Recent B physics results from CLEO II

Nuclear Instruments and Methods m Physics Research A 351 (1994) 19-30

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Section A

FI SFVIFR

Recent B physics results from CLEO II Dan Payne

Wilson Laboratory Cornell University, Ithaca, NY 14850, USA

For the CLEO Collaboration Abstract We report on recent results obtained using the large sample of B mesons collected with the CLEO II detector at CESR . We have made the first observation of the rare decay modes: B° -+ -7r+,ir- and K+7r - and the electromagnetic penguin B --> K*y. We have also reconstructed two other modes for the first time : the Cabibbo and colour-suppressed decay B - -> qllT - and a mode with baryons in the final state: B° -+ AC p7r+ ,ir- . In the hadronic decays we find that colour suppression is important. Several tests of the factorization hypothesis are described which show it is a valid approximation at least up to the mass of the at meson. In the framework of factorization, we use our two-body hadronic branching fraction results to extract the parameters da I and Ja2l in the BSW model. We have also measured the polarization in B - t/lK* decays ; the result shows that this mode will be useful for future CP violation studies. l

1. Introduction This article describes recent B physics results from the CLEO II experiment which operates at the Comell Electron Storage Ring (CESR) . This machine was designed to investigate the Y states via e+e - annihilation and discovered both the Y (3S) and Y (4S) resonances around 1980 [ 1 ] . The Y (4S) is just above the BB threshold and is believed to decay 550% of the time to B+B - pairs and 50% of the time to B°B° meson pairs. In 1983 CLEO fully reconstructed B mesons for the first time [ 2] . Unfortunately, as seen in Fig. 1, the Y(4S) sits on a large continuum background and only 1/3 of e+ e- annihilations result in a BB pair. However, this source of background can * Fax: +1 607 255 8062, e-mail dgp@Ins717 .lns.cornell.edu.

z 20

0 w 0 Û m

rn

T ( IS)

16

12 ô

4

T

(2S)

T(33)

T (4S)

I n

9.45 9.50 10.00 10 .05

10 .40

10.50

W, CENTER OF MASS ENERGY

10.60

(GeV)

Fig. 1 . The Y resonances . 0168-9002/94/$07 .00 © 1994 Elsevier Science B.V. All rights reserved SSDI0168-9002(94)00919-8

be studied by collecting data at energies in the continuum just below the Y(4S) . The peak luminosity achieved by CESR is typically 2 x 1032 cm -Z S -1 . Since the successful installation of the CLEO II detector in 1989 we have collected an integrated luminosity of 2 fb -1 on-resonance (which corresponds to approximately 4 million B mesons) and 1 fb -1 in the continuum. The analyses described here have, for the most part, used approximately half of this data . 2. The CLEO II detector A detailed description of the CLEO II detector can be found in Ref. [3] . It is designed to detect both charged and neutral particles with excellent resolution and efficiency. The tracking system consists of a 6-layer straw-tube drift chamber and a 10-layer drift chamber which combine to provide precise coordinates close to the interaction region . These are surrounded by the main tracking device : a 51layer drift chamber which also provides ionization energy loss measurements (dE/dx) used for particle identification . The combined momentum resolution can be parameterized as : (8p/p) 2 = (0 .0015p) 2 + (0 .005) 2 (p in GeV/c) . Particle identification is also provided by a scintillator time-offlight system which surrounds the drift chambers. Outside the tracking system is an electromagnetic calorimeter which consists of 7800 thallium-doped CsI crystals which provide an energy resolution of: (SE/E) (%) = 0.35/e 75 + 1 .9 0. 1 E (E in GeV) . Both the tracking chambers and the electromagnetic calorimeter are located inside a superconducting coil which produces a magnetic field of 1 .5 T. The iron II . e+e- EXPERIMENTS

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D. PaynelNucl. Instr. and Meth . in Phys Res. A 351 (1994) 19-30

return yoke of this magnet is interleaved with three layers of gas wire chambers for muon identification .

a, (/i) =cl (li)

3. B decay theory

where e = 1/Ncu,ours and Ncolours = 3. However, because they were introduced to absorb the unknown effects of soft gluons, they are intended to be extracted from experimental results. The BSW model can be used to predict many twobody branching fractions as a function of a, and a2 . By measuring a sample of these two-body B branching fractions, we have determined these parameters .

Before describing our experimental results, a brief discussion of B decay theory is appropriate since it is relevant to much of what follows. In the Standard Model the B meson decays predominantly via an external spectator diagram in which a virtual W boson is emitted. For two-body decays, if strong effects are ignored and the approximation Vud = Vcs = 1 is made, the Hamiltonian which describes this process at the quark level is [4] : H= 7Veb {

[du) +

(sc ] (cb)} ,

(1)

where GF is the Fermi coupling constant, Veb is the appropriate CKM matrix element, and (q,q,) is the weak-current giYt. (1 - Y5) qj . However, the exchange of hard gluons between the initial and final state quarks leads to QCD corrections. The result is an effective Hamiltonian which contains the original term multiplied by a factor cl (lc) and an additional term multiplied by c2 (A) [5] : Herr= GFVb

{cl(/i) [(du) + (Sc)J (cb)

+C2 (/1) [ (cu) (ab)

