A direct measurement of |Vcs| in hadronic W decays using a charm tag
Introduction
The data collected by ALEPH in the years 1996 and 1997, at average centre-of-mass energies of 172 GeV and 183 GeV, respectively, are used to study the properties of hadronic W boson decays. The total luminosity collected in the two periods is 67.7 pb−1 of which 57.0 pb−1 was collected at the higher energy. The identification of charm jets in these decays leads to a direct measurement of the fraction RWc=Γ(W→cX)/Γ(W→hadrons), where X stands for . Unless explicitly stated, the charge-conjugate modes are always implied throughout this paper. Within the Standard Model, this branching ratio can be expressed as a function of the different Cabibbo-Kobayashi-Maskawa (CKM) matrix elements through the following relation:Assuming the unitarity of the CKM matrix, RWc is expected to be equal to 0.5. Therefore, a measurement of RWc is a direct test of this assumption. Furthermore, Eq. (1)can be used to extract a value of the least well known CKM matrix element |Vcs| which is currently measured to be 1.01±0.18 using D→Kℓνℓ decays [1].
A charm jet tagger (called in the following) was developed to identify W→cX decays. This is based on a neural network with 12 variables as input and uses mainly information from charm lifetime, jet-shape properties, reconstruction of D mesons and lepton identification. The procedure of extracting RWc and from the output is described in this paper. Another analysis based on a Fisher Discriminant technique is also presented as a crosscheck of the method.
Section snippets
The ALEPH detector
The ALEPH detector [2] and its performance [3] are described in detail elsewhere. Only a brief account of the parts of the apparatus relevant for this analysis is given here. Charged particles are detected over the range |cosθ|<0.95 by an inner drift chamber and a large time projection chamber (TPC), complemented by a silicon strip vertex detector (VDET) made of two layers each providing measurements in the rφ and rz coordinates, with a resolution of 12 μm in rφ and 12 μm in rz, for tracks at
Monte Carlo samples
The value of RWc is extracted by comparing the output distribution in the data to the corresponding Monte Carlo distribution, where generated events are processed through a full simulation of the ALEPH detector response and through the same reconstruction chain. The KORALW [5] event generator with the complete set of WW-like four-fermion diagrams was used to produce two samples of 200,000 WW events at centre-of-mass energies of 172 GeV and 183 GeV, generated with reference W masses MWref of
Event selection
The event selection was performed separately for the purely hadronic events, W+W−→4q, and for the semileptonic events, (ℓ=e,μ).
For the semileptonic events and , the neural network described in Ref. [11] is used for the selection. The signal efficiency is about 90% for a purity of 95%. The selected events are then forced into two jets using the DURHAM-P [12] algorithm, the lepton being removed when doing the clusterisation. Events corresponding to decays
Result
To extract RWc, a binned maximum likelihood fit is performed to the shape of the output distribution of NNcmax. In each bin k of this distribution, the expected number of Monte Carlo events is defined aswhere ProbW→cX(k), and Probbkg(k) are the probability density functions, determined from Monte Carlo, in the bin k of the distribution for the W→cX, and non-WW background events, respectively. The total number NW of
Studies of systematic uncertainties
The following sources of systematic errors are considered:
- 1.
An uncertainty of 5% is applied to the normalization of the background as estimated in Ref. [22].
- 2.
The error resulting from the particular choice of QCD Monte Carlo generator is estimated by replacing JETSET by HERWIG for both the background and the decays, and the full difference between the two results is taken as the systematic error. Since HERWIG is known to provide a very approximate representation of exclusive
Checks of the analysis
The consistency of the distribution in data and Monte Carlo is tested by means of a Kolmogorov test. This gives a consistency confidence level of 98.1% if the fitted value of RWcis used in the simulation, while this confidence level becomes smaller than 1% if RWcis set to zero.
The number of fully reconstructed D mesons in the most charm-like jet of a pair is 21 in the data; 22.6 are expected in the simulation using the fitted value of RWc, while only 9.8 would be expected without
Conclusion
Using a charm tag based on the properties of jets produced in W decays, the inclusive charm production rate in W decays, RWc, is measured. The analysis of the 172–183 GeV data collected by ALEPH in 1996 and 1997 leads to the valuefrom which the valueis derived. The measured value of RWcis in agreement with the Standard Model expectation of 0.5 assuming the unitarity of the CKM matrix and with the result obtained by the
Acknowledgements
We are indebted to our colleagues of the accelerator divisions for the outstanding performance of the LEP accelerator. Thanks are also due to the many engineering and technical personnel at CERN and at the home institutes for their contributions toward the success of ALEPH. Those of us not from member states wish to thank CERN for its hospitality.
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