3 2

EXCLUSIVE HADRONIC DECAYS OF B MESONS

And H , .,

PI fier Kern

The recent experimental results on exclusive hadronic decays of B mesons obtained by the ARGUS collaboratione presented in the talk. The results include exclusive hadronic decays involving a b --+ c transition, namely B

decays with a D, D` plus several pins and B decays to J /c', or ik~' mesons plus Kaons have been studied. Themeasurements of branching ratios for body B decays involving a J /t, or i" ' meson are of wide interest in thelight of proposals for the study of CP violation in future experiments . The branching ratios are compared withthe predict

of the model of Bauer, Stech and Wirbel and with a model of A.V. Dobrovolskaya . Using thecleanest

ay channels, the masses and mass difference of the charged and neutral B meson are obtained . Thismass difference is then compared with the mass splitting in other isospinmultipletts and with theoretical models.

Introduction

ysik, Heidelberg

easurements of exclusive B meson decay rates testtheoretical models of the k decays of heavy quarks .Dare to the large mass of the b quark, the influence ofthe strong interaction can be calculated more reliablythan for the decays of lighter mesons. There are sev-eral models available which describe hadronic B decays1.2.3,4,5 . In addition, reconstruction in hadronic chan-nels provides the only means of determining the Bo andB- masses . A measurement of their mass splitting isa test of the quark model predictions, and an essentialinput to estimates of the ratio of charged-to-neutral Bmeson production on the T(4S) .

This paper summarizes ARGUS measurements ofexclusive hadronic B decays involving b -+ c transi-tions . B mesons (B- = (bû) and Bo = (bd)) are pro-duced through the reaction e+e- --+ T(4S) , BB andare reconstructed in hadronic decay modes with multi-particle final states . In particular, channels with a D,D', J / v, or v,' meson have been studied .

The ARGUS detector is a magnetic 4 7r spectrom-eter 6 at the e+c- storage ring DORIS II at DESY .The data sample used for this analysis comprises 229pb -1 on the T(4S) resonance, corresponding to about192000 T(4S) decays .

0920-5632/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-11olland)

Nuclear Physics B (Proc . Suppl .) 21 (1991) 392-399North-Holland

2.

Data AnalysisMeasurements of inclusive charm and charmonium

rates on the T(4S) resonance show that the majorityof B decays produce D mesons 7.8,9, as expected fromthe dominance of b --+ c transitions . The momentumspectra of the D, D' and J /V" mesons are found to besoft, reflecting a substantial multibody component inhadronic decays.

The D and D' mesons are reconstructed in thefollowing decay modes ;

D.+ -~ Do,r+Drto -+ D0,ro

po --, K-,r+

p+ --+ KS,r+Do

KS ,r+,r-

D+ -~ K-ir+r+Do -4 K-7r+ 7r- 7r+

D+ -~ Kor+r+w-

Do -+ K_r+ro

The charmonium mesons are reconstructed in theirleptonic decay modes and in case of the y" ' as well inthe dominant hadronic decay into a J/u, and two pions .

e _

,V,' -4 e+e_

~,' --* J/ ~1,7r' 7r _

KS mesons are reconstructed from 7r+r - combina-tions forming a secondary vertex . gyr o mesons are recon-structed by their decay into two photons . High-energy

B- .-, D';' p-

seen

V/,I T0

BO ---, 6'K

A. Hôlscher/Exclusive hadronic decays of B mesons

393

ü

< 0.11% . at 90% C.L .

BO .-* ps- ~(_

-

< 0.1 3c7

BO -+ D-+ DS

i,

j

(0.75 ~ 0.38)%BO --i, D"

< 0.10%. at 90°1( C.L .

Table 1: B meson branching ratios

(1 .5 -± 1.1)%

B- --~ D''7r- ir-7r- 7r- < 1 .0% at 90% C.L. d

B- -~ J/OK-r

',(0.07 = 0.03 = 0.01)ac (0.08 ± 0.02±0.02)/c

B- -i ty'K- ! (0.18 = 0.08 = 0.04)% < 0.05%

B- --~ J/4,K*- °! (0 .16 ± 0.11 = 0.03)% ; (0.13 = 0-09 ::t. 0.0.03)"/-,-

i -i V,'K'- 1 < 0.49%, at 90% C.L . < 0.35%

B- -+ J/iOK-7r+x- < 0.16%, at 90% C.L. ; (0.12 = 0.06 = 0.03)(7

B- -~ v'K-T'7r' Ij (0.19± 0.11 ±0.04)

