- Email: [email protected]
Exclusive hadronic decays of B mesons
3 2
Nuclear Physics B (Proc . Suppl .) 21 (1991) 392-399 North-Holland
EXCLUSIVE HADRONIC DECAYS OF B MESONS And
H ,.,
PI fier Kern
ysik, Heidelberg
The recent experimental results on exclusive hadronic decays of B mesons obtained by the ARGUS collaboration e 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. The body B decays involving a J /t, or i" ' meson are of wide interest in the measurements of branching ratios for light of proposals for the study of CP violation in future experiments . The branching ratios are compared with the predict of the model of Bauer, Stech and Wirbel and with a model of A.V. Dobrovolskaya . Using the ay channels, the masses and mass difference of the charged and neutral B meson are obtained . This cleanest mass difference is then compared with the mass splitting in other isospinmultipletts and with theoretical models. Introduction easurements of exclusive B meson decay rates test theoretical models of the k decays of heavy quarks . Dare to the large mass of the b quark, the influence of the strong interaction can be calculated more reliably than for the decays of lighter mesons. There are several models available which describe hadronic B decays 1.2.3,4,5 . In addition, reconstruction in hadronic channels provides the only means of determining the Bo and B- masses. A measurement of their mass splitting is a test of the quark model predictions, and an essential input to estimates of the ratio of charged-to-neutral B meson production on the T(4S). This paper summarizes ARGUS measurements of exclusive hadronic B decays involving b -+ c transitions . B mesons (B- = (bû) and Bo = (bd)) are produced through the reaction e+e - --+ T(4S) , BB and are reconstructed in hadronic decay modes with multiparticle 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 spectrometer 6 at the e+c - storage ring DORIS II at DESY. The data sample used for this analysis comprises 229 pb -1 on the T(4S) resonance, corresponding to about 192000 T(4S) decays .
2.
Data Analysis Measurements of inclusive charm and charmonium rates on the T(4S) resonance show that the majority of B decays produce D mesons 7.8,9, as expected from the dominance of b --+ c transitions . The momentum spectra of the D, D' and J /V" mesons are found to be soft, reflecting a substantial multibody component in hadronic decays. The D and D' mesons are reconstructed in the following decay modes; D.+ -~ Do,r+ Drto -+ D0,ro po --, K-,r+ Do KS ,r+,r Do -4 K-7r+7r -7r+ Do -+ K_r+ro
p+ --+ KS ,r+ D+ -~ K-ir+r+ D+ -~ Kor+r+w-
The charmonium mesons are reconstructed in their leptonic decay modes and in case of the y"' as well in the 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 - combinations forming a secondary vertex . gyr o mesons are reconstructed by their decay into two photons . High-energy
0920-5632/91/$03 .50 © 1991 - Elsevier Science Publishers B.V. (North-11olland)
A. Hôlscher/Exclusive hadronic decays of B mesons B-decay
:.
B- --i Dow-
; (0.20 t 0 .08 = 0.06)°7(
B-
ARGU511
B- --+ D'O7rD'opB- --~
- 0.4 )% (0 .26 :± 0.14 t 0.07)%.
B- --" D; 1 irB- --, D'-7r-,r-rO B- .-, D';' p-
: ( 1 .8 = 0.7 =0.5 )% seen
B- -~ J/OK-
r ',
-
< 1 .0% at 90% C.L. (0 .07 = 0.03 = 0.01)ac
(0.08 ± 0.02 ±0 .02)/c
! (0 .18 = 0.08 = 0.04)%
< 0.05%
1 < 0.49%, at 90% C.L .
