Charlton, D G

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I uitvd Kingdom OCR OutputB15 2TTBirminghamEdgbastonThe- lvuixersity of BirmirighamSchool of Physics and Space ResearchBrussels. 27 july — 2 August 1995)]lL[8f7l.(1.liOII.(ll E rrrop/rysics Corference on High Energy Physics,(invited talk given at thethe Standard Model at the 0.1% level.Rc = 0.1540 i 0.0074- are obtained. These results together are consistent withvalue of Rc is also taken from experiment, Rb = 0.2219 :1; 0.0017 andabove the Standard Model expectation. Tor ntmp = 180 *1.; 12 GeV. If theStandard Model prediction. This value of Rb lies three standard deviationscorrelated sources of error, yields Rb == 0.2205 rl; 0.0016 if RC is fixed to itsCombination of the latest published and preliminary results, taking into accountand SLC. The results are dominated by double lifetime tag measurements.Precise measurements of the Z partial decay width to bb have been made at LEPSli") °Wt>i0AbstractRoyal Society University Researcl1 FellowD.C. CharltonMeasurements of Rb using Lifetime Tags()l*`HTlilV1lN(}H¢\lV1T H E 1ll\1\YERSlTYNovember 1995BHAM-HEP/95-01analyses. probability density function for each reconstructed track OCR Outputging approaches. The discussion here focusses on these It has also been adopted by DELPHI2 and SLD3. Thetion. The measurements are dominated by lifetime tag inally developed by ALEPH1 for the Rb measurement.differing only by a very small photon-exchange correc assumption of no lifetime in that hemisphere, was origthe impact parameters seen in a hemisphere, under theO I`(Z —~ hadrons)Rb E (I) A tag based on the joint probability for obtainingr(z° -» bE)ses.practically identical to the quantity algorithms are used by the experiments in the Rb analyat the Z0 peak which contain primary bb pairs. This is high efficiency samples. Two different lifetime taggingmeasurements are made ofthe fraction of hadronic events times is the most effective way of achieving high purity,high mass and long lifetime of b hadrons. In practice, SLC experiments, tagging b hadrons by their long lifeaccessible ofthe hadronic decay widths, because ofthe With the precise silicon vertex detectors of the LEP andThe Z0 to bb width is also the most experimentally3 Lifetime TagsFigure 1: Diagrams contributing specially to the Z°bb vertexMonte Carlo modelling.obtained from lower energy measurements and throughtematic errors arise from the knowledge of ec, éuds and Cb...._......-..T3 .... . .......... { yt sion, dominated by Nb, in this method. Remaining syshadron decays, although at the cost of statistical preciadvantage that it is independent ofthe modelling of bends and Cb are known. The method has the majorEquations 2 and 3 can be solved for Rb and eb if cd,tions for charm and lighter flavours can be neglected).and the correlation Cb is close to 1 (analogous correlaspheres in bb, cE and lighter-flavour events, respectively,where Eb, eb and end, are the efficiencies for tagging hemiNt, I CbeiRb + ein, s e?,ds(l — Rb - RC) (3)sive particles might affect this vertex more than others. N, : 6bRb + ebRb -%— ends(1— Rb — RC) (2)be seen first in Pb; because new physics coupling to masModel, it is possible that indications of new physics might tagged, Nd, can be written:widths. Further, beyond the framework ofthe Standard Nt, ind the number of events with both hemispherespendence on the top mass from those ofthe other quark each hemisphere. The number of tagged hemispheres,such as those shown in Fig. 1 give Fbg a different de— thrust hemispheres, and separately looking for a b tag indiffer from those for other quarks and leptons. Diagrams technique. This consists of dividing each event into twomeasurements of Rb now all employ the “double—tagging”ticularly interesting because the Zobb vertex correctionsWith the high statistics available at the zu, competitiveThe partial decay width of the Z0 to bb, Pkg, is par1 Introduction 2 Experimental Method: Double Taggingobtained. These results together are consistent with the Standard Model at the 0.1% level.If the value of RC is also taken from experiment, Rb : 0.2219 t 0.0017 and Rc : 0.1540 j; 0.0074 arevalue of Rb lies three standard deviations above the Standard Model expectation, for mm? : 180t12 GeV.correlated sources of error, yields Rb = G.2205—:0.0016 if RC is fixed to its Standard Model prediction. Thisare