Showing papers by "D. M. Strom published in 1995"

A search for lepton flavour violating Z0 decays

Abstract: We have searched for lepton flavour violating Z0→eμ, Z0→eτ and Z0→μτ decays in a sample of 4.0×106 visible Z0 decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z0→eμ. The samples of selected Z0→eτ and Z0→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level: $$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$

A comparison of b and uds quark jets to gluon jets

Abstract: Symmetric three-jet events are selected from hadronic Z0 decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For thek ⊥ jet finder withy cut=0.02, we find $$\begin{gathered} \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle b quark}} = 1.089 \pm 0.024(stat.) \pm 0.024(syst.) \hfill \\ \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle uds quark}} = 1.390 \pm 0.038(stat.) \pm 0.032(syst.) \hfill \\ \end{gathered} $$ as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.

Observations of π-B charge-flavor correlations and resonant Bπ and BK production

Abstract: Evidence is presented for kinematic and charge correlations of B mesons with charged pions and kaons. Using a new technique, a sample of over 80 000 partially reconstructed B mesons is obtained in 3.5·106 hadronic Z0 decays recorded using the OPAL detector at LEP. The invariant mass distributions of B+π− and B+K− combinations show enhancements consistent with the decays of P-wave resonances of a b antiquark and a light quark. We observe an excess of 1738±195 B+π− pairs with invariant masses in the range 5.60-5.85 GeV and an excess of 149±31 B+K− pairs with invariant masses in the range 5.80–6.00 GeV. Labeling the observed enhancements generically as B** we find $$\begin{gathered} \frac{{BR(Z^0 \to \bar b \to B^{**0} \to B^{(*) + } \pi ^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.18 \pm 0.04, \hfill \\ \frac{{BR(Z^0 \to \bar b \to B_s^{**0} \to B^{(*) + } K^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.026 \pm 0.008, \hfill \\ \end{gathered} $$ where B(*)+ indicates the sum of B+ and B*+ and the errors include statistical and systematic contributions. From a study of π-B charge-flavor correlations we conclude that the production flavor of a B meson can be tagged with the charge of a pion in an appropriately chosen kinematic region, and that the performance of this flavor tag compares favorably in the\(Z^0 \to q\bar q\) environment with lepton-based tags.

A measurement of the production of D*± mesons on the Z0 resonance

Abstract: We have studied the production of D*± mesons in a sample of 1.25 million multihadronic decays of the Z0, in which 1969 candidates have been identified. We have determined the total multiplicity of charged D* mesons in multihadronic Z0 decays to be

Measurement of single photon production in e+e− collisions near the Z0 resonance

Abstract: A measurement of the single photon production cross-section is presented based on a data-sample of 40.5 pb−1 collected with the OPAL detector at centre-of-mass energies within 3 GeV of the Z0 mass. Single photon events arise from initial state radiation and the production of an “invisible” final state consisting of neutrinos or possibly particles such as sneutrinos or photinos. The single photon topology is also sensitive to new Z0 decays such as $$Z^0 \to \bar vv* \to \bar vv\gamma $$ orZ 0→γX, X→invisible particles. A total of 447 single photon candidates were observed with energy exceeding 1.75 GeV in the polar angle region |cosθ|<0.7. The estimated background from processes with visible reaction products, mainly e+e−→e+e−γ, is 37±6 events. Interpreting the cross-sections as being solely due to Z0 decay to invisible particles and the expected W-contributions, the Z0 invisible with is determined to be 539±26±17 MeV corresponding toN v=3.23±0.16±0.10 light neutrino generations. The differential cross-section with photon energy is presented. Upper limits are set on additional invisible contributions to the Z0 width, on possible non-resconant processes, and on Z0 decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative twobody decay of the Z0 to a new particle X, with mass below 64 GeV and an invisible signature, has a Z0 branching ratio of less than 4.3×10−6 at 95% confidence level.

