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1 `< ( J ;> leli LAP?-EXP JANVIER 1996 Scmilcptcmic Dccays of thc B masons at LEP Sylvie Rosicr-Lees LAPP IN2P3-CNRS, F Annccy Lc Vicux Cedex Sw Contribution to advanced study conference on Production and Decay of Hyperons Charm and Beauty Hadrons, Strasbourg, Sept. 5-8, B.P. 110 F ANNECY-LE-VIEUX CEDEX TELEPHONE TELBCOPIE OCR Output

2 Semileptzonic Decays of the B mesons at LEP Sylvie R0sier Lces LAPP, IN2P3-CNRS, F Annecy lc Vieux Cedex We review the scmileptonic b-hadron branching ratio Br(b 1uX) measurements from LEP. Both the inclusive and exclusive measurements are presented and different methods for the inclusive measurements (decay Model dependent or not) are contrasted. From those measurements we extract 5 value of V,b. An updated measurement ofthe branching ratio Br(b» TUX) is also given. 1. INTRODUCTION a good starting point for predictions.the naive spectator model predicts Br(b ~» lux) :0.16. One of the most important motivations in the measurement of Br(b 1vX) is that it can be used to extract the CKM matrix element Kb. 0z5(mZ) ] Parton ] HQEI3] HQE [4] This can be performed using two independent Model [2] methods: I 0.13 I I By combining the inclusive measurements of I I I Br(b + 1uX) and the mean b hadron lifetime n, Br(b + l14x) Gpms i : Tgj X (f 9 VCi» + f 9uV.3t)(1) Table 1: Theoretical predictions for the semilep tonic branching ratio Br(b + lux) depending on C15(TTlZ) Equation (1) becomes if we neglect Vub : As the exchange of hard gluons between quarks enhances the non leptonic rates, the introduction Br(b + ll/[x) = 13 >< Kt (2) of perturbative QCD corrections decreases signif icantly the naive spectator model prediction. The In the exclusive decay BO > D*+ Di, prediction for Br(b + 1uX) is further reduced by 2 I/Qb -rg,is estimated by extrapolating the par 2% by introducing non perturbative corrections tial width dbr(bo + D*+ '17l)/dw to 4.::1: of order O(1/mg) in HQE. Theoretical predictions for Br(b + 1uX) are dbr BO D*+ `17 < lci,tb >< F( l2 (3) quoted in table 1 for pole masses values mb:4.8 GeV and mczl.33 GeV. where w is the scalar product ofthe four veloci T conclude, predictions on the semileptonic ties ofthe BO and D*+ mesons: w : vb vd + branching fraction depend on our knowledge and F(w) is the Isgur Wise function with F(1) of the effective quark masses and on a pre equal to according to Therefore cise estimate of the QCD corrections, which as we will give a review ofthe inclusive and exclusive it was shown are important and tend to push Br(b lux) measurements. Br(b ~ ll/x) down. In a complete diferent domain, the measure ment ofthe branching ratio Br(b 1*uX) allows 1.2. Measurements of Br(b + 1uX) to explore new physics, as explained in the last The semileptonic branching fraction is measured section. at the T(4s) experiments and at LEP experi ments. Assuming the semileptonic decay width 1.1. Theoretical predictions FSL is the same for all b flavored hadrons, Since the lifetimes ofthe BO and Bi mesons are Br(b >lux) should be different at LEP com equal within 10%, the spectator model provides pared to 'I`(4s) since different particles are proocr Output

