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CLEO RESULTS ON B MESON DECAY Cleo Collaboration
B. Gittelman
To cite this version:
B. Gittelman. CLEO RESULTS ON B MESON DECAY Cleo Collaboration. Journal de Physique
Colloques, 1982, 43 (C3), pp.C3-110-C3-113. �10.1051/jphyscol:1982325�. �jpa-00221878�
JOURNAL DE PHYSIQUE
ColZoque C3, suppZe'ment au n o 12, Tome 43, de'cembre 1982 page C3- 1 10
CLEO RESULTS ON B MESON DECAY
CLEO Collaboration Presented by B. Gittelman
Department o f Physics, Cornell U n i v e r s i t y , Ithaca, NY 14853, U.S.A.
I n t r o d u c t i o n
The T(4S) s t a t e a t a mass of 10.548 G ~ v ( ' ) is the f o u r t h member o f the-T f a m i l y . I t has a measured w i d t h o f T = 20 MeV and i s thought t o decay i n t o a B and B meson.
The cross s e c t i o n a t t h e peak f o r e"e- + T(4S) i s 1.0 nb. The resonance s i t s on t o p o f a 2.5 nb continuum cross section. P r o p e r t i e s o f t h e T(4S), o r e q u i v a l e n t l y , BB mesons, a r e i n f e r r e d from data recorded a t t h e T(4S) peak a f t e r making an a p p r o p r i a t e s u b t r a c t u s i n g data from nearby energies below t h e resonance. With t h e CLEO DetectodP1, we have measured some general p r o p e r t i e s o f t h e T(45) decays: t h e branching f r a c t i o n i o e p t ~ n s ' ~ ) ; t h e i n c l u s i v e y d o f charged and n e u t r a l kaons, protons, and l a m b d a ~ Y ~ * ~ j ; t h e charged mu1 t i p 1 i c i t y i p j ; and t h e average charged energy f r a c t i o n . These r e s u l t s may be considered independent o f e x p l i c i t models o f B decay. I n t h i s note, we r e p o r t on t h e t h e o r e t i c a l i m p l i c a t i o n s o f these measure- ments. B meson decays a r e found t o be c o n s i s t e n t w i t h t h e standard SU(2) x U ( l ) model. Other models a r e r u l e d o u t by t h e data. We present an upper l i m i t on t h e amplitude f o r b + u r e l a t i v e t o b + c. A program f o r r e c o n s t r u c t i n g B mesons i s discussed. P r e l i m i n a r y r e s u l t s of t h i s program have y i e l d e d an upper l i m i t f o r t h e branching f r a c t i o n B + ( Y + anything) and t h e f i r s t d i r e c t evidence f o r B + Do.
Models o f b Quark Decay
I n t h e Standard SU(2) X U(1) Model, t h e b quark i s t h e Q = -1/3 member o f t h e t h i r d generation o f weak isodoublets. I t mixes w i t h t h e s and t h e d quark and t h e r e f o r e , can decay through t h e charged c u r r e n t i n t o a n u o r c quark. Current data support t h i s model t o t h e e x t e n t : a) One has observed l e p t o n s and t h e r e f o r e be- 1 ieves t h e B meqon deca s weakly; b) The l e p t o n branching f r a c t i o n s , Br(B + evx) =
7
~ r ( ~ + ~ v x ) = 0 . 1 2 4 + 0 . 0 2 5 ( 3
,
are approxi a e l y equal t o values one c a l c u l a t e s f o r f r e e quark decay i t h i n t h e Standard M o d e l b j ; c ) There are %1.4 kaons i n t h e f i n a l s t a t e of a B decayY4y5). T h e o r e t i c a l estimates suggest b-x should dominate and t h e s t r o n g kaon y i e l d i s c o n s i s t e n t w i t h t h i s . However, s i n c e no d i r e c t evidence has been presented f o r t h e existence o f t h e Q = 2/3 member o f t h e isodoublet, t h e t quark, a number o f a l t e r n a t i v e models have been suggested. Our data a l r e a d y p e r m i t us t o r u l e o u t most o f these. We present a b r i e f d e s c r i p t i o n o f f o u r types o f models and t h e main reason f o r t h e i r r e j e c t i o n . The reader w i l l f i n d 8,gyt-e complete discussion o f t h e reasons i n CLEO r e p o r t s submitted t o t h e conference(.
1. The b quark i s s t a b l e . This we r u l e o u t immediately, since we observe many p a r t i c l e s i n t h e T(4S) f i n a l decay products.