< 0.35%
°! (0 .16 ± 0.11 = 0.03)% ; (0 .13 = 0-09 ::t. 0..0 03)"/-,-
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 D O B _ --~ DT pD' BO --~ ir
~! (0 .9 t 0.5 f 0.3 )%
(0 .48 f 0.11 ± 0.11)%
(0 .27 y 0.08 = 0.U5y c F
(0 .28 ± 0 .09 ± 0 .06)% 1(0.33 t 0.09 .~ 0.06)%
BO --+ D'+Tr-7ro BO --> D'-p-
< 0.4%
seen
B- --~ D''7r- ir-7r- 7r-
B- --~ J / 4,K* -i V,'K'i
(0 .30 :t 0.06 - 0.04)El"c
~' (1 .0 i 0.6
B- -~ D'Yr-7r-
B- -i ty'K -
OLEO
0.4 )%, ! (1 .3 t 0.4 ! (0-40 ± 0.14 i 0.12)7(
Dop-
BO -+ D'+x-7r-x-
, (1 .8 ± 0.4 ±0.5 )% ! (0.7 t 0.3 ± 0.3 )% ~
(1 .9 j 0 .9 = 1 .3)%,
jt (1 .2 y 0.3 = 0.4 )% ;
(1 .5 ± 0.4 y 1.0)%
BO -' DOpO
< 0.1 %
!~ BO --~ D' + Ir 7r a+7r 0 l; (4 .1 t 1.5 - 1.6 )% BO _ --~ J /4'KS BO - 'KS_ _ B0 -+ J/?,K'O BO --~ ?,'K.O
BO --+ J/V"7rO V/,I T0
BO ---, 6' K BO .-* p s- ~(_
D-+ D S BO --i, D" BO -+
393
ij (0 .04 t 0.03 y 0.01)% ; (0.03 ± 0.02 ± 0.41)% < 0.08% ii < 0.14% at 90% C.L . . ' (0 .11 ± 0.05 :-- 0.02)%, ' (0.11 s 0.05 = 0.03)% '! < 0.23% at 90% C.L . I (0.14 ± 0.08 :t 0.04)% , ; < 0.10% at 90% C .L . ü < 0.11% . at 90 % C . L . < 0.10%. at 90 °1( C .L . -
i,
< 0.1 3c7
j
(0 .75 ~ 0. 38) % (1 .5 -± 1.1)%
Table 1: B meson branching ratios
d
3,44
A. Bnlscher/F.eclusi%v hadronic decays of B mesons
ARGUS
B o -,
D'
z5-
~ D' ;,+ 4
h
T1
r2 7
"r3
Figure 1 : Out- cvent with a Bb candidate . which was rccurxstructcd au tlic B~ --+ D' + 7r- dc(-ay channel.
335
A. Hôlscher/Exclusive hadronic decays of B mesons
7rOs, whose decay photons may merge into a single cluster in the calorimeter, are included in the sample by considering all clusters with energy greater than 800 MeV as 7ro candidates . The momentum resolution is improved by applying a mass constraint fit on all these intermediate resonances . Now the analysis steps are briefly described. Details can befound in 10,11 . Particle combinations originating from B decays must have the beam energy. Therefore B candidates are selected by requiring that the combinations have an energy only 2-3 of away from the beam energy. Furthermore there are additional requirements for some channels. A kinematic fit is applied to these B meson candidates, which constrains the energy to the beam energy. This improves the mass resolution by roughly one order of magnitude to about 4 to 4.5 MeV/c2 for combinations in the B mass region, depending slightly on the decay channel . All combinations with a mass greater than 5.17 GeV/c2 are considered as B candidates. There are events with more than one candidate per event in the same B decay channel, especially in the channels with 7ro's, where the combinatorial background is high. Only one candidate per decay channel and per event is accepted, by choosing the candidate with the highest total probability calculated for the sum of all k2 contributions from particle identification, kinematical fits and the beam energy constraint fit. Fig. 1 shows a candidate for the decay mode B~ -4 D'+7r - . The fast pion from the B decay is emitted in the one hemispere and the decay products of the D°+ are all on the opposite side. Fig. 2 shows the combined B signal in the D'+n7r(n < 3) decay channels . A check has been made that the analysis procedure does not bias the mass distribution . For this check, wrongcharge combinations have been used, i.e. combinations where the charge of one pion differs from the charge of the corresponding particle in the analysed candidate channels . As can be seen from the hatched histogram, the mass distribution of wrong-charge combinations, normalized in the region below 5.25 GeV/c 2 , provides
an excellent description of the background shape can be parameterized by the function '1
-
2
Ebear r
-exp(-b (1 - -
whflch
2
cn