3 duced, which leads to the following relation : BI(b -» 1)(z ); LL X Blu) ~1(r 4s) O ) ( ) Tg + Tg Single Tag- Pbb dependent method According to equation (4) Including the LEP lifetime measurements, Nhad Pima rb+ : 1.55 zi: 0.06 ps, rb : 1.63 ;+: 0.06 ps and Br(b + lz/x) and also Pbb or Br(b > c > 1) can 17, : 1.57;}; 0.02 ps, it becomes : be extracted from a fit on the momentum p and the transverse momentum pq spectra. In this Br(b» l)(z0)/br(b» l)('i`(4s)) : 0.98 ;t 0.03 case, the product Br(b -+ 1uX) ><Pbb is measured, Compared to the T(4s) measurement, the ad where Pbb is either assumed from the standard ditional contribution of other hadrons like (Ab model [6] or measured [7],[8],[9]. As the single or B,) reduces slightly the expected average lepton tag events are used, this method has the Br(b > lux) at the Z0. advantage to be not statistically limited. A recent analysis [6], using different tech 2. INCLUSIVE MEASUREMENTS nique was performed by the L3 experiment. The branching ratio Br(b > 1/X) is mea We distinguish two approaches in this measure sured using the missing energy spectrum of the ment. In the traditional approach the measure jets in the two jets events assuming E, : ment depends on the phenomenological models Ebmm Eb",. This measurement with ener describing the momentum spectrum of the lepton getic neutrino is complementary to the traditional while in the other approach the measurement is Br(b» l1/x) measurement (looking for electrons, independent of the knowledge of these models. muons) as it is sensitive to Br(b» 1-VX). The total semileptonic branching fraction measured is 2.1. Traditional approach as shown on Figure 1. To select the semileptonic decays ofthe b quark, these well known properties are used : The fragmentation ofthe b quark Br(b ~ 1/X) : i 0.77,m,d;1.24,y,,% 0.70) is hard which provides a high momen Br(b 1uX) can be extracted if we assume a con tum to the lepton ( usually the momentum tribution of 1:1:0.25 for e, p, r in the 1/ spectrum. p has to be greater than 2-4 GeV/c ) b is heavy which provides to the lepton a Double Tag-Pbb independent method high transverse momentum pt with respect In the limit of no background, Br(b > lux) is to the jet ( typically pt has to be greater to proportional to the ratio of the number of dilep GeV/c) ton events to the number of single lepton events. After applying these selection criteria, an over all purity of 80 % with an efficiency varying from W q{ [u 2 [ Bfsr. X (1 BrSL) 30 to 60 % is achieved. (5) To enrich the sample in b events, the long life This method [11] has the advantage to be inde time property ofthe b hadrons is used by tagging pendent at the first order of Pbb but is statisti the bb with the microvertex detector. A purity of cally limited as it is based on the dilepton events 90 % is achieved with an efficiency varying from sample. 30 to 60 % depending on detector performances. In a recent measurement, performed by the The main experimental problem comes from ALEPH experiment [10], the z" decays into b the leptonic cascade (b - c I) contamina quarks are selected with high efficiency tion which is the main background at high pgp; so (~ 60%) and a very good purity ( ~ 95%) on usually both Br(b + lux) and Br(b» c -+1) are the basis of the long b-hadron lifetimes. The lep measured simultaneously. ton yield and spectra in hemispheres containing OCR Output

4 10' Data * 0000 Y.?'. O u l,%o 10 E rn }_ C] Fu navx 2 10 \\. (GeV Jet energy [GeV] M t. BDO I ;_ ** Y < > ><, 4».;»~ 1200 if... _' l. e 7 1 I_ lim I _ (cn//e) ZODB (bh lmao.