2. The b uark i s i n a weak l e f t handed i s o s i n g l e t b u t mixes w i t h t h e s and d
quai-ks?lO). Hence i t can decay through t h e usual charged c u r r e n t . Since t h e r e i s no t quark, t h e r e i s no mechanism t o suppress t h e n e u t r a l c u r r e n t and one expects
+ -
t o observe b -+ qZO -+ qe e o r
q2u-.
M. Peskin and G. Kane have p u t a lower l i m i t on t h e branching f r a c t i o n f o r these decay modes ( l l ) . They f i n d Br(B + 1'1-x)/Br(B -+ 1-vx) must be g r e a t e r than 118. U n l i k e s i g n d i l e p t o n events can a r i s e from several sources. However, a non zero s i g n a l i n t h e q u a n t i t y
+
(Nee
+
Nvu-
New - Nve), where N i s t h e number o f events c o n t a i n i n g an e and a evp-, e t c , i s evidence o f a n e u t r a l c u r r e n t . We f i n d no s i g n a l i n t h i s q u a n t i t y
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982325
5 . Gittelman C 3 - 1 1 1
and a r e a b l e t o s e t a 90% confidence upper l i m i t of 0.09 f o r t h e r a t i o of branching f r a c t i o n s ( 8 ) .
3. The b quark always deca s t o a lepton plus a p a i r o f l i g h t a n t i q u a r k s ( b l q i ) . Theories of t h i s class(") a r e disproved p r i m a r i l y because we do not observe baryons a s s o c i a t e d with t h e T(4S). An attempt t o salvage t h e s e models by sug- g e s t i n g t h e baryons might be neutrons i s r u l e d o u t by t h e observed charged energy f r a c t i o n .
4. The b quark always decays t o two l e p t o n s and a quark, ( b + l l q ) ( 1 3 ) . This hypothesis i s unable t o account simultane u s l y f o r t h e measured lepton branching f r a c t i o n s and t h e charged energy f r a c t i o n P 9 ) .
An Upper Limit on Br(b + u)/Br(b + c )
Within t h e context of t h e Standard Model, one wants t o know t h e r e l a t i v e s t r e n g t h o f t h e coupling o f b t o u versus b t o c . Data on t h e kaon y i e l d from t h e T(4S) and t h e lepton momentum spectrum i n d i c a t e t h a t t h e b quark decays primarily t o t h e c quark. We have obtained l i m i t s on t h e magnitude of b + uW- d e c a y s ( 4 9 5 9 1 4 ) . The kaon y i e l d s a r e given i n t a b l e 1 . To reduce systematic e r r o r s , we consider t h e r a t i o * p, of kaons per
BE
event t o kaons per continuum event. The mean value of p i s (p- + p0)/2 =((1.76 k 0.16) + (1.39 k 0.24))/2 = 1.58 k 0.15. From a Monte Carlo c a l c u l a t i o n based on t h e standard model o f b decay(15), we expect p = 1 . 8+
0.1 f o r b + c and p = 0.95+
0.1 f o r b -t u. Assuming t h e branching f r a c t i o n s Br(b + u) + Br(b + c ) = 1 , we can w r i t e 1.58 ? 0.15 = (0.95+
O.l)Br(b + u) + (1.8 k 0.1)Br(b + c ) . Solving f o r t h e r a t i o , R = Br(b + u)/Br(b + c),we obtain t h e 1 Std Devia- t i o n l i m i t s , 0.09 < R < 0.79.
Table 1 - Kaon Yields from T(4S) and Nearby Continuum
K+
+
K- K O tP1
(event-' ) (event )
T(4S) =
BB
1.58+0.13 1.24k0.21Continuum 0.90k0.04 0.89k0.04
(14) A more r e s t r i c t i v e l i m i t on R is derived from t h e lepton momentum s p e c t r a
.
The measured s p e c t r a a r e shown i n f i g u r e 1 . The s o l i d curves a r e t h e expected d i s - t r i b u t i o n f o r B + 1vD and B + IvD* with equal frequency. The two dashed curves a r e c a l c u l a t i o n s f o r B -+ lvx where x i s a hadronic system having mass of 0.14 and 1 . 0 GeV. These correspond t o t y p i c a l hadronic masses one expects f o r b + lvu decays.
We have obtained an upper l i m i t t o t h e branching f r a c t i o n r a t i o , R , by f i t t i n g t h e measured s p e c t r a with t h e functional form, F(p) = A ( 0 . 5 f(p,MD) + 0.5 f(p,MD,) + R f(p,M ) ) , where f(p,M ) i s a t h e o r e t i c a l spectrum f o r B -+ l v x , M i s t h e mass of t h e hadl(onic system, an6 A i s a normalization c o n s t a n t . F i t s were hiade f o r A and R
Figure 1 b) Muon Momentum c ) Upper Limit on
a ) Electron Momentum Spectrum Spectrum B r ( b - w ) / B r ( k ) vs Mass of t h e
1 2 - Hadronic Spectrum
f o r b u decays.
