Recent heavy flavour results from the LEP experiments are presented. These
include a search for new physics in rare B decays, a new model-independent
measurement of the b-quark fragmentation function at the Z peak, updated
measurements of |Vcb|, results on DeltaGamma_s, searches for B0s oscillations,
as well as a new measurement of sin(2beta). Many combined results, obtained by
dedicated working groups are also given...Comment: 6 pages, 5 figure

Schneider, O.

English

arXiv

Recent heavy flavour results from the LEP experiments are presented. These include a search for new physics in rare B decays, a new model-independent measurement of the b-quark fragmentation function at the Z peak, updated measurements of |Vcb|, results on DeltaGamma_s, searches for B0s oscillations, as well as a new measurement of sin(2beta). Many combined results, obtained by dedicated working groups are also given..

Schneider, O

CERN Document Server

IPHE 2000-009May 18, 2000HEAVY FLAVOUR PHYSICS RESULTS FROM LEP 1O. SCHNEIDERInstitut de Physique des Hautes Energies,Universite´ de Lausanne, CH-1015 Lausanne, Switzerlande-mail: Olivier.Schneider@iphe.unil.chRecent heavy flavour results from the LEP experiments are presented. These include a searchfor new physics in rare B decays, a new model-independent measurement of the b-quarkfragmentation function at the Z peak, updated measurements of |Vcb|, results on Γs, searchesfor B0s oscillations, as well as a new measurement of sin(2β). Many combined results, obtainedby dedicated working groups, are also given. The LEP measurements of Vcb from B0 →D−`+ν decays average to |Vcb| = (39.7 ± 3.0) × 10−3, while inclusive measurements yield|Vcb| = (40.8 ± 2.0) × 10−3 dominated by theoretical uncertainties. Charmless semileptonicdecays have been observed inclusively, BR(b → u`−ν) = (1.67 ± 0.55) × 10−3, correspondingto |Vub| = (4.04+0.62−0.74) × 10−3. Signicant progress has been made in the B0s sector, wherethe width dierence is now close to being measured with a combined result of Γs/Γs =0.24+0.16−0.12 or Γs/Γs < 0.53 at 95% CL. However, despite continuing improvements, ms isstill unmeasured, with a lower limit of 14.6 ps−1 at 95% CL. Improved heavy flavour resultsare expected from LEP by Summer 2000. The current status of electroweak heavy flavourphysics is summarized in another presentation.Talk presented at the XXXVth Rencontres de Moriond,Electroweak Interactions and Unified Theories,Les Arcs, France, March 11–18, 2000.1It is not possible to review here all the heavy flavour results produced by the ALEPH, DELPHI,L3 and OPAL experiments based on their LEP 1 data, taken until 1995 at an e+e− center-of-massenergy equal or close to the Z mass. Although the peak activity of LEP 1 analysis is behind us, thesedata are still being analyzed, producing new and improved measurements.The focus of this presentation is on new or updated b-physics results released since the 1999Summer Conferences, as well as on the latest averages produced by various LEP heavy flavour workinggroups.1 Many of these results can be related to the magnitude of the least well known CKM matrixelements jVcbj, jVubj, jVtdj and jVtsj, which are in turn related to the lengths of the sides of the CKMunitarity triangle. These measurements will be (together with the forthcoming results from the Bfactories and the Tevatron) important ingredients of future tests of the CKM picture within theStandard Model, where an inconsistency may indeed be an indirect indication of new physics.New physics may also be responsible for unexpectedly high branching ratios in rare B decaysinvolving flavour-changing neutral currents. For example the branching ratio of the decay B !K−K−pi+, for which the OPAL collaboration has recently released2 an upper