BO -' DOpO !~ < 0.1%

BO --~ D'+ Ir 7r a+7r0 l ; (4 .1 t 1.5 - 1.6 )%

BO --~ J/4'KS ij (0.04 t 0.03 y 0.01)% ; (0.03 ± 0.02 ± 0.41)%_BO - 'KS_ ii < 0.14% at 90% C.L . < 0.08%._B0 -+ J/?,K'O ' (0 .11 ± 0.05 :-- 0.02)%, ' (0.11 s 0.05 = 0.03)%

BO --~ ?,'K.O '! < 0.23% at 90% C.L . I (0.14±0.08 :t 0.04)%

BO --+ J/V" 7rO , ; < 0.10% at 90% C.L .

B-decay : . ARGU511 OLEO

B- --i Dow- ; (0.20t 0.08 =0.06)°7( (0.30 :t 0.06 - 0.04)El"c

B- Dop- ! (1 .3 t 0.4 0.4 )%,

B- --+ D'O7r- ! (0-40 ± 0.14 i 0.12)7(

B- --~ D'op- ~' (1 .0 i 0.6 - 0.4 )%B- -~ D'Yr-7r- (0.26 :± 0.14 t 0.07)%. < 0.4%

B- 1--" D; ir- seenB- --, D'-7r-,r-rO : ( 1 .8 = 0.7 =0.5 )%

BO D

BO --~ DTp-_BO --~

D'ir-

(0.48f 0.11 ± 0.11)%

~! (0 .9 t 0.5 f0.3 )%

(0.28 ± 0.09 ± 0 .06)%

(0.27 y 0.08 =0.U5y c

F

1(0.33 t 0.09 .~ 0.06)%

BO --+ D'+Tr-7ro , (1 .8 ± 0.4 ±0.5 )%

BO --> D'-p-!

(0.7 t 0.3 ± 0.3 )% ~ (1 .9 j 0.9 = 1.3)%,BO -+ D'+x-7r-x- jt (1 .2 y 0.3 =0.4 )% ; (1 .5 ±0.4 y 1.0)%

3,44

A. Bnlscher/F.eclusi%v hadronic decays of B mesons

ARGUS

B o -, D' z5-

D'

~ ;,+4h T1 7r2 "r3

Figure 1 : Out- cvent with a Bb candidate . which was rccurxstructcd au tlic B~ --+ D'+ 7r- dc(-ay channel.

7rOs, whose decay photons may merge into a singlecluster in the calorimeter, are included in the sampleby considering all clusters with energy greater than 800MeV as 7ro candidates . The momentum resolution isimproved by applying a mass constraint fit on all theseintermediate resonances .

Now the analysis steps are briefly described . Detailscan befound in 10,11 . Particle combinations originatingfrom B decays must have the beam energy. ThereforeB candidates are selected by requiring that the com-binations have an energy only 2-3 of away from thebeam energy. Furthermore there are additional require-ments for some channels. A kinematic fit is applied tothese B meson candidates, which constrains the energyto the beam energy. This improves the mass resolu-tion by roughly one order of magnitude to about 4 to4.5 MeV/c2 for combinations in the B mass region, de-pending slightly on the decay channel . All combinationswith a mass greater than 5.17 GeV/c2 are consideredas B candidates. There are events with more than onecandidate per event in the same B decay channel, espe-cially in the channels with 7ro's, where the combinato-rial background is high. Only one candidate per decaychannel and per event is accepted, by choosing the can-didate with the highest total probability calculated forthe sum of all k2 contributions from particle identifica-tion, kinematical fits and the beam energy constraintfit .

Fig . 1 shows a candidate for the decay modeB~ -4 D'+7r- . The fast pion from the B decay is emit-ted in the one hemispere and the decay products of theD°+ are all on the opposite side . Fig. 2 shows the com-bined B signal in the D'+n7r(n < 3) decay channels . Acheck has been made that the analysis procedure doesnot bias the mass distribution . For this check, wrong-charge combinations have been used, i .e . combinationswhere the charge of one pion differs from the chargeof the corresponding particle in the analysed candidatechannels . As can be seen from the hatched histogram,the mass distribution of wrong-charge combinations,normalized in the region below 5.25 GeV/c2 , provides

