Showing papers by "German Martinez published in 2000"

Photonic events with missing energy in e+e- collisions at √s = 189 GeV

Abstract: Photonic events with large missing energy have been observed in $\rm e^+e^-$ collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process $\mathrme^+\mathrme^-\to u\overline{ u}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)$ and $\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)$ are derived for the case of stable and invisible $\mathrm{Y}$ . These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = u^*, \mathrm{Y} = u$ ), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$ ) and to supersymmetric models in which $\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}={\tilde{\mathrm{G}}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $\mathrme^+\mathrme^- \to \mathrm{XX}$ , $\rm X \to Y \gamma$ , when $M_{\mathrm Y}\approx 0$ . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

QCD analyses and determinations of αs in e+e− annihilation at energies between 35 and 189 GeV

Abstract: We employ data taken by the JADE and OPAL experiments for an integrated QCD study in hadronic e $^+$ e $^-$ annihilations at c.m.s. energies ranging from 35 GeV through 189 GeV. The study is based on jet-multiplicity related observables. The observables are obtained to high jet resolution scales with the JADE, Durham, Cambridge and cone jet finders, and compared with the predictions of various QCD and Monte Carlo models. The strong coupling strength, $\alpha_s$ , is determined at each energy by fits of ${\cal O}(\alpha_s^2)$ calculations, as well as matched ${\cal O}(\alpha_s^2)$ and NLLA predictions, to the data. Matching schemes are compared, and the dependence of the results on the choice of the renormalization scale is investigated. The combination of the results using matched predictions gives \[\alpha_s(M_{Z^0})=0.1187^{0.0034}_{0.0019}.\] The strong coupling is also obtained, at lower precision, from ${\cal O}(\alpha_s^2)$ fits of the c.m.s. energy evolution of some of the observables. A qualitative comparison is made between the data and a recent MLLA prediction for mean jet multiplicities.

Precision luminosity for Z $^0$ lineshape measurements with a silicon-tungsten calorimeter

Abstract: The measurement of small–angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects electrons from small–angle Bhabha scattering at angles between 25 and 58 mrad. At LEP center-of-mass energies around the Z $^0$ , about half of all Bhabha electrons entering the detector fall within a 79 nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length tungsten absorber plates are sampled by 608 silicon detectors with 38,912 radial pads of 2.5 mm width. The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45 GeV electrons or photons to be determined with a total systematic error of less than 7 microns. We have quantified all significant sources of systematic experimental error in the luminosity determination by direct physical measurement. All measured properites of the luminosity event sample are found to be in agreement with current theoretical expectations. The total systematic measurement uncertainty is $3.4 \times 10^{-4}$ , significantly below the theoretical error of $5.4 \times 10^{-4}$ currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of $\Gamma_{\rm inv}/ \Gamma_{\ell^+\ell^-}$ , a quantity of basic physical interest which depends crucially on the luminosity measurement.

Search for R-parity violating decays of scalar fermions at LEP

Abstract: A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb\(^{-1}\) at a centre-of-mass energy of \(\sqrt{s}=\) 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and stop quarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield multi-leptons, jets plus leptons or multi-jets, with or without missing energy, in the final state. No significant excess of such events has been observed. Limits on the production cross-sections of scalar fermions in R-parity violating scenarios are obtained. Mass exclusion regions are also presented in the framework of the Constrained Minimal Supersymmetric Standard Model.

Transverse and longitudinal Bose-Einstein correlations in hadronic Z0 decays

Abstract: Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z $^0$ hadronic decays are studied as a function of the three components of the momentum difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r $_{t_{side}}$ , and longitudinal, r $_l$ , dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y $_{cut}$ = 0.04, are r $_{t_{side}}$ = (0.809 $\pm$ 0.009 (stat) $^{+0.019}_{-0.032}$ (syst)) fm, r $_l$ = (0.989 $\pm$ 0.011 (stat) $^{+0.030}_{-0.015}$ ({\it syst})) fm and r $_l$ /r $_{t_{side}}$ = 1.222 $\pm$ 0.027 (stat) $^{+0.075}_{-0.012}$ (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.