limit of 1.29  10−4 at90% CL, is predicted to be 10−11 at most in the Standard Model (box diagram) but is practicallyunconstrained in certain supersymmetric models with R-parity violation (tree diagram possible).1 b-fragmentation studiesUnderstanding the production of b-hadrons in Z decays is important for many heavy flavour analyses.The b-quark hadronisation can be described in terms of the variable xE = Eb-hadron/Ebeam, the fractionof the beam energy retained by the weakly-decaying b-hadron produced in a b jet. Being not knownaccurately, the distribution of xE is simulated by the LEP experiments using the JETSET generatortogether with phenomenological models that relate the energy of the b-hadron with that of the initialb-quark. The most commonly used of these models, from Peterson et al., relies on a single parameterwhich has merely been tuned to reproduce the experimental spectra of high transverse momentumleptons originating mostly from the decay b ! c`−ν. This tuning corresponds to a mean xE ofhxEi = 0.702 0.008, which is the value recommended up to now for heavy flavour analyses at LEP.3The ALEPH collaboration has submitted to this conference a new measurement4 of the shape ofthe xE distribution (see Fig. 1) based on approximately 3000 B ! Dlν decays, where the B mesonenergy has been estimated in a model-independent way from an identied lepton, a fully reconstructedD meson and missing energy information. The energy spectrum is found to be somewhat harderthan assumed before, with hxEi = 0.7198 0.0045stat  0.0053syst, consistent with a previous ALEPHmeasurement. This conrms a recent result from SLD,5 hxEi = 0.7140.005stat0.007syst0.002model,which has small model-dependent systematics, although based on an inclusive sample of b-hadrons.These direct measurements of the shape of the xE distribution have now sucient precision toenvisage tests of the b-fragmentation model predictions and to discriminate amongst these models forthe rst time. For example, both ALEPH and SLD data favour the description of Kartvelishvili et al.over the one from Peterson et al.2 Measurements of |Vcb| and |Vub|The study of the B0 ! D−`+ν decay kinematics allows the extraction of jVcbj. The dierential decayrate as a function of the boost ω of the D− in the B0 rest frame is predicted by the Heavy QuarkEective Theory to bedΓdω= K(ω)F2D(ω)jVcbj2 , (1)where K(ω) is a known phase-space function and FD(ω) a single form factor which, in the heavyquark limit, is equal to unity at zero recoil. The interesting observable is thus the decay rate at zerorecoil, but since the phase space vanishes at ω = 1, the quantity FD(1)jVcbj must be extracted froman extrapolation of the measured dierential rate at ω > 1. At LEP, this extrapolation relies on aspecic parametrization6 of the shape of FD(ω) in terms of the slope ρ2 at ω = 1.2xE1/N dN/dx EALEPH data (preliminary)Peterson et al.Kartvelishvili et al.Collins, SpillerPhys. Rev. D27, 105 (1983)Phys. Lett. B78, 615 (1978)J. Phys. G11, 1289 (1985)00.511.522.533.50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Figure 1: xE distribution of B → Dlν mesons measuredby ALEPH4 compared with predictions from various b-fragmentation models tted to the data.283032343638400 0.4 0.8 1.2 1.6 2ρ2F(1)|Vcb|/10-3ALEPHDELPHIOPALAverageLEP PreliminaryFigure 2: LEP measurements of FD∗(1)|Vcb| and ρ2 fromB0 → D−`+ν decays, and their average. The error ellipses(39% CL) include systematic uncertainties.The OPAL collaboration has recently updated their result obtained with fully reconstructed decaysand performed a new analysis7 based on an inclusive D− reconstruction relying on the identicationof the slow pion from the D− decay. Their new combined result is displayed on Fig. 2, togetherwith earlier and similar