A. Hôlscher/Exclusive hadronic decays of B mesons

an excellent description of the background shape whflch

can be parameterized by the function

2' 1 -

-exp(-b (1 - -Ebear r

cn

2

where a and b are free parameters . The firstscribes the threshold behaviour, while the exponentialfactor is an empirical nmxW of the drop in the bground toward smaller masses. This functionperfectly the background in the right charge di rtion for masses smaller than the B mass . It isgood agreement with the wr

-charge distributes

dfurthermore supported by Monte Carlo stud'

In total the ARGUE and CLEO collaborations havereconstructed a few hundered B decays 9,11 . In Tathe measured branching ragas in the exclusive decaymodes of both experiments are summarized. The agree-

ment between both experiments is quite good. Thebranching ratios for the B --} D(')n7r and B -+ D(')Dsmodes are in the order of a few percent . They are in-creasing with increasing number of pions in the finalstate. On the other hand, B decays involving J/u" or 0'mesons have only rates of the order of a tenth percent

and smaller .Searching for subsystems in the B -* D'' mr de-

cays the ARGUE collaboration recently discovered ev-idence for P wave D meson production 10,11 . The Pwave D mesons were predicted 12.13.14 to have maser

335

around 2.4-2 .5 GeV/c2 . Two of the four predictedstates were found by the ARGUE and CLEO collabo-rations 15,16,17,18, namely the Dî(2414) being a 1+and the D2(2459) a 2+ P wave D meson . In theB- --+ D'+r-7r- and B- --> D`+rr- 7r-xo decay chan-

nels the invariant mass distribution of the D'7 - com-binations shows an enhancement in the region 2.40-2.47GeV/c2 . In these exclusive decay modes, however, the

statistics are poor and as both resonances are overlap-ping one cannot differentiate between the 1- and 2+

state . Therefore the resonant D'+ 7r- combinations are

called D (;)O ,

z

Figure 2 : Mass distribution of B candidates in the decay channels D`'na(n < 3) .

15

10

A . H5ischer/Exclusive hadronic decays ofB mesons

5.20 5.25 5.30

15

10

b)B°

- ~4,5.20 5.25 5.30 5.20 5.25 5.30

M

(GC' V/C2'1V1

l ~.r

V

C:7

Figure 3: Mass distribution of the charged and neutral B meson candidates .

3.

Masses of the charged and neutral B Mesons

The masses of the charged and neutral B mesonscan only be obtained in exclusive hadronic decays . Fortheir determination the ARGUS collaboration uses onlydecay modes with a small background (see figure 3) .These are usually two-body Bdecay modes without neu-tral pions 10,11 .

Table2 shows the masses of the B mesons as deter-mined by the ARGUS and CLEO collaborations . Bothexperiments measure within the errors equal masses ofthe charged and neutral B mesons . Table 3 compares

ARGUS MeV/c2] 11 6 CLEO lMeV/c2 j 9 ~ms_ 5280.5 i 1.0 : 2.0

5278.9 f 0.4 f 2.01MHo 1

5279.6 t 0.7 i 2.0

15279.3 -- 0.4 ± 2.00.9f1 .2t0.5 -0.4±0.6

Table 2: B meson masses as determined by the AR-GUS and CLBO collaborations .

the experimentally measured mass splittings in differentmesonic isospin multipletts . In contrast to the B systemall other mesonic multipletts show a mass difference of3-5 MeV/c2 between the charged and neutral meson.Also the predictions of two theoretical models are given.One model calculates the mass splittings with the helpof the MIT-BAG model 20 and the other with a poten-tial model 21 . Both models describe the mass splittingswell and even predict a smaller mass difference in the B

system compared to the others. This is due to correc-tions in the B system compared to the other systems

and by the fact that the hadronic and electromagneticcorrections have a negative relative sign . However, the

theoretical predictions of the mass difference are slightly

too large .

4.-

Discussion of the results

With the large number of observed exclusive decaymodes some interesting observations can be made re-

garding global properties of two-body decays involving

a D or D' plus a it or p in the final state. Assuming

that the ratios of vector-to-pseudoscalar production are

A. Hôlscher/Exclusive hadronic decays ofB mesons

.1, t MrV c2 t

data 19

Ti 85 20

K' 85 21

Table 3 : Comparison of theoretical modelsezperimentally measured mass splittings of differentisospinmultipletts . The values marked (*I are was input.

independent of t

B mescm charge . the f,ages for the ratio of D' to D production

e obta

BR(B --* D'ir )/BR(B --+ Dir) =1.0 _- 0.3

BR(B --" D°p)/BR(B -~ Dp) = 0.8 - 0.4

These results indicate that foe two-body decays wherethe four-momentum squared of the offshell

