cleo - recent results and future prospects

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Nuclear Physics B (Proc. Suppl.) 111 (2002) 40-47 CLEO - Recent Results and Future Prospects A. Bornheim a For the CLEO Collaboration* %alifornia Institute of Technology, Pasadena, California 91125, USA We report the recent results from the CLEO collaboration on measurements of the photon energy spectrum for b + sy and hadronic recoil mass in B + XJz?. Using HQET we extract IVcbl from semileptonic decay rates and II&l from the lepton endpoint spectrum of B + X,ZV with reduced theoretical uncertainties compared to previous measurements. We give an outlook on plans of the CLEO collaboration to transform CESR and CLEO into a charm factory. The physics potential in this regime is discussed. 1. Introduction The precise determination of the CKM matrix elements and the verification of the flavor sec- tor of the standard model down to the percent level is the main goal of modern experimental heavy flavor physics. To achieve this goal vari- ous experimentally determined quantities have to be linked to the fundamental parameters of the theory. This procedure is in many cases compli- cated by non-perturbative effects which can not be calculated from first principles. However, by using HQET [l] expansions many of these effects can be treated in a systematic way. By combin- ing different b decay modes and using HQET ex- pansions we can reduce theoretical uncertainties in the extraction of standard model parameters. In chapter 2 of this paper we present the latest results from the CLEO collaboration on this ef- fort. The results on the b + sy energy spec- trum and the extraction of IVubl and IVcbl were obtained from the CLEOII/II.V data set which was recorded between 1989 and 1999. After a brief period of further data taking on the T(4S) with an upgraded detector, CLEO III, the CLEO collaboration has entered a new era as of fall 2001. CESR, the e+e- collider provid- ing CLEO with luminosity, will be transformed to operate at center of mass energies down to 3.5 GeV. With data to be taken between 2002 and *This work is supported by the DOE and the NSF. 2005 CLEO will perform high precision measure- ments on the T-resonances lS, 2s and 3S and on and around the !4j-resonances in the charm sec- tor. Among other physics topics of great interest we will be able to extract many parameters such as decay constants and absolute branching ratios which will further help to test and improve the de- scription of non-perturbative effects in the heavy flavor sector. The progress on this effort to date and some key measurements are discussed in the chapter 3 of this paper. 2. Recent results from CLEOII/II.V 2.1. The b + sy Photon Spectrum The b -+ sy transition is only permitted via loop diagrams in the standard model. Our new analysis aims, besides reducing the error on the inclusive branching ratio, for an extraction of the photon energy spectrum and its moments. The major challenge is to reduce the dominant con- tinuum background by a factor of one hundred. This is achieved by a combination of neural net techniques using event shapes, kinematics of de- tected leptons, and a pseudo-reconstruction seek- ing the best K(n7r) combination. The raw photon energy spectrum obtained with this approach is shown in the upper plot of Fig. 1. To extract the b + sy contribution to this spectrum BB back- ground has to be subtracted which is determined based on monte car10 simulations of generic BB 0920-5632/02/$ - see front matter 0 2002 Elsevier Science B.V All rights reserved. PI1 SO920-5632(02)01682-l

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Page 1: CLEO - recent results and future prospects

Nuclear Physics B (Proc. Suppl.) 111 (2002) 40-47

CLEO - Recent Results and Future Prospects

A. Bornheim a For the CLEO Collaboration*

%alifornia Institute of Technology, Pasadena, California 91125, USA

We report the recent results from the CLEO collaboration on measurements of the photon energy spectrum for b + sy and hadronic recoil mass in B + XJz?. Using HQET we extract IVcbl from semileptonic decay rates and II&l from the lepton endpoint spectrum of B + X,ZV with reduced theoretical uncertainties compared to previous measurements. We give an outlook on plans of the CLEO collaboration to transform CESR and CLEO into a charm factory. The physics potential in this regime is discussed.

1. Introduction

The precise determination of the CKM matrix elements and the verification of the flavor sec- tor of the standard model down to the percent level is the main goal of modern experimental heavy flavor physics. To achieve this goal vari- ous experimentally determined quantities have to be linked to the fundamental parameters of the theory. This procedure is in many cases compli- cated by non-perturbative effects which can not be calculated from first principles. However, by using HQET [l] expansions many of these effects can be treated in a systematic way. By combin- ing different b decay modes and using HQET ex- pansions we can reduce theoretical uncertainties in the extraction of standard model parameters. In chapter 2 of this paper we present the latest results from the CLEO collaboration on this ef- fort. The results on the b + sy energy spec- trum and the extraction of IVubl and IVcbl were obtained from the CLEOII/II.V data set which was recorded between 1989 and 1999. After a brief period of further data taking on the T(4S) with an upgraded detector, CLEO III, the CLEO collaboration has entered a new era as of fall 2001. CESR, the e+e- collider provid- ing CLEO with luminosity, will be transformed to operate at center of mass energies down to 3.5 GeV. With data to be taken between 2002 and

*This work is supported by the DOE and the NSF.