measurements from ALEPH and DELPHI. A combination of these results,performed by the LEP Vcb working group,1 takes into account all correlations and yieldsFD(1)jVcbj = (34.9  0.7 1.6)  10−3 and ρ2 = 1.13  0.08  0.16 . (2)Systematic uncertainties are dominated by the limited knowledge of the D recoil spectrum for B !D`νX background events. With FD(1) = 0.88  0.05 from theoretical calculations taking intoaccount nite quark masses and QCD corrections,8 this leads to the combined LEP estimate jVcbj =(39.7  0.8stat  1.8syst  2.2theory) 10−3 from exclusive decays.Current theoretical calculations based on heavy quark symmetry relate jVcbj and jVubj to theinclusive b! c`−ν and b! u`−ν decay widths,jVcbj = 0.0411√BR(b! c`−ν)0.105√1.55 psτband jVubj = 0.00445√BR(b! u`−ν)0.002√1.55 psτb, (3)with total uncertainties estimated to be  5%.8 While the measurements of the inclusive b ! `−branching ratio and b-hadron lifetime are well established since several years, with current averages8of BR(b! `−) = (10.58 0.07 0.17)% and τb = 1.564 0.014 ps, analyses measuring the b! u`−νbranching ratio are quite recent and unique to LEP. They face the diculty of dealing with a verylarge b ! c`−ν background, but have the advantage to be sensitive to the whole lepton spectrum(rather than only to the its end-point where the b ! c`−ν decays are supressed). L3, ALEPH, andDELPHI have now all published evidence for b ! u`−ν transitions, and their measurements averageto8 BR(b ! u`−ν) = (1.67  0.36  0.37  0.20)  10−3, where the rst uncertainty summarizesstatistics and experimental systematics, the second uncertainty reflects the limited knowledge of b!c`−ν transitions, and the third one results from the modelling of b ! u`−ν. Using Eq. 3, the LEPaverages from inclusive semileptonic b decays are jVcbj = (40.8 0.4exp  2.0theory) 10−3 and jVubj =(4.04+0.62−0.74)  10−3. The former can be combined1 with the less precise but consistent LEP estimatefrom exclusive decays to yield jVcbj = (40.5  1.8)  10−3.300.10.20.30.40.50.60.7∆Γs / ΓsLEP + CDF (prel.)00.10.20.30.40.50.60.71.1 1.2 1.3 1.4 1.5 1.6 1.71 / Γs  (ps)∆Γs / ΓsLEP + CDF (prel.)Figure 3: Combined 68%, 95% and 99% CLcontours in the plane (1/Γs,Γs/Γs).8 Thetop (bottom) plot is obtained without (with)the constraint 1/Γs = τB0 .-1-0.500.511.522.530 2 4 6 8 10 12 14 16 18 20∆ms (ps-1)Amplitudedata ± 1 σ 95% CL limit   14.6 ps-11.645 σ sensitivity    14.6 ps-1data ± 1.645 σdata ± 1.645 σ (stat only)LEP + SLD + CDF (prel.)Figure 4: Combined measurements of the B0s oscillation amplitude as afunction of ms, obtained by the LEP B oscillations working group.1The measurements are dominated by statistical uncertainties. Neigh-bouring points are statistically correlated.3 Results on the B0s decay width differenceInformation on Γs, the decay width dierence between the two mass eigenstates of the B0s{ B0ssystem, can be obtained by studying the proper time distribution of untagged data samples enrichedin B0s mesons. In the case of an inclusive or a semileptonic B0s decay selection, both the short-and long-lived components are present, and the proper time distribution is a superposition of twoexponentials with decay constants Γs Γs/2. In principle, this provides sensitivity to both Γs and(Γs/Γs)2. Ignoring Γs and tting for a single exponential leads to an estimate of Γs with a relativebias proportional to (Γs/Γs)2. An alternative approach, which is directly sensitive to rst order inΓs/Γs, is to determine the lifetime of B0s candidates decaying to CP eigenstates; measurements existfor B0s ! J/ψφ,9 and now also for B0s ! D()+s D()−s ,10 which are predicted to be mostly CP-evenstates. Recently, ALEPH has also obtained for the rst time an estimate of Γs/Γs directly froma measurement of the B0s ! D()+s D()−s branching