+ (Ec) (sb) ] } ,

(2)

where cl and c2 are the Wilson coefficients evaluated at the appropriate QCD mass scale /t,. It is this additional term which contains the quark couplings necessary for the internal spectator diagram. Although the Wilson coefficients can be calculated in the framework of perturbative QCD, the predictions for decay rates are uncertain because it is unclear exactly what mass scale should be used . The usual approach is to assume It - Mb . In addition, because the quarks are inside hadrons, some knowledge of the form-factors involved is required. Most phenomenological models of B decay assume that the W fragmentation products are moving sufficiently fast that they do not interact with the other quarks in the decay. This allows the decay amplitude to be expressed as the product of two independent hadronic currents, one describing the decay of the W and the other the formation of the charmed meson. This is known as the "factorization" hypothesis [6] . We have performed several tests of factorization which are described later. Although the effective Hamiltonian accounts for perturbative hard gluons, it is difficult to predict the effects that nonperturbative soft gluons may have . One approach, which was first suggestedby Bauer, Stech and Wirbel (B SW) [7] and assumes factorization, is to replace the Wilson coefficients with two new coefficients a, (la) and a2(/l) . In the framework of perturbative QCD, these are related to the Wilson coefficients :

a2 (A)

+ec2(/1),

=C(/-0 +6c,(A),

4. Reconstruction tools As described above, the Y (4S) resonance sits on a large continuum background . However, this background can be suppressed by exploiting the difference in topologies between Y(4S) -+ BB and continuum e+e- ~ qq events (where q = u, d, s, c) . Because the B mesons are produced almost at rest, these events tend to have a spherical structure in the laboratory frame while the continuum events are jetlike . The most effective event-shape variable is the second Fox-Wolfram parameter, R2 [8], which is zero for a completely spherical event and unity for a perfectly collimated jet. In general we require that candidates satisfy R2 < 0.5 . Continuum events are further suppressed by placing a requirement on the angle between the sphericity axis of the tracks forming the B candidate and the sphericity axis of the remainder of the event. This cut is dependent on the particular decay mode being studied, but in general we require I cos 0, 1 < 0.7 . At the Y(4S) the B mesons are produced back-to-back, each with a momentum of approximately 300 MeV/c and an energy equal to the beam-energy . Because the energy of candidates can be accurately measured, this provides a powerful tool which is unique to threshold machines for rejecting combinatorial background . Candidates are required to satisfy the beam-energy constraint Jt1EJ < 8-46 MeV, where DE = Ecand - Ebeam . The value of the cut depends on the decay mode . Because the B mesons are emitted with the beamenergy, the invariant mass resolution can be improved by using the CESR beam-energy when calculating the invariant mass . This gives the beam-constrained mass : M2 = (E2eam - I E P,1 2 ) 1 ~ 2 , where the momentum sum is over all charged and neutral tracks in the candidate. This reduces the width of the invariant mass peak by an order of magnitude and provides a resolution of approximately 2.6 MeV/c2 which is dominated by the spread in the beam-energy . 5. Masses of the B mesons As an example of how well we can fully reconstruct B mesons, Fig. 2 shows the invariant mass distributions of both charged and neutral B mesons t9] . Because this analysis is

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D. Payne/Nucl . Instr. and Meth m Phys. Res . A 351 (1994) 1 9-30 80

40

w'

40

0 520

5.22

525

M 6 (GeV)

I 527

I

I

11

~.

I

I 530

Fig 2. Invariant mass of (a) B - candidates and (b) B0 candidates

Fig. 3. External and internal spectator diagrams for B decays to baryons . N = p, n, A or N', Y= A, YO , Y + or E - , O c = A+ , E0, Eô or $é + and ':1 c = ~ C or E0 .

191 .

limited by systematic uncertainties, we have only used those modes for which the signal to background ratio is large 1 B - - OK-, B0 - t/,K*0, B- -a DO ir , B_ - -> D0p-, B- ~ D'° ir - , B - -., D'0p-, B0 -, D+ar-, B0 -, D+p-, B° --4D' + or_ - and B° -r D'+p- . There are a total of 362 B - and 340 B° signal events . After applying a correction for initial-state radiation, as described in Ref. [ 10], the values for the B- and B° masses are (5278.8 t 0.2 f 0.5 f 2.0) and (5279 .2 f 0.2 f 0.5 f 2.0) MeV/c2 respectively . The first error is statistical, the second is due to the uncertainty in the correction for initial-state radiation and the last is from the uncertainty in the beam-energy. The mass difference between these states is : M(B0 ) - M(B - ) = (0 .41 t 0.25 t 0.19) MeV/C2 . This can be determined more accurately because the uncertainty in the beam-energy cancels, as do many of the other systematic uncertainties. It is interesting to compare this result with the values obtained in both the charm and strange sectors: M(D+ ) - M(D0 ) _ (4 .77 f 0.27) MeV/CZ and M(K0 ) - M(K- ) _ (4 .024 f 0.032) MeV/CZ [11] . Naively, one would expect similar mass differences in the B sector, although some models do claim to account for the smaller value [ 12] . 6. Decays to baryons We have fully reconstructed a sample of B mesons which contain baryons in the final state [ 13] . This is the first time that such decays have been observed . We search for the decays B- A'p(n7r) which have n charged pions in the final state (n = 0, 1, 2, 3) . The spectator diagrams for processes of this type are shown in Fig. 3. The AC was re1 Throughout this article, reference to a particular charge state implies the inclusion of the charge conjugate state.