- up was *... _. M I.._... :1 te i ZOO P. Lcevy Figure 1. The jet energy spectrum in the L3 data and the results ofthe fit for the measurement of Br(b > 1/X) Figure 2. ALEPH data. Comparison between measured spectra and fits results for Single lepton spectrum (top) and dilepton sample (bottom) one lepton candidate or two opposite charge lep tons candidates are used. Both Br(b > l1/x) and Br(b > c > 1) are extracted from the fit on the pq spectrum of the single lepton and lower pp of dilepton events, the result on the fit 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 +lux) measurement depends on the effi ciency which is highly dependent on the lepton momentum spectrum. If we soften the lepton mo mentum spectrum, we decrease the efliciency and consequently the Br(b l1/x) increases. The momentum spectrum ofthe lepton in the b de cays at LEP depends on: The momentum spectrum ofthe b hadrons itself; for a harder fragmentation the Br(b -+11/X) is lower: typically by vary ing the fragmentation from 0.68 to 0.72 the Br(b + 11zX) decreases by about 0.05 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 1vX). According to the recom mendations of the LEP electroweak work ing group [12] all LEP experiments use the same models: ACCMM, ISGW, ISGW* [13[,[14] for b > l decays with parameters extracted from a fit on the CLEO data [15] and DELCO and MARK III[16] concerning the c + I decays Summary of the model dependent LEP results In Table 2 all the measurements are quoted with as a first error the statistical error, the second one is the systematic uncertainty due to the de tector performances and the inclusive lepton anal ysis proper to each experiment and the last error is due the modelling ( the central value is ob tained 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 Rb : 0.22 and < mg > :0.70, the LEP average Br(b - + ll/x) is (the last measurement quoted in Table 2 is not included in this average as it is sta tistically correlated to the third measurement): Br(b --» 1) : zh 0.07smg zh 0.16,,,. zh 0.37mC,d% OCR Output

5 < mz > Br(b + I1/X) (%) Experiment ;& 0.30,,,,,1 0.32,,,,,1 0.27,,,,,,; ALEPH (90-91) [7] iO.39,,,,,j;0.19,,,,ti0.19,,,,,4 DELPHI (91-92)[8] ;i; 0.11,,,,,1 0.30,,,,,1 0.35,,,,,,; L3 (91-92) [6] ,,,,,1 0.60,,,,,1 0.40,,,,,,; OPAL (90-91)[9] (indcp) ,,,,,1 0.28,,,,,1 0.35,,,,,,; ALEPH (92-93)[10] (indcp) ,,,,, ,,,,,1 0.31@ L3 (90-91) [11] Table 2:LEP Br(b» 11JX) measurements dependent on the models This measurement is clearly systematics limited unknowns :Br(b -+1uX), Br(b» c -> l) the ex as ABr(b -+1)/(Br(b» 1) ~ 3 4%; the weak tracted Br(b -» lux) is then : ness in those analyses is the dependence on lepton for model spectrum. Br(b > 1) :: ,,,, ,,.,,1 0.11,,,,,,% 2.2. 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 sec ondary lepton without relying on model depen dent shapes. This method is also used by the CLEO experiment [18]. A recent measurement described in [10] uses no prediction for pq spectra: Direct and cascade leptons can be distinguished based on the correlation of their charge with that ofthe b quark. Two independent measurements are needed to separate them, for that two inde pendent samples of bb events are selected. - In the first sample bb events are selected by ap plying 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 -+1) 1 Br(b lux) In the second sample, opposite side dilepton events are selected. One tags one side with a high p, lepton or by using the jet charge tech nique (to increase the statistics), therefore the charge of the tag lepton gives b quark charge. Opposite sign lepton on the other side are se lected 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 lep ton charge and the tag lepton charge are oppo site, FOS is the number of those events. FO, Br(b + lvx) /(Br(b > lvx) -[-Br(b -+ c -+1) ) Two quantities are measured F1 and FOS for two Value consistent with the model dependent mea surement; Here the model dependence has been further reduced at the cost of enlarging the sta tistical error by using charge correlations rather than fits to the pq- spectra Comparison with measurements at the 'I`(4s) To compare T(4s) to LEP results we apply the factor