Showing papers by "E. L. Barberio published in 2001"

A simultaneous measurement of the QCD colour factors and the strong coupling

Abstract: Using data from $e^+e^-$ annihilation into hadrons, taken with the OPAL detector at LEP at the Z pole between 1991 and 1995, we performed a simultaneous measurement of the colour factors of the underlying gauge group of the strong interaction, $C_F$ and $C_A$ , and the strong coupling, $\alpha_s$ . The measurement was carried out by fitting next-to-leading order perturbative predictions to measured angular correlations of 4-jet events together with multi-jet related variables. Our results, \[ C_A=3.02\pm 0.25\mathrm{(stat.)} \pm 0.49\mathrm{(syst.)}\:,\quad C_F=1.34\pm 0.13\mathrm{(stat.)} \pm 0.22\mathrm{(syst.)}\:, \] \[ \alpha_s (M_Z)=0.120\pm 0.011\mathrm{(stat.)} \pm 0.020\mathrm{(syst.)}\:, \] provide a test of perturbative QCD in which the only assumptions are non-abelian gauge symmetry and standard hadronization models. The measurements are in agreement with SU(3) expectations for $C_F$ and $C_A$ and the world average of $\alpha_s(M_{Z})$ .

Precise determination of the Z resonance parameters at LEP: "Zedometry"

Abstract: This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in ${\mathrme}^+{\mathrme}^-$ collisions with the OPAL detector makes use of the full LEP 1 data sample comprising $161 \mathrm{pb}^{-1}$ of integrated luminosity and $4.5\times10^6$ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from $\gamma/$ Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: $m_{\mathrm{Z}} = (91.1852 \pm 0.0030)$ GeV, $\Gamma_{\mathrm{Z}} = (2.4948 \pm 0.0041)$ GeV, $\sigma^0_{\mathrm{h}} = (41.501 \pm 0.055)$ nb, $R_{\ell} = 20.823 \pm 0.044$ , and $A_{\mathrm{FB}}^{0,\ell} = 0.0145 \pm 0.0017$ . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, $\Gamma_{\mathrm{inv}}/\Gamma_{\ell\ell} = 5.942 \pm 0.027$ . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, $N_{ u} = 2.984 \pm 0.013$ . Interpreting the data within the context of the Standard Model allows the mass of the top quark, $m_{\mathrm{t}} = (162 ^{+29}_{-16})$ GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of $m_{\mathrm{t}}$ as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: $\alpha_{\mathrm{s}}(m_{\mathrm{Z}}) = 0.127 \pm 0.005$ and $m_{\mathrm{H}} = (390^{+750}_{-280})$ GeV.

Two Higgs Doublet Model and model independent interpretation of neutral Higgs boson searches

Abstract: Searches for the neutral Higgs bosons $\mathrm{h}^0$ and $\mathrm{A}^0$ are used to obtain limits on the Type II Two Higgs Doublet Model (2HDM(II)) with no CP–violation in the Higgs sector and no additional particles besides the five Higgs bosons. The analysis combines approximately 170 pb $^{-1}$ of data collected with the OPAL detector at $\sqrt{s} \approx 189$ GeV with previous runs at $\sqrt{s} \approx m_Z$ and $\sqrt{s} \approx 183$ GeV. The searches are sensitive to the $\mathrm{h}^0, \mathrm{A}^0\rightarrow\mathrm{q}\bar{\mathrm{q}}, \mathrm{g}\mathrm{g}, \tau^+\tau^-$ and $\mathrm{h}^0\rightarrow\mathrm{A}^0\mathrm{A}^0$ decay modes of the Higgs bosons. For the first time, the 2HDM(II) parameter space is explored in a detailed scan, and new flavour independent analyses are applied to examine regions in which the neutral Higgs bosons decay predominantly into light quarks or gluons. Model–independent limits are also given.

Particle identification using the time-over-threshold method in the ATLAS Transition Radiation Tracker

Abstract: Test-beam studies of the ATLAS Transition Radiation Tracker (TRT) straw tube performance in terms of electron–pion separation using a time-over-threshold method are described The test-beam data are compared with Monte Carlo simulations of charged particles passing through the straw tubes of the TRT For energies below 10 GeV , the time-over-threshold method combined with the standard transition-radiation cluster-counting technique significantly improves the electron–pion separation in the TRT The use of the time-over-threshold information also provides some kaon–pion separation, thereby significantly enhancing the B-physics capabilities of the ATLAS detector

Measurement of W boson polarisations and CP-violating triple gauge couplings from W+W- production at LEP

