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Nuclear Physics B (Proc. Suppl.) 75B (1999) 201-207

Recent Heavy Meson Results from CLEO Mark Dickson a &Cornell University, Wilson Laboratory, Ithaca N.Y., U.S.A. Recent results from CLEO in the study of raxe, semileptonic mad hadronic B decays will be presented, as well as measurements of charm meson properties.

1. I n t r o d u c t i o n The C L E O detector is located at the Cornell Electron Storage Rin K (CESR) which operates on (or slightly below) the production threshold of the T(4S) resonance. These energies provide a dataset containing B ° B ° and B + B - pairs which are nearly at rest in the lab frame and (from the data taken slightly below the resonance) the means to statisticallysubtract the events arising from continuum events (e+e - --, q~ where q is a u, d, s or c quark). Between 1990 and 1995 C L E O accumulated 4.7 fb-I of data with the C L E O II detector [1]. After a briefshutdown, during which a siliconvertex detector was installed and the drift gas used by the primary charged particletracking chamber was changed, C L E O II.5 started taking data. As of the time of this conference ~ 6 fb-I has been accumulated with the new detector. These datasets contain more than ten million B-mesons and charmed mesons, allowing C L E O to conduct ever more precise measurements and sensitivesearches. The analyses I will review here are ones which have, at the earliest, been presented for the first time this spring and are restricted to the area of heavy meson physics. I will therefore be ignoring the m a n y interesting C L E O results which were presented lastsummer, as well as all those involving r , 2-photon and baryon physics. 2. R a r e B d e c a y s T h e rare decays of B-mesons fall into two distinct categories, those suppressed by the smallness of the relevant C K M matrix element and

those suppressed by loop diagrams (some decays, such as B --* lr~r, proceed via both mechanisms). These processes are characterized by branching fractions of the order 10 -4 or smaller. In the past C L E O has observed such processes and recently updated the measurement of the loop process b --, ~ . In addition, a new search for the baryonic decays of B-mesons which proceed via a b -~ u transition or a gluonic penguin has been completed. 2.1. I n c l u s i v e b ---, s7 It has been three years since CLEO reported [2] the first observation of the radiative penguin b --* s% The measurement of this inclusive branching fraction provoked a great deal of interest as it provided strong constraints on physics beyond the Standard Model [4]. This important analysis has just been repeated using the full CLEO II dataset. At the T ( 4 S ) center-of-mass energy, the signal for this process is a single photon, with between 2.2 GeV and 2.7 C,eV of energy, which originated from a B-meson decay. However, there are two very large backgrounds, both involving continuum events. In one, a photon produced by initial-state-radiation (ISR) is combined with a generic continuum event (i.e. e+e - --. qqT). In the the second, the decay of a ~r°, T/or w from a continuum event produces a photon whose energy falls in the signal region. Even after vetoing photons which, when paired with others, form a 7r° or ~/mass, these backgrounds remain a substantial problem. Therefore CLEO has used two powerful and largely independent methods to reduce these pernicious backgrounds and obtain a measurement of B ( b ---, sT). One method uses eight vaxiablcs, which de-

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scribe the event shape, to discriminate between the more collimated continuum events and the signal events where a B-meson has decayed essentially at rest. Unfortunately, it was found that no simple set of cuts on these variables could yield the needed discrimination. Cutting on an optimized linear combination of these variables also proved inadequate, so a neural network was used to combine the eight variables into a single variable "r". Figure 1 shows the distributions found for signal and background Monte Carlo (MC) samples. These distributions are then used to assign a weight to each event which is proportional to the probability that the event is from b --, sT. Figure 2 shows the photon energy spectrum for the weighted events. The data shown correspond to ~,, 3.3 x 106 B B pairs. A clear excess is seen in the signal region, consistent with that expected from b --, s? transitions. The second method used to dig the signal out from the enormous continuum background is based on vetoing any events which are not kinematically consistent with a B-meson decay. In this method a X~ variable defined as [3],

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is used to veto events which are not consistent with B-meson decays. In addition, a cut is made on the angle between the thrust axis of the candidate B and that found for the remaining event. In continuum events these two axes tend to align, while in B-meson decays the two axes are distributed randomly with respect to one another. This method has a much lower efficiency than the first (only 10% as opposed to 30% for the event shape analysis), but the background rejection is about 4 times that of the first, so the two methods have approximately equal statistical significance. Figure 3 shows the photon energy spectrum, both before and after subtraction, for the data after the continuum veto has been applied. Restricting the photon energy to be between 2.2 GeV and 2.7 GeV (and then correcting the yield by a factor of 0.85) we find the preliminary CLEO II inclusive branching fraction/3(b --, sT) to be (2.11 + 0.68 + 0.13 ± 0.36) × 10 -4 (Eventshape method) and (2.89 ± 0.50 ± 0.15 ± 0.35) x 10 -4 (B Reconstruction method), where the uncertainties are statistical, back~ound subtraction and systematic respectively. Combining both methods, CLEO obtains a preliminary result of

