b and c semileptonic decays in OPAL

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PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 75B (1999) 216-220

b and c semileptonic decays in OPAL. Pauline Gagnon, OPAL Collaboration a* aCentre for Research in Particle Physics, Ottawa, Canada For several years, experimental results for BR(b --+ gX) obtained near the T(4S) and Z ° resonances have disagreed at the few sigma level. A new O P A L result for BR(b -~ pX) b a s e d o n the full data sample collected near the Z ° resonance is presented here. The result is extracted using m u o n m o m e n t u m and jet shape information in b-tagged events. The inclusive charm hadron semileptonic branching ratio is measured in a charm enriched sample of hadronic Z ° decays. N e w analysis methods used by O P A L to extract (b -+ p) and (c -~ t) are discussed in more detail,and the O P A L results are compared to those from other experiments. Both resultsare preliminary.

1. I n t r o d u c t i o n

2. The B R ( b --~/~X) measurement

Measurements of the semileptonic decays of heavy hadrons are important tests of our understanding of the dynamics of heavy quark physics. The BR(b -4 ~X) and BR(c --+ £X) measurements are also needed inputs to other related measurements. While theoretical calculations for BR(b -~ gX) agree with the experimental measurements within errors [1,2], the measurements obtained at different center-of-mass energy have disagreed for several years at a few sigma level. The semileptonic branching fraction for B mesons has been measured at the T ( 4 S ) resonance to be BR~L = (10.45 4- 0.21)% [3]. Using all results obtained above the B meson production threshold, one obtains BR~L = (11.12 4- 0.20)% [4] taking into account the correlated errors with all other electroweak measurements. The b superscript indicates that the high-energy data correspond to a mixture of B ±, B °, Bs and b baryons as opposed to B + and B ° only as at the T(4S) resonance. The new method presented here aims at measuring BR(b -+ £X) directly, instead of extracting this quantity from a global fit to electroweak parameters as was the case for all previously published results such as [5]. The correlation with Rb, the fraction of Z° events decaying into bb, is removed and the agreement with the T(4S) much improved.

We use data with center-of-mass energies within 3 GeV of the Z° peak collected during the 1992 - 1995 running period when the silicon microvertex detector was fully operational [6]. Each event is divided into two hemispheres by the plane perpendicular to the thrust axis. Using the thrust and its direction, we select 3.73 million multihadronic events well contained within the central barrel region. Lifetime tagging of b-flavoured events is done by a neural network algorithm and is used to reduce contamination from other quark flavours. The neural network has seven input parameters, the most important of which are the decay length, its uncertainty and the vertex multiplicity. Using double tags, one can measure the efficiency for b/) events in data to be 0.2956 4- 0.0004, with a b-purity at 0.9302 :l: 0.0051. Events containing one b-tagged hemisphere have their other hemisphere searched for a muon candidate 2. By using only muons found in the hemisphere opposite the b-tagged hemisphere, we do not introduce correlations between b-flavour tagging and muon selection. Muons are identified by associating central detector tracks with track segments in the muon detectors and requiring a position match in two orthogonal coordinates. The muon candidates are required to have momenta greater than 3 GeV/c.

*CERN PPE division, CH-1211 Geneve 23, Suisse

2The analysis presented here is preliminary. Electrons will also be included in an upcoming final publication

0920-5632/99/$ - see front matter © 1999 ElsevierScienceB.V. All rights reserved. PII S0920-5632(99)00355-2

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P Gagnon/Nuclear Physics B (Proc. Suppl.) 75B (1999) 216-220

Jet shape and momentum variables are used in a second neural network trained to distinguish between (b -+ #) and (b -~ c -~ #) decays. The neural network has five input parameters. The most powerful variables are the muon full and transverse momenta. In (b ~ / ~ ) decays, the muon momentum spectrum reflects the hard fragmentation of the primary b hadron and is thus particularly efficient at separating these muons from other sources. Similarly, the high mass of the b hadron induces a high muon momentum in the rest frame of the b hadron decay, which, once boosted along the b jet direction, gives a harder Pt spectrum for (b -~ p) than (b -~ c -~ #) decays. Combining the information from these five variables through a neural network allows not only the inherent separation power of each variable to be used, but also to take into account the correlations between them. 2.1. E x t r a c t i n g t h e s a m p l e c o m p o s i t i o n The muon sample described above has contributions from many sources. The distribution of the o u t p u t variable in the data is compared to the Monte Carlo to determine the fraction of events from (b -+ #) and (b ~ c --~/~) decays. Contributions from (c -~ #) decays are suppressed due to the b-tagging requirement. To determine the fraction of muon candidates coming from (b --+/~) decays, we perform a X2 fit to the shape of the neural network output variable. The Monte Carlo sample is divided in three subsampies. Event types having similax neural network output distributions are put within the same subsample. The three subsamples are: 1. (b --+ p) and (b -~ J/~I' ~ #+/~-) 2. (b ~ c ~ ~u) and (b ~ e ~ / ~ ) , where the came from the W - decay 3. all other background events. A total of 38617 muons are selected in the data. The results of the fit axe/([b, J / ~ ] --~ p) = 0.5316 :t: 0.0025 and f ( b --+ [c, ~] ~ / ~ ) = 0.2710 =t= 0.0080 where the uncertainties are purely statistical. The correlation coefficient between f([b, J/g2] -+ p) and f ( b -4 [c,~] -4/z) is -0.394. The background fraction and the