correction estimated in section 1.2 Model dependent Method Model independent Method VVe observe that for both measurements, Br(b lux) is measured at LEP and in `1`(4s) experiments with equivalent accuracy; we also no tice that Br(b + lux) measurements at LEP are slightly higher than those obtained at the 'I`(4s) bv 1 to 1.5 0*. Br(b + 1) : % 'I`(4s) Br(b + 1) : % LEP Br(b -» 1) TZ % 'I`(4s) Br(b > 1) : % LEP 2.4. [VQ,] extraction from inclusive mea surement To calculate [Vc,] from Br(b + lux), we first combine the branching fraction and the b life times at LEP (n,) and then use the theoretical calculations which predict the constant 7C (eq. 2). A good summary of all the theoretical models OCR Output

6 Model ACCMM Shifman ct al 7C (ps l) ;}; i 4. Table 3: V,b\ from inclusive Br(b» 11xX) measurement at LEP VCI,] >< ,,,, 1 44;.,, ,,,,, :k 2.,;,, Ball etal ,,,,1 2.,;,, giving 7, is given in [19]. For different theoretical models, we quote in Table 3 [Kg,} values obtained with the measurement of inclusive Br(b + lr/x). This inclusive measurement yield : DELPHI experiment; After that a low momen tum pion with an opposite charge to the lepton is added to D to build D*. Finally the candidates are selected by cutting on Am : m(d*) m(d ) V,b{ : ( ,,,,, :k 3,;,,) >< 10` The B; + D*luX background The main background source and therefore tl e 3. THE EXCLUSIVE MEASUREMENTS main contribution to the systematic uncertainty is the contribution of the D" decaying into D*vr 3.1. Br (B3 + D*lu) measurement resonant or non resonant. In the analysis done by ALEPH [21] and DELPHI [22] measure this the ALEPH experiment this background is mini branching ratio while extracting IQ;,. mized as much as possible by : D*lv selection First in the final state, a high p,pt lepton is required then D are exclusively reconstructed for both experiments into K+1r', K+1r'1r+1r` or K?1r`1r+ modes as illustrated on Figure 3. An using the vertex information where there should be no additional pions consistent with the D*I vertex demanding a low missing mass (less than 1GeV2/c4) as M3 ~ Mzm, : (PB PD~;)2 DELPHI PREIJMINAR Y In the DELPHI analysis, the contribution of the D" is directly measured..m ` [ All X 9 " 20 J ii Z iz Ei >f 773 I 4.. ' rj: XV; >; f,, 5/ W.>; 2 r»~4 O x.;, n;4.;.lq{4k44i Z4.»2»2 rfzz.; 0 4.,1, 4.. i.1; xl.1-l1zf1kzlu, Figure 3. Example of exclusive DO reconstruction in DELPHI a} '. 2 vg lll) Hm _Q '. # z_ _.. i' ; 7 V-. 3.,. V l *-1/ rxr; K *1 =i %#? 6 G >/ 014 (L15 (Llc (L17 16 L8 Z A.\{ Al,K_m.WNK (GeV/H).\lK>,n (Ge\Ye ; inclusive DO analysis is also performed by the Results Assuming a fraction ofb + B0 equal to the total branching fraction Br(B3» D*l1/) is determined to be in ALEPH and DELPHI (re spectively): Br(Bg» D*lu) : ,,,, ,,.,,% [21] Br(Bg + D*lv) : ,,,, ,,.,,% [22] Both measurements are consistent within the er rors and are clearly dominated by the systemat ics. This leads to the LEP average value consis tent with the measurement obtained at the 'I`(4s) experiments. Br(B» D"h/) : % LEP Br(B + D*l1») : % T(4s) OCR Output

7 }K;, determination From fits to the distributions of transfer momen tum (q2) (for more details see [21],[22]) the prod uct of HQ,] times the normalization ofthe form factor F(q3mw ):F(1) are extracted. [VCb >< F(1):( ,m, i 2.5sySt)10 3 [21] vcb X F(1) : (37.4 at 2.1,.,,. i 3.4,,,.)10 * [22] Using the value F(1) : O.93j;0.03 [1], the follow ing values for HQ.,] are obtained: Vcb : (33.8 ;t 2.5,.8. i 2.7,,, h,0)10`3 [21] Vcbl = (40.2 i 2.3,.8. zh 3.7,,, h,,)10`3 [22] Both results are consistent within the errors and are consistent with the value obtained with the inclusive measurement of