2005 CLEO will perform high precision measure- ments on the T-resonances lS, 2s and 3S and on and around the !4j-resonances in the charm sec- tor. Among other physics topics of great interest we will be able to extract many parameters such as decay constants and absolute branching ratios which will further help to test and improve the de- scription of non-perturbative effects in the heavy flavor sector. The progress on this effort to date and some key measurements are discussed in the chapter 3 of this paper.

2. Recent results from CLEOII/II.V

2.1. The b + sy Photon Spectrum The b -+ sy transition is only permitted via

loop diagrams in the standard model. Our new analysis aims, besides reducing the error on the inclusive branching ratio, for an extraction of the photon energy spectrum and its moments. The major challenge is to reduce the dominant con- tinuum background by a factor of one hundred. This is achieved by a combination of neural net techniques using event shapes, kinematics of de- tected leptons, and a pseudo-reconstruction seek- ing the best K(n7r) combination. The raw photon energy spectrum obtained with this approach is shown in the upper plot of Fig. 1. To extract the b + sy contribution to this spectrum BB back- ground has to be subtracted which is determined based on monte car10 simulations of generic BB

0920-5632/02/$ - see front matter 0 2002 Elsevier Science B.V All rights reserved.

PI1 SO920-5632(02)01682-l

Page 2: CLEO - recent results and future prospects

A. Bornheim/Nuclear Physics B @‘roe. Suppl.) III (2002) 4&47 41

‘--% BB Prediction

2 3 4 5 E, (QW

Figure 1. Observed photon spectrum shown in a) with the scaled continuum background predic- tion from off-resonance data, and after continuum subtraction in b), where the BB background is now displayed.

processes. The BB contributipn to the E7 spec- trum is shown in the lower plot of Fig. 1. Af- ter subtracting this contribution we get the pure b -+ s y photon energy spectrum shown in Fig. 2.

This spectrum, naively a sharp line, is smeared out by the b quark Fermi motion and varying re- coil mass (i.e., QCD effects). There are also small contributions from the B boost (@ M 0.06) and detector resolution effects. We extract the mo- ments, accounting for efficiencies, resolution and boost smearing. We estimate the model depen- dency of the moment extraction using the Ali- Greub [2] and the Kagan-Neubert [3] approach. We find, for ET > 2.0 GeV [4]:

(ET) = (2.346 f 0.032 f 0.011) GeV

(,?$ - (EJ2 = (0.0226 f 0.0066 f 0.0020) GeV2

where the brackets (...) denote the average value. To lowest order, (ET) = 4 [MB - ii], where A measures the energy of the light degrees of free- dom of the B meson, sometimes referred to as ‘brown muck’. Second order terms in ~/MB are absent in this particular expansion and third or- der terms are estimated as 0(0.5GeV3). From first order alone we find A = (0.35 f 0.08 f 0.10) GeV [4] with the first error entirely due to ex- perimental uncertainties and the second related to the theoretical uncertainties in the extraction

- Spectator Model

Figure 2. Measured photon spectrum for b -+ s y events. Scaled continuum background prediction from off-resonance data and BB background has been subtracted. The data is compared to a spec- tator model prediction.

procedure. Furthermore, we avoid using second moments which give poorer determinations of the parameters and, as in the case of hadronic moments to be discussed later, have expressions which do not converge as rapidly. It should be pointed out that all calculations where done in the MS scheme to order l/M; and @so:. The extracted moments are only meaningful if treated consistently in scheme and order.