dominated by double lifetime tag measurements. Combination of the results, taking into accountPrecise mcasluemcms of the Z partial decay width to bb have been made at LEP and SLC. The resultsP.O.Boz 363, Birmingham B15 2TT, United KingdomRoyal Society University Research Fellow, School of Physics and Space Research, University of Birmingham,DG. CHARLTONMEASUREMENTS OF Rb USING LIFETIME TAGScorrelations in the tagging efficiencies between the hemi relations, discussed above, and various aspects of charm OCR OutputThere are several types of effects which can introduce periments, the main systematics arise from efficiency coreffects, which are essentially uncorrelated between the ex4 Correlations in Tagging Efficiencies perimental precision on Rb (Fig. 3). Aside from detectormethod is acceptable.0.0017 0.0014statistical I 0.0022 0.0040precision resulting from the use ofthe double—tagging0.0027 0.0022total systematic I 0.0026 0.0037less excellent, above 20%. so that the loss of statistical0.0006 0.0003light quarks I 0.0002 0.0001background low. The b tagging efficiencies are nonethe0.0003 0.0005gluon splitting I 0.0003 0.0004ties lie above 90% — crucial for keeping systematics from0.0005 0.0008c fragmentation I 0.000]. 0.00120.0006 0.0004c lifetimes I 0.0005 0.0003(* the OPAL numbers include also lepton tags)0.00100.0010 0.0018c decay mult. I 0.00060.00090.0016 0.0013c fractions I 0.0009SLD I 31% i 94%0.0011 0.0007eff. correlation l 0.0016 0.0010OPAL* l 23% l 94%detector effects l 0.0015 0.0009 0.0012 0.0025DELPHI l 21% 92%OPALALEPH 1 DELPHI SLDALEPH I 26% l 96%eb l b purity Monte Carlo statistics.lifetime tag Rb mcasiu•ements1·2*3‘°. "Detector effects” includeTable lz Surnrnary of Lifetime Tag PerforrnancesTable 2: Surnmary of main systematic uncertainties on the precisethe combined figures are quoted. The b hemisphere puri These measurements together dominate the current exondary vertex or a high—pt lepton to tag a hemisphere. lifetime tag Rb measurements are shown in Table 2.summarised in Table 1. Since OPAL* allow either a sec The main systematic uncertainties affecting the precisedifferent detectors. as used for the Rb measurements. isThe performance ofthe tagging algorithms in the 5 Systematic Uncertairitiesdecay lengths to reduce resolution uncertainties.tical subtraction of vertices reconstructed with negativearound +0.2% for the decay length tag.divided by its error is greater than 8. OPAL use a statis—are -1% to -2% for the impact parameter lifetime tag,tices qualify as b tags if the reconstructed decay lengthexpressed in terms of the variable p : (C1, - 1)eb/(1- eb)ondary vertex reconstruction in each jet of an event. Vertematic error. The typical sizes ofthe correlations, reOPAL use a different algorithm*, which employs secmodelled, and to derive an estimate of the residual syssimulation to ensure that the correlating effects are wellGHC CVCHL h€HIiSph¢Y¢ Bu COHIC ftOIH Y.hC pTlH1&Py VEYCCKrelations obtained can be checked against Monte Carlolighter events, where PN is the joint probability that the tracks inFigure 2: Expected distributions of —— loggP_v) for SLD for b, c and relatively minor corrections for background. The corcorrelations to be estimated directly in the data, with·l0g10(Px)0 1 z 1 1 s 1. 1 g ·> dominated by correct b tags, this procedure allows themomentum correlations. Since tagged hemispheres are4%* Hints.tor for geometrical correlations, or the thrust value for_ ,_ »* ¤for example, the position ofthe thrust axis in the detecwt ‘·. '·°‘·. variable reflecting a possible efficiency correlating effect:A x` i A ` N5 measure the tagging eHiciency as a function of an event10 uds A'c ••,whether a source of correlation is well understood is togm ent collaborations. A common technique used to evaluateW-‘ data and Monte Carlo studies, differently by the differThe correlations are estimated with a mixture ofE10 Q_Eg Atwo hemispheres ofthe event.SLDmary vertex information, which is shared between thec 1the ends ofthe vertex detector acceptance; and the priobtained by SLD, is shown in Fig. 2. metrical correlations in tagging emciency, for example atare used to tag b events. The performance of this tag, as and b quarks to travel into the same hemisphere; geono lifetime information is constructed, and small values tagging efhciency, or in extreme cases causing both bmomentum