We review the semileptonic b-hadron branching ratio Br(b! lx) measurements from LEP. Both the inclusive

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1 Semileptonic Decays of the B mesons at LEP Sylvie Rosier-Lees a a LAPP, IN2P3-CNRS, F-7419 Annecy le Vieux Cedex We review the semileptonic b-hadron branching ratio Br(b! lx) measurements from LEP. Both the inclusive and exclusive measurements are presented and dierent methods for the inclusive measurements (decay Model dependent or not) are contrasted. From those measurements we extract a value of jv cbj. An updated measurement of the branching ratio Br(b! X) is also given. 1. INTRODUCTION One of the most important motivations in the measurement of Br(b! lx) is that it can be used to extract the CKM matrix element V cb. This can be performed using two independent methods: - By combining the inclusive measurements of Br(b! lx) and the mean b hadron lifetime b Br(b! l l X) b = G Fm 5 b (f cg c V 2 cb + f u g u V 2 ub)(1) Equation (1) becomes if we neglect V ub : Br(b! l l X) b = 2 c V 2 cb (2) - In the exclusive decay B! D?+`?`, jv cb j 1=2 B is estimated by extrapolating the partial width dbr(b! D?+`?`)=d! to!=1: dbr(b! D?+`?`) d! / V 2 cb B F (!)2 (3) where! is the scalar product of the four velocities of the B and D?+ mesons:! = v B v D?+ and F (!) is the Isgur Wise function with F (1) equal to :93 :3 according to [1]. Therefore we will give a review of the inclusive and exclusive Br(b! lx) measurements. In a complete dierent domain, the measurement of the branching ratio Br(b! X) allows to explore new physics, as explained in the last section Theoretical predictions Since the lifetimes of the B and B mesons are equal within 1%, the spectator model provides a good starting point for predictions.the nave spectator model predicts Br(b! lx) =.16. S (m Z ) Parton HQE[3] HQE [4] Model [2] Table 1: Theoretical predictions for the semileptonic branching ratio Br(b! lx) depending on S (m Z ) As the exchange of hard gluons between quarks enhances the non leptonic rates, the introduction of perturbative QCD corrections decreases significantly the nave spectator model prediction. The prediction for Br(b! lx) is further reduced by 2% by introducing non perturbative corrections of order O(1=m 2 b ) in HQE. Theoretical predictions for Br(b! lx) are quoted in table 1 for pole masses values m b =4.8 GeV and m c =1.33 GeV. To conclude, predictions on the semileptonic branching fraction depend on our knowledge of the eective quark masses and on a precise estimate of the QCD corrections, which as it was shown are important and tend to push Br(b! lx) down Measurements of Br(b! lx) The semileptonic branching fraction is measured at the (4s) experiments and at LEP experiments. Assuming the semileptonic decay width? SL is the same for all b avored hadrons, Br(b! lx) should be dierent at LEP compared to (4s) since dierent particles are pro- 1

2 duced, which leads to the following relation : Br(b! l)(z ) = 2 b + B + Br(b! l)((4s)) B Including the LEP lifetime measurements [5], B + = 1:55 :6 ps, B = 1:63 :6 ps and b = 1:57 :2 ps, it becomes : Br(b! l)(z )=Br(b! l)((4s)) = :98 :3 Compared to the (4s) measurement, the additional contribution of other hadrons like ( b or B s ) reduces slightly the expected average Br(b! lx) at the Z. 2. INCLUSIVE MEASUREMENTS We distinguish two approaches in this measurement. In the traditional approach the measurement depends on the phenomenological models describing the momentum spectrum of the lepton while in the other approach the measurement is independent of the knowledge of these models Traditional approach To select the semileptonic decays of the b quark, these well known properties are used : The fragmentation of the b quark (hx b ei :7) is hard which provides a high momentum to the lepton ( usually the momentum p has to be greater than 2-4 GeV/c ) b is heavy which provides to the lepton a high transverse momentum p t with respect to the jet ( typically p t has to be greater to GeV/c) After applying these selection criteria, an overall purity of 8 % with an eciency varying from 3 to 6 % is achieved. To enrich the sample in b events, the long lifetime property of the b hadrons is used by tagging the b b with the microvertex detector. A purity of 9 % is achieved with an eciency varying from 3 to 6 % depending on detector performances. The main experimental problem comes from the leptonic cascade (b! c! l) contamination which is the main background at high p T ; so usually both Br(b! lx) and Br(b! c! l) are measured simultaneously Single Tag-? b b dependent method According to equation (4) N l N had = 2Br SL (1? Br SL )? b b? had (4) Br(b! lx) and also? b b or Br(b! c! l) can be extracted from a t on the momentum p and the transverse momentum p T spectra. In this case, the product Br(b! lx)? b b is measured, where? b b is either assumed from the standard model [6] or measured [7],[8],[9]. As the single lepton tag events are used, this method has the advantage to be not statistically limited. A recent analysis [6], using dierent technique was performed by the L3 experiment. The branching ratio Br(b! X) is measured using the missing energy spectrum of the jets in the two jets events assuming E = E beam? E jets. This measurement with energetic neutrino is complementary to the traditional Br(b! lx) measurement (looking for electrons, muons) as it is sensitive to Br(b! X). The total semileptonic branching fraction measured is as shown on Figure 1. Br(b! X) = 23:8 :77 stat 1:24 syst % Br(b! lx) can be extracted if we assume a contribution of 1:1:.25 for e; ; in the spectrum Double Tag-? b b independent method In the limit of no background, Br(b! lx) is proportional to the ratio of the number of dilepton events to the number of single lepton events. N ll Br SL / N l 2 (1? Br SL ) (5) This method [11] has the advantage to be independent at the rst order of? b b but is statistically limited as it is based on the dilepton events sample. In a recent measurement, performed by the ALEPH experiment [1], the Z decays into b quarks are selected with high eciency ( 6%) and a very good purity ( 95%) on the basis of the long b-hadron lifetimes. The lepton yield and spectra in hemispheres containing

3 1 4 Data Fit Fit b νx Events/GeV Jet energy [GeV] Figure 1. The jet energy spectrum in the L3 data and the results of the t for the measurement of Br(b! X) Figure 2. ALEPH data. Comparison between measured spectra and ts results for Single lepton spectrum (top) and dilepton sample (bottom) one lepton candidate or two opposite charge leptons candidates are used. Both Br(b! lx) and Br(b! c! l) are extracted from the t on the p T spectrum of the single lepton and lower p T of dilepton events, the result on the t is shown on Figure 2. Compared to the dilepton technique,this measurement is not statistically limited Models dependence In all the methods described previously, the Br(b! lx) measurement depends on the eciency which is highly dependent on the lepton momentum spectrum. If we soften the lepton momentum spectrum, we decrease the eciency and consequently the Br(b! lx) increases. The momentum spectrum of the lepton in the b decays at LEP depends on: The momentum spectrum of the b hadrons itself; for a harder fragmentation the Br(b! lx) is lower: typically by varying the fragmentation from.68 to.72 the Br(b! lx) decreases by about.5 The momentum of the lepton in the b,c rest frame. Addition of D softens the lepton spectrum and we expect a higher Br(b! lx). According to the recommendations of the LEP electroweak working group [12] all LEP experiments use the same models: ACCMM, ISGW, ISGW [13],[14] for b! l decays with parameters extracted from a t on the CLEO data [15] and DELCO and MARK III[16] concerning the c! l decays Summary of the model dependent LEP results In Table 2 all the measurements are quoted with as a rst error the statistical error, the second one is the systematic uncertainty due to the detector performances and the inclusive lepton analysis proper to each experiment and the last error is due the modelling ( the central value is obtained when using ACCMM model, the ISGW and ISGW are used as extremes). We took into account this last correlated uncertainty in the estimation of the average value. If we assume R b = :22 and < x b e > =.7, the LEP average Br(b! lx) is (the last measurement quoted in Table 2 is not included in this average as it is statistically correlated to the third measurement): Br(b! l) = 11:7 :7 stat :16 syst :37 mod %