A model independent measurement of quark and gluon jet properties and differences

Abstract: Three jet events are selected from hadronic Z0 decays with a symmetry such that the two lower energy jets are produced with the same energy and in the same jet environment. In some of the events, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet, with an estimated purity of about 93%. Comparing these gluon jets to the inclusive sample of lower energy jets from the symmetric data set yields direct, model independent measurements of quark and gluon jet properties, which have essentially no bias except from the jet definition. Results are reported using both thek⊥ and cone jet definitions. For thek⊥ algorithm, we find $$\frac{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle gluon}}{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle quark}} = 1.25 \pm 0.02(stat.) \pm 0.03(syst.)$$ for the ratio of the mean charged particle multiplicity of gluon to quark jets, while for the cone algorithm, we find $$\frac{{\left\langle {n_{cone}^{ch.} } \right\rangle gluon}}{{\left\langle {n_{cone}^{ch.} } \right\rangle quark}} = 1.10 \pm 0.02(stat.) \pm 0.02(syst.)$$ using a cone size of 30°. We also report measurements of the angular distributions of particle energy and multiplicity around the jet directions, and of the fragmentation functions of the jets. Gluon jets are found to be substantially broader and to have a markedly softer fragmentation function than quark jets, in agreement with our earlier observations.

Search for heavy charged particles and for particles with anomalous charge in e+e− collisions at LEP

Abstract: Using the OPAL data accumulated in 1991–1993 amounting to 74 pb−1 of integrated luminosity, corresponding to 1.64×106 selected multi-hadronic events, a search has been performed for charged particles with unusual mass or unusual charge. The mass was determined from a combination of momentum and ionization energy loss measurements. No isolation criteria were applied to the tracks examined, so that both isolated particles and particles produced in jets were valid candidates. For particles with chargeQ/e=−1, one candidate with a mass of approximately 4.2 GeV/c2 was found, which is compatible with the background rate expected according to a Monte Carlo simulation. The implications of this search for the mass limits of a conjectured stable or quasi-stable charged gluino composite $$(\tilde gq\bar q')^ \pm $$ are discussed. Limits are also presented for the production of fractionally-charged particles withQ/e=±2/3 and ±4/3 as well as for particles withQ/e=±2.

The production of neutral kaons in Z0 decays and their Bose-Einstein correlations

Abstract: The production of neutral kaons in e+e− annihilation at centre-of-mass energies in the region of the Z0 mass and their Bose-Einstein correlations are investigated with the OPAL detector at LEP. A total of about 1.26×106 Z0 hadronic decay events are used in the analysis. The production rate of K0 mesons is found to be 1.99±0.01±0.04 per hadronic event, where the first error is statistical and the second systematic. Both the rate and the differential cross section for K0 production are compared to the predictions of Monte Carlo generators. This comparison indicates that the fragmentation is too soft in bothJetset andHerwig. Bose-Einstein correlations in Ks0Ks0 pairs are measured through the quantityQ, the four momentum difference of the pair. A threshold enhancement is observed in Ks0Ks0 pairs originating from a mixed sample of\(K^0 \bar K^0\) and K0K0 (\(\bar K^0 \bar K^0\)) pairs. For the strength of the effect and for the radius of the emitting source we find values of λ=1.14±0.23±0.32 andR0=(0.76±0.10±0.11) fm respectively. The first error is statistical and the second systematic.

Inclusive strange vector and tensor meson production in hadronic Z0 decays

Abstract: Measurements have been made in the OPAL experiment at LEP of the inclusive production of strange vector φ(1020) and K*(892)0 mesons, and the tensor meson K2*(1430)0. The overall production rates per hadronic Z0 decay have been determined to be 0.100±0.004stat.±0.007syst. φ(1020) mesons, 0.74±0.03stat.±0.03syst. K*(892)0 mesons and (forxE<0.3) 0.19±0.04stat.±0.06syst. K2*(1430)0 mesons. The measurements for the vector states update previously published results based on lower statistics, while the K2*(1430)0 rate represents the first direct measurement of a strange tensor state in Z0 decay. For the vector states, both the overall production rates and normalised differential cross sections, with respect to the scaled energy variablexE, have been compared to JETSET and HERWIG predictions. The peak positions in the ζ=ln(1/xp) distributions have been measured and compared to measurements of other hadron states.

Measurement of the tau lepton polarization and its forward - backward asymmetry from Z0 decays

Abstract: We report on a measurement of the tau lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector. This measurement is based on analyses of the $$\tau \to e\bar v_e v_\tau $$ , $$\tau \to \mu \bar v_\mu v_\tau $$ , τ→π(K)v τ and τ→ρ v τ from a sample of 30663 e+e+→τ+τ- events collected during the years 1990 to 1992. Assuming that the tau lepton decays according to V-A theory, we measure the average τ polarization to be

=(−14.9±1.9±1.3)% and the τ polarization forward-backward asymmetry to be A =(−8.9±2.2±0.9)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality. When combined under the assumption of universality, our results can be interpreted as a measurement of sin2 θ eff lept =0.2321±0.0023 within the context of the Standard Model.