of b and b quarks in the event and so theirjoint probability that all tracks in the hemisphere haveportance include: the radiation of gluons. correlating thelifetime in that hemisphere, is obtained from data. Theto have a. given impact parameter. in the event of no spheres of an event. Those estimated to be of most imance matrix of the final averaged electroweak parameters. OCR Outputslightly with centre-0f—mass energy. The value of this rasurements. The procedure gives directly the full covarislightly lower than Rb (by 0.0003) and that it variesing the full covariance matrix between the input measection ratio c·(e+e' —» bb)/o·(e+e` —· hadrons) isminimisation is used to find the final averages, employPhoton-exchange contributions mean that the crossresult gives a full breakdown of systematic errors. A X26 Energy Dependence of the bb Fraction systematic parameter variation, and requiring that eaching effects into account, by providing standard ranges foriiavour electroweak working group, takes these correlatshower and other Monte Carlo programslThe averaging procedure developed by the LEP heavywith, and slightly higher than, the predictions of partondifferent ways on external input assumptions and ranges.measured by OPAL9. The measured rate is consistentsure more than one parameter at a time, and depend induction by this process in Z0 decays has recently beenelectroweak results is not trivial. The analyses often meaefficiencies to primary heavy quarks. The rate of cE proA consistent combination of the different heavy quarksidered separately because they have different taggingnot included in the definition of Rb, and must be con8 Combined Heavy Quark Electroweak Resultsrectly from Z0 decay. These secondary heavy quarks areof heavy quarks in the parton shower, rather than diAn systematic error also arises from the production considered in the averaging procedure, discussed next.used in the derivation of results here. extent on the value of RC assumed: this dependence istent with, and more precise than, the older averagesg ton p and pt spectrau. The results all depend to somecontinue to improve, and new measurements are consis tag; and from all ofthe LEP experiments using fits to lepperimental measurements of these quantities, however, using a “mixed” analysis of lepton spectra and a lifetimethe knowledge ofc lifetimes, and ofc fragmentation. Ex reduced charm modelling uncertainties; from DELPHIRemaining systematics in the charm sector arise from tags; from DELPHI using a multivariate analysis withthe shorter c baryon lifetimes). there are results from ALEPH and L3 using event shapepredictions, with a large error (unimportant because of PHI, OPAL and SLD. Apart from the lifetime analyses,ing c baryons: they are instead taken from Monte Carlo this conference are the preliminary numbers from DELthe topological decay branching ratios for weakly decay The latest results on Rb are shown in Fig. 3. New forarises from this difference. There are no measurements ofor impact parameter probability at LEP: a further error Standard Model value of 0.172.which are usually not included in the secondary vertex Figure 3: Latest results for Rb, in the case where Rc is fixed to itsThat analysis included tracks from KO —· mr decay.and Dj are taken from a measurement by MARK IHS Rc = {L17] (fixed) 0.2 mit 0.22 0.13 h0.l4distributions for the weakly decaying c mesons D0, D'0;2us;o.¤m1¤Averageical in b hadron decays. The charged decay multiplicitymultiplicities are tagged preferentially, since they are typ...wSLJ2§`$i;3`t§’ifl2‘"°“'“° H·•·H-decays affects the tagging efficiency, because high decayThe charged decay multiplicity produced in c hadron EP lepton tits 0.22l9:0.0039be used to improve further this source of uncertainty.02'.t0.003t0.00791 GeV is reasonable. In future, LEP measurements can‘ mmshapeL 9 3lftassumption of energy independence between 10.55 and ozzsstouzoosLEPH 90 9 ’¤Z·· A igii°""“"""° =f*"°*iat lower energy, and are consistent, indicating that the|).1l97·;0.00l-$:0.0022ing ratios]. The results are of similar precision to those OPAL (92-94 prel) lifetime ~ lepton ‘·made at LEP of these production fractions times branch ......°*%2‘i5;L2?$;3i·t;$;*+-·—++derive the Rb errors. Measurements have now also been0.223l:0.0029t0.003Smeasurements ofthe c production fractions are used to DELPHI (92+93 prel) mixedworld averagesg. The uncertainties on the low energy °*%E‘§}1it‘?.tH%§53Q7 +*•··?