Br(b» 1uX) Br(Bg > D*""Iv) measurement If we add the previous measurement Br(B3 + D*lv) : % to Br(Bg Dlv) : % measured at the 'I`(4s) and com pare to the inclusive Br(b + lvx), 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 ofthe D*'* compared to the ISGW model). In fact we expect four neutral and four charged states D* mesons. Two narrow states (D1 with a mass equal to MeV and D; with a mass equal to MeV) can be observed as mass peaks; the wide resonances are treated as non resonant four body decays (D*lwr(X)) E 2.1. Narrow D" search First D"l candidates are selected as it was de scribed in the previous section, then a pion is as sociated to the D*Z system and finally one looks for peaks in Am : md,, mp. The D; decays into D*vr or Dvr while the D. decays only into D* r. In other words, D; and D. contribute to the D"1r channel in one peak due to the resolu tion ofthe detectors while in the channel Dr, we expect to see two peaks: The first peak is a mixture of D; and D. states decaying into D*vr with a D" decaying in Doro where the 1rO is unseen 20 ll) 0 L U., ,,-m-11,$.m1_, mmm oa w. Arr (G \//c') , Amt. (Gov/e') OPAL D "rc' Am (GeV) 1)* Am (GeV) D n* ALEPH prellmlncry ( 1.. ] g Arr (GeV/c') m._.l_.-n.in I Am ((}cv) Figures 4: Am" distributions for the right sign, wrong-sign and anti-tagged sign samples for ALEPH [21] (top) and in different channels for OPAL [23] (bottom) The second peak is the pure contribution of the D;. From the Figures 4 we clearly see a peak in the D*vr channel the pure D; state is less clearly iden tified, due to the low statistics Wide D" or non resonant D*vr The four body decays D*1rlu(X) analysis is based on vertex topology. Again at a starting point the D*lu analysis is applied, then a second pion so called 77,..* with the same lepton charge is selected OCR Output

8 the standard model prediction. Therefore an up Assuming as previously a Br(b + BO,B`) per limit of 0.50 is put for tanb/mh+ at 90 % equal to 0.39 zl; 0.02, the results obtained by the CL for all the models. LEP experiments are listed in Table 4 and are consistent within the errors. Applying the isospin rules it leads to : Br(B` + D*1rlu) : 1.41i 0.37% Br(B -» 1)*7:11/) : % If we sum all the exclusive branching ratios and compare this sum to the inclusive measurement we get : Br(B + D, D*, D ""lu) : 8 ;l; 0.7% 5. SUMMARY Two LEP inclusive results are available; the first one which is clearly dominated by systematic due to the decay model. Br(b > 1uX) I j; 0.07,.,. ;t 0.16,,,. ;k 0.35,..0.1% The recent one obtained only by the AIEPH ex periment where the dependence on the model is significantly reduced. This new approach seems leaving some room for unidentified decays ( to be promising and will be certainly applied by 25% of the total inclusive Br(b > 1vX) We the other experiments in the future. can perhaps suspect some other contributions like higher resonant states or non resonant D*lwr Br(b + ll/x) : ;l: 0.24,.,.,. i 0.28,,,. i 0.11,,,.,,;% with n > 1 which are experimentally very dihi Compared to the results obtained at the T(4.s) cult to measure. experiments with equivalent accuracies, those re sults are slightly higher by 1 to THE Br (b + rux) MEASUREMENT Concerning the exclusive semileptonic b decay, the branching fraction Br(B - + D*I1/) is deter The branching ratio Br(b + rvx ) is expected mined to be: to be equal to 2 to 3 % according to the Stan dard Model predictions. A higher Br(b + r1zx ) Br(BO D*l1/) : 4.16 ;i: 0.41% may indicate new physics : for instance models with two Higgs doublets enhance this branching Competitive and consistent with the T(4s) mea fraction by %. surement. The