]½d/½,,I

¢ 1.0 0.9 0.8

IV~d/V~I Upper Limit (90% CL) 0.56 0.58 0.62

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