ratio,10 under the assumption that these decayspractically account for all the CP-even nal states.Figure 3 shows condence contours in the plane (1/Γs,Γs/Γs) obtained from a combined likeli-hood built with all the available information from LEP and CDF, including dedicated Γs studies aswell as B0s lifetime measurements. The corresponding results for Γs/Γs are8Γs/Γs = 0.24+0.16−0.12 or Γs/Γs < 0.53 at 95% CL (4)without external constraint, andΓs/Γs = 0.17+0.09−0.10 or Γs/Γs < 0.31 at 95% CL (5)when constraining 1/Γs to the current world average of the B0 lifetime. Such a constraint is wellmotivated theoretically, since the B0 and B0s decay widths are predicted to dier by  1% at most,400.10.20.30.40.50.60.70.80.91-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1Figure 5: Results of two dierent CKM ts shown in the (ρ, η) plane, using the Wolfenstein parametrization. The twodark closed curves represent the 68% and 95% CL contours obtained using the procedure and assumptions of Caravaglioset al.14 with md = 0.484 ± 0.015 ps and the ms results of Fig. 4. The two lightest closed curves are the result ofthe same t, except that the experimental information on ms is ignored. The circles centered on (1,0) indicate theconstraints corresponding to various values of ms, with the hadronic uncertainty indicated as the dashed band.but the current experimental check of this assumption, τB0s/τB0 = 0.937  0.040,8 is still of limitedprecision. These combined results on Γs/Γs are not yet precise enough to test the Standard Modelpredictions, which typically lie between 5% and 20%.4 Search for B0s oscillationsB0s{ B0s oscillations have been the subject of many studies from ALEPH, DELPHI and OPAL, aswell as SLD and CDF. No oscillation signal has been found so far. Because of the limited statisticsavailable, the most sensitive analyses are currently the ones based on inclusive lepton samples, andon samples where a lepton and a Ds meson have been reconstructed in the same jet. However, withlarger samples, the most promising approach would be to use fully reconstructed B0s mesons, whichhave a much better proper time resolution suitable to resolve higher oscillation frequencies.DELPHI11 have fully reconstructed 44 B0s candidates in the D0K−pi+, D0K−a+1 , D()−s pi+ andD()−s a+1 , channels, whereas ALEPH have recently reported 50 candidates in the latter two channels.The number of signal events is estimated to be  20 in each experiment, but with a proper timeresolution of  0.08 ps, more than two times better compared to more inclusive selections. As aresult, these analyses, which have very poor sensitivity by themselves due to the lack of statistics, dononetheless have a non-negligible impact on the average measurement of the oscillation amplitude Aat high values of ms, the mass dierence between the two mass eigenstates of the B0s system.All results have been combined, including the latest ones from ALEPH12 released for this conferenceand based on D−s `+ correlations and fully reconstructed B0s candidates, to yield the amplitudes Ashown in Fig. 4 as a function of ms. In the combination procedure,8 the sensitivities of the inclusivelepton analyses, which depend directly on the assumed fraction fB0s of B0s mesons in an unbiasedsample of weakly-decaying b hadrons, have been rescaled to a common value of fB0s = 0.096  0.012.This value is obtained from direct production measurements, measurements of the time-integratedmixing probability χ of b-hadrons at LEP, as well as the new world average of the B0 oscillationfrequency, md = 0.484  0.015 ps. The combined sensitivity for 95% CL exclusion of ms values isfound to be 14.6 ps−1, which is also the actual limit below which all values of ms are excluded by thedata at 95% CL. No oscillation