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B(b --, s7) = (2.50 + 0.47 ± 0.39) × 10 -4. It is important to note that A L E P H has also observed this process and measures [5] the branching ratio to be B(b --, s7) = (3.11 ± 0 . 8 0 ± 0.72) × 10 -4, in good agreement with CLEO. There have also been advances in the theoretical understanding of this process; a recent NLO calculation [6] finds B(b --* sT) = (3.28 :]: 0.33) × 10 -4 , in good agreement with the measurements. 2.2. R a r e B --, B a r y o n s CLEO has also searched for rare hadronic decays of B-mesons. In the case of the two body decays to mesons such as B --, r~r, B -~ K~r, B --, ~7'K observations have been reported last summer, and while there have not been any recent developments in these meson modes, a new search has been completed for decays involving protons and A's. Figures 4(a-f) show the dominant diagrams expected to contribute to the decays B --, AA, pA, / ~ or pA~r. As in the case of B --, K~, most predictions assume the penguin contribution is small. However, CLEO's observed rate for B -* KTr implies a large penguin contribution to these meson

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modes, so it was interesting to search for similar B decays to baryons. This analysis utilized the entire CLEO II and first portion of the CLEO II.5 datasets which contain 5.75 × 106 B B pairs. The signal yield was determined by a likelihood fit to five variables which distinguish continuum background from B decays. However, as is often the case in such a search, no clear signal was observed. Therefore CLEO has determined the 90% CL upper-limits for these decay modes of the B-meson. These limits and the theoretical expectations are given in Table 1.

Table 1 (CLEO II Preliminary) 90% CL upper-limits for the listed modes. Mode CLEO Theory B °-,AA < 3 . 9 × 1 0 -8 0 . 1 3 x 1 0 -6 [7] B +~p-A < 2 . 6 x 1 0 -6 - - ~ 3 x 1 0 -8 [8] B ° --~ pATr- < 13 x 10 -6 ~ 0.5 x 10 -6 [9] B °~p~ < 7 . 0 x 1 0 -6 4 . 8 x 1 0 -6 [7]

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Table 2 (CLEO II Preliminary) The helicity amplitudes found from the likelihood fit to the angular distribution ~)(0D-, cosOp,X). The first uncertainties are statistical and the second are systematic. Bo - , D . _ p + B + - , ~.Up+ magnitude phase,' magnitude phase [Ho[ 0.936 0 0.932 0 IH+I 0.152 =t: 0.058 ± 0.037 1.47+0.37±0.32 0.228±0.069±0.036 0.95±0.31±0.19 ]H_] 0.317±0.052±0.013 0.19±0.23±0.14 0.283±0.068±0.039 1.13±0.27±0.17

3. H a d r o n i c B Decays: B ---, D*p The study of rare decays of B-mesons are only one of the many tools we have at our disposal in understanding the heavy meson sector of the standard model. Of equal importance are the precision measurements which are made possible by the large numbers of B's in the CLEO dataset. A perfect example of this is the measurement of the helicity amplitudes of the decay B ° ~ D * - p + and B + --, -fl0p+ (charge-conjugate reactions are implied throughout this paper). Hadronic weak decays of the B-meson are complicated by the strong interactions described by QCD. To a large extent, however, exact solutions to these QCD equations do not exist. Understanding these interactions in B decays has become increasingly important as the search for CP violation in the b-sector of the standard model is approaching a new era with the construction of the B factories. This is because direct CP viola-