15

10

5

0 5200

5.225 5.250 5.275 Ac p (I +2+3)7E Beam Const Mass (GeV/c2 )

5 300

Fig. 4 Invariant mass distribution of A, p(a+2-rr+3ir). The contributions from each of these final states are indicated .

constructed using the decay modes Ac- - pK -,7r+, pK0 and Air+ . To reduce the combinatoric background, both the kaon and proton candidates were required to satisfy stringent particle identification requirements. Fig. 4 shows the invariant mass distribution when all these modes are combined . Among these modes, only 130 -4 Ac pvr+ .7r has a sufficient number of candidates to claim a signal . In this mode we observe 12 .4±39 events, which gives: 8(13 0 -4 A- p7r+ vr- ) _ (0 .193 f 0.062 f 0.058 10 .044)%, where the first error is statistical, the second systematic and the third is due to the uncertainty in the A. branching fractions. 11 . e+e- EXPERIMENTS

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Table 1 Ratios of branching fractions [9] . In each case the numerator is expected to be colour suppressed

Table 2 B- 0, 0' and Xcl branching fractions [9] Upper limits are quoted at the 90% confidence level.

Branching-fraction ratio

Upper limit (90% CL)

B mode

Number of candidates

Branching fraction (%)

ß(13O -. DO7r0 )/%3(B - DO7r -) S(BO_ DOp)/13(B --, Dop) D071)/5(B -" DO7r ) 13(BO _ t3(Bo - DO n')/C3(B -+

0.09

B- -OK13 0 -OKO BO - (bK *o B- -iPK * -

58 .7±79 10 .0±3 .2 290±5 .4 126±3.6

0110±0 .015±0009 0.075±0024±0008 0.169±0031±0.018 0178±0 .051±0023

B- -+y~KBO -iP KO Bo -O ' K*O

7.0±26

0.061±0 .023±0 .009

42±2 .3

<0.19

B - Xc, K BO ~ Xc I KO Bo - X,IK *o B ~XclK*

6± 24 I±1 1 .2±15 0

AT-)

O 13(BO Do p) _ - D w)113(BB(B0 --. D*0 7r0 )/13(B- - D*07r -) 13(130 -. D*O p)/B(B - D*( ) p -) 13(BO -. D*0,7)/13(B -D *0 7r ) 13(BO --, D*071')/B(B- -" D*07r) D*Op 13(BO -. D*Ore)/13(B )

0.05 0.12 0.16 0.05 0.20 0.07

0.14

0.54 0.09

7. Colour suppression When a B decays through an internal spectator diagram, the colours of the quarks from the virtual W must match those in the original B meson in order to produce final state particles which are colour singlets . For example, if one considers the two decays B - ---> D07T - and TO -} D° vro , the first can only proceed via an external spectator diagram, while the second occurs only through an internal spectator diagram which should be "colour suppressed" . One expects this second decay amplitude to contain a factor of 1/3 to account for the allowed colour combinations and an additional factor of 1/f because the W- decays to tid and can only couple to the dd part of the 7rO wavefunction. While this simple picture gives a suppression factor of 1/18, more refined QCD based calculations [ 14] predict factors of order 1/50. However, these ideas do not work in the charm sector where l3(D ° --> K° 7r0 )/i3(D0 --> K-,7r+ ) = 0.57±0.13, for example [ 11 ] . By measuring ratios of branching fractions for which the numerator is expected to be colour suppressed, it has been possible to test this idea in the B sector [9] . Table 1 summarizes our results. Although we have not yet reached the 1/50 level, it appears that colour suppression is important in B decays . One possible explanation is that the running of as reduces the number of soft gluons exchanged at the b mass scale, which in the charm sector may act to resolve the colour discrepancies. 8. Decays to charmonium Decays which contain a +A, 0', or X,I in the final state are the only colour-suppressed B decays which have ever been reconstructed. These modes have the advantage that they are extremely clean, especially when the di-muon final state is used to reconstruct the 0 . This makes them ideal candidates for tagging b6 events at hadron colliders .

B- - tb' K* -

0

2± 1 4

< 0 08 < 0 30

0.097 ± 0 040 ± 0.009 <0 .27 <0 .21 <021

Among this class of decays, the most important are Bo -> 0K° and O'Kso because the final states are CP eigenstates . The decay B o -* 0K*0 may also be used for CP violation measurements provided K*0 -> K°or° (which is a CP eigenstate) is used to reconstruct the K*O . However, there is an added complication here : because B o -+ 0K* 0 is a P-> VV decay, the parity of the final state has an additional contribution from the helicities of the ip and K* O . Therefore, the final state has definite CP only if it is dominated by a single helicity state, otherwise the CP asymmetry will be diluted . A measurement of the polarization in this decay is described in Section 8.2 . 8.1 . Branching fractions

Table 2 summarizes our branching fraction results for B decays which contain a iA, t/i' or X,I in the final state. The charmonium states were reconstructed in the modes: X,I t/ry, +p' - Oir+ ir - and (t(i',t/~) -> e+ P -. In addition, K* candidates were identified using K*° ~ K-.7r+ and K* K- ~r° , K°7r -. By using isospin symmetry to combine the charged and neutral B results, we measure the ratio of vector to pseudoscalar production : Li(B ~K*) 1 .71 . B(B -> OK) -

The BSW model predicts a value of 1 .61 for this quantity, which uses the somewhat counter-intuitive assumption that factorization is valid for internal spectator decays . 8 .2. Polarization in B -+ OK* aecays

After integrating over the azimuthal angle between the and K* decay planes, the differential decay rate for the process B --4 t/1K* can be written [15 ] :

D. PaynelNucl. Instr. and Meth. m Phys. Res. A 351 (1994) 19-30

23

aD

CD w

a

5 300

MB (GeV/c') 30

2

b) ro

w

20

10

1 25

E

-020

I

-0 .10

I

1

or

-0 040

AE (GeV)

0.00

I . ~_~ -_

.