where a and b are free parameters. The first scribes the threshold behaviour, while the exponential factor is an empirical nmxW of the drop in the b ground toward smaller masses. This function perfectly the background in the right charge di r tion for masses smaller than the B mass. It is good agreement with the wr -charge distributes d furthermore supported by Monte Carlo stud' In total the ARGUE and CLEO collaborations have reconstructed a few hundered B decays 9,11. In Ta the measured branching ragas in the exclusive decay modes of both experiments are summarized. The agree-
ment between both experiments is quite good. The branching ratios for the B --} D(')n7r and B -+ D(')Ds modes are in the order of a few percent. They are increasing with increasing number of pions in the final state. 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 decays the ARGUE collaboration recently discovered evidence for P wave D meson production 10,11 . The P wave D mesons were predicted 12.13.14 to have maser around 2.4-2.5 GeV/c2 . Two of the four predicted states were found by the ARGUE and CLEO collaborations 15,16,17,18, namely the Dî(2414) being a 1+ and the D2(2459) a 2+ P wave D meson . In the B- --+ D'+r-7r- and B- --> D`+ rr-7r-xo decay channels the invariant mass distribution of the D'7 - combinations shows an enhancement in the region 2.40-2 .47 GeV/c2 . In these exclusive decay modes, however, the statistics are poor and as both resonances are overlapping one cannot differentiate between the 1- and 2+ state. Therefore the resonant D'+ 7r - combinations are called D(;)O,
A . H5ischer/Exclusive hadronic decays of B mesons
5.25
5.20
5.30
Figure 2 : Mass distribution of B candidates in the decay channels D`' na(n
15
15
10
10
b)B°
z
5.20
5.25
5.30
< 3).
5.20
M (G C' V /C 2 '1V1
- ~4,
5.25
l ~ .r
V
5.30 C: 7
Figure 3: Mass distribution of the charged and neutral B meson candidates .
A. Hôlscher/Exclusive hadronic decays of B mesons 3.
Masses of the charged and neutral B Mesons
.1, t MrV
data 19
c2 t
Ti 85 20
K' 85 21
The masses of the charged and neutral B mesons can only be obtained in exclusive hadronic decays . For their determination the ARGUS collaboration uses only decay modes with a small background (see figure 3) .
These are usually two-body B decay modes without neutral pions 10,11 .
Table 2 shows the masses of the B mesons as deter-
mined by the ARGUS and CLEO collaborations . Both experiments measure within the errors equal masses of
the charged and neutral B mesons . Table 3 compares ARGUS MeV/c2] 11 6 C LEO lMeV/c 2 j 9 ~
1
ms_ MHo
1
5280.5 i 1.0 : 2.0 5279 .6 t 0.7 i 2.0 0.9f1 .2t0.5
5278.9 f 0.4 f 2.0 15279.3 -- 0.4 ± 2.0 -0 .4±0.6
Table 2: B meson masses as determined by the AR-
GUS and CLBO collaborations .
the experimentally measured mass splittings in different mesonic isospin multipletts . In contrast to the B system
all other mesonic multipletts show a mass difference of 3-5 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 help of the MIT-BAG model 20 and the other with a potential model 21 . Both models describe the mass splittings
well 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 electromagnetic corrections 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 decay
modes some interesting observations can be made re-
garding global properties of two-body decays involving or p in the final state. Assuming that the ratios of vector-to-pseudoscalar production are a D or D' plus a
it
Table 3: Comparison of theoretical models ezperimentally measured mass splittings of different isospinmultipletts . The values marked (*I are w as 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 where the four-momentum squared of the offshell 'small, corresponding to the mass of the sr or p, the rate 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, representing small fractions of the sum of the inclusive Do (70 f 11)°7 7. and D- meson branching ratios, 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
A . H51scher j&elusive hadronic decays o[ B mesons
001
0.04
010
040
1.00
4.00
M00
branching ratio (Z)
Figure
Comparison of of the model of Bauer. Stech and
irbel with the experimental data .
lee .o-ID-T _ .-Wo
B_-D°n' 001
004
010
0.40
1.00
4.00
1000
branching ratio (7 .)
