A direct measurement of |Vcs| in hadronic W decays using a charm tag

doi:10.1016/S0370-2693(99)01088-6

Physics Letters B

Volume 465, Issues 1–4, 21 October 1999, Pages 349-362

https://doi.org/10.1016/S0370-2693(99)01088-6 Get rights and content

Abstract

The inclusive charm production rate in W decays is measured from a study of the properties of final state particles. The sample of W pairs is selected from 67.7 pb⁻¹ collected by ALEPH in 1996 and 1997 at centre-of-mass energies near 172 and 183 GeV in the channels W⁺W⁻→4q and $W^{+} W^{−} →ℓνq q ̄$ . The branching fraction of hadronic W decays to a final state containing a c quark, R^W_c= Γ(W→cX)/Γ(W→hadrons), is measured to be 0.51±0.05_stat±0.03_syst. This allows a direct determination of the CKM matrix element |V_cs|=1.00±0.11_stat±0.07_syst.

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 R^W_c=Γ(W→cX)/Γ(W→hadrons), where X stands for $d ̄, s ̄ or b ̄$ . 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: $R^{W}_{c} = |V_{cd} |^{2} +|V_{cs} |^{2} +|V_{cb} |^{2} |V_{ud} |^{2} +|V_{us} |^{2} +|V_{ub} |^{2} +|V_{cd} |^{2} +|V_{cs} |^{2} +|V_{cb} |^{2} .$ Assuming the unitarity of the CKM matrix, R^W_c is expected to be equal to 0.5. Therefore, a measurement of R^W_c 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 |V_cs| which is currently measured to be 1.01±0.18 using D→Kℓν_ℓ decays [1].

A charm jet tagger (called $NN_{c}$ 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 R^W_c and $|V_{cs} |$ from the $NN_{c}$ 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 R^W_c is extracted by comparing the $NN_{c}$ 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 M_W^ref of

Event selection

The event selection was performed separately for the purely hadronic events, W⁺W⁻→4q, and for the semileptonic events, $W^{+} W^{−} →ℓνq q ̄$ (ℓ=e,μ).

For the semileptonic events $W^{+} W^{−} →eνq q ̄$ and $W^{+} W^{−} →μνq q ̄$ , 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 $W^{+} W^{−} →τνq q ̄$ decays

Result

To extract R^W_c, a binned maximum likelihood fit is performed to the shape of the output distribution of NN_c^max. In each bin k of this distribution, the expected number of Monte Carlo events is defined as $N_{k}^{MC} = R^{W}_{c} N_{W} Prob_{W→cX} (k)+(1−R^{W}_{c})N_{W} Prob_{W↛cX} (k) + N_{bkg} Prob_{bkg} (k)$ where Prob_W→cX(k), $Prob_{W↛cX} (k)$ and Prob_bkg(k) are the probability density functions, determined from Monte Carlo, in the bin k of the $NN_{c}^{max}$ distribution for the W→cX, $W↛cX$ and non-WW background events, respectively. The total number N_W 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 $q q ̄ (γ)$ 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 $q q ̄ (γ)$ background and the $W→q q ̄$ 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 $NN_{c}^{max}$ 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 R^W_cis used in the simulation, while this confidence level becomes smaller than 1% if R^W_cis 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 R^W_c, while only 9.8 would be expected without $W→cX$

Conclusion

Using a charm tag based on the properties of jets produced in W decays, the inclusive charm production rate in W decays, R^W_c, is measured. The analysis of the 172–183 GeV data collected by ALEPH in 1996 and 1997 leads to the value $R^{W}_{c} =Γ(W→cX)/Γ(W→hadrons)=0.51±0.05_{stat} ±0.03_{syst},$ from which the value $|V_{cs} |=1.00±0.11_{stat} ±0.07_{syst}$ is derived. The measured value of R^W_cis 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.

References (27)

M. Skrzypek et al.
Comp. Phys. Commun.
(1996)
T. Sjöstrand
Comp. Phys. Commun.
(1994)
G. Marchesini
Comp. Phys. Commun.
(1992)
L. Bellantoni
Nucl. Instr. and Meth. A
(1991)
Particle Data Group, Review of Particle Physics, Phys. Rev. D 54 (1996)...
Nucl. Instr. and Meth. A
(1990)
Nucl. Instr. and Meth. A
(1995)
Phys. Lett. B
(1997)
Phys. Lett. B
(1991)
Phys. Rev. Lett.
(1979)

Phys. Rev. Lett.