2.2. Hadronic Moments in B + XC@ We use both e and p in the momentum range

1.5 < pe < 2.5 GeV/c, reconstructing the neutrino properties via four-momentum balance and the hermeticity of the detector [8]. Using cuts on charge balance, a multiple-lepton veto and &&,, = (Eki,, - pki,,) guarantee that all observed particles are accounted for exactly once. This helps to ensure that the missing four- momenta, which is measured with a resolution of o(pmiss) = llOMeV/ c, is due to a single miss- ing neutrino. After checking consistency with zero missing mass, we use E, = lpmissI rather than Emiss since it has better resolution. We

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42 A. Bornheim/Nuclear Physics B (Proc. Suppl.) III (2002) 4&47

use B decay kinematics to determine Mj$ = Mi -I- M& - 2EBEeg + 2p’B . p’ep without need- ing to observe the hadrons or group them into those from the B vs. the B decay. The dot prod-

uct 2p’B . &, which averages zero, is neglected. Thus, four-momentum balance of the entire Bl? event measures the neutrino properties, leaving the hadronic recoil system unobserved. Thereby we avoid systematic uncertainties related to ex-

plicitly reconstructing the hadrons. After contin- uum subtraction we expect a small 5% contam- ination from c -+ s& secondaries and b + u& which is subtracted using the monte carlo. Final results for the moments are determined from the Mi distributions corresponding to the mixture of DCV, D*Cfi, and X&v spectra which best fit the data as shown in Fig. 3.

- Fit +

0 -4 -2 0 2 4 6 6 1

% WV*)

Figure 3. Observed recoil mass in B + X,G showing data as points along with a fit to D, D* and heavier charm meson contributions.

We take moments of the generated Mj$ distribu- tions while fitting the data to reconstructed quan- tities passed through the full physics and detec- tor simulation and analysis, hence accounting for the B boost, resolution, efficiency and neglecting of the p’B . p’en term. Heavy states XH beyond the D and D* include D** states modeled with ISGW2 [9] d an non-resonant D(*)T as described by Goity-Roberts [lo] with their normalization

fixed by data. We extract [ll]:

(MS - &fk) = (0.251 f 0.023 f 0.062) GeV2

((Mg - @,)2) = (0.639 f 0.056 f 0.178) GeV4

((M$ - (Mi))2) = (0.576 f 0.048 f 0.163) GeV4

where &f~ denotes the spin-averaged D, D* mass.

The main systematic uncertainties are due to the neutrino reconstruction efficiency and the model dependency of the XH state simulation. The the-

0.2 0.4 0.6 0.8 1.0 X

Figure 4. Constraints from measured b + s y photon energy and B + X,CV recoil mass first moments in the A-Xi plane. The ellipse indicates

AX2 = 1, including systematic errors.

oretical expressions for the moments [12-141 use

a consistent scheme and include most of the ef- fects of the lepton energy cut. Unlike the mean photon energy discussed earlier, the second order HQET expansion parameters appear here. These are Xi, related to the Fermi motion energy of the b quark, and X2, measuring the QCD hyper- fine splitting; the latter is fixed from mB* - mg [15]. The third-order terms are again estmated estimated as 0(0.5GeV3). Combining with the b --+ sy result, we find [ll]:

;i = (0.35 * 0.07 Z!Z 0.10) GeV

x1 = (-0.236 f 0.071 f 0.078) GeV”

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A. Bornheim/Nuclear Physics B (Proc. Suppl.) III (2002) 4&47 43

The errors have the same meaning as for the b + s y moments. The results are best viewed in the li - Xi plane as shown in Fig. 4.

2.3. Extracting lVubl from the B + X,&S Endpoint

The extraction of IV&l from the B + X,& Endpoint is complicated by the very large b + c backgrounds as shown in Fig. 5. We can therefore only measure the portion of the rate near the lep- ton momentum endpoint above some phi,. Un- fortunately it is difficult to calculate the fraction of the lepton spectrum above a certain momen- tum cut. We rely on local duality and terms of order l/(iPf~ - 2p$,) can enter, endangering the convergence of the expansion.

We can get information about the details of the endpoint b -+ uCV rate from the observed b -+ s y spectrum, since they are smeared by a common non-perturbative structure function [18,19], up to corrections of order hoco/rnb [20]. We can extract the structure function from b + sy and then use this to predict the fraction of b + utr? rate above the experimental lepton momentum cut. Details of this method are still under discus- sion [21].

0 0.5 1.0 1.5 2.0 2.5 3.0

p,(GeVIc)

Figure 5. Example of a prediction of the lepton spectra for b + c.G and b + ulv. Note the that the b + ulv contribution is scaled by a factor of 10.

2.00 2.25 2.50 2.75 3.00 Momentum (GeV/c)

Figure 6. Inclusive lepton spectrum in data near the endpoint. The upper plot gives the raw spec- trum for the continuum (shaded) and b + c (open histogram) contributions; the lower plot shows the efficiency-corrected b + td5 rate.