Figure 5 : Comparison of the model of of Dobrovolskaya et al . with the experimental data .
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 Vc b = 0 .045 22 is assumed . A global fit for the al and a2 parameters using all the measured two body decay modes where
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
Given the sizeable uncertainty on
of the branching ratios for most of the low-multiplicity B decays _ involving D, D`, J :" or z.'. ' mesons . The masses of the B~ and B - mesons were found to be equal within
the branching ratios and the fact that the B - decay
the experimental errors . The two-body decay modes are
theoretical predictions exist is made . Only the B - decay modes depend on an interference between the a l J~ and a2 amplitudes .
A . H61scher/Exclusive hadronic decays of B mesons
rates depend only weakly on a2, we obtain, by fitting all 12 branching ratios for which theoretical predictions exist, two possible solutions: al 1 0.87 ±0 .08
a2
i Xa /DoF 0.19±0.03 9.2/10
prob j 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 second solution . 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 decays
are by far not saturated by the two body decays, as calculated 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 decay into the multipion final states . The evidence for B decays into P wave charm mesons is one example for such
higher 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 experimental 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 parameters ..~ r... ... . .___ . of the model and 'vvnoei. Values for t:+
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 .
1. F.Hussain and M.Scadran . Physical Review° (1984) 1492 . 2. J.G .K6rner, Proc . Int . Symp . on Production a Decay of Heavy Hadrons . Hei g 1 3.
.Bauer, B.Stech, (1987) 103.
.Wi
. Z . Phys . Cr34
4. M.J .Sav e and .B .Wi- , Phys . (1989) 3346 . 5. A.V.Dobrovolskaya et at ., Phys . L (1989)293.
22
AM. Dobrovotskaya . private communication (I 6 . H .Albrecht et al. (ARGUS at` ), , . Instruments Meth . JL276 (1989) 1 .
7. H.Schriïder . Proceedings of the Int. Sy High Energy Physics, Munich 19 8. G.H der (ARGUS collaboration) . Ph .D. T University Hamburg. DESY F15-89/01 . 29, 9. Y.Kubota. Proceedings of the Int. Symposium
Heavy Quark Physics . Ithaca Y 1989 . . A.Hâlscher (ARGUS collaboration) . Ph .D .T 10
University Heidel g (199±1). 11 . H.Albrecht et at. (ARGUS collaboration). DESY 90-046 (1990) . 12. E .Eichten et al. . Phys . Rev . 1821 (198±1) 203 . 13. S .Godfrey and . Isgur . Phys. Rev. 8832 (19,f5)
14. A.B .Kaidalov and A.V .Nogteva . Sov. Journa l of Nuclear physics 47 (1988) 512. 15.
16. 17. 18. 19 . 20.
21 .
. J.C .Anjos et al . (Tagged Photon collaboration) Phys. Rev. Lett . 62 (1989) 1717 . H .Albrecht et al. (ARGUS collaboration), Phys . Lett . B221 (1989) 422 . H .Albrecht et al. (ARGUS collaboration), Phys . Lett . B232 (1989) 398 . P.Avery et al . (CLEO collaboration) . Phys. Rev . 1841 (1990) 774. M.Aguilar-Benitez et al ., Review of particle properties. Phys .Lett. B204 (1988) 1 . K.P.Tiwari et al ., Phys . Rev. 8331 (1985) 642. D.Y .Kim and S.N .Sinha, Ann. Phys. (N .Y .) 42 (1985) 47 .
