CUSB RESULTS ON THE WEAK DECAY OF THE B MESON

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CUSB RESULTS ON THE WEAK DECAY OF THE B MESON

P. Franzini

To cite this version:

P. Franzini. CUSB RESULTS ON THE WEAK DECAY OF THE B MESON. Journal de Physique Colloques, 1982, 43 (C3), pp.C3-114-C3-116. �10.1051/jphyscol:1982326�. �jpa-00221879�

JOURNAL DE PHYSIQUE

CoZZoque C3, suppZ6ment au n o 12, Tome 43, ddcembre 1982 page C3- 1 14

CUSB RESULTS ON THE WEAK DECAY OF THE B MESON

CoZwnbia-Stony Brook Experiment, WiZson Laboratory, ComeZZ U n i v e r s i t y , I t h a c a , NY 14853, U.S.A.

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

Since t h e d i s c o v e r y o f t h e f i r s t b-flavored p a r t i c l e , t h e B-meson [ I ] a t CESR, a major p a r t of i t s running time d u r i n g t h e l a s t two y e a r s h a s been devoted t o t h e s t u d y of B decay a s a means t o l e a r n about t h e weak i n t e r a c t i o n of t h e b-quark. I n t h e " s t a - ndard model" t h e b-quark i s t h e I3 = -1 component of a weak, l e f t handed, i s o s p i n d o u b l e t t o g e t h e r w i t h t h e s o f a r undiscovered t o p quark. The s t a n d a r d model d e s c r i b e s t h e weak c o u p l i n g s of b t o u and c quarks i n terms o f two elements of t h e quark mix- i n g m a t r i x [21 V: Vub(bu)W + Vuc (bc)W where Vub and Vuc can have a non z e r o r e l a t i v e phase which would induce CP v i o l a t i o n . C o n s t r a i n t s have been d e r i v e d [3] f o r t h e mag- n i t u d e of VUb and Vuc from t h e experimental knowledge of o t h e r weak decays and t h e use of Cabibbo U n i v e r s a l i t y . While t h e s t a n d a r d model h a s s o f a r p a s s e d many t e s t s , one must measure t h e b-couplings (V's) t o c o n f r o n t and complete t h e model.

Since t h e t o p h a s n o t been d i s c o v e r e d s o f a r , many new e x o t i c c o u p l i n g s of t h e b-quark t o o t h e r q u a r k s and l e p t o n s [41 have been proposed. I n t h e f o l l o w i n g we p r e - s e n t t h e CUSB r e s u l t s on e x c l u d i n g e x o t i c models, t h e s e m i l e p t o n i c branching r a t i o and r e p o r t t h e f i r s t s i g n i f i c a n t l i m i t on t h e r e l a t i v e magnitude of t h e Vbu and Vbc elements o f t h e quark mixing m a t r i x based on o u r measurement of t h e e l e c t r o n spectrum from B-decays. We a l s o b r i e f l y p r e s e n t l i m i t s on t h e B-meson mass, which i s i m p o r t a n t t o t h e c a l c u l a t i o n of t h e end p o i n t of t h e e l e c t r o n spectrum and i t s smearing.

B-Mass

The crude l i m i t f o r t h e B-mass:

I$,,,<2MB<I$,,. , i m p l i e s t h a t B-mesons

could be produced i n T n ' d e c a y with a a P H O T O N SIGNAL

- ^CO

l a b o r a t o r y momentum o f O<p<1100 MeV/c, , ^{FROM B*DECAY}

r e s u l t i n g i n a l a b o r a t o r y Doppler smea- 2: 5 0 0 r i n g o f t h e e l e c t r o n spectrum end p o i n t s g

of 0 t o 40% (f.w.1 i n a d d i t i o n t o a o w

U z

s m a l l u n c e r t a i n t y of t h e end p o i n t i t s e l f ^IW i n t h e B r e s t frame ( <2.2%) . ^li l a r g e