decays B» D**l1z and B Methods D(*)7\ ll/ are clearly seen at LEP. However it re The lifetime tags method is applied by three ex mains some missing decays as the sum of the ex periments (ALEPH [24], OPAL[26], L3 [6],[25l); clusive branching fraction are lower than the in then events with inclusive leptons (b - + e,;1) clusive Br(b > lvx). which are sources of neutrinos are rejected (ex An updated value also of Br(b + rux ) is given to be: cept in L3 analysis). The Br(b + rux ) is ex tracted from a fit on the missing energy spectrum Br(b ~ Tl/X) : 2.62 j; 0.10,.,,. i 0.53,,,.% or from an excess of events at large missing energy [24]. This measurement is in perfect agreement with The systematic errors are dominated by the the standard model prediction. contamination of D, decaying into rvx, the knowledge of the fragmentation the knowl ACKNOWLEDGEMENTS edge ofthe branching fraction Br(b» l1/x) and the missing energy reconstruction. ln preparing this review, I have profited from many helpful discussions with colleagues from 4.2. Results the LEP experiments: J.P Lees, M. Schmitt, Preliminary results are listed in Table 5. lt leads D.Bloch, P.Roudeau, T.S Dai, G.Rahal-Callot, F. to a LEP average value on perfect agreement with Behner, M. Wadhwa and M.Kobel. Many thanks OCR Output

9 ALEPH [20] DELPHI [22] Br(B -~ D*+ y1 X) % B»(B» D*»+»1 X) % Table 4: Exclusive resonant Br(B + D*m/IX) LEP measurements LEP Average % % % Br(b» 1uX) (%) Br(b > T1/X)(%) Experiment :1: 0.30,m 3E 0.37,,,,, ALEPH (91-93) [24] ;}; 0.46,,,, ;b 1.04,y,t L3 (91 92)[6] ;&0.11,,,,,10.52,y,, OPAL (91-94) [26] ;{; 0.10,,,,,1 0.53,,,,, LEP Average Table 5: LEP Br(b > rux) measurements. for their help. I am also very grateful to the or 14 N.Isgur,D.Scora,B.Grinstein and M.Wise, ganizers ofthe conference for giving me the op Phys. reb. D39 (1989) 799. portunity t0 participate to such a nice conference 15 CLEO Collaboration, Phys. Rev. D 45 (1992) in the city of Strasbourg DELCO Collaboration, Phys. Rev. Lett. 43 REFERENCES (1979) The Mark HI Collaboration, Phys. Rev. Lett. 54 (1985) 1976 M.Neubert, Phys.Lett B338 (194) ARGUS Collaboration, Phys. Lett. B 318 G.A1tarel1i and S.Petrarca, Phys. Lett. B261 (1993) (1991) 303 Recent results on b and c physics from CLEO I.Bigi et al, Phys. Lett. B323 (1994) 408., V.Jain (these proceedings). 19 E.Bagan et al., Phys. Lett. B342 (1995) 362. Leptonic and Semileptonic Decays of Charm B Lifetime measurements at LEP, S.Braibant. and Bottom Hadrons, J.D. Richman and P.R. (these proceedings). Burchat,Stanford-HEP Measurement of the Branching ratio b» lux ALEPH Collaboration, Phys. Lett. B345 with electrons, muons and neutrinos. L3 Col (1995) 103 and EPS-HEP 95 eps laboration submitted to The EPS Conference, ALEPH Collaboration EPS-HEP 95 eps0635 Brussels and CERN/ ALEPH Collaboration, 1994,Z.Phys. C 62, 22 DELPHI Collaboration EPS-HEP 95 eps and DELPHI internal note Phys Measurement of Pb;/Thad using impact pa OPAL Collaboration, CERN-PPE/95-02 sub rameter measurements and lepton iden mitted to Zeit. fiir Physik. tification. DELPHI Collaboration. CERN 24 ALEPH Collaboration, Phys. Lett. B 298 PPE/ (1993) 479., Phys. Lett. B 343 (1995) OPAL Collaboration, 1993, Z.Phys C 60, 199. L3 Collaboration, Phys. Lett. B 332 (1994) Measurement of the semileptonic b branching 26 OPAL Collaboration EPS-HEP 95 eps0282 ratios from inclusive leptons in Z decays.the ALEPH Collaboration, EPS-HEP 95 eps404. and note PN 186 (1995). 11 L3 results on Rb and Br(b >l1/x) for the Glasgow Conference (1994) and L3 internal note HF of the LEP Electroweak working group. LEPHF/ G.Alterelli et al, Nucl. Phys. B 208 (1982) 365.