22 . M .Danilov, Proceedings of the XIV . Int. Symposium on Lepton Photon Interactions, Stanford 1989 .
Nuclear Physics B (Proc . Suppl .) 21 (1991) 392-399 North-Holland
EXCLUSIVE HADRONIC DECAYS OF B MESONS And
H ,.,
PI fier Kern
ysik, Heidelberg
The recent experimental results on exclusive hadronic decays of B mesons obtained by the ARGUS collaboration e 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. The body B decays involving a J /t, or i" ' meson are of wide interest in the measurements of branching ratios for light of proposals for the study of CP violation in future experiments . The branching ratios are compared with the predict of the model of Bauer, Stech and Wirbel and with a model of A.V. Dobrovolskaya . Using the ay channels, the masses and mass difference of the charged and neutral B meson are obtained . This cleanest mass difference is then compared with the mass splitting in other isospinmultipletts and with theoretical models. Introduction easurements of exclusive B meson decay rates test theoretical models of the k decays of heavy quarks . Dare to the large mass of the b quark, the influence of the strong interaction can be calculated more reliably than for the decays of lighter mesons. There are several models available which describe hadronic B decays 1.2.3,4,5 . In addition, reconstruction in hadronic channels provides the only means of determining the Bo and B- masses. A measurement of their mass splitting is a test of the quark model predictions, and an essential input to estimates of the ratio of charged-to-neutral B meson production on the T(4S). This paper summarizes ARGUS measurements of exclusive hadronic B decays involving b -+ c transitions . B mesons (B- = (bû) and Bo = (bd)) are produced through the reaction e+e - --+ T(4S) , BB and are reconstructed in hadronic decay modes with multiparticle 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 spectrometer 6 at the e+c - storage ring DORIS II at DESY. The data sample used for this analysis comprises 229 pb -1 on the T(4S) resonance, corresponding to about 192000 T(4S) decays .
2.
Data Analysis Measurements of inclusive charm and charmonium rates on the T(4S) resonance show that the majority of B decays produce D mesons 7.8,9, as expected from the dominance of b --+ c transitions . The momentum spectra of the D, D' and J /V" mesons are found to be soft, reflecting a substantial multibody component in hadronic decays. The D and D' mesons are reconstructed in the following decay modes; D.+ -~ Do,r+ Drto -+ D0,ro po --, K-,r+ Do KS ,r+,r Do -4 K-7r+7r -7r+ Do -+ K_r+ro
p+ --+ KS ,r+ D+ -~ K-ir+r+ D+ -~ Kor+r+w-
The charmonium mesons are reconstructed in their leptonic decay modes and in case of the y"' as well in the 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 - combinations forming a secondary vertex . gyr o mesons are reconstructed by their decay into two photons . High-energy
0920-5632/91/$03 .50 © 1991 - Elsevier Science Publishers B.V. (North-11olland)
A. Hôlscher/Exclusive hadronic decays of B mesons B-decay
:.
B- --i Dow-
; (0.20 t 0 .08 = 0.06)°7(
B-
ARGU511
B- --+ D'O7rD'opB- --~
- 0.4 )% (0 .26 :± 0.14 t 0.07)%.
B- --" D; 1 irB- --, D'-7r-,r-rO B- .-, D';' p-
: ( 1 .8 = 0.7 =0.5 )% seen
B- -~ J/OK-
r ',
-
< 1 .0% at 90% C.L. (0 .07 = 0.03 = 0.01)ac
(0.08 ± 0.02 ±0 .02)/c
! (0 .18 = 0.08 = 0.04)%
< 0.05%
1 < 0.49%, at 90% C.L .