A neural net is used for continuum suppression and the signal region in lepton momentum com- prises 2.2 < pe < 2.6 GeV/c. We have lowered our cut from 2.3 GeV/c to increase the rate and reduce model dependence. The data are shown in Fig.6. We observe good subtraction for pl > 2.6 GeV/c, and extract (1874 f 123 f 326) B + X,.@ events, resulting in a partial branching ra- tio (before radiative corrections) of A&,(2.2 - 2.6GeV/c) = (2.35 f 0.15 f 0.45) x 10P4. Sys- tematics uncertainties include variations of form factors and heavy charm states in Monte-Carlo modeling of b + & backgrounds. The extrac- tion of 1 V&, ) is based on an expression for the in- clusive rate derived in the upsilon expansion [22]:

l&l = [(3.06 f 0.08 f 0.08) x 1O-3] x

[(&b/0.001). (1.6ps/r~)]~‘~

The required &, = B(B -+ X,&) is related to the observed rate A&, in momentum window (p)

by A&(p) = Fu(p) &,. The b + s y photon energy spectrum will provide our prediction for F,(p), which is simply a properly normalized in- tegral of the observed portion of the solid curve of Fig. 5. Using b + s y data with 1.5 < E7 < 2.8

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44 A. Bornheim/Nuclear Physics B (Proc. Suppl.) 11 I (2002) 4&47

GeV, we fit the shape function [3] to various pa- mass energy range between 3.5 and 5 GeV. Mod- rameterizations. We use this to determine [23] ifications needed for this change in energy are in- F,(2.2 - 2.6 GeV/c) = 0.138 f 0.034. Consis- stallations of wigglers into CESR to maintain low tent results are obtained by the more model- beam emittance at low energies and a replace- dependent method of fitting the spectrum to pa- ment for the CLEO silicon detector. This device rameters in the Ali-Greub spectator model [2] and suffered from unexpected radiation damage and feeding this information into the ACCMM model will be replaced by a low mass drift chamber in [24,25]. Our preliminary result is: fall 2002.

IVubl = (4.09 f 0.14 f 0.66) x 1O-3

This is in good agreement with CLEO’s exclusive (~/p/w)& analyses [8,26]:

3.2. Physics opportunities on the ‘r-resonances lS, 2s and 3S

IV&l = (3.25 f O.l4_t;:;; f 0.55) x 1O-3

3. Recent results from CLEOIII and out- look on CLEO-c

3.1. The CLEO III detector : Performance and modifications for CLEO-c

The CLEO detector was upgraded in 1998 and 1999 with a new silicon tracker, a RICH detector and a new DAQ system to handle event rates up to 1 kHz. Together with the magnet, muon cham- bers and the electro magnetic calorimeter from the previous incarnation of CLEO this setup is referred to as CLEOIII. It was used to record 9.2fb-1 of data around the Y(45) resonance be- tween fall 2000 and summer 2001. Branching ra- tios for several rare B decay modes have been measured with this data set [36]. Some modes like B + DK are measured with with better preci- sion than from the CLEO II/II.V data set because of the tremendous improvement in K/T separa- tion from the new RICH detector. Results from CLEOIII are in good agreement with those ob- tained from CLEOII/II.V in modes where they are available for both data sets. With two high luminosity e+e- colliders operat- ing on the Y’(4S) the CLEO collaboration decided to discontinue operating above the BB thresh- old and focus instead on physics at lower ener- gies. During a transition period lasting from fall 2001 until late 2002 CLEO will take data on the Y-resonances lS, 2S and 3s. Starting in early 2003 CLEO will take high statistics data sam- ples, typically more than two orders of magni- tude bigger than existing ones, in the center of

Data currently being taken by CLEO on the T- resonances lS, 2S and 3S will increase the world data sets in this range by an order of magnitude. This will provide precision data for development of phenomenological models and rigorous tests of lattice &CD, as well as a good prospects for dis- covering qa(lS) in hindered Ml transitions. A survey of radiative decays of T(lS) which com- plements J/q data in glueball searches is also on the list of interesting topics. Further details of the physics program can be found in [37].