8

Q v a l u e f o r T'" ^-t BB would imply, how-

e v e r a s i n t h e D c a s e , l a r g e production a :z 2

of B*'S which a r e , q u i t e r e l i a b l y p r e d i c - ¹⁰⁰ t e d t o decay a s B -t B + y w i t h Mg* -

" 50 MeV. We have searched [51 f o r 50 M~

5 0 100 500 1000

MeV photons from 7''. decays i n o u r detec- E ⁽ MeV1

t o r and o b t a i n e d a n u l l r e s u l t . We have

r e c e n t l y improved o u r s e n s i t i v i t y and Figure 1. CUSB i n c l u s i v e photon spectrum o b t a i n a 90% co2fidence l e v e l upper l i m i t from t h e T" , background sub- f o r B.R. ( Tr9+ B B) <9%. Figure 1 shows t r a c t e d . See t e x t f o r shaded t h e o b s e r v e d ~ n c l u s ~ v e photon spectrum from a r e a .

T'" a f t e r background s u b t r a c t i o n and t h e expected s i g n a l f o r B.R. =l. Combining t h e B. R. l i m i t and t h e measured T'" width w i t h p o t e n t i a l model c a l c u l a t i o n s by Eichten [ 61, we d e r i v e l i m i t s f o r t h e l a b K.E. of t h e B mesons from T'* o f 10<TB<25 MeV, w i t h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982326

P. F r a n z i n i C3-115

1 ^{, ,} , , , , , , , , , , ,

I

9 5 10.0 ~n ⁵

*

E x o t i c Models

I n t h e s t a n d a r d model t h e c o u p l i n g s o f W t o quarks and l e p t o n s l e a d t o

b+qqs

b+qR3 with w e l l d e f i n e d r a t i o s , i n p a r t i c u l a r , t h e decay b + qRv ( k e , p ) accounts f o r 26% of a l l decays w i t h experiments and t h e o r y i n good agreement. For t h e s e de- c a y s t h e average n e u t r i n o energy i s 1.35 GeV, and f o r t h e T" t h e t o t a l energy i n v ' s i s on t h e average 6.4% of t h e c. m. energy. Models have been proposed f o r t h e b decay without c o u p l i n g t o W, such a s I ) b -+ qRR and 11) b -t qqR where 33% and 17% r e s p e c t i - v e l y o f t h e energy would be c a r r i e d away by n e u t r i n o s . I n a d d i t i o n , i n model I1 bary- ons could appear i n t h e quark e v o l u t i o n l e a d i n g t o an a d d i t i o n a l a p p a r e n t missing en- e r g y . CUSB i s an e x c e l l e n t and extremely s t a b l e c a l o r i m e t e r which measures a l l t h e e l e c t r o m a g n e t i c ( T O ) energy, and about h a l f of t h e charged p a r t i c l e energy o v e r 60%

o f 417. Thus, on t h e average we e x p e c t t o measure 40% of t h e beam energy. Figure 2 shows t h e f r a c t i o n of t h e t o t a l energy

.- -

away from t h e measurement, and f o r model Ecm (GeV) 11, l e s s t h a n 34%, i . e . more t h a n 140

Figure 2. away. A l t e r n a t i v e l y , Model I c o u l d account

F r a c t i o n of t o t a l energy measured f o r o n l y 3.3% of b-decays, and Model I1 in CUSB. ( n o t e o f f s e t z e r o . ) f o r l e s s t h a n 7.5%. F i n a l l y , I want t o

mention t h a t i f models I and I1 were cor- r e c t , our d e t e c t i o n e f f i c i e n c y f o r T'"

decays would be d e c r e a s e d by a f a c t o r o f approximately t h r e e , implying t h a t i t s l e p - t o n i c width i s " 0.9 KeV, i n s t r o n g c o n t r a d i c t i o n with p o t e n t i a l models and any n a i v e e x p e c t a t i o n s .