0 000

a 0 040

0 10

Fig 5 (a) AE vs invariant mass for B- - 0Tr- candidates. The ellipses show the predicted 3o contours for the B- - 07r- signal and the B- -+ 0K_ background. (b) Projection onto the AE axis for candidates in the signal mass region. The dashed histogram shows the simulated prediction for the B - - OK - profile. The arrows indicate the aG-rr - signal region chosen .

d'`l'

dcos9,odcosOK"

oc

1 sinz 9K . (1 + COS2 B,y ) 4

8.3 . First observation of B - t/wr

X(IH+l1 2 +IH_11 2 ) +Cos2 9K " sin2 o0IH0I 2 ,

(5)

where H0,ft are the helicity amplitudes for the three possible helicity alignments in the (tp, K' ) system: I0, 0) and I±1,±1). In the subsequent decay K" -r Kar, 9K " is the angle between the K or ar and the B meson in the K' rest frame, the so-called helicity-angle . o,& is the equivalent angle for the tG decay. In total we observe 29 Bo - 0K»o and 13 B - --* OK' - candidates with negligible background. The helicity amplitudes are extracted by performing a 2dimensional unbinned maximum likelihood fit to the efficiency corrected helicity angle distributions . The longitudinal polarization can be calculated from these amplitudes :

C

1Hol z r

/B-ftK "

IHo1 2

+IH+t12+IH-112

=0 .80±0.08±0.05 .

(6)

The first error is statistical and the second an estimate of the systematic error which is dominated by the uncertainty in the acceptance. This shows that while B~ OK' is not completely polarized, it is dominated by a single CP eigenstate and will be useful for future CP violation measurements .

Using the full on-resonance data sample of 2 fb -1 , we have searched for the colour- and Cabibbo-suppressed modes B- - V17r - and Bo , sparo. We have seen the first evidence for thedecay B - --~ Oar As for the Cabibbo-favored B- tiK modes, the predominant production mechanism is via an internal spectator diagram . For this reason B-> t/tar is expected to be suppressed relative to B - t/1K by a factor tanz o~, where-0, is the Cabibbo angle. However, because of this suppression, interference from other rare or exotic processes may be large enough to produce observable effects. For B-> tp-;ir, the largest source of background is from the Cabibbo-favored decay B- -> t/iK - . However, because of the excellent energy resolution of the CLEO lI detector, the beam-energy constraint (see Section 4) can be used to distinguish these modes. Fig. 5 shows a plot of AE vs invariant mass for these events . The B --> 4r7r - candidates are clustered around DE = 0, while those from B - - r/rKdecays are distributed at negative values . The B- - Viar signal region is defined to be -20 < 4E < 40 MeV. The residual background from the tail of the B- -> iPK - distribution has been estimated by using a high statistics Monte Carlo and is found to be less than 1 event. After subtracting this background we are left with 4.2 events which gives: 13(B - - Viar- ) = (4.8 ± 2.6) X 10 -5 . A comparison with Il . e+e- EXPERIMENTS

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hys Res. A 351 (1994) 19-30

_Table 3 BO branching fractions used to test factorization [9] . For D*+-rr-7r-7T+, the three pion mass is required to lie between 1.0 GeV and 1 .6 GeV, for consistency with the al meson hypothesis . BO mode D* + 7r _ D*+

D*+ ir - iT - 7r+

Number of candidates

Branching fraction (%)

718±9.4 763±100 49 .1±8 .3

026±0.03±0.04 0741010±014 0.63±010±0.11

0100 " v

>

DATA

- KS model

--- ISGW model

0075

WSB model

v is

0050

*o

13(B - -+ t//K - ) shows that this result is consistent with that expected from Cabibbo suppression . We also find one B° -> t/17r° candidate with negligible background . If the conservative approach is adopted and this single event is treated as background, it corresponds to 1 an upper limit: B(B ° -> rp7r° ) < 6.9 x 10 -5 at the 90% confidence level.

Because most phenomenological models of B decay assume factorization, it is important to test this hypothesis . As described in Section 3, the factorization ansatz allows an analogy to be drawn between hadronic and semileptonic decays because in both cases the decay amplitude is expressed as the product of two independent currents, one representing the decay of the virtual Wand the otherdescribing the formation of the charmed hadron . We have used this fact to test factorization in various ways . 9.1 . Branching fraction tests If factorization is valid: (Bu - D* + P_vt) i7

1 q2 =, ~

m`

.0025

0

1

I 2 .0

1 ,

4 .0

1

6.0

'

8 .0

'

10 .0

12 .0

g2(GeV 2) Fig. 6. The BD - D*+ e-v decay rate as a function of q2. The curves represent fits to various semileptonic decay models [ 17].