B- evO(5O%l
O 1'0 2 0 3 0 Pm,.(GeV/Cl
P ,
- (GeVlcl U l v l O t Y l FOR 0 - l e v
JOURNAL DE PHYSIQUE
a t d i s c r e t e values o f Mx between 0.14 and 1.5 GeV. We f i n d no evidence f o r any b+u component t i e , a l l f i t s were c o n s i s t e n t w i t h R=O). The 90% confidence upper l i m i t on R i s p l o t t e d i n f i g u r e l c . We conclude, i f t h e mean mass M, i n B+lvx f o r b w i s l e s s than 1.0 GeV, t h e branching f r a c t i o n r a t i o , U r ( h ) / E r ( ~ m ) is l e s s t r ~ a n 10%.
Using t h e s p e c t a t o r model, we compute t h e phase space f a c t o r f o r t h e branching f r a c t i o n , B r ( h ) l B r ( b - . c ) = 2.5 x 1V
v I*,
and o b t a i n a l i m i t on t h e m a t r i x elements/ v ~ ~ / v ~ 1
< 0.2 (90% c o k ~ J e n c e % v e l ) .Reconstruction o f B Mesons
Reconstruction o f B mesons from the f i n a l charged p a r t i c l e t r a c k s i s needed t o p r o v i d e a p r e c i s e measurement o f t h e B mass. To r e c o n s t r u c t B mesons a t t h e ~ ( 4 s ) i s d i f f i c u l t . Since B and B a r e produced simultaneously and s i n c e t h e i r momentum i s low, t h e i r decay products do n o t separate i n phase space. Hence t h e r e i s no way t o d i s t i n g u i s h which t r a c k t o associate w i t h B and which w i t h B. The average charged m u l t i p l i c i t y o f ~ ( 4 s ) f i n a l s t a t e s i s 11.5. Hence, one must consider many combina- t i o n s i n a s s o c i a t i n g t r a c k s w i t h B and B. For example, f o r a f i n a l s t a t e having 10 charged t r a c k s and no photons, t h e r e a r e 325 t r a c k combinations t o be considered.
One approach c o n s i s t s o f a t t e m p t i n g t o r e c o n s t r u c t B and B i n the same event w i t h o u t imposing o v e r a l l momentum c o n s t r a i n t . Then one i s l o o k i n g f o r a simultaneous peak a t M = M-. Because o f t h e l a r g e number o f wrong combinations, good momentum r e s o l u - tBon
it
r quired. Simple estimates i n d i c a t e one needs 6p/p < 10-3 t o o b t a i n a s i g n a l w i t h %l o 0 BB
decays. For t h i s reason, we have been pursuing s t r a t e g i e s i n which we f i r s t search f o r events w i t h a d i s t i n c t i v e signature, and then attempt reconstruc- t i o n . We discuss two approaches, events c o n t a i n i n g Y ' s and events c o n t a i n i n g D's.We have searched f o r Y's i n a sample o f 13667 nb-' o f data recorded a t t h e peak o f t h e T(4S). The Y's a r e recognized by t h e i r l e p t o n i c decay i n t o e'e- o r p+p-. A t l e a s t one o f t h e daughter l e p t o n s i s i d e n t i f i e d t o reduce t h e number of wrong combinations t h a t have t o be considered. Mass spectra o f i d e n t i f i e d muons t o g e t h e r w i t h an o p p o s i t e l y charged t r a c k a r e shown i n f i g u r e 2. I n 2a, a l l o p p o s i t e l y charged t r a c k s a r e considered, whereas i n 2b, o n l y t r a c k s t h a t produced a s i g n a l i n t h e i n n e r muon d r i f t chambers are used. The dash-dot curve i s t h e ex- pected background. I t s shape i s computed by combining t h e i d e n t i f i e d muon from one event w i t h a p p r o p r i a t e o p p o s i t e l y charged t r a c k s from a l l o t h e r events. The back- ground curve i s normalized t o have t h e same area as t h e measured number of events.
The d o t t e d curves represent t h e expected s i g n a l f o r a branching f r a c t i o n f o r B -."
(Y
+
anything) of 1.9% i n 2a and 1.4% i n 2b. A s i m i l a r a n a l y s i s was made f o r t h e e+e- f i n a l s t a t e s . We conclude t h a t t h e branching f r a c t i o n B + ( Y + anything) i s l e s s than 1.4%.30
-
3$
2 0 P0 5
D
.