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tion in B decays depends on interference between diagrams which differ by a strong phase, due to final state interactions (FSI). A means to search for these FSI's (and also test the validity of the often used assumption of factorization in hadronic decays) is the measurement of the helicity states of the D* and p emerging from the decay of a B-meson. The angular distribution of the decay products can be described by three angles, OD., cos Op and X which are defined in Fig. 5. The angular distribution, Z)(OD., cos0p, X), can be written as the coherent sum of the helicity states in terms of three complex amplitudes (H0, H+ and H_): [ Ho]O,O > +//+]1, I > + H - l - 1,-1 > 12 where the numbers denote the D* and p helicity states and ]0, 0 > = ~.~cOSOD.cOsOp I1, 1 > = 4---~sinOo.sinOpeiX~'-: ] - 1 , - 1 > = 4--~2sinOD.sinOpe-~X with the coherent sum normalized to unity. The results of an unbinned likelihood fit to the B ° ~ D * - p + and B + ~ -flop+ data are given in Table 2. The small difference between the amplitudes found for the two decays could be due to the extra inner spectator diagram which contributes to the B + --, ~.0p+ process. The longitudinal polarization found in this measurement, 0.8?8 ± 0.034 ± 0.040, is consistent with the theoretical expectation of 0.85-0.88 for B ° ---* D * - l % , at q2 _- rap2 as well as an earlier estimate obtained by CLEO from the measurement of the form factors for the semileptonic decay [11] as expected from factorization. The fits also yield nontrivial phases for the various amplitudes, perhaps proriding us with our first evidence for final state interactions in B decay.

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4. Semileptonic B Decays: B ---+ D l v Another example of a detailed analysis which was only recently possible to complete at CLEO, is the study of the form factor relevant to the semileptonic decay B --+ D l u . Semileptonic decays of the B-meson, when combined with Heavy Quark Effective Theory (HQET), provide us with our best determinations of the CKM matrix element Vcb. In order to determine Vcb from the semileptonic decay of the B-meson, one must extrapolate the differential cross-section to the point of zero-recoil of the D-meson. This is troublesome as the cross-section drops to 0 at this point in phase space, and the theory cannot predict the form factor away from this point. It is therefore desirable to use the data to constrain the form factor in addition to determining ]Vcb] and the branching fractions. In this analysis, events containing a D and a lepton are selected. After all selection criteria have been applied, the sample contains four classes of events, B --, D h , , B ---, D ' I v , B ---, D**h~ a n d B --, D(*)~rh, (the presence of the last three processes is further reduced by requiring E o > 3.4 GeV in the determination of the

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B branching ratios). The distribution of a variable (cos0B-D~ [12]) which is sensitive to the invariant mass of the particle or particles recoiling against the D - l pair is then fit, allowing the normalization of the four shapes, taken from Monte Carlo, to float. An additional constraint (based on isospin symmetry), that the ratio of vector to pseudo-scalar decays should be the same for charged and neutral B-mesons, is also applied. Figure 6 shows the results of fitting the CLEO II dataset. From these fits (plus the world averages for the B-meson lifetimes), CLEO finds (preliminary) B(B- --* D°l-~) = (2.12 4- 0.18 4- 0.17)% and B(-B ° ---* D + l P ) = (2.05 4- 0.18 4- 0.20)%, where the first uncertainty is statistical, the second is systematic. To obtain a measurement of IVcbJ, CLEO uses the differential decay width dF/dw (where w = 0*B • ~TD), and parameterizes the form factor as FD(W) = Fp(1)(1 -- p2D(w -- 1) + CD(W -- 1)2). Allowing p~) and Co to float, CLEO finds (preliminary) IV~lFo(1) = (4.07+L52 1.04) x 10 -2. The values obtained for the slope (p~) and the curvature (CD) of the form factor are shown in Fig. 7. Using theory [13] to further constrain the form factors, CLEO finds ]V~blFD(1) = (4.74-0.64-0.4) × 10 -2. This result for IV~bl is consistent with that previously obtained using B --, D*lr, data.

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Other potential handles on the shape of the form factors in B decays rely on the study of the charmed mesons. One example of this is the synthesis of chiral and heavy-quark symmetries into an effective theory (HHCPT) which, once several parameters are constrained by data, provides strong constraints on the form factors in the bottom sector [14I. 5.1. D* w i d t h One such experimental constraint comes from S(D'+-.D%) CLEO's published ratio l~(D,+._,D+Tro ) .~ 0.055 + 0.017' [15]. This branching ratio, under very loose assumptions, allows the determination of the three branching fractions of the D *+. These branching fractions are particularly important to the study of B and charmed mesons as the decay of the D *+ is frequently used to tag the event. Its measurement also allows, when combined with the previously measured ratios B ( D ; + ..-*D+~ °)

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framework of heavy hadron chirai perturbation theory (HHCPT) [16]. This theory describes the data well and predicts F(D *+) = (22 - 39) keV or (281 - 1157) keV (depending on which of the two solutions for g and ~ is used). A true test of the theory would be the measurement of the width of the D* meson. It is tempting to believe that CLEO's recently installed silicon vertex detector might make such a direct measurement possible. Figure 9 shows the mass difference, 6 M - M ( K r 7 ~ +) - M ( K 7 0 - M,~+, found in an early analysis of CLEO II.5 data. One sees that while excellent resolution has been achieved, more than a factor of two improvement over CLEO II, pushing the limit on F(D *+) (currently at 131 keV from the ACCMOR collaboration) will have to wait until there is an extremely solid understanding of the charged particle tracking resolution. However, even ignoring the effect of detector resolution (which can only act to widen the mass difference peak), CLEO's result reinforces the conclusion implied by ACCMOR's upper-limit on F(D*+), that the correct solution from the HHCPT calculation is F(D *+) = (22 - 39) keV. If this is correct, a direct measurement of F(D *+) will be impossible at CLEO.