9. Tests of factorization

I'(B° -, D*+h-)

m

2 2 -62 iT cl fhIVudI 2 ,

(7)

where h - represents the meson from the decay of the W. We have tested factorization for the three cases h = it-, p - and a, by measuring the left hand side of Eq . (7) experimentally (Rexp) and comparing it with the theoretical prediction for the right hand side (Rlheory) . We assume cI = 1 .1 f 0.1, as derived from perturbative QCD (the error reflects the uncertainty in the QCD mass scale), f r = 131 .74 ± 0.15 MeV, fv = 215 ± 4 MeV from e + e - - po, fa, = 205 ± 16 MeV [ 16]W_and Ud = 0.975 ± 0.001 . Table 3 summarizes the - D* + h - branching fractions which form part of the experimental input. In order that the kinematics for the hadronic and semileptonic processes are the same, the semileptonic rate must be evaluated at the appropriate value of the virtual W mass, q2 = mh . Predictions for these semileptonic rates are derived by extrapolating the observed differential decay rate to the required q2 value.

Table 4 Comparison of ReXp with Rtheory as defined in Eq (7) [9] BO mode sO -D *+ vr BO-D *+p BO -D *+a,

Rexp (GeV2)

Rtheory (GeV2)

1 1 ±01 ±02

1 2±0.2

30±0 .4±0 .6 4 .0±0 .6±05

3.3±0 .5 30±0 .5

Until more data are available, theoretical models are required to fit the form of this distribution . Fortunately, the model dependence is small, as seen in Fig. 6. We use the predictions of the BSW model since it gives the central value in all cases. A summary of our results is given in Table 4. By considering the ratios of these hadronic branching fractions, many of the experimental systematic errors cancel, as does the uncertainty in cl from the QCD mass scale. Table 5 compares these ratios with the expectations from factorization, as defined above, and the theoretical predictions of Bauer, Stech and Wirbel (BSW) and Reader and Isgur (RI) [ 16] . In each case the predicted ratios agree with our results within the experimental errors . 9.2 . Polarization tests Another test of factorization is to compare the polarization when a B decays to two vectors with the polarization in the equivalent semileptonic process_ . For example, if factorization is valid, the polarization in B° D* + p- decays should be the same as that in B 5 -+ D* + Q - v decays evaluated at q2 = mP . This method has the advantage that it is not affected by QCD corrections [ 18 ] . _ We have measured the polarization in BO --~ D* + p - using the same technique that was used for B° t#K* o decays . -4

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Table 5 _ Ratios of BO branching fractions compared to theoretical expectations 191 . Ratio

Measured

Factorization

RI model

BSW model

5(BO-D "+p- ) /S(BO-.D *+-rr -)

29±05±05

29±005

2.2-23

28 3.4

8(BO - D'+a1 ) /8(BO - D'+vr-)

5.0± 1 .0±0.6

However, in this case the spin structure of the final state particles is different so the angular distribution has the form : dz r a 1 sin2 0D. sin2 0p( I H+i1 2 + IH-1I2) dcosOD " dcos0p 4 +Cos 2 OD.cos2 0p1Ho1 2 .

(8)

After subtracting the background using the B mass sideband and correcting for the detector acceptance, a 2-dimensional unbinned maximum likelihood fit to the helicity angle distributions of the D" + and the p- yields r L/F = (93±5±5)%, where the first error is statistical and the second is systematic . This systematic error is dominated by the uncertainty in the background parameterization and the detector acceptance. This agrees with the prediction of Neubert [19] for the polarization in Bo --> D' +B- v evaluated at q2 = M2 which is approximately 85% . The tests we have performed suggest that factorization is valid up to the mass of the at meson. In the_ near future we will extend these tests to decays of the type Bo -> D'+DSwhich will probe a higher q2 region . It will be interesting to see if factorization is still a valid approximation when the W decay product has a softer momentum distribution . 10. Determination of al and a2 As described in Section 3, the parameters at (A) and a2 (/-t) in the BSW model are introduced to absorb the unknown effects of soft gluons and are intended to be extracted from experimental measurements . Two-body b--4c hadronic B decays fall into three categories : those which can only occur via external spectator decays and depend on at, those which occur only through internal spectator diagrams and depend on a2, and those which can occur via either mechanism and depend on both at and a2 . Table 6 gives the BSW predictions for some of these branching fractions along with our measured values [9] . A fit to the branching fractions which depend only on at or a2 gives: l ail=1 .15±0.04±0.10, I a2 l

=0 .26±0.01±0.02.

The systematic errors include contributions from the charm and charmonium branching fractions, the detector acceptance, background shapes and the value of Vcb. It also in-

3.4±0 .3

20-2 .1

cludes the uncertainty from the B meson production fractions at the Y(4S) and their individual lifetimes z . By considering the third class of decays it is possible to define ratios of branching fractions which allow the relative sign of at and a2 to be established : B(B- --> Do7r - ) _ B(BO -> D+ir -) C3(B- --, Do p- ) R2 ~ 8(BO -> D+p-)

2

R1 =

R3= 4=

2

S(B- --, D*o ir ) _ _ (1 + 1 .29a2/at) 2 , 8(B0 --> D`+,r -) 8(B- --, D'op-) 2 = (1 +0 .75a2/at) . f3(BO -> D"+p-)

(12) (13) (14)