Search for neutral Higgs bosons in e+e- collisions at √s ≈ 189 GeV

Abstract: Searches for the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb\(^{-1}\) of \(\mathrm{e}^+ \mathrm{e}^-\) collision data collected at \(\sqrt{s} \approx 189\) GeV were used to search for Higgs boson production in the SM process \({\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{H}^{0}\mathrm{Z}^{0}\) and the MSSM processes \({\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{h}^{0} \mathrm{Z}^{0}\) and \({\mathrm{e}}^+ {\mathrm{e}}^- \rightarrow \mathrm{A}^{0} \mathrm{h}^{0}\). The searches are sensitive to the \(\mathrm{b}\bar{\mathrm{b}}\) and \(\tau^+\tau^-\) decay modes of the Higgs bosons, and also to the MSSM decay mode \(\mathrm{h}^{0} \rightarrow \mathrm{A}^{0} \mathrm{A}^{0}\). OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are \(m_{\mathrm{H}} >74.8\) GeV and \(m_{\mathrm{A}} >76.5\) GeV, assuming \(\tan\beta > 1\), that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of \(\tan\beta\) between 0.72 and 2.19 are excluded.

Leading particle production in light flavour jets

Abstract: The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the five quark flavours making only minimal model assumptions. The analysis is based on a large statistics sample of hadronic $\rm Z^0$ decays collected with the OPAL detector at the LEP $\rm e^+e^-$ collider. These results provide a basis for future studies of light flavour production at other centre-of-mass energies. We use our results to study the hadronisation mechanism in light flavour jets and compare the data to the QCD models JETSET and HERWIG. Within the JETSET model we also directly determine the suppression of strange quarks to be \[ \rm \gamma _s = 0.422 \pm 0.049 (stat.) \pm 0.059 (syst.) \] by comparing the production of charged and neutral kaons in strange and non-strange light quark events. Finally we study the features of baryon production.

Measurement of the low-cursive greek chi behaviour of the photon structure function Fγ2

Abstract: The photon structure function $F_2^\gamma(x,Q^2)$ has been measured using data taken by the OPAL detector at $\rm e^+e^-$ centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in $Q^2$ ranges of 1.5–30.0 GeV $^2$ (LEP1), and 7.0–30.0 GeV $^2$ (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in $F_2^\gamma$ at low-x as expected from QCD.

Multiplicities of Π0, η, K0 and of charged particles in quark and gluon jets

Abstract: We compared the multiplicities of $\pi^0, \eta, \rm K^0 $ and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale $\rm Q_{jet} $ which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of $\rm Q_{jet}$ for $ \pi^0, \eta$ or $\rm K^0 $ was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of $ \eta $ meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, $\rm C=2.27\pm 0.20(stat.+syst.),$ is about one standard deviation higher than the perturbative prediction.

Measurement of the B+ and B-0 lifetimes and search for CP(T) violation using reconstructed secondary vertices

Abstract: The lifetimes of the $\rm B^+$ and $\rm B^0$ mesons, and their ratio, have been measured in the OPAL experiment using 24 million hadronic $\rm Z^0$ decays recorded at LEP $\rm Z^0 \rightarrow \rm b\overline{b}$ decays were tagged using displaced secondary vertices and high momentum electrons and muons The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample The results are \begin{eqnarray*} \tau_{B^+} & = & 1643 \pm 0037\pm 0025 \rm\,ps \tau_{B^0} & = & 1523 \pm 0057 \pm 0053 \rm\,ps \tau_{B^+}/ \tau_{B^0} & = & 1079 \pm 0064 \pm 0041 \,, \end{eqnarray*} where in each case the first error is statistical and the second systematic A larger data sample of 31 million hadronic $\rm Z^0$ decays has been used to search for CP and CPT violating effects by comparison of inclusive b and $\rm\overline{b}$ hadron decays No evidence for such effects is seen The CP violation parameter $\rm Re(\epsilon_B)$ is measured to be \[ \rm Re(\epsilon_B) = 0001 \pm 0014 \pm 0003 \] and the fractional difference between b and $\rm\overline{b}$ hadron lifetimes is measured to be \[ \left(\frac{\Delta\tau}{\tau}\right)_{\rm b} \equiv \frac{\rm\tau (b \mbox{hadron}) - \tau \mbox{(\rm\overline{b} hadron)}} {\rm\tau (average)} = -0001\pm 0012\pm 0008 \, \]

Search for trilinear neutral gauge boson couplings in Zγ production at √s = 189 GeV at LEP

Abstract: The data recorded at a centre-of-mass energy of 189 \GeV by the OPAL detector at LEP are used to search for trilinear couplings of the neutral gauge bosons in the process ${\rm e}^+{\rm e}^-\rightarrow{\rm Z\gamma}$ . The cross-sections for multihadronic events with an energetic isolated photon, and for events with a high energy photon accompanied by missing energy are measured. These cross-sections and the photon energy, polar angle and isolation angle distributions are compared to the Standard Model predictions and to the theoretical expectations of a model allowing for ${\rm Z\gamma Z}$ and ${\rm Z\gamma\gamma}$ vertices. Since no significant deviations with respect to the Standard Model expectations are found, constraints are derived on the strength of neutral trilinear gauge couplings.