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5.2. D Lifetimes While width measurements appear to be out of the question, lifetimes are not. CLEO hopes to be able to compete with the huge statistics currently available at fixed target experiments because the backgrounds at the T(4S) are much smaller. In a preliminary analysis just recently completed, CLEO has measured the lifetimes of the D °, D + and Ds+ to be (0.403 ~: 0.009)ps, (1.034:1=0.033)ps and (0.475 + 0.024)ps respectively. These results were obtained using ~ 1.6 fb -1 or 1.9 million c2 pairs of CLEO II.5 data. By the end of the year it is expected that CLEO II.5 will have recorded more than 8 fb - t , and as the systematic uncertainties are determined from the data, it is expected that the total uncertainty will scale with the luminosity. Therefore it is not unreasonable to expect that the uncertainties will decrease by a factor of two by this time next year, putting CLEO in striking distance of the best charm lifetime measurements.

REFERENCES

6. Conclusions

8. V. Chernyak and I. Zhitnitsky, Nucl. Phys. B345, 137 (1990). 9. Spectator model calculationfrom B(Bh+yr~ - ) = (6.2 + 2.7) x 104 [10]. Does not include penguin effects. 10. CLEO Collaboration, X. Fu et hi. Phys. Rev. Lett. 79, 3125 (1997). 11. CLEO Collaboration, J. E. Duboscq et al., Phys. Rev. Lett. 76, 3898 (1996). 2EBEDI M~ - M~t 12. COSas-DI =2lpsl[PDtl

1. CLEO Collaboration, Y. Kubota et al., Nucl. Instrum. Methods Phys. Res., Sect. A 320, 66 (1992). 2. CLEO Collaboration, M.S. Alam et hi., Phys. Rev. Lett. 74, 2885 (1995). 3. E is the reconstructed B-meson energy, Eb is the beam energy, MB is the B-meson mass and M B c =-- j E ~ - [ Z ~ [ 2 where/Y, are the mometa of the B daughters. 4. See for example, F. Borzumati and C. Greub, hep-ph/9802391; W. de Boer et ed.,hepph/9805378; H. Baer et al., Phys. Rev. D58 15007 (1998). For a review see J. Hewett, SLAC-PUB-6521 (1994). 5. ALEPH Collaboration, R. Barate et hi., Phys. Lett. B429, 169 (1998). 6. Chetyrkin, Misiak, and Miinz, Phys.Lett. B 4 0 0 206 (1997); Erratum-ibid.B425 414 (1998). 7. See for example, M. Jarii et hi., Phys. Lett.

B237, 513 (1990). The physics program at CLEO continues to be very active. I have reviewed only the latest results from CLEO (restricting myself further to only those applicable to heavy meson physics), attempting to demonstrate the wide scope of physics topics which can be addressed at T(4S) energies. Currently CLEO has a great many analyses in progress. For example, one can expect that in the very near future (the end of this summer) new resuits on eharmless two body decay modes of the B will be available. In addition, it is expected that charm measurements such as branching fractions and lifetimes of the D o to K K , 7r~r and K~¢ will be available by the end of the year. It should also be noted that CLEO is nearing the installation of a major upgrade to the CESR storage ring, as well as to the CLEO detector. CLEO III is scheduled to be installed early next year, and with its ring imaging Cherenkov detector, 4-layer silicon vertex detector and new drift chamber, it is anticipated that CLEO will continue to provide major contributions to the study of heavy mesons well into the next century.

-

13.

14.

15. 16.

where FD, = PD + ~ (the sum of D and lepton momenta),/~B is the B momentum. I. Caprini, L. Lellouch and M. Neubert, CERN-TH/97-91. C. G. Boyd, B. Grinstein and R. F. Lebed, Phys. Rev. D56, 6895 (1997). For example see; Gnstavo Burdman and Joachim Kambor, Phys. Rev. D55 2817 (1997). CLEO Collaboration, J. Bartelt et hi., Phys. Rev. Lett. 80, 3919 (1998). Iain W. Stewart, hep-ph/9803227, (to appear in Nucl. Phys. B).