A least squares fit to these ratios gives a2/al =0 .23±0.04± 0.11, where the first error is statistical, the second is systematic and we have ignored the theoretical uncertainties in the predicted ratios . This result disagrees with the extrapolation from charm meson results which predicts a negative value for a2/at at the B mass scale. 11 . Charndess hadronic decays : B0

-> Tr*Tr ,

K+a-

The existence of b->u transitions was established some time ago by examining the lepton end-point spectrum in semileptonic decays [ 21 ] . However, until now, the only decays to have been fully reconstructed contained a charmed meson in the final state. We have been able to reconstruct modes without a charmed meson by searching for Bo -> 7r+ 7r , K+7r - and K+K- [221 . The decay Bo --> 7r + 7r- involves a b->u spectator diagram with a possible small contribution from a b->d penguin, as shown in Fig. 7. It is expected to have a branching fraction - 1 x 10 -5 [71 and, because it is a CP eigenstate, could be used to observe CP violation directly [23] . The decay Bo --> K+a- is expected to occur via both a b->s penguin and a doubly-Cabibbo-suppressed spectator diagram. Theoretical predictions for this branching fraction are in the range 1-2x 10 -5 [24] . In this case, although it is 2 hhis ratio is constrained by the CLEO measurement of the B- D'Iv branching fractions : (f+ ,r+1f0 ,r0) = 13(B - - D' 02 - v)/l3(BO -. D' + p - v) = 1 .20 ± 0.20 ± 0 19 [201 11 . e+e- EXPERIMENTS

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Table 6 Predictions for B branching fractions m terms of the BSW parameters at and a2 [9] . We have used Vc1, = 0 041, TB = 1 .44 ps and assumed fD, = fD, 220 MeV. Our experimental results are also shown Mode

BSW prediction (%)

Measured BF (%)

B0 -" D+-rr-

0 264a2

019±004±0 .06

BO - D+p_BO -+ D * +-rr

0.62laj 0 .254a2

B0 --~ D * +p-

BO

t~K*O

BBB_ B_

- DOIT- DOP -. WO .rr--, D*Op_

0.26±003±0.04

0.702aj

B- - #PK_ B - OK*0 -0KO

0

081±0 .11±018 074±010±0.14

1 .819a2 2.932a2

0110±0 .015±0009 0 178 ± 0.051 ± 0 023

1 817a2 2 .927a2

0.075±0024±0008

0.169±0 .031±0 .018

0.265[a1 + 1 230a2] 2 0 622[ al + 0 .662a2 12 0255[at + 1 292a2] 2 0.703[a2 + 0.635a2 + 1 .487at a2 l

7r, K 7r, K Bo

Table 7 Branching fractions for charmless BO decays [22] . The upper limits are quoted at the 90% confidence level. Mode

Nfit

Lî(10-5 )

UL (10-5 )

Ir + ir-

7.2 +43 -3 5

6.4+39

1 3+0.8 ± 0 2 ' -0.6 1 ., +0 7 ± 0.2

2.9

+00 13 .6±3 9

00±00 4±pj ± 0.2 2

K+ 7r -

Fig. 7 (a) Spectator diagrams for BO -+ 7r + ar - , K+ 7r - and (b) penguin diagrams for the same modes [22].

not a CP eigenstate, CP violation may arise from the interference of the amplitudes for the two diagrams [25] . The decay B o -+ K+ K - is expected to be strongly suppressed since it involves a b-->u transition and W-exchange . In all cases the signature is two back-to-back tracks, each with a momentum of approximately 2.6 GeV/c. Because these are high momentum tracks, the background from Y(4S) ~ BB events is negligible and the majority of the background comes from continuum e+ e - - qq events which must be efficiently suppressed . This is partially achieved by using the standard tools described in Section 4. However, this is not sufficient and a Fisher discriminant technique [26] is used to reduce the background further. The discriminant, F a,y is a linear combination of input variables, y which are weighted to provide maximum separation between Monte Carlo signal and background samples. The 11 input variables are the direction of the candidate thrust axis, the flight direction of the candidate and nine variables which measure the momentum flow of the rest of the event in nine angular bins about the candidate's thrust axis . Both the signal and background distributions in this variable are well fitted by Gaussian functions whose means are separated by 1 .5v. The most challenging aspect of this analysis is to separate pions from kaons at momenta of 2 .6 GeV/c. This

055±004±0.05 135±012±0.15 0.52±0.07±0.07 16ßf021±028

K+K -

7r + 7r - or K+ 7r -

00

26 07

is achieved by exploiting the beam-energy constraint, DE, and the dE/dx information . For example, if both tracks are treated as pions, the AE shift for K7r events provides a separation from irir of 1 .7vA E . The dE/dx separation between pions and kaons at 2.6 GeV/c is 1 .ßv. In order to maximize the detection efficiency, we do not explicitly cut on any of these variables, but instead perform a maximum-likelihood fit which exploits the information available in the DE, candidate mass, F and dE/dx distributions. This fit yields the level of background and the number of candidates in the individual modes. Fig . 8 shows the results of this fit displayed as a contour plot on the N, and NK, axes . The best fit values are: NKK = 0 .0±0.0, N,r,r = 7.2+4 3 and NK,r = 6.4+39 . Although the number of 7r-7r or K7r events taken individually are not sufficiently significant to claim signals in the individual modes, N,r  + NK,r = 13 .6 which excludes zero at the level of 5.4v. This indicates that either .7r.7r, Kvr or both are present in the data . Table 7 gives the branching fractions for each mode along with the 90% confidence level upper limits wherewe do not claim a signal . These results are consistent with theoretical expectations . We have also searched for the decays B + -+ 7r4 7r 1 , K + 7r° and B 0 - .7r° -7r° . These are similar to the modes described above, except that the presence of 1r° ' s in the final states reduce the detection efficiencies . The first of these modes is