'-small, corresponding to the mass of the sr or p, therate of D and D' production is very similar . This is

marked contrast to inclusive D and D' production 7"8.9,

where the production ratio of directly produced charm

mesons D : D' was measured to be about 1:3 . The

other vector-to-pseudoscalar ratios :

BR(B --+Dp)/BR(B --+Drr)=3.2 .E1.2

BP'B -R D'p);1SR(B - Dir = 2.5=1.2

show that there is a significant enhancement of p over 7r

production . Altogether, only about 8% of the B- and

11% of BQ decay nodes have been observed through

the exclusive hadronic modes investigated here, repre-

senting small fractions of the sum of the inclusive Do

and D- meson branching ratios, (70f 11)°7 7.

The two-body decays can also be used to perform

tests of theoretical models of weak decays . One model

supplying a substantial list of exclusive B meson decays

is that due to Bauer, Stech and Wirbel 3. Decays are

grouped into three classes, which are described by pa-

rameters al, a2 or a combination of both . Most modes

are described by only one of the parameters, making it

Figure

A . H51scherj&elusive hadronic decays o[B mesons

001 0.04 010 040 1.00

branching ratio (Z)

Comparison of of the model of Bauer. Stech and

irbel with the experimental data .

001

_.o-ID-T.-Wo

B_-D°n'

mple to determine their absolute value . For the er-

raction of results . the lifetime of the B meson is taken

to be _1B = (1.15 =0.14) _10-12 s 7 and Vcb = 0.045 22

is assumed . A global fit for the al and a2 parameters

using all the measured two body decay modes wheretheoretical predictions exist is made . Only the B- de-

cay modes depend on an interference between the a l

and a2 amplitudes . Given the sizeable uncertainty onthe branching ratios and the fact that the B- decay

004 010 0.40 1.00

branching ratio (7.)

4.00 M00

lee

4.00 1000

Figure 5 : Comparison of the model of of Dobrovolskaya et al . with the experimental data .

the number of peons increasing c)ranching ratios in the

B - D' - n7r decays are well described .

5 . Summary

In conclusion, measurements have been presented

of the branching ratios for most of the low-multiplicity B

decays involving D, D`, J_J~ :" or z.'. ' mesons . The masses

of the B~ and B- mesons were found to be equal within

the experimental errors . The two-body decay modes are

rates depend only weakly on a2, we obtain, by fitting

all 12 branching ratios for which theoretical predictionsexist, two possible solutions :

al

a2

i Xa/DoF prob j

1 0.87 ±0.08

0.19±0.03

9.2/10

51%

8 1 .03 ± 0.09 1! -0.20 ± 0.03 ( 6.5/10

X 77%

Thus, both solutions are almost equally probable . The

theoretical prediction for the two parameters is erg = 1 .1

and a2 = -0.24 3, in good agreement with the secondsolution . This is illustrated in fig. 4. where the 12

measurement for the two-body B decay channels are

compared to the predictions of the Bauer-Stech-Wirbel

model, using the second solution.

In contrast to D meson decays, the B meson decaysare by far not saturated by the two body decays, as cal-

culated in the model of Bauer, Stech and Wirbel 3 . This

is seen in the large branching ratios for the multi-body

B ---, D°'mr decay modes. A part of these can perhaps

be attributed to higher resonances, which then decayinto the multipion final states . The evidence for B de-

cays into P wave charm mesons is one example for suchhigher resonances . However such decays are up to now

not calculated in this model. The only model describing

multi-body decays so far is the model of Dobrovolskaya

et al . 5 . In this model the heavy quark b decays into

a c quark by emitting a LF boson, with the VI' boson

then fragmenting into several pions. The remaining c`q

state on the other hand then hadronizes into a D or D'

meson. Fig. 5 shows the comparison of the experimen-

tal data with this model. The model describes the data

Quite well . Especially the similar branching ratios for

:he B -4 Dar and B --> D`7r decay modes and the with

found to be consistent with the model of Bauer, Stech

and

parameters of the model'vvnoei . Values for t:+..~ r.. . .. . ..___ .

are extracted, with good agreement on the absolute

value of the predictions . The multi body decay modes

on the other hand are well described in the model of

Dobrovolskaya et al .

A. H61scher/Exclusive hadronic decays of B mesons

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3.

22