4 R b < x b e > Br(b! lx) (%) Experiment :2 :3 stat :32 syst :27 mod ALEPH (9-91) [7] :6 :39 stat :19 syst :19 mod DELPHI (91-92)[8] :73 :11 stat :3 syst :35 mod L3 (91-92) [6] :6 :6 stat :6 syst :4 mod OPAL (9-91)[9] (indep) :34 :13 stat :28 syst :35 mod ALEPH (92-93)[1] (indep) :44 :48 stat :43 syst :31 mod L3 (9-91) [11] Table 2:LEP Br(b! lx) measurements dependent on the models This measurement is clearly systematics limited as Br(b! l)=(br(b! l) 3? 4%; the weakness in those analyses is the dependence on lepton for model spectrum Model Independent method. Charge Correlation method in ALEPH The ARGUS collaboration [17] has introduced a new method using dilepton events that allowed to separate the contributions of primary and secondary lepton without relying on model dependent shapes. This method is also used by the CLEO experiment [18]. A recent measurement described in [1] uses no prediction for p T spectra: Direct and cascade leptons can be distinguished based on the correlation of their charge with that of the b quark. Two independent measurements are needed to separate them, for that two independent samples of b b events are selected. - In the rst sample b b events are selected by applying impact parameter tag to one hemisphere, therefore a purity of 99% is achieved: after that the number F1 of events containing a single lepton in the opposite hemisphere is measured F1 Br(b! c! l) + Br(b! lx) - In the second sample, opposite side dilepton events are selected. One tags one side with a high p t lepton or by using the jet charge technique (to increase the statistics), therefore the charge of the tag lepton gives b quark charge. Opposite sign lepton on the other side are selected with looser cuts on the momentum (p > 1GeV =c) in order to be less sensitive to the model. Then one counts how often the signal lepton charge and the tag lepton charge are opposite, F S is the number of those events. F os Br(b! lx) /(Br(b! lx) +Br(b! c! l) ) Two quantities are measured F1 and FOS for two unknowns :Br(b! lx), Br(b! c! l) the extracted Br(b! lx) is then : Br(b! l) = 11:1 :23 stat :28 syst :11 mod % Value consistent with the model dependent measurement; Here the model dependence has been further reduced at the cost of enlarging the statistical error by using charge correlations rather than ts to the p T spectra Comparison with measurements at the (4s) To compare (4s) to LEP results we apply the factor correction estimated in section 1.2 Model dependent Method Br(b! l) = 1:3 :4% (4s) Br(b! l) = 11:7 :4% LEP Model independent Method Br(b! l) = 1:2 :4% (4s) Br(b! l) = 11:1 :38% LEP We observe that for both measurements, Br(b! lx) is measured at LEP and in (4s) experiments with equivalent accuracy; we also notice that Br(b! lx) measurements at LEP are slightly higher than those obtained at the (4s) by 1 to jv cb j extraction from inclusive measurement To calculate jv cb j from Br(b! lx), we rst combine the branching fraction and the b lifetimes at LEP ( b ) and then use the theoretical calculations which predict the constant c (eq. 2). A good summary of all the theoretical models

5 Model c (ps?1 ) jv cb j 1 ACCMM 4: 8: 42: 1: exp 4: the Shifman et al 41:3 4: 41:4 1: exp 2: the Ball et al 43:2 4: 4:5 1: exp 2: the Table 3: jv cb j from inclusive Br(b! lx) measurement at LEP giving c is given in [19]. For dierent theoretical models, we quote in Table 3 jv cb j values obtained with the measurement of inclusive Br(b! lx). This inclusive measurement yield : jv cb j = (41:2 1 exp 3 the ) 1?3 3. THE EXCLUSIVE MEASUREMENTS 3.1. Br(B d! D? l) measurement ALEPH [21] and DELPHI [22] measure this branching ratio while extracting jv cb j D? l selection First in the nal state, a high p,p T lepton is required then D o are exclusively reconstructed for both experiments into K +?, K +? +? or Ks? + modes as illustrated on Figure 3. An DELPHI experiment; After that a low momentum pion with an opposite charge to the lepton is added to D o to build D?. Finally the candidates are selected by cutting on m = m(d? )?m(d o ) The B d! D? lx background The main background source and therefore the main contribution to the systematic uncertainty is the contribution of the D?? decaying into D? resonant or non resonant. In the analysis done by the ALEPH experiment this background is minimized as much as possible by : using the vertex information where there should be no additional pions consistent with the D? l vertex demanding a low missing mass (less than 