-

-

w

\ 4 0 - o W

weak Couplings

In t h e s t a n d a r d model t h e b s e m i l e p t o n i c decay i s g i v e n , i g n o r i n g t h e Fermi cons- t a n t , by an e f f e c t i v e c o u p l i n g (ev) (Vbu(bu)CL + ^{V (bc) )} where (abk = iaya(1-y5)ub

CL bc a

e t c . and VbU, Vbc a r e t h e r e l e v a n t elements o f t h e quark mixing m a t r i x . The q u e s t i o n

T T l T~ CONT,NUUM

0 0 ⁰

,

T"' -

i s what can we l e a r n about Vbu/Vbc from B -t evX. QCD e v o l u t i o n of q u a r k s i s a some- observed a t t h e f o u r resonances and con- tinuum. Note t h e extreme s i m i l a r i t y of t h e f i r $ t f o u r v a l u e s which average t o 40.94% - 0.06%. The Monte Carlo r e s u l t i s 42% which we renormalize t o t h e ob- s e r v e d v a l u e . For t h e T'" assuming 100%

9

what mysterious p r o c e s s and t h e q u e s t i o n i s , what i s X f o r h and b+c? On t h e o t h e r hand, t h e s p e c t a t o r model f o r computing t h e B. R.'s i s expected t o be r e l i a b l e f o r t h e o r e t i c a l r e a s o n s and is s o f a r e x p e r i m e n t a l l y confirmed. Sometime ago Cabibbo, Corbo and Maiani [8] had c a l c u l a t e d t h e D b e t a decay spectrum i n t h e s p i r i t of t h e s p e c t a t o r model by assuming t h a t t h e bound c-quark moves with a Fermi motion i n t h e D meson. They a l s o f o l d e d i n t h e D motion i n t h e l a b and computed QCD r a d i a t i v e cor- r e c t i o n s , which a r e n o t c r i t i c a l f o r M v X where t h e e spectrum v a n i s h e s a t t h e kine- m a t i c l i m i t . The agreement with t h e D d a t a i s e x c e l l e n t . Encouraged by t h i s we a p p l i e d t h e method t o B decay i n t o XcZ0 and X u s i n g t h e same ~ e r m i motion and t h e above-

c#O

BB decay, s t a n d a r d b-decay and u s i n g t h e measured BR's we o b t a i n t h a t 38% of t h e energy should be observed, which compares

mentioned B motion. We ignored r a d i a t i v e c o r r e c t i o n s , w a i t i n g f o r A l t a r e l l i , Cabibbo, Corbo, Maiani and M a r t i n e l l i 191 t o do it r i g h t . T h e i r j u s t o u t r e s u l t s a r e b a s i c a l l y i n v e r y good agreement w i t h o u r simple c a l c u l a t i o n s which a r e shown i n f i g u r e 4. There a r e c l e a r d i f f e r e n c e s in t h e b+u and b+c s p e c t r a , and i n t h e e n d p o i n t s . C a l c u l a t i o n s by E i g i and E v e r t z 1101 i n c l u d e resonance e f f e c t s and t h e a u t h o r s conclude t h a t t h e y s t e e p e n t h e s p e c t r a but do n o t a l t e r t h e e n d p o i n t s .

In our d e t e c t o r we can o b t a i n a very c l e a n e l e c t r o n s i g n a l f o r energy above 1 GeV. A t = 2.2 GeV, t h e end p o i n t f o r B -t evXc, o u r r e s o l u t i o n i s approximately 3%.

JOURNAL DE PHYSIQUE

1 '

I 2 3

et ENERGY (GeV)

~ i g u r e 3. Uncorrected e x p e r i m e n t a l s p e c t r a . CUSB

e * ENERGY (GeVl

F i g u r e 4. Comparison o f d a t a , background s u b t r a c t e d and c o r r e c t e d f o r e f f i c i e n c y , and c a l c u l a t i o n s . Our energy s c a l e i s a l s o s i m i l a r l y a c c u r a t e , by u s i n g 5 GeV e+e- Bhabha s c a t t e r i n g a s c a l i b r a t i o n . These u n c e r t a i n t i e s a r e s m a l l compared t o t h e = 17% d i f f e r e n c e i n t h e computed end p o i n t s . In o t h e r words, r e s o l u t i o n u n f o l d i n g i s a minor e f f e c t , Dop- p l e r smearing i s i n f a c t dominant. The main complication comes from t h e f a c t t h a t t h e T'. i s o n l y ~ 3 0 % of ~ ( h a d r o n ) and t h e l a r g e continuuin does g i v e some h i g h energy e l e c t r o n s from decays o f f a s t D ' s . Figure 3 g i v e s t h e raw e l e c t r o n spectrum a t t h e