< 0.35%
°! (0 .16 ± 0.11 = 0.03)% ; (0 .13 = 0-09 ::t. 0..0 03)"/-,-
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 D O B _ --~ DT pD' BO --~ ir
~! (0 .9 t 0.5 f 0.3 )%
(0 .48 f 0.11 ± 0.11)%
(0 .27 y 0.08 = 0.U5y c F
(0 .28 ± 0 .09 ± 0 .06)% 1(0.33 t 0.09 .~ 0.06)%
BO --+ D'+Tr-7ro BO --> D'-p-
< 0.4%
seen
B- --~ D''7r- ir-7r- 7r-
B- --~ J / 4,K* -i V,'K'i
(0 .30 :t 0.06 - 0.04)El"c
~' (1 .0 i 0.6
B- -~ D'Yr-7r-
B- -i ty'K -
OLEO
0.4 )%, ! (1 .3 t 0.4 ! (0-40 ± 0.14 i 0.12)7(
Dop-
BO -+ D'+x-7r-x-
, (1 .8 ± 0.4 ±0.5 )% ! (0.7 t 0.3 ± 0.3 )% ~
(1 .9 j 0 .9 = 1 .3)%,
jt (1 .2 y 0.3 = 0.4 )% ;
(1 .5 ± 0.4 y 1.0)%
BO -' DOpO
< 0.1 %
!~ BO --~ D' + Ir 7r a+7r 0 l; (4 .1 t 1.5 - 1.6 )% BO _ --~ J /4'KS BO - 'KS_ _ B0 -+ J/?,K'O BO --~ ?,'K.O
BO --+ J/V"7rO V/,I T0
BO ---, 6' K BO .-* p s- ~(_
D-+ D S BO --i, D" BO -+
393
ij (0 .04 t 0.03 y 0.01)% ; (0.03 ± 0.02 ± 0.41)% < 0.08% ii < 0.14% at 90% C.L . . ' (0 .11 ± 0.05 :-- 0.02)%, ' (0.11 s 0.05 = 0.03)% '! < 0.23% at 90% C.L . I (0.14 ± 0.08 :t 0.04)% , ; < 0.10% at 90% C .L . ü < 0.11% . at 90 % C . L . < 0.10%. at 90 °1( C .L . -
i,
< 0.1 3c7
j
(0 .75 ~ 0. 38) % (1 .5 -± 1.1)%
Table 1: B meson branching ratios
d
3,44
A. Bnlscher/F.eclusi%v hadronic decays of B mesons
ARGUS
B o -,
D'
z5-
~ D' ;,+ 4
h
T1
r2 7
"r3
Figure 1 : Out- cvent with a Bb candidate . which was rccurxstructcd au tlic B~ --+ D' + 7r- dc(-ay channel.
335
A. Hôlscher/Exclusive hadronic decays of B mesons
7rOs, whose decay photons may merge into a single cluster in the calorimeter, are included in the sample by considering all clusters with energy greater than 800 MeV as 7ro candidates . The momentum resolution is improved by applying a mass constraint fit on all these intermediate resonances . Now the analysis steps are briefly described. Details can befound in 10,11 . Particle combinations originating from B decays must have the beam energy. Therefore B candidates are selected by requiring that the combinations have an energy only 2-3 of away from the beam energy. Furthermore there are additional requirements for some channels. A kinematic fit is applied to these B meson candidates, which constrains the energy to the beam energy. This improves the mass resolution by roughly one order of magnitude to about 4 to 4.5 MeV/c2 for combinations in the B mass region, depending slightly on the decay channel . All combinations with a mass greater than 5.17 GeV/c2 are considered as B candidates. There are events with more than one candidate per event in the same B decay channel, especially in the channels with 7ro's, where the combinatorial background is high. Only one candidate per decay channel and per event is accepted, by choosing the candidate with the highest total probability calculated for the sum of all k2 contributions from particle identification, kinematical fits and the beam energy constraint fit. Fig. 1 shows a candidate for the decay mode B~ -4 D'+7r - . The fast pion from the B decay is emitted in the one hemispere and the decay products of the D°+ are all on the opposite side. Fig. 2 shows the combined B signal in the D'+n7r(n < 3) decay channels . A check has been made that the analysis procedure does not bias the mass distribution . For this check, wrongcharge combinations have been used, i.e. combinations where the charge of one pion differs from the charge of the corresponding particle in the analysed candidate channels . As can be seen from the hatched histogram, the mass distribution of wrong-charge combinations, normalized in the region below 5.25 GeV/c 2 , provides
an excellent description of the background shape can be parameterized by the function '1
-
2
Ebear r
-exp(-b (1 - -
whflch
2
cn
where a and b are free parameters. The first scribes the threshold behaviour, while the exponential factor is an empirical nmxW of the drop in the b ground toward smaller masses. This function perfectly the background in the right charge di r tion for masses smaller than the B mass. It is good agreement with the wr -charge distributes d furthermore supported by Monte Carlo stud' In total the ARGUE and CLEO collaborations have reconstructed a few hundered B decays 9,11. In Ta the measured branching ragas in the exclusive decay modes of both experiments are summarized. The agree-
ment between both experiments is quite good. The branching ratios for the B --} D(')n7r and B -+ D(')Ds modes are in the order of a few percent. They are increasing with increasing number of pions in the final state. 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 decays the ARGUE collaboration recently discovered evidence for P wave D meson production 10,11 . The P wave D mesons were predicted 12.13.14 to have maser around 2.4-2.5 GeV/c2 . Two of the four predicted states were found by the ARGUE and CLEO collaborations 15,16,17,18, namely the Dî(2414) being a 1+ and the D2(2459) a 2+ P wave D meson . In the B- --+ D'+r-7r- and B- --> D`+ rr-7r-xo decay channels the invariant mass distribution of the D'7 - combinations shows an enhancement in the region 2.40-2 .47 GeV/c2 . In these exclusive decay modes, however, the statistics are poor and as both resonances are overlapping one cannot differentiate between the 1- and 2+ state. Therefore the resonant D'+ 7r - combinations are called D(;)O,
A . H5ischer/Exclusive hadronic decays of B mesons
5.25
5.20
5.30
Figure 2 : Mass distribution of B candidates in the decay channels D`' na(n
15
15
10
10
b)B°
z
5.20
5.25
5.30
< 3).
5.20
M (G C' V /C 2 '1V1
- ~4,
5.25
l ~ .r
V
5.30 C: 7
Figure 3: Mass distribution of the charged and neutral B meson candidates .
A. Hôlscher/Exclusive hadronic decays of B mesons 3.
Masses of the charged and neutral B Mesons
.1, t MrV
data 19
c2 t
Ti 85 20
K' 85 21
The masses of the charged and neutral B mesons can only be obtained in exclusive hadronic decays . For their determination the ARGUS collaboration uses only decay modes with a small background (see figure 3) .
These are usually two-body B decay modes without neutral pions 10,11 .
Table 2 shows the masses of the B mesons as deter-
mined by the ARGUS and CLEO collaborations . Both experiments measure within the errors equal masses of
the charged and neutral B mesons . Table 3 compares ARGUS MeV/c2] 11 6 C LEO lMeV/c 2 j 9 ~
1
ms_ MHo
1
5280.5 i 1.0 : 2.0 5279 .6 t 0.7 i 2.0 0.9f1 .2t0.5
5278.9 f 0.4 f 2.0 15279.3 -- 0.4 ± 2.0 -0 .4±0.6
Table 2: B meson masses as determined by the AR-
GUS and CLBO collaborations .
the experimentally measured mass splittings in different mesonic isospin multipletts . In contrast to the B system
all other mesonic multipletts show a mass difference of 3-5 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 help of the MIT-BAG model 20 and the other with a potential model 21 . Both models describe the mass splittings
well 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 electromagnetic corrections 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 decay
modes some interesting observations can be made re-
garding global properties of two-body decays involving or p in the final state. Assuming that the ratios of vector-to-pseudoscalar production are a D or D' plus a
it
Table 3: Comparison of theoretical models ezperimentally measured mass splittings of different isospinmultipletts . The values marked (*I are w as 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 where the four-momentum squared of the offshell 'small, corresponding to the mass of the sr or p, the rate 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, representing small fractions of the sum of the inclusive Do (70 f 11)°7 7. and D- meson branching ratios, 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
A . H51scher j&elusive hadronic decays o[ B mesons
001
0.04
010
040
1.00
4.00
M00
branching ratio (Z)
Figure
Comparison of of the model of Bauer. Stech and
irbel with the experimental data .
lee .o-ID-T _ .-Wo
B_-D°n' 001
004
010
0.40
1.00
4.00
1000
branching ratio (7 .)