1GeV 2 =c 4 ) as M 2 M 2 miss = (P B?P D? l) 2 DELPHI PRELIMINARY In the DELPHI analysis, the contribution of the D is directly measured. Entries /.8 MeV/c a) Entries / 15 MeV/c b) Results Assuming a fraction of b! B equal to :39:2 the total branching fraction Br(Bd! D? l) is determined to be in ALEPH and DELPHI (respectively): 4 6 Br(B d! D? l) = 5:8 :1 stat :63 syst % [21] Br(B d! D? l) = 3:51 :7 stat :53 syst % [22] M=M (Kπ)π -M Kπ (GeV/c 2 ) M K3π (GeV/c 2 ) Figure 3. Example of exclusive D reconstruction in DELPHI Both measurements are consistent within the errors and are clearly dominated by the systematics. This leads to the LEP average value consistent with the measurement obtained at the (4s) experiments. Br(B d! D? l) = 4:16 :4% LEP inclusive D o analysis is also performed by the Br(B d! D? l) = 4:66 :4% (4s)

6 jv cb j determination From ts to the distributions of transfer momentum (q 2 ) (for more details see [21],[22]) the product of jv cb j times the normalization of the form factor F(q 2 max )=F(1) are extracted. jv cb j F(1) = (31:4 2:3 stat 2:5 syst )1?3 [21] jv cb j F(1) = (37:4 2:1 stat 3:4 syst )1?3 [22] Using the value F (1) = :93:3 [1], the following values for jv cb j are obtained: jv cb j = (33:8 2:5 stat 2:7 syst 1:3 theo )1?3 [21] jv cb j = (4:2 2:3 stat 3:7 syst 1:3 theo )1?3 [22] Both results are consistent within the errors and are consistent with the value obtained with the inclusive measurement of Br(b! lx) Br(Bd! D?? l) measurement If we add the previous measurement Br(Bd! D? l) = 4:16 :41% to Br(Bd! Dl) = 1:8 :41% measured at the (4s) and compare to the inclusive Br(b! lx), clearly some exclusive decays are missing. Furthermore there is clear indications of B decays into D?? (The agreement between the CLEO data and ISGW is better when increasing the contribution of the D?? compared to the ISGW model). In fact we expect four neutral and four charged states D?? mesons. Two narrow states (D 1 with a mass equal to MeV and D 2? with a mass equal to MeV) can be observed as mass peaks; the wide resonances are treated as non resonant four body decays (D? l(x)) Narrow D?? search First D? l candidates are selected as it was described in the previous section, then a pion is associated to the D? l system and nally one looks for peaks in m = m D?? m D? The D 2? decays into D? or D while the D 1 decays only into D?. In other words, D 2? and D 1 contribute to the D? channel in one peak due to the resolution of the detectors while in the channel D, we expect to see two peaks: - The rst peak is a mixture of D 2? and D 1 states decaying into D? with a D? decaying in D where the is unseen Events/2 MeV Events/2 MeV Events/2 MeV a) b) c) OPAL D *+ π m (GeV) D + π m (GeV) 2 1 D π m (GeV) Figures 4: m distributions for the rightsign, wrong-sign and anti-tagged sign samples for ALEPH [21] (top) and in dierent channels for OPAL [23] (bottom) - The second peak is the pure contribution of the D? 2. From the Figures 4 we clearly see a peak in the D? channel the pure D? 2 state is less clearly identied, due to the low statistics Wide D?? or non resonant D? The four body decays D? l(x) analysis is based on vertex topology. Again at a starting point the D? l analysis is applied, then a second pion so called?? with the same lepton charge is selected

7 only if it is consistent with the B decay point. Assuming as previously a Br(b! B ; B? ) equal to :39 :2, the results obtained by the LEP experiments are listed in Table 4 and are consistent within the errors. Applying the isospin rules it leads to : Br(B?! D? l) = 1:41 :37% Br(B! D? l) = 2:47 :37% If we sum all the exclusive branching ratios and compare this sum to the inclusive measurement we get : Br(B! D; D? ; D?? l) = 8 :7% leaving some room for unidentied decays ( 25% of the total inclusive Br(b! lx) ). We can perhaps suspect some other contributions like higher resonant states or non resonant D? l with n > 1 which are experimentally very dicult to measure. 