The energy calibration of LEP in the 1993 scan

Abstract: This report summarizes the procedure for providing the absolute energy calibration of the LEP beams during the energy scan in 1993. The average beam energy around the LEP ring was measured in 25 calibrations with the resonant depolarization technique. The time variation of this average beam energy is well described by a model of the accelerator based on monitored quantities. The absolute calibration of the centre of mass energies of the off-peak points is determined with a precision of 2 parts in 105 resulting in a systematic error on the Z-mass of about 1.4 MeV and on the Z-width of about 1.5 MeV.

Measurement of the longitudinal, transverse and asymmetry fragmentation functions at lep

Abstract: The fragmentation function for the process e+e−→h+X, whereh represents a hadron, may be decomposed into transverse, longitudinal and asymmetric contributions by analysis of the distribution of polar production angles. A number of new tests of QCD have been proposed using these fragmentation functions, but so far no data have been published on the separate components. We have performed such a separation using data on charged particles from hadronic Z0 decays atOpal, and have compared the results with the predictions of QCD. By integrating the fragmentation functions, we determine the average charged particle multiplicity to be\(\overline {n_{ch} } = 21.05 \pm 0.20\). The longitudinal to total cross-section ratio is determined to be σL/σtot=0.057±0.005. From the longitudinal fragmentation function we are able to extract the gluon fragmentation function. The connection between the asymmetry fragmentation function and electroweak asymmetrics is discussed.

Measurement of {Mathematical expression}using a double tagging method

Abstract: The fraction of\(Z^0 \to b\bar b\); events in hadronicZ0 decays has been measured using the data collected by OPAL in 1992 and 1993. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used together to obtain an event sample enriched in\(Z^0 \to b\bar b\) decays. To reduce the systematic error on the measurement of the\(Z^0 \to b\bar b\) fraction, the efficiency of the\(b\bar b\) event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. A value of $$\frac{{\Gamma (Z^0 \to b\bar b)}}{{\Gamma (Z^0 \to hadrons)}} = 0.2171 \pm 0.0021 \pm 0.0021$$ was obtained, where the first error is statistical and the second systematic. The uncertainty on the decay width\(\Gamma (Z^0 \to c\bar c)\) is not included in these errors. A fractional variation of this width by ±8% about its Standard Model prediction would result in a variation of the measured\(Z^0 \to b\bar b\) fraction of ±0.0015.

A measurement of the QCD colour factor ratios CA/CF and TF/CF from angular correlations in four-jet events

Abstract: From 1 105 045 hadronic Z0 decays observed with the OPAL detector at the LEP e+e− collider, 21 732 four-jet events are selected. A simultaneous fit of three selected angular variables from these events by the second order QCD matrix element calculation yieldsCA/CF=2.11±0.16(stat.)±0.28(syst.)TF/CF=0.40±0.11(stat.)±0.14(syst.) for the ratios of colour factors, in agreement with SU(3) expectations ofCA/CF=9/4 andTF/CF=3/8.

Improved measurements of the B0 and B+ meson lifetimes

Abstract: Updated measurements of the B0 and B+ meson lifetimes are presented. From a data sample of 1.72 million hadronic Z0 decays recorded during the period 1991 to 1993, a sample of approximately 1000 semileptonic B meson decays containing a D0, D+ or D*+ has been isolated. From the distribution of decay times in the different samples the lifetimes of the B0 and B+ mesons are determined to be 1.53±0.12±0.08 ps and 1.52±0.14±0.09 ps, respectively, where the first error is statistical and the second systematic. The ratio of the B+ to B0 lifetimes is measured to be 0.99±0.14−0.04+0.05, confirming expectations that the lifetimes are similar.

A measurement of the forward-backward asymmetry of e + e - →bb by applying a jet charge algorithm to lifetime tagged events

Abstract: The forward-backward asymmetry of\(e^ + e^ - \to Z^0 \to b\bar b\) has been measured using approximately 2.15 million hadronicZ0 decays collected at the LEP e+e− collider with the OPAL detector. A lifetime tag technique was used to select an enriched\(b\bar b\) event sample. The measurement of the\(b\bar b\) asymmetry was then performed using a jet charge algorithm to determine the direction of the primary quark. Values of: $$\begin{gathered} A_{FB}^b = 0.062 \pm 0.034 \pm 0.002 - 0.082\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 89.52GeV, \hfill \\ A_{FB}^b = 0.0963 \pm 0.0067 \pm 0.0038 - 0.471\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 91.25GeV, \hfill \\ A_{FB}^b = 0.172 \pm 0.028 \pm 0.007 - 0.055\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 92.94GeV, \hfill \\ \end{gathered}$$ were measured where, in each case, the first error is statistical, the second is systematic and the third term gives the variation due to a change\(\Delta (\Gamma _{b\bar b} /\Gamma _{had} )\) in the value of\(\Gamma _{b\bar b} /\Gamma _{had} = 0.216\) assumed. The dependence on the assumed charm asymmetry at the same energy is Δ(AFBb)≈+0.07Δ(AFBc). Assuming the Standard Model form for the couplings, these measurements correspond to an effective weak mixing angle of: $$\sin ^2 \theta _W^{eff,e} = 0.2313 \pm 0.0012 \pm 0.0006$$ giving Mtop=196−38−19+33+16GeV/c2, where the first error is statistical and the second is systematic. The Higgs mass assumed is 300 GeV/c2. A variation in the assumed mass of the Higgs boson between 60 and 1000 GeV/c2 corresponds to an uncertainty in sin2θWeff,e of ±0.00006 and on Mtop of−26+20GeV/c2.

A study of charm meson production in semileptonic B decays

Abstract: The results of a study of charm meson production in semileptonic B hadron decays are presented. Based on a sample of 1.72 million hadronic Z0 decays the following product branching ratios (averaged over electrons and muons) are obtained: $$\begin{gathered} B(b \to D^ + \ell X) \cdot B(D^ + \to K^ - \pi ^ + \pi ^ + ) \hfill \\ = (1.82 \pm 0.20 \pm 0.12) \times 10^{ - 3} , \hfill \\ B(b \to D^0 \ell X) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (2.52 \pm 0.14 \pm 0.17) \times 10^{ - 3} \hfill \\ B(b \to D^* \ell X) \cdot B(D^{* + } \to D^0 \pi ^ + ) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (7.53 \pm 0.47 \pm 0.56) \times 10^{ - 4} \hfill \\ \end{gathered} $$ where the first error is statistical and the second systematic.

Measurement of the hadronic decay current in τ−→π− − + v τ

Abstract: The decay τ−→π−−+vτ has been studied using data collected with the OPAL detector at LEP during 1992 and 1993. The hadronic structure functions for this decay are measured model independently assuming G-parity invariance and neglecting scalar currents. Simultaneously the parity violating asymmetry parameter is determined to be\(\gamma VA = 1.08 _{ - 0.41 - 0.25}^{ + 0.46 + 0.14} \), consistent with the Standard Model prediction of γVA=1 for left-handed tau neutrinos. Models of Kuhn and Santamaria and of Isgur et al. are used to fit distributions of the invariant 3π mass as well as 2π mass projections of the Dalitz plot. The model dependent mass and width of thea1 resonance are measured to be\(m_{a_1 } = 1.266 \pm 0.014_{ - 0.002}^{ + 0.012} \) GeV and\(\Gamma _{a_1 } = 0.610 \pm 0.049_{ - 0.019}^{ + 0.053} \) GeV for the Kuhn and Santamaria model and\(m_{a_1 } = 1.202 \pm 0.009_{ - 0.001}^{ + 0.009} \) GeV and\(\Gamma _{a_1 } = 0.422 \pm 0.023_{ - 0.004}^{ + 0.033} \) GeV for the Isgur et al. model. The model dependent values obtained for the parity violating asymmetry parameter are γVA=0.87±0.27−0.06+0.05 for the Kuhn and Santamaria model and γVA=1.10±0.31−0.14+0.13 for the Isgur et al. model. Within the Isgur et al. model the ratio of theS-andD-wave amplitudes is measured to beD/S=−0.09±0.03±0.01.

A study of B meson oscillations using dilepton events

Abstract: The time dependence of B meson oscillations is studied using hadronic Z0 decays with identified leptons in both thrust hemispheres. Decay times are reconstructed for each of the semileptonic B decays by forming vertices which include the lepton and by estimating the B meson momentum. The mass difference of the two mass eigenstates in the Bd0 system, Δmd, is measured to be\(0.462_{ - 0.053 - 0.035}^{ + 0.040 + 0.052} ps^{ - 1}\), where the first error is statistical and the second systematic. For the Bs0 system, a lower limit of Δms>2.2 ps−1 at 95% C.L. is derived.

Measurement of the τ-→h-h+h-v τ and τ-→h-h+h-≥1π 0vτ branching ratios

Abstract: The branching ratios of the τ−→h − h + h − v τ and τ−→h − h + h −≥1π0 v τ decays, whereh is either a charged π orK meson, are measured using a data sample of 87861 τ+τ− pairs collected with the OPAL detector at LEP. The two branching ratios are extracted simultaneously from a sample of three charged particle decays and found to be: $$\begin{gathered} B(\tau ^ - \to h^ - h^ + h^ - u _\tau ) = (9.87 \pm 0.10 \pm 0.24)\% \hfill \\ B(\tau ^ - \to h^ - h^ + h^ - \geqslant 1\pi ^0 u _\tau ) = (5.09 \pm 0.10 \pm 0.23)\% \hfill \\ \end{gathered}$$ where the first error is statistical and the second systematic. The branching ratio of the τ lepton into three charged particles is measured to be: $$B(\tau ^ - \to 3 - prong) = (14.96 \pm 0.09 \pm 0.22)\%$$

Measurement of the leptonic branching ratios of the τ lepton

Abstract: The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e+e−→τ+τ− candidates we find 7322\(\tau \to e u \bar u\) and 7941\(\tau \to \mu u \bar u\) candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios\(B(\tau \to e u \bar u )\)=(18.14±0.20±0.28)% and\(B(\tau \to \mu u \bar u )\)=(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992: $$\begin{gathered} B(\tau \to e u \bar u ) = (18.04 \pm 0.33)\% , \hfill \\ B(\tau \to \mu u \bar u ) = (17.36 \pm 0.27)\% . \hfill \\ \end{gathered} $$ These leptonic branching ratios are used with other properties of the muon and tau-lepton to test the universality of charged current leptonic couplings in these decays. The ratio\(R_\tau = B(\tau \to hadrons + u _\tau )/B(\tau \to e\bar u _e u _\tau )\) is calculated using our measured values of the leptonic branching fractions of the tau and tau lifetime from which a value of αs(Q2=Mτ2) is extracted. The value of αs(Q2=MZ2) is obtained byQ2 evolution and agrees with the value from the Z0 line shape analysis.

A test of CP-invariance in Z0→τ+τ- using optimal observables

Abstract: Using 27 490 Z0→τ+τ− decays, accumulated in 1991, 1992 and 1993 with the OPAL detector at LEP, a direct test ofCP-invariance in the neutral current reactione+e−→τ+τ− is performed by measuringCP-odd observables which are proportional to the weak dipole moment of the τ-lepton. A new method based on optimalCP-odd observables constructed from the τ flight and spin directions is employed. More sensitive measurements of the real and, for the first time, the imaginary part of the weak dipole moment with highest possible signal to noise ratio are obtained. No evidence for a non-zero expectation value of the considered observables and hence forCP-violation is observed. An upper limit on the weak dipole moment of |Re(dτω)|<7.8×10−18e·cm and |Im(dτω)|<4.5×10−17e·cm with 95% confidence level is obtained.

Study of 18-cm long single-sided AC-coupled silicon microstrip detectors

Abstract: The SSC GEM silicon Central Tracker design incorporated 18-cm long single-sided AC-coupled silicon microstrip ladders. Compared to the 12-cm long ladders considered in the preliminary stages of the tracker design, the 18-cm long ladders have the advantage of reduced cost, channel count and overall power consumption, and led to a simplified tracker assembly. However, such long ships also present the challenge of maintaining satisfactory performance. The increased capacitance and series resistance contribute to lower signal-to-noise ratios, longer time walk, higher power consumption per channel and increased probability of crosstalk to neighboring channels. In this paper, an accurate method to calculate the geometric capacitance of the AC-coupled microstrips is presented and the calculated results are compared with measurements, SPICE simulations are performed to predict the noise, the extent of interstrip capacitive coupling and the dispersion of the detector signal due to the finite series resistance of the metal strips and the long length of the detector. The influence of the preamplifier current and the shaping time on the signal and noise levels is also presented. The study concludes that the 18-cm long ladders can successfully satisfy the performance goals of the GEM silicon Central Tracker. >