Abstract: Measurements are presented of the polarisation of W\(^{+}\)W\(^{-}\) boson pairs produced in e\(^{+}\)e\(^{-}\) collisions, and of CP-violating WWZ and WW\(\gamma\) trilinear gauge couplings. The data were recorded by the OPAL experiment at LEP during 1998, where a total integrated luminosity of 183 pb\(^{-1}\) was obtained at a centre-of-mass energy of 189 GeV. The measurements are performed through a spin density matrix analysis of the W boson decay products. The fraction of W bosons produced with longitudinal polarisation was found to be \(\sigma_{\rm L}/\sigma_{\rm total} = (21.0 \pm 3.3 \pm 1.6)%\) where the first error is statistical and the second systematic. The joint W boson pair production fractions were found to be \(\sigma_{\rm TT}/\sigma_{\rm total} = (78.1 \pm 9.0 \pm 3.2)%\), \(\sigma_{\rm LL}/\sigma_{\rm total} = (20.1 \pm 7.2 \pm 1.8)%\) and \(\sigma_{\rm TL}/\sigma_{\rm total} = (1.8 \pm 14.7 \pm 3.8)%\). In the CP-violating trilinear gauge coupling sector we find \(\tilde{\kappa}_{\rm z} = -0.20^{+0.10}_{-0.07}\), \(g^{\rm z}_{4} = -0.02^{+0.32}_{-0.33}\) and \(\tilde{\lambda}_{\rm z} = -0.18^{+0.24}_{-0.16}\), where errors include both statistical and systematic uncertainties. In each case the coupling is determined with all other couplings set to their Standard Model values except those related to the measured coupling via \(SU(2)_{\rm L}\times U(1)_{\rm Y}\) symmetry. These results are consistent with Standard Model expectations.

Measurement of triple gauge boson couplings from W+W- production at LEP energies up to 189 GeV

Abstract: A measurement of triple gauge boson couplings is presented, based on W-pair data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass energy of 189 GeV with an integrated luminosity of 183 pb(-1). After combining with our previous measurements at centre-of-mass energies of 161-183 GeV we obtain kappa = 0.97(-0.16)(+0.20), g(1)(z) = 0.991(-0.057)(+0.060) and lambda = -0.110(-0.055)(+0.058), where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to their Standard Model values. These results are consistent with the Standard Model expectations.

Determination of the b quark mass at the Z mass scale

Abstract: In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using \({\cal O}(\alpha_{\mathrm{S}}^2)\) calculations for massive quarks, this difference was used to determine the b quark mass in the \({\overline{\mathrm{{MS}}}}\) renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be \({\overline{m}_{b}}({m_{Z}}) = (2.67 \pm 0.03{\mathrm{(stat.)}} ^{+0.29}_{-0.37}{\mathrm{(syst.)}} \pm 0.19{\mathrm{(theo.)}}){\mathrm{ GeV}}\). Evolving this value to the b quark mass scale itself yields \({\overline{m}_{b}}({\overline{m}_{b}}) = (3.95 ^{+0.52}_{-0.62}){\mathrm{ GeV}}\), consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations.

Precision neutral current asymmetry parameter measurements from the Tau polarization at LEP

Abstract: Measurements of the $\tau$ lepton polarization and forward-backward polarization asymmetry near the Z $^0$ resonance using the OPAL detector are described. The measurements are based on analyses of $\tau \rightarrow{\rm e} u_e u_{\tau}, \tau\rightarrow \mu u_{\mu} u_{\tau}, \tau\rightarrow \pi u_{\tau}, \tau \rightarrow \rho u_{\tau}$ and $\tau\rightarrow{\rm a}_1 u_{\tau}$ decays from a sample of 144,810 $\rm e^+e^-\rightarrow \tau^+\tau^-$ candidates corresponding to an integrated luminosity of 151 pb $^{-1}$ . Assuming that the $\tau$ lepton decays according to V–A theory, we measure the average $\tau$ polarization near $\sqrt{s} ={\rm M}_{\mathrm{Z}}$ to be $\langle P_{\tau}\rangle= (-14.10 \pm 0.73 \pm 0.55)\%$ and the $\tau$ polarization forward-backward asymmetry to be $\rm A_{\mathrm{pol}}^{\mathrm{FB}} = (-10.55 \pm 0.76 \pm 0.25)\%$ , where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z $^0$ interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: \begin{eqnarray} {\cal A}_{\tau} & = & 0.1456 \pm 0.0076 \pm 0.0057, onumber {\cal A}_{\mathrm e}& = & 0.1454 \pm 0.0108 \pm 0.0036. onumber \end{eqnarray} These measurements are consistent with the hypothesis of lepton universality and combine to give ${\cal A}_{\ell} = 0.1455 \pm 0.0073$ . Within the context of the Standard Model this combined result corresponds to $=0.23172 \pm 0.00092$ . Combing these results with those from the other OPAL neutral current measurements yields a value of $=0.23211 \pm 0.00068$ .