Figure 5 : Comparison of the model of of Dobrovolskaya et al . with the experimental data .
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 Vc b = 0 .045 22 is assumed . A global fit for the al and a2 parameters using all the measured two body decay modes where
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
Given the sizeable uncertainty on
of the branching ratios for most of the low-multiplicity B decays _ involving D, D`, J :" or z.'. ' mesons . The masses of the B~ and B - mesons were found to be equal within
the branching ratios and the fact that the B - decay
the experimental errors . The two-body decay modes are
theoretical predictions exist is made . Only the B - decay modes depend on an interference between the a l J~ and a2 amplitudes .
A . H61scher/Exclusive hadronic decays of B mesons
rates depend only weakly on a2, we obtain, by fitting all 12 branching ratios for which theoretical predictions exist, two possible solutions: al 1 0.87 ±0 .08
a2
i Xa /DoF 0.19±0.03 9.2/10
prob j 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 second solution . 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 decays
are by far not saturated by the two body decays, as calculated 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 decay into the multipion final states . The evidence for B decays into P wave charm mesons is one example for such
higher 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 experimental 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 parameters ..~ r... ... . .___ . of the model and 'vvnoei. Values for t:+
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 .
1. F.Hussain and M.Scadran . Physical Review° (1984) 1492 . 2. J.G .K6rner, Proc . Int . Symp . on Production a Decay of Heavy Hadrons . Hei g 1 3.
.Bauer, B.Stech, (1987) 103.
.Wi
. Z . Phys . Cr34
4. M.J .Sav e and .B .Wi- , Phys . (1989) 3346 . 5. A.V.Dobrovolskaya et at ., Phys . L (1989)293.
22
AM. Dobrovotskaya . private communication (I 6 . H .Albrecht et al. (ARGUS at` ), , . Instruments Meth . JL276 (1989) 1 .
7. H.Schriïder . Proceedings of the Int. Sy High Energy Physics, Munich 19 8. G.H der (ARGUS collaboration) . Ph .D. T University Hamburg. DESY F15-89/01 . 29, 9. Y.Kubota. Proceedings of the Int. Symposium
Heavy Quark Physics . Ithaca Y 1989 . . A.Hâlscher (ARGUS collaboration) . Ph .D .T 10
University Heidel g (199±1). 11 . H.Albrecht et at. (ARGUS collaboration). DESY 90-046 (1990) . 12. E .Eichten et al. . Phys . Rev . 1821 (198±1) 203 . 13. S .Godfrey and . Isgur . Phys. Rev. 8832 (19,f5)
14. A.B .Kaidalov and A.V .Nogteva . Sov. Journa l of Nuclear physics 47 (1988) 512. 15.
16. 17. 18. 19 . 20.
21 .
. J.C .Anjos et al . (Tagged Photon collaboration) Phys. Rev. Lett . 62 (1989) 1717 . H .Albrecht et al. (ARGUS collaboration), Phys . Lett . B221 (1989) 422 . H .Albrecht et al. (ARGUS collaboration), Phys . Lett . B232 (1989) 398 . P.Avery et al . (CLEO collaboration) . Phys. Rev . 1841 (1990) 774. M.Aguilar-Benitez et al ., Review of particle properties. Phys .Lett. B204 (1988) 1 . K.P.Tiwari et al ., Phys . Rev. 8331 (1985) 642. D.Y .Kim and S.N .Sinha, Ann. Phys. (N .Y .) 42 (1985) 47 .
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