D. PaynelNucl. Instr. and Meth . in Phys. Res. A 351 (1994) 19-30

27

7

20

6 15

5 4

z

Y

10

M

w 5

0

3 2

0 5.200

5.220

5 240

5.260

5.280

5 300

M(K *)' )(GeV) Fig. 8. Likelihood contours from the fit to N,r r and NKr . The central value is indicated by the cross. The dotted line is the 90% confidence level contour [221 Table 8 13- ,ir +7ro, K+aro and iro-rro branching-fraction upper hmits Mode

UL (10-5 ) (90% CL)

7r+7r o K+ao vro7ro

43 2.5 2.6

Fig. 10. The K*y mass distribution . The contributions from the three K* decay modes are indicated [271

this inclusive branching fraction are in the range 2-4x 10 -° [28] . If the observed rate differs significantly from these predictions it would indicate physics beyond the Standard Model [29] . There are two possible strategies : the first is to reconstruct a particular final state and the second is to study the inclusive momentum spectrum of photons in BB events . We have investigated both approaches . 12 .1 .

W Fig. 9 Penguin diagram for b-sy. The photon may be radiated from any of the quark lines [271 .

expected to occur predominantly through either an external or internal b--+u spectator diagram, while the second has an additional contribution from a b--*d penguin process. The mode B ° --+ oro iro should occur predominantly through an internal b->u spectator and a b-->d penguin diagram. We do not see signals in any of these channels . Table 8 summarizes our upper limits for these branching fractions at the 90% confidence level. 12. Electromagnetic penguin decays We have seen the first direct evidence for electromagnetic penguin decays. The radiative transition b~ sy (Fig . 9) provides the best signature for decays of this type. In the framework of the Standard Model, theoretical predictions for

B --+ K*y

We have searched for the exclusive final states B ° -> K*°y and B- -4 K* - y [27], where the modes K* ° --> K- ,7r + and K* - -> Ko ir-, K- iro are used to reconstruct the secondary particles. Unfortunately, the fraction of b--> sy which hadronizes to a particular final state is difficult to predict. Estimates for K*y are in the range 4-40% [301 . Because the photon has a large momentum, the background from BB events is small. The majority of background comes from e+ e - --+ q4 events, either from the decay of iro 's, or from initial-state radiation (ISR) in which the electron or positron radiates a photon before they annihilate . In addition to the usual techniques for suppressing the qq background, the ISR background is reduced by using variables evaluated in the e + e - rest-frame after the photon is radiated . In this boosted frame, continuum events still look jet-like . We require R'2 < 0.3 and I cos O' l > 0.5, where 0' is the angle between the photon and the thrust axis of the rest of the event. The invariant mass distribution is shown in Fig. 10 . We find 8 B o -> K* oy and 5 B- -* K* - y candidates in the signal region 5.274-5 .286 GeV/c2. The backgrounds are estimated by considering a two-dimensional "grand sideband" : 2 JAEJ < 280 MeV and MK* y > 5 .2 GeV/c , excluding the signal region ~AEJ < 100 MeV and MK*,. > 5.274 GeV/c2. II. e+e- EXPERIMENTS

28

D PaynelNucl. Instr and Meth. i n Phys Res. A 351 (1994) 19-30

Table 9 B- K*y branching fractions [271 Bo - K*oy K*O -. K+irSignal events Sideband background _ Residual BE background Branching fraction (10-5 )

K*

8 1 .1± 0.2 0 .30 ± 0.15 4.0 ± 1 7 ± 0 8

The scale factor between the number of background events in this sideband and the number falling in the signal region is estimated from a Monte Carlo simulation tuned to match the continuum data. Residual sources of background from BB events were also investigated using a high statistics Monte Carlo. This included a study of the possible feeddown from other b- sy modes. 1 Table 9 summarizes our results. If we make the reasonable assumption that the B ° -* K*° y and B - -r K* - y branching fraction are equal, we find : 13(B , K* y) = (4 .5f 1 .5f 0 .9) x 10 -5 , where the systematic error accounts for uncertainties in the acceptance and background estimates. This is consistent with Standard Model predictions for electromagnetic penguin decays . 12.2. b ~ sy inclusive We have also used the alternative approach of searching for an excess in the photon momentum spectrum [31 ] . The majority of photons from b-4 sy should appear in the momentum range 2.2-2 .7 GeV/c, which is considered the signal region . This is beyond the end-point of photonsproduced in b-* c events . The inclusive rate is particularly interesting because it can be calculated more accurately . Again, the dominant background is from continuum events which are suppressed using the event shape variables described above. After subtracting the remaining background using data collected at energies just below the B threshold, we observe an excess of 69 f 43 events in the signal region. From this we derive the upper limit: B(b - sy) < 5 .4 x 10 -4 at the 95% confidence level. This includes the systematic error from the uncertainty in the composition of the final states which determines the fraction of photons from b --+ sy which fall in the signal region . This result is consistent with theoretical expectations . The inclusion of more data will soon allow us to measure this inclusive branching fraction . 12 .3 . B - Kt * >e+eWe have also searched for the processes B-+ Kl*le+ e -. Within the Standard Model these are expected to occur either through an electromagnetic penguin decay in which the virtual photon materializes as an e+ e - pair, or via a weak box-diagram . In contrast to B--> K*y, angular momentum conservation allows the e + e - pair to be accompanied by either a K or a K* . Table 10 summarizes our results . The up-