Production rates of bb̄ quark pairs from gluons and bb̄bb̄ events in hadronic Z0 decays

Abstract: The rates are measured per hadronic $\mathrm{Z}^0$ decay for gluon splitting to $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{b\overline{b}}}$ , and of events containing two $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{4b}}$ , using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to $\mathrm{b\overline{b}}$ quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two $\mathrm{b\overline{b}}$ quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topology is combined in a likelihood fit to extract the values of $g_{\mathrm{b\overline{b}}}$ and $g_{\mathrm{4b}}$ , namely \begin{eqnarray*} g_{\mathrm{b\overline{b}}} & = & (3.07 \pm 0.53 \mathrm{(stat)} \pm 0.97\mathrm{(syst)})\times 10^{-3},\\ g_{\mathrm{4b}} & = & (0.36\pm 0.17 \mathrm{(stat)} \pm 0.27\mathrm{(syst)})\times 10^{-3}. \end{eqnarray*}

A study of B0s meson oscillation using D-s-lepton correlations

Abstract: From data collected around the Z resonance by the OPAL detector at LEP, a sample of B $_s^0$ decays was obtained using D $_s^-}\ell^+$ combinations, where the $D_s^-$ was fully reconstructed in the $\phi\pi^-, \mathrm{K}^{*0}\mathrm{K}^-$ and $\mathrm{K}^0_{\mathrm{S}}\mathrm{K}^-$ decay channels or partially reconstructed in the $\phi\ell^-\bar{ u}(\mathrm{X})$ decay channel. These events were used to study $\mathrm{B_s^0}$ oscillation. The flavor (b or $\bar{b}$ ) at decay was determined from the lepton charge while the flavor at production was determined from a combination of techniques. The expected sensitivity of the experiment is 4.1 ps $^{-1}$ . The experiment was not able to resolve the oscillatory behavior, and we deduced that the B $_s^0$ oscillation frequency $\Delta m_s > 1.0$ ps $^{-1}$ at the 95% confidence level.

A search for a narrow radial excitation of the D∗± Meson

Abstract: A sample of 3.73 million hadronic Z decays, recorded with the OPAL detector at LEP in the years 1991–95, has been used to search for a narrow resonance corresponding to the decay of the $\mathrm{D}^{*\prime\pm}(2629)$ meson into $\mathrm{D}^{*\pm}\mathrm{\pi}^{+}\mathrm{\pi}^{-}$ . The $\mathrm{D}^{*+}$ mesons are reconstructed in the decay channel $\mathrm{D}^{*+} \to \mathrm{D}^{0} \mathrm{\pi}^{+}$ with $\mathrm{D}^{0} \to \mathrm{K}^{-}\mathrm{\pi}^{+}$ . No evidence for a narrow $\mathrm{D}^{*\prime\pm}(2629)$ resonance is found. A limit on the production of narrow $\mathrm{D}^{*\prime\pm}(2629)$ in hadronic Z decays is derived: \[ f(\mathrm{Z} \to \mathrm{D}^{*\prime\pm}(2629)) \times Br(\mathrm{D}^{*\prime +} \to \mathrm{D}^{*+} \mathrm{\pi}^{+} \mathrm{\pi}^{-}) < 3.1 \times 10^{-3} \mathrm{(95\% C.L.)} \]

Measurement of |Vub| using b hadron semileptonic decay

Abstract: The magnitude of the CKM matrix element $|{\rm V}_{\rm ub}|$ is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on $b \to{\rm X}_{\rm u} \ell u $ event topology and kinematics This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays We measure Br( $b \to{\rm X}_{\rm u} \ell u$ ) to be ( $163 \pm 053^{+055}_{-062}) \times 10^{-3}$ The first uncertainty is the statistical error and the second is the systematic error From this analysis, $|{\rm V}_{\rm ub}|$ is determined to be: \[|{\rm V}_{\rm ub}| = (400 \pm 065 ({\rm stat}) ^{+067}_{-076} ({\rm sys}) \pm 019 ({\rm HQE})) \times 10^{-3} \] The last error represents the theoretical uncertainties related to the extraction of $|{\rm V}_{\rm ub}|$ from Br( $b \to{\rm X}_{\rm u} \ell u$ ) using the Heavy Quark Expansion

Charged multiplicities in Z decays into u, d, and s Quarks

Abstract: About 4.4 million hadronic decays of Z bosons, recorded by the OPAL detector at LEP at a centre-of-mass energy of around \(\sqrt{s}= 91.2\) GeV, are used to determine the mean charged particle multiplicities for the three light quark flavours. Events from primary u, d, and s quarks are tagged by selecting characteristic particles which carry a large fraction of the beam energy. The charged particle multiplicities are measured in the hemispheres opposite to these particles. An unfolding procedure is applied to obtain these multiplicities for each primary light quark flavour. This yields

Production rates of b.hivin.b quark pairs from gluons and b.hivin.bb.hivin.b events in hadronic Z0 decays.

Abstract: The rates were measured per hadronic Z0 decay for gluon splitting to b.hivin.b quark pairs, gb.hivin.b, and of events contg. two b.hivin.b quark pairs, g4b, using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to b.hivin.b quarks were selected if two of the jets are close in phase space and contain detached secondary vertexes. For the event sample contg. two b.hivin.b quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topol. was combined in a likelihood fit to ext. the values of gbb and g4b, i.e. gb.hivin.b = (3.07 +- 0.53(stat) +- 0.97(syst)) * 10-3 and g4b = (0.36 +- 0.17(stat) +- 0.27(syst)) * 10-3. [on SciFinder (R)]