(1985)

Phys. Lett. B

(1998)

Yu.L. Dokshitzer

J. Phys. G

(1991)

Cited by (12)

Review of particle physics
2004, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
The minimal model of nonbaryonic dark matter: A singlet scalar
2001, Nuclear Physics B
We propose the simplest possible renormalizable extension of the Standard Model—the addition of just one singlet scalar field—as a minimalist model for nonbaryonic dark matter. Such a model is characterized by only three parameters in addition to those already appearing within the Standard Model: a dimensionless self-coupling and a mass for the new scalar, and a dimensionless coupling, λ, to the Higgs field. If the singlet is the dark matter, these parameters are related to one another by the cosmological abundance constraint, implying that the coupling of the singlet to the Higgs field is large, λ∼O(0.1–1). Since this parameter also controls couplings to ordinary matter, we obtain predictions for the elastic cross section of the singlet with nuclei. The resulting scattering rates are close to current limits from both direct and indirect searches. The existence of the singlet also has implications for current Higgs searches, as it gives a large contribution to the invisible Higgs width for much of parameter space. These scalars can be strongly self-coupled in the cosmologically interesting sense recently proposed by Spergel and Steinhardt, but only for very low masses (≲1 GeV), which is possible only at the expense of some fine-tuning of parameters.
Constraining models with vector-like fermions from FCNC in K and B physics
2001, Nuclear Physics B
In this work we update the constraints on tree level FCNC couplings in the framework of a theory with n isosinglet vector-like down quarks. In this context, we emphasize the sensitivity of the B→J/ψK_S CP asymmetry to the presence of new vector-like down quarks. This CP asymmetry, together with the rare decays $B→X_{s,d} l l ̄$ and $K→πν ν ̄$ are the best options to further constrain the FCNC tree level couplings or even to point out, in the near future, the possible presence of vector-like quarks in the low energy spectrum, as suggested by GUT theories or models of large extra dimensions at the TeV scale.
The determination of the CKM matrix
2001, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
A general discussion of the Cabibbo–Kobayashi–Maskawa (CKM) matrix is given and the importance stressed of determining the matrix elements as an essential part of understanding CP violation in and beyond the Standard Model. The status of knowledge of the matrix elements connecting the first and second generation quarks is reviewed. A perspective on determinations of the full CKM matrix is presented as an introduction to the separate contributions to the panel discussion that follows.
W properties and decays at LEP2
2001, Nuclear Physics B - Proceedings Supplements
Preliminary results from W-pair events measured by the four LEP experiments are presented. These results are based on data taken during years 1996–2000 at e⁺e⁻ centre-of-mass energies between 161–208 GeV and corresponding to a total integrated luminosity of 560 pb⁻¹. A total number of about 7500 W-pair candidates per experiment is used to determine the W decay branching fractions, which are used to test lepton universality to a 1.6% accuracy level. Hadronic decays of W into charm are identified and the fraction of charm decay is 0.494±0.044, corresponding to the CKM matrix element value |V_cs|=0.96±0.08. The W mass is measured to be 80.427±0.046 GeV, in agreement with other determinations of this value. The measured value of the W width is Γ_W=2.12±0.11 GeV. All results agree with the Standard Model expectations.
A measurement of the rate of charm production in W decays
2000, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
Using data recorded at centre-of-mass energies around 183 and 189 GeV with the OPAL detector at LEP, the fundamental coupling of the charm quark to the W boson has been studied. The ratio $R_{c}^{W} ≡Γ (W → c X)/Γ (W → hadrons)$ has been measured from jet properties, lifetime information, and leptons produced in charm decays. A value compatible with the Standard Model expectation of 0.5 is obtained: $R_{c}^{W} =0.481±0.042 (stat.) ±0.032 (syst.)$ . By combining this result with measurements of the W boson total width and hadronic branching ratio, the magnitude of the CKM matrix element |V_cs| is determined to be |V_cs|=0.969±0.058.

View all citing articles on Scopus

²: Supported by CICYT, Spain.

¹: Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain.

View full text

A direct measurement of |Vcs| in hadronic W decays using a charm tag

Abstract

Introduction

Section snippets

The ALEPH detector

Monte Carlo samples

Event selection

Result

Studies of systematic uncertainties

Checks of the analysis

Conclusion

Acknowledgements

Comp. Phys. Commun.

Comp. Phys. Commun.

Comp. Phys. Commun.

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Phys. Lett. B

Phys. Lett. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Lett. B

J. Phys. G

A direct measurement of |V_cs| in hadronic W decays using a charm tag