B

-, K0ar-

2 0.05±0.03 0.01±0.01

- K* _Y

57±31±1 .1

K*-

K- vr

3 08±03 010±005

Table 10 B- K(*)e+e branching-fraction upper limits. Mode

UL (10-5 ) (90% CL)

K-e+eK* oe+e-

1 .3 17

K- p+ p+A_ K *o w

.5 2.8

Table 11 B-, p, m branching-fraction upper linuts . Mode

UL (10-5 ) (90% CL)

p- y

1.8 3.1 1.4

PY O)y

per limit of 1 .7 x 10-5 for B ° --> K*°e + e - is approaching the theoretical prediction of 0.5 x 10 -5 [32] . 12 .4. B -

(p, co) y

Using techniques similar to those in the B-+ K*y analysis, we have searched for the decays B - -4 p- y and B° --> (p°,(o)y [33] which are also mediated by an electromagnetic penguin, but in this case involve a b --+ dy transition . Because of the large natural width of the p and the difficulty of distinguishing pions from kaons at these momenta, there is a considerable background to the B° -r p9 y mode from B° -+ K*° y. A technique based on a neural network is used to suppress this . We do not find signals in any of these channels . Table 11 summarizes the upper limits we are able to place on their branching fractions. Because the top quark dominates these electromagnetic penguin loops, the relative values of the B-r (p, w) y and B-4 K*y branching fractions depend on the ratio of CKM matrix elements Vd/vt, : B(B --+ (p, a)) y) a B(B --+ K*y)

lid ya

,

(15)

where the constant of proportionality depends on the ratio of form factors involved . By combining these upper limits with our results for the B ~ K* y branching fraction we find :

D PaynelNucl. Instr. and Meth in Phys. Res. A 351 (1994) 19-30

I Ud/vts I < 0.56-0.62 at the 90% confidence level. The range of possible values reflects the spread in the predictions for the form factor ratio [ 34] . This agrees with the recent result from the ALEPH collaboration: I id/V, I < 0 .63 [ 35 ] . 13 . Summary Using our large sample of B mesons we have continued to expand our knowledge of the B-sector. We find that, in contrast to the charm sector, colour suppression is important in B decays . In fact, the only coloursuppressed decays we observe are those with charmonium in the final state. These have been used to measure the polarization in B --> iPK' decays which is found to be dominated by a single helicity state and will therefore be useful for CP violation studies. We have also seen several B decay modes for the first time . These include the colour-suppressed and Cabibbosuppressed decay B - -> 0,7r - , a mode with baryons in the final state Bo -4Ac- pvr+ 7r- , the charmless hadronic decays Bo --* a+ ,7r- and K+.7r - and the electromagnetic penguin decay B --+ K'y. We have performed several tests of the factorization hypothesis . Our findings indicate that this approximation is valid at least up to the mass of the al meson. More data will allow us to extend this study to higher q2 values . By comparing the branching fractions we have measured with the predictions of the BSW model, we have extracted values for the phenomenological parameters jai I and ja2I . We find that a2/al is positive at the B mass scale while an extrapolation from charm results predicts a negative value. The proposed CLEO/CESR upgrade has been approved [36] . This will involve a substantial increase in the number of electron and positron bunches circulating in CESR and should provide us with an integrated luminosity of 20 fb -1 by the year 1998, an order of magnitude more than we currently have. We also plan to improve the particle identification capabilities of the detector. Coupled with the increase in luminosity, this should open a new window on rare B decays, and perhaps provide the first observation of CP violation in the B sector towards the end of this century. Acknowledgements We gratefully acknowledge the CESR staff in providing us with excellent luminosity and running conditions .

[2] [3] [4] [5] [6] [7] [8] [9] [101 [111 [12] [13] [ 14] [151 [16] [17] [18] [191 [20] [211 [22] [231 [24] [251 [26] [27] [28]

[29] [30]

References 11] Discovery of the Y(3S) was reported by D. Andrews et al., Phys. Rev Lett . 44 (1980) 1108 and T Bohringer et al ., Phys Rev. Lett. 44 (1980) 1111 . Discovery of the Y(4S) was published by D. Andrews et al ., Phys. Rev Lett . 45 (1980) 219 and G. Finocchiaro et al., Phys. Rev . Lett. 45 (1980) 222.

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[341 J.M . Soares Phys. Rev. D 49 (1994) 283, S. Narison, CERN-TH-7166/94, Ali, Braun and Simma, CERN-TH-7118/93 . 1351 ALEPH collaboration, Phys. Lett . B 322 (1994) 441 .

[36] The CESR/CLEO upgrade, CLNS-93-1265 ; See also S. Gray, these Proceedings (2nd Int. Workshop on B-Physics at Hadron Machines, Le Mont Saint Michel, France, 1994) Nucl Instr. and Meth . A 351 (1994) 43