/ ( b ~ [c, ~] -~ #) are highly correlated. These fractions are slightly corrected to take into account systematic shifts due to possible differences between d a t a and Monte Carlo. Several sources are investigated, such as the b hadron species content, the fake muon rate, and the (b --+ r --~ p) content. Using these corrected fractions, we can adequately reproduce the shape of the neural network output variable distribution observed in data for all selected muons. This can be seen in figure 1 where we have superimposed the neural network o u t p u t variable distributions from the three types of Monte Carlo decays used to perform the fit. OPAL Preliminary ~2500

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Figure 1. Results of the fit after all systematic corrections have been applied. The three categories (b --+ p) and (b -+ J / ~ -~ p+/~-) events, (b ~ c -~ p) and (b -~ e ~ / J ) events, and all other backgrounds are described in the text. The X2 is calculated including statistical errors only as an indicator of the goodness-of-fit. 2.2. R e s u l t s for B R ( b - - + / z X ) After removing the contribution from (b -~ J / ~ -~ # + p - ) , we obtain: f ( b -~ #) = 0.5222 + 0.0024 (stat.) + 0.0056 (syst.).

P Gagnon /Nuclear Physics B (Proc. Suppl.) 75B (1999) 216-220

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The f ( b -4 #) fraction times the number of selected b-flavoured hemispheres with a muon in the opposite hemisphere gives the number of b hadrons that decayed semileptonically. The original number of semileptonic b decays is calculated using the efficiency for muon selection derived from the Monte Carlo. The total number of b hemispheres is extracted from the number of all b-tagged hemispheres using the sample purity extracted from the data. After including all shifts and systematic uncertainties, we obtain: BR(b -4 ~X) = (10.864-0.08 (stat.)4-0.22 (s'-+ ~+0.46 (model))% Y~'I-o.31 for the semileptonic branching ratios for muons. The largest systematic errors are the modeldependent errors, which include the uncertainty due to the modelling of the muon decay spectrum, fragmentation models and extrapolation to the muon low-momentum region. All errors are shown in table 1. We use the ACCMM lepton decay model for the central value and the IGSW and IGSW** to evaluate the uncertainty. To compare this result with the T(4S) result, we need to apply a correction to account for the presence of b baryons and Bs in the sample. Assuming similar semileptonic widths for all b hadrons, we get

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where B denotes an equal mixture of B ° and B ~: as found at the T ( 4 S ) resonance, and b, the b hadron mixture found near or at the Z° resonance. The current world average is 1.027 ± 0.018 [3] where the error on ~ dominates. Even with this correction applied, this result is in good agreement with the value found at the T(4S), namely, (10.45 :t: 0.21)%. 3. T h e (e ---+ ~) m e a s u r e m e n t This analysis is of particular interest since no measurement of (c --4 g) at the Z° resonance has been published so far. One difficulty is to obtain a high-purity sample of primary c~ events. We first enrich our sample in Z° --4 c~ events by reconstructing D *+ mesons in one hemisphere. Then we look for leptons in the hemisphere opposite to

[

Sources ~ BR.(b -4/~X) systematic sources Rb for b-purity -~0.021 5=0.022 P~ for b-purity 4-0.042 other b-purity inputs 4-0.156 muon id efficiency detector resolution effects 5=0.086 Monte Carlo statistics 4-0.041 b hadron species 4-0.041 5=0.098 b decay table fake muon rate 4-0.004 fake muon spectrum 4-0.002 (b -4 r -4/~) content 4-0.016 (b --+ J / ~ - 4 / ~ + # - ) content 5=0.013 total systematics 5=0.219 model-dependent sources (b -4 #) model -o.~ +0.413 (b -~ c -4/~) model q:0.038 fragmentation models 5=0.204 total model-dependence +0.462 -0.30~ Table 1 Summary of all absolute systematic and modeldependent errors for BR(b -4/~X) given in percent.

the one containing the D *+ meson. These leptons come from primary charm (c -4 g) as well as from primary bottom (b -4 g) and cascade (b -4 c -4 g) decays. All data collected with the OPAL detector near the Z° resonance between 1990-95 is used for this analysis. We need to extract N~).+,t_ , the number of events where both a D °+ meson from Z° -4 c~ decay is reconstructed in one hemisphere, and a lepton coming from a semileptonic charm decay is found in the opposite hemisphere. The charge correlation between the D *+ meson and the lepton is used to form two samples: the right sign sample, ND.+,t-, where the D *+ meson and the lepton have opposite charge, and the wrong sign sample, ND-+,t+, where they have the same charge. The ND.+,e- sample mostly contains c -4 g and b ~ g - r g events, but also b ~ c -~ g decays due to mixing (and vice versa

P Gagnon/Nuclear Physics B (Proc. Suppl.) 75B (1999) 216-220 for the ND.+,t+ sample). Both samples have contributions from the combinatorial background. The number of leptons opposite a D *+ meson in Z° --~ c~ events, N~).+,t_, can be expressed as the difference between ND-+,e- and ND-+,t+, plus some contributions from bottom and background events, ANb and ANbgd. The first correction, ANb, can be derived directly from analytic expressions for ND.+,t- and ND.+,t+ , [7] and reflects the fact that mixing affects both samples differently. It is calculated from the known branching ratios and the mixing parameter. The second correction, ANb$d, is given by ANbgd = N ~ d - N ~ +, where N ~ d ( N ~ +) is the number of combinatorial background events in the right sign (wrong sign) sample. This number is determined using both data and Monte Carlo simulations. Hence, the signal is:

N~).+e- = (ND.+,t- - ND-+,t+) -- ANb -- ANbgd . The inclusive semileptonic branching ratio of charm hadrons, B(c --+ e), is then given by B(c -~ ~) = ND'+,t¢ a r~ tD*+_¢--4t , - ,D*-I- J¢ el where ND.+ is the number of events with a D *+ meson, feD'+, the fraction of D *+ mesons produced in primary c~. decays, and e~-~t, the lepton reconstruction efficiency in the D *+ sample. 3.1. T h e D*+£ + The D *+ mesons are reconstructed in the following five D O decay channels: D *+ --r D°zr + K-lr + , K-e+ve , K-p+u.

t--r K-Tr+Tr - , K-Tr+Tr-rr + ,

"3-prong" "electron" "muon" "satellite" "5-prong" .

The identification algorithm and the method to separate the different sources contributing to the observed D *+ signal is described in [10]. The selected sample of D *+ candidates has contributions from: D *+ mesons produced in Z° --+ c~ events; D *+ mesons produced in Z° ~ bl~ events; a small contribution from D *+ mesons produced in events where a c~ pair is produced

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in the splitting of a gluon; and from combinatorial background. The combinatorial background in the sample of D *+ mesons is subtracted on a statistical basis using an independent sample of background candidate events, selected based on a hemisphere mixing technique first introduced in [8]. The candidate for the pion in the D *+ -+ DOn + decay is selected in the hemisphere opposite to the rest of the candidate, and reflected through the origin. This sample is known to be an unbiased estimator of the combinatorial background [8,9], providing a reliable modelling of the background shape. The remaining two sources of D *+ production, Z° ~ bb --+ D*+X and Z° -+ c~ ~ D*+X, are separated by applying three different flavour tagging methods, based on lifetime, jet shape and hemisphere charge information, as described in [10]. Combining all D *+ channels, the overall charm fraction is determined to be: feD*+ = 0.774 + 0.008 (stat.) + 0.022 (syst.). The D*+g - sample is formed by searching the hemisphere opposite the identified D *+ meson for a lepton with a charge opposite to that of the D *+ candidate. Electrons are identified using a neural network technique Muons are selected based on the X2 of the matching between track segments in the central tracking chambers and in the muon chambers. Both leptons are selected with a momentum above 2 GeV/c.

3.2. Results for (c -+ £) The momentum and transverse momentum spectra for electrons and muons are shown in figure 2 after subtracting the wrong sign sample from the right sign sample. Both data and Monte Carlo are shown before applying the residual corrections for bottom events and combinatorial background charge asymmetry. The total number of leptons from charm c hadron decays is N~).+, e_ = 378 + 31 and N D*+,pc = 476 =t: 40, respectively, or N Dc* + , l - -- - 854 5= 51 D*+/?- events. Using the number of selected D *+ mesons, ND.+ = 15784 :h 99, and the charm fraction in the D *+, the inclusive semileptonic branching ratios of charm hadrons in Z ° --~ cd events are found to be

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