T "' and t h e measured continuum c o n t r i b u t i o n t o it. A s expected, t h e continuum f a d e s

away a t t h e end o f t h e spectrum, t h u s t h e r e s u l t s a r e q u i t e independent from t h e sub- t r a c t i o n . The s u b t r a c t e d spectrum h a s been compared with t h e o u t l i n e d c a l c u l a t i o n , u s i n g mb = 5.2 GeV and mc f r e e . We o b t a i n mc = 2.0 GeV a s a b e s t f i t , we a r e f u l l y c o n s i s t e n t w i t h mc = 1.8 GeV and a r e 1 0 0 ' s away from mq<500 MeV. F i g u r e 4 shows t h e d a t a and t h e c a l c u l a t i o n s f o r mb= 5.2 GeV, mc=1.8 GeV (curve B) , f o r mq=O (curve A ) . Curve C i s t h e c o n t r i b u t i o n from secondary (D's from B decays) D decays, which should be added t o curve B when comparing w i t h d a t a . Use quark masses from r e f . 11.

The e x p e r i m e n t a l l y measured spectrum i s c o n s i s t e n t with z e r o c o n t r i b u t i o n from b+u, curve A. Col t h e b a s i s of o u r r e s u l t s and t h e s p e c t r a o f f i g u r e 4, j u s t i f i e d by t h e complete c a l c u l a t i o n s of r e f e r e n c e 9, we o b t a i n a 90% c . 1. upper l i m i t of

B R ( B + ~ V X ~ ) / B R ( B + S V X ~ ) < 9.3%. Using t h e f a c t o r from i n t e g r a t i n g o v e r a v a i l a b l e phase space f o r b+u and b+c from r e f . 9 , t h e above l i m i t corresponds t o

[ v ~ ~ / v ~ ~ ~ ~ < 0.04 o r (vbul /[vbc < 0.20. I n t h e Maiani n o t a t i o n f o r t h e K-M m a t r i x t h i s imposes a bound

sinf3/siny < 0.20, which r e p r e s e n t s a s i g n i f i c a n t improvement on o u r knowled- ge o f t h e K-M m a t r i x 1121. F i n a l l y , from t h e same d a t a we o b t a i n an improved r e s u l t f o r t h e B s e m i l e p t o n i c branching r a t i o of BR(B+eVX)= 0.131*0.012, i n good agreement with c a l c u l a t i o n s in t h e s t a n d a r d model w i t k ~ c o r r e c t i o n s f o r s t r o n g i n t e r a c t i o n e f f e c t s s e e f o r i n s t a n c e , r e f e r e n c e 1 3 .

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3. ^{S .} Pakvasa, t h i s conference.

4. For ex. E. = m a n Phys. Rev. =(1979)317, H. Georgi and S. lasho ow Nucl. Phys.

B167 (1980)173.

5. R.D. Schamberger e t a l . Phys. Rev. D26(1982)720.

6. E. Eichten Phys. Rev. ~ 2 2 ( 1 9 8 0 ) 1 8 1 9 7 7. 1.1. B i g i and S. Ono, Nucl. Phys. =(1979)93.

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9. G. A l t a r e l l i e t a l . Univ. of Rome p r e p r i n t , p r e s e n t e d a t t h i s conf. by M a r t i n e l l i . 10. 1.1. B i g i and H.G. E v e r t z , p r e p r i n t PITHA 82/08.

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12. L. Maim$, r a p p o r t e u r t a l k , t h i s conference.

13. J. L e v e i l l e , Proc. XVIIth Moriond Conference, (1982) .