* V0.2.1920.0)22:0.0026The decay branching ratios have been taken from the LEPH 92 ` ' A flMmmnificant energy dependence between there and the Z0ARGUS measurements° at 10.55 GeV, assuming no sigtion fractions employed have been taken from CLEO and7 Results of the Measurementsintroduces an uncertainty on Rb via ec. The producbilities ofthe different species also varies a lot. Thishadrons vary substantially, the lifetime tagging proba with expectation, which is a shift ofjust ~ -0.3%.Since the lifetimes cf different weakly-decaying c and DELPHI2. The results are found to be consistenttio has been measured ::2 GeV off the Z0 peak by OPAL4lifetime tags has reached the 1.2% level. Combination ofThe precision of individual measurements of Rb using 95-03; D. Bardin, these proceedings.W. Hollik and G. Passarino, CERN yellow report9 Conclusion culations for the Z Resonance”, Eds. D. Bardin.13. “Reports of the Working Group on Precision Cal12.top mass of 180;t12 GeV. T. Behnke, J. Huber, A. Passeri, these proceedings.11.The value of Rb predicted is Rb : 0.2155 : 0.0005 for a P. Perret, these proceedings.10.group on precision calculations for the Z resonance M.H. Seymour, Nucl. Phys. B436 (1995) 163.includes radiative corrections as derived by the working B353 (1995) 595.obtained from the ZFITTER program version 4.9, which OPAL Collaboration, R. Akers et al., Phys. Lett.The standard model prediction shown in Fig. 4 is Lett. B263 (1991) 135.is obtained. MARK lll Collaboration, D. Coffman et al., Phys.model value of Rc : 0.172,a value Rb : 0.2205 :& 0.0016 M. Hauschild, these proceedings.When the fit is repeated with RC fixed to its standard D45 (1992) S1.35%. The average Rb and RC values are shown in Fig. 4. Particle Data Group, K. Hikasa et al., Phys. Rev.below 15%, except for that between Rb and RC, which is toletto et al., Phys. Rev. D3'7 (1988) 1719.between the measured parameters in Equations 4 are all Phys. C52 (1991) 353,CLEO Collaboration, D. Borprobability for the overall fit is 80%. The correlations ARGUS Collaboration, H. Albrecht et al., Zeit.E1') : (7.76 i 0.36)% and Y : 0.1145 : 0.0061. The X al., Zeit. Phys. C65 (1995) 17.and also Brfb —> K) : (11.12 1 0.23)%, Br(b —» c —» this conference; OPAL Collaboration, R. Akers etOPAL Collaboration, paper eps0278 contributed toFB,pol; 0.606 ; 0.090 (4) this conference.FB,pol: 0.841 ; 0.053 SLD Collaboration, paper eps0222 contributed toet al., Zeit. Phys. C66 (1995) 323.: (7.29; 0.58)%A23to this conference; DELPH1 Collaboration, P. AbreuI (9.97; 0.31)%A21?DELPHI Collaboration, paper eps0570 contributed: 0.1540; 0.0074 Lett. B313 (1993) 535.; 0.2219; 0.0017 ALEPH Collaboration, D. Buskulic et al.. Phys.Referencesboth LEP and SLDHJZ. The results obtained are:and c partial width and asymmetry measurements fromThe averages are performed including the latest b the OPAL Rb experts, J.R. Batley and M. Morii.Su Dong and H.A. Nealjr. from SLD. My thanks also toModel prediction for A range of cop masses lar T. Behnke, K. Monig and P.S. Wells from LEP andFigure 4: Combined result for Rb and Rc compared to Standardworking group. Thanks belong to them all, in particugroup, together with members of the SLD heavy flavour`18.114 0.210 0.218 0.22 0.122 0.124 0.226 0.228members of the heavy-iiavour LEP electroweak workingrious task requiring patience and care. lt was done by theThe preparation of the averages presented here is a labo0.14 - 68% LL / lAcknowledgments1/ l0.1s two.' lsample which should be completed in the next year orbe answered by the final analyses ofthe full LEP I data0.16 the standard model, is a question which hopefully willproblems, invalid input assumptions, or physics beyondWhether this deviation is due to unforeseen experimentalthe value of Rb disagrees more with the expectation.0.17 }—prediction. If Rb is also determined from experiment,WL / 95% CL § E E X"Swlies three standard deviations above the Standard Modelow ci }—~\ \99.·>% ciits Standard Model value. The central result obtainedU0.18 “0.7% measurement, if the value of Rb is taken to haveofthe method. and SLD, taking into account correlated errors, gives aVarious simple checks are made t0 ensure the reliability the latest preliminary and published results from LEP

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