3.3. Physics opportunities at CLEO-c Data to be taken by CLEO in the center-

of-mass energy range between 3.5 and 5.0 GeV will enable us to extract valuable information on decay constants, absolute branching ratios and semileptonic decay form factors in the charm sec- tor. It will also allow a direct determination of the CKM matrix elements Vcd and V,,. More gen- erally speaking, it will provide deeper insight in the transition region between light quark physics and bottom quark physics. The latter benefits from HQET which allows the extraction of fun- damental parameters form experimental measure- ments as shown earlier with the CLEO II/II.V re- sults. However, some measurements are still lim- ited by uncertainties coming from our poor under- standing of non-perturbative strong interactions, some of which we hope to reduce by studying the transition region around the !l? resonances. The theoretical framework which will be used to understand and control the non-perturbative ef- fects down to the charm quark scale and possibly even lower is lattice &CD. Recent advances in this field have provided a larger variety of non- perturbative results at an accuracy level of 10

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A. Bornheim/Nuclear Physics B (Proc. Suppl.) 111 (2002) 4&47 45

- 20 % for B and D mesons as well as T and @. High precision data with uncertainties at the 1 -2 % level is needed to validate these calcu- lations experimentally. Once validated this will help to reduce uncertainties in measurements in

the b quark sector in particular and the verifica- tion of the CKM mechanism in general. Extensive

Figure 7. Monte carlo simulation for the measured Kern invariant mass spectrum in $(3770) -+ Dfl events where the opposite D has been reconstructed (Double Tag).

monte car10 studies have shown that this level of precision can be achieved with CLEO-c. In Fig. 7 the invariant mass spectrum of D+ from $(3770) -+ DD decays is shown. The opposite D in the event has been reconstructed as well. This

double tag method provides an extremely clean sample needed to achieve 1 % errors on the abso- lute branching ratio. A high quality calibration sample for lattice QCD will be provided by ab- solute form factor measurements for every charm meson semileptonic pseudoscalar to pseudoscalar and every pseudoscalar to vector transition. A demonstration of the cleanliness of this analysis is shown in Fig. 9 where U = Emiss-Pmiss is plot- ted for the decays D -+ nlu and D + Klu. Close to perfect separation is achieved. The extraction of weak decay constants is provided from a mea- surement of leptonic decays such as D, + pv. Very good signal to background enhancement is achieved by cutting on the missing mass in tagged D,D, production at fi = 4100MeV as shown in

0.1 0.2 0.3

'J=E,iru-P,,s.(~V)

Figure 8. Separation between the Cabibbo sup- pressed D -+ dv (shaded) and the Cabibbo al- lowed D + Klu (non-shaded) which is produced

ten times more often in $(3770) decays. The dis- tribution shows the observable U = Emiss - Pmiss and is based on a monte carlo simulation of the CLEO-c detector.

Fig. 9.

4. Summary

Heavy flavor physics and the exploration of the CKM mechanism benefits increasingly from very high statistics measurements with small experi- mental errors. Theoretical uncertainties in the

extraction of fundamental parameters from the experimental results become more and more a limiting factor. In an effort to overcome these

limits the CLEO collaboration focuses on new techniques, using HQET, to better control non- perturbative effects that hinder the interpretation experimental results. We presented here new results from the CLEO collaboration on the measurement of the photon spectrum b + sy and the hadronic mass mo- ments from fi + XJV. These results are used to extract IV,bl with more controlled systematic errors. Using studies of the lepton endpoint spec- trum combined with constraints from the b -+ s y photon spectrum we extract l&,1 with reduced uncertainties. In the future the CLEO collaboration will expand

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46 A. Bornheim/Nuclear Physics B (Proc. Suppl.) 111 (2002) 4lw7

00

f”

h 6 zm I $ z O20

0 - 0.4 -0.2 hl(Y)2(:ev2k4) a2 0.4

Figure 9. Monte car10 simulation of the missing mass distribution for D,D, tagged pairs at fi = 4100MeV. The shaded area shows D, + ~1” events, t,he non-shaded area shows the summed background processes, demonstrating the excel- lent separation with the CLEO-c detector.

its efforts by undertaking a large variety of mea- surements in the charm quark sector. This will be done by transforming CLEO and CESR into a charm-factory. In the same way that we are able to better understand non-perturbative effects in the beauty sector by utilizing HQET, we hope to improve our knowledge in the charm quark sec- tor using lattice &CD. A number of key measure- ments to be carried out with CLEO-c are pre- sented in this paper.

5. Acknowledgments

I would like to thank the organizers of the KEKTC5 conference for a very inspiring event. The great effort of all my colleagues at CLEO and CESR is highly appreciated.

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