0* z
-
I!
2
22 0
5 0
2
I0 fo 8
I
M r p Q . V /cal MII,,lG*YIca1
F i g u r e 2 Mass spectrum o f i d e n t i f i e d muons of momentum g r e a t e r than 1.0 GeV/c w i t h : a) a1 1 o p p o s i t e l y charged t r a c k s b) o p p o s i t e l y charged muons i d e n t i f i e d i n
e i t h e r t h e i n n e r o r o u t e r muon chambers.
B. Gittelman C3-113
- A7 TI451
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OFF T1451 25D1.80 2 00 2 2C
IA..IG~YIC' 1
-.
We have searched f o r P o ' s i n t h e same T(4S) data sample. The k'n-, k - n s t a t e s were reconstructed.
To enhanc t e s i g n a l , a c u t on t h e Fox Wolfram parameteryl6! H I H < 0.3 was made. Only i d e n t i - f i e d kaons were2us?d and t h e Do momentum was r e - s t r i c t e d t o be g r e a t e r than 1 GeVIc. The kn mass p l o t i s shown i n F i g u r e 3. One observes an en- hancement a t 1.86 G ~ V / C ~ . There a r e 35
+
11 events i n t h e peak. An a n a l y s i s o f continuum data recorded below t h e T(4S) g i v e s 6.7+
8.3 events f o r e q u i v a l e n t l u m i n o s i t y , thus p r o v i n g t h e Do s i g n a l i s coming from t h e BB decqys. P r e l i m i n a r y c a l c u l a t i o n g i v e s 6+3 Do's p e r BB decay. One ex- pects between 1 and 2 depending upon t h e amount of D* p r o d u c t i o n i n B decay. T h i s r e s u l t i s consis- t e n t w i t h t h e conclusions we have drawn from t h e l e p t o n spectra.Summary
General ~ r o ~ e r t i e s o f B meson decav have been Figure 3 measured. t h e ' data i s c o n s i s t e n t wi t i t h e Standard
from decay SU(2) X U(1) Model and i n c o n s i s t e n t w i t h o t h e r suggested models. Our measured l e p t o n momentum spectrum i n d i c a t e s a B meson u s u a l l y decays t o D meson. We have a l s o shown t h e f i r s t d i r e c t e v i - dence f o r D mesons i n f i n a l s t a t e s o f B decay.
References
1. T h i s energy i s quoted i n CESR u n i t s . A r e p o r t submitted t o t h e conference, "High P r e c i s i o n Measurement o f t h e T Meson Mass", A.S. Artamonov, e t a l , P r e p r i n t 82-94 I n s t i t u t e o f Nuclear Physics, Novosibirsk, USSR, gives a new value f o r t h e T(1S) mass. Using t h i s value, we o b t a i n 10.579 GeV f o r t h e ~ ( 4 s ) mass.
2. The CLEO Detector, D. Andrews e t a l , Cornell P r e p r i n t CLNS 821538. Submitted t o Nuclear Instruments and ~ethods'.
3. Semileptonic Decay o f B Mesons, CLEO C o l l a b o r a t i o n . Submitted t o Physical
Review+.
++
4. Charged Kaon and Proton Production a t t h e T Resonances and Nearby Continuum
.
5. Neutral Kaon and Lambda Production i n t h e Upsilon Region*'.
6. M.S. Alam, e t a l , Phys. Rev. L e t t . 49, 357 (1982).
7. C. Quigg and J.L. Rosner, Phys. ~ e v r 1532 (1979).++ ~ , 8. An Upper L i m i t on B Meson Decay Via t h e Neutral Current
.
9. Proof t h a t b Quark Decay i s n o t ~ x o t i c ' .
10.V. Barger and S. Pakvasa, Phys. L e t t .
81,
195 (1979).11 .G.L. Kane and M.E. Peskin, Nucl. Phys.
B
195, 29 (1982).12.H. Georgi and M. Machacek, Phys. Rev. LE'fE-", 1639 (1979).
13. E. Derman, Phys. Rev.
D,
319 (1979) ; R.N. tbhaptra, Phys. L e t t .82,
101 (1979); H. Georgi and S.L. Glashow, Nucl. Phys.B,
173 (1980).1 4 . l i m i t s on ( b + ~ ) / ( b - t c ) i n Semileptonic B ~ecay++.
15.5. Levei 1 le , Laboratory o f Nuclear Studies Report CLNS 511505 ( 1 981 )
,
Cornel 1 U n i v e r s i t y , Ithaca, NY 14853 USA.16.G.C. Fox and S. Wolfram, Phys. Rev. L e t t . 41, 1581 (1978).