4. THE Br(b! X) MEASUREMENT The branching ratio Br(b! X ) is expected to be equal to 2 to 3 % according to the Standard Model predictions. A higher Br(b! X ) may indicate new physics : for instance models with two Higgs doublets enhance this branching fraction by 1-2 % Methods The lifetime tags method is applied by three experiments (ALEPH [24], OPAL[26], L3 [6],[25]); then events with inclusive leptons (b! e; ) which are sources of neutrinos are rejected (except in L3 analysis). The Br(b! X ) is extracted from a t on the missing energy spectrum or from an excess of events at large missing energy [24]. The systematic errors are dominated by the contamination of D s decaying into X, the knowledge of the fragmentation hx b ei, the knowledge of the branching fraction Br(b! lx) and the missing energy reconstruction Results Preliminary results are listed in Table 5. It leads to a LEP average value on perfect agreement with the standard model prediction. Therefore an upper limit of.5 is put for tan =m H + at 9 % CL for all the models. 5. SUMMARY Two LEP inclusive results are available; the rst one which is clearly dominated by systematic due to the decay model. Br(b! lx) = 11:7 :7 stat :16 syst :35 mod % The recent one obtained only by the AlEPH experiment where the dependence on the model is signicantly reduced. This new approach seems to be promising and will be certainly applied by the other experiments in the future. Br(b! lx) = 11:1 :24 stat :28 syst :11 mod % Compared to the results obtained at the (4s) experiments with equivalent accuracies, those results are slightly higher by 1 to 1.5. Concerning the exclusive semileptonic b decay, the branching fraction Br(B! D? l) is determined to be: Br(B! D? l) = 4:16 :41% Competitive and consistent with the (4s) measurement. The decays B! D?? l and B! D (?) l are clearly seen at LEP. However it remains some missing decays as the sum of the exclusive branching fraction are lower than the inclusive Br(b! lx). An updated value also of Br(b! X ) is given to be: Br(b! X) = 2:62 :1 stat :53 syst % This measurement is in perfect agreement with the standard model prediction. ACKNOWLEDGEMENTS In preparing this review, I have proted from many helpful discussions with colleagues from the LEP experiments: J.P Lees, M. Schmitt, D.Bloch, P.Roudeau, T.S Dai, G.Rahal-Callot, F. Behner, M. Wadhwa and M.Kobel. Many thanks

8 ALEPH [2] DELPHI [22] LEP Average Br(B?! D?+? l? X) :95 :31% :91 :53% :94 :25% Br(B! D? + l? X) 1:65 :44% 1:65 :44% Table 4: Exclusive non resonant Br(B! D? lx) LEP measurements Br(b! lx) (%) Br(b! X)(%) Experiment :75 :3 stat :37 syst ALEPH (91-93) [24] 1.8 1:67 :46 stat 1:4 syst L3 (91-92)[6] :66 :11 stat :52 syst OPAL (91-94) [26] :62 :1 stat :53 syst LEP Average Table 5: LEP Br(b! X) measurements. for their help. I am also very grateful to the organizers of the conference for giving me the opportunity to participate to such a nice conference in the city of Strasbourg. REFERENCES 1 M.Neubert, Phys.Lett B338 (194) G.Altarelli and S.Petrarca, Phys. Lett. B261 (1991) 33 3 I.Bigi et al, Phys. Lett. B323 (1994) E.Bagan et al., Phys. Lett. B342 (1995) B Lifetime measurements at LEP, S.Braibant. (these proceedings). 6 Measurement of the Branching ratio b! lx with electrons, muons and neutrinos. L3 Collaboration submitted to The EPS Conference, Brussels ALEPH Collaboration, 1994,Z.Phys. C 62, Measurement of? b b=? had using impact parameter measurements and lepton identication. DELPHI Collaboration. CERN- PPE/ OPAL Collaboration, 1993, Z.Phys C 6, Measurement of the semileptonic b branching ratios from inclusive leptons in Z decays.the ALEPH Collaboration, EPS-HEP 95 eps L3 results on R b and Br(b! lx) for the Glasgow Conference (1994) and L3 internal note HF of the LEP Electroweak working group. LEPHF/ G.Alterelli et al, Nucl. Phys. B 28 (1982) N.Isgur,D.Scora,B.Grinstein and M.Wise, Phys. reb. D39 (1989) CLEO Collaboration, Phys. Rev. D 45 (1992) DELCO Collaboration, Phys. Rev. Lett. 43 (1979) 173. The Mark III Collaboration, Phys. Rev. Lett. 54 (1985) ARGUS Collaboration, Phys. Lett. B 318 (1993) Recent results on b and c physics from CLEO, V.Jain (these proceedings). 19 Leptonic and Semileptonic Decays of Charm and Bottom Hadrons, J.D. Richman and P.R. Burchat,Stanford-HEP ALEPH Collaboration, Phys. Lett. B345 (1995) 13 and EPS-HEP 95 eps ALEPH Collaboration EPS-HEP 95 eps635 and CERN/ DELPHI Collaboration EPS-HEP 95 eps569 and DELPHI internal note Phys OPAL Collaboration, CERN-PPE/95-2 submitted to Zeit. fur Physik. 24 ALEPH Collaboration, Phys. Lett. B 298 (1993) 479., Phys. Lett. B 343 (1995) L3 Collaboration, Phys. Lett. B 332 (1994) OPAL Collaboration EPS-HEP 95 eps282 and note PN 186 (1995).