signal can be claimed based on the deviation from A = 0 seen in Fig. 4around 17 ps−1: a fast Monte Carlo study13 shows indeed that statistical fluctuations can produced amore signicant deviation (anywhere in the explored range in ms) in  3% of the samples generatedwith a very large true value of ms.The information on jVtsj obtained, in the framework of the Standard Model, from the combinedms limit is hampered by the hadronic uncertainty, as is the case when extracting jVtdj from md.5However, many uncertainties cancel in the frequency ratiomsmd=mB0smB0ξ2∣∣∣∣VtsVtd∣∣∣∣2 , (6)where ξ is currently known to  6% from lattice QCD. This relation can be used in ts of the CKMmatrix, together with the experimental results on ms, md, jVub/Vcbj and K , as well as theoreticalinputs and unitarity constraints. Examples of such ts,14 shown in Fig. 5, illustrate the fact that thecombined ms results from Fig. 4 provide, together with the measured value of md, a signicantconstraint on the CKM matrix, favouring positive values of the Wolfenstein parameter ρ.5 CP violation in B0 → J/ψK0S decaysALEPH has recently released a new measurement15 of the CP asymmetry in B0, B0 ! J/ψK0S decays,A(t) =NB0(t)−NB¯0(t)NB0(t) +NB¯0(t)= − sin(2β) sin(md t) , (7)where NB0(t) and NB¯0(t) are the number of events produced as B0 and B0 as a function of theproper time t, and β is one of the angles of the CKM unitarity triangle. From a sample of 23fully reconstructed candidates, selected with a signal eciency of (28  2)% and an estimated purityof (71  12)%, ALEPH measures sin(2β) = 0.93+0.64−0.88(stat)+0.36−0.24(syst). The systematic uncertaintyis dominated by the limited knowledge of the probability of mistagging the initial state, measuredto be (25  6)% using B ! J/ψK events. This sin(2β) result can be combined with previousmeasurements from OPAL and CDF to yield15 sin(2β) = 0.91  0.35 or sin(2β) > 0 at 98.5% CL,increasing the condence that CP violation has been observed in the B sector.AcknowledgmentsI would like to thank D. Abbaneo and R. Forty for useful comments and careful reading of this writeup,A. Stocchi for providing Fig. 5, and the organizers of these \Rencontres" for an enjoyable conference.References1. The LEP jVcbj, jVubj, b-lifetimes, Γs and B oscillations working groups are coordinated by theLEP Heavy Flavour Steering Group; see http://www.cern.ch/LEPHFS/ and the links therein.The combination procedures used by these groups are described in a public document.82. G. Abbiendi et al., OPAL coll., Phys. Lett. B476, 233 (2000).3. The LEP heavy flavour working group, LEPHF 98-01, Sept. 1998.4. ALEPH coll., ALEPH 2000-020 CONF 2000-017, March 2000.5. K.Abe et al., SLD coll., SLAC-PUB-8316, hep-ex/9912058, Dec. 1999, subm. to Phys.Rev. Lett.6. I. Caprini, L. Lellouch and M. Neubert, Nucl. Phys. B530, 153 (1998).7. G. Abbiendi et al., OPAL coll., CERN-EP/2000-032, Feb. 2000, subm. to Phys. Lett. B.8. ALEPH, CDF, DELPHI, L3, OPAL et al., SLD coll., LEPHFS note 99-02, April 2000, to besubm. as CERN-EP preprint, and references therein.9. F. Abe et al., CDF coll., Phys.Rev. D57, 5382 (1998).10. R.Barate et al., ALEPH coll., CERN-EP/2000-036, Feb. 2000, subm. to Phys. Lett. B.11. DELPHI coll., DELPHI 99-109 CONF 296, June 1999, contribution 4 520 to Int. EurophysicsConf. on High Energy Physics, Tampere, 1999.12. ALEPH coll., ALEPH 2000-029 CONF 2000-024, March 2000.13. D. Abbaneo and G.Boix, J. of High Energy Physics 8, 4 (1999).14. F. Caravaglios et al., hep-ph/0002171, presented by A. Stocchi at the 3rd Int. Conf. on B physicsand CP violation, Taipei, Taiwan, Dec. 1999.15. ALEPH coll., ALEPH 99-099 CONF 99-054, Nov. 1999, and references therein.6

Heavy flavour physics results from LEP1

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