Three-pion interferometry results from central Pb + Pb collisions at 158 A GeV/c

Abstract: Three-Pion Interferometry Results from Central Pb+Pb Collisions at 158 A GeV/c M.M. Aggarwal, 1 A. Agnihotri, 2 Z. Ahammed, 3 A.L.S. Angelis, 4 V. Antonenko, 5 V. Arefiev, 6 V. Astakhov, 6 V. Avdeitchikov, 6 T.C. Awes, 7 P.V.K.S. Baba, 8 S.K. Badyal, 8 C. Barlag, 9 S. Bathe, 9 B. Batiounia, 6 T. Bernier, 10 K.B. Bhalla, 2 V.S. Bhatia, 1 C. Blume, 9 R. Bock, 11 E.-M. Bohne, 9 Z. B¨ r¨ cz, 9 D. Bucher, 9 o o 12 H. B¨ sching, 9 L. Carlen, 13 V. Chalyshev, 6 S. Chattopadhyay, 3 R. Cherbatchev, 5 T. Chujo, 14 A. Buijs, u 9 A.C. Das, 3 M.P. Decowski, 18 H. Delagrange, 10 V. Djordjadze, 6 P. Donni, 4 I. Doubovik, 5 A. Claussen, S. Dutt, 8 M.R. Dutta Majumdar, 3 K. El Chenawi, 13 S. Eliseev, 15 K. Enosawa, 14 P. Foka, 4 S. Fokin, 5 M.S. Ganti, 3 S. Garpman, 13 O. Gavrishchuk, 6 F.J.M. Geurts, 12 T.K. Ghosh, 16 R. Glasow, 9 S. K.Gupta, 2 B. Guskov, 6 H. ˚ .Gustafsson, 13 H. H.Gutbrod, 10 R. Higuchi, 14 I. Hrivnacova, 15 M. Ippolitov, 5 H. Kalechofsky, 4 A R. Kamermans, 12 K.-H. Kampert, 9 K. Karadjev, 5 K. Karpio, 17 S. Kato, 14 S. Kees, 9 C. Klein-B¨ sing, 9 o 9 B. W. Kolb, 11 I. Kosarev, 6 I. Koutcheryaev, 5 T. Kr¨ mpel, 9 A. Kugler, 15 P. Kulinich, 18 S. Knoche, u M. Kurata, 14 K. Kurita, 14 N. Kuzmin, 6 I. Langbein, 11 A. Lebedev, 5 Y.Y. Lee, 11 H. L¨ hner, 16 L. Luquin, 10 o 19 V. Manko, 5 M. Martin, 4 G. Mart´ 10 A. Maximov, 6 G. Mgebrichvili, 5 Y. Miake, 14 D.P. Mahapatra, inez, 8 G.C. Mishra, 19 Y. Miyamoto, 14 B. Mohanty, 19 M.-J. Mora, 10 D. Morrison, 20 D. S. Mukhopadhyay, 3 Md.F. Mir, H. Naef, 4 B. K. Nandi, 19 S. K. Nayak, 10 T. K. Nayak, 3 S. Neumaier, 11 A. Nianine, 5 V. Nikitine, 6 S. Nikolaev, 5 P. Nilsson, 13 S. Nishimura, 14 P. Nomokonov, 6 J. Nystrand, 13 F.E. Obenshain, 20 A. Oskarsson, 13 I. Otterlund, 13 M. Pachr, 15 S. Pavliouk, 6 T. Peitzmann, 9 V. Petracek, 15 W. Pinganaud, 10 F. Plasil, 7 U. von Poblotzki, 9 M.L. Purschke, 11 J. Rak, 15 R. Raniwala, 2 S. Raniwala, 2 V.S. Ramamurthy, 19 N.K. Rao, 8 F. Retiere, 10 K. Reygers, 9 G. Roland, 18 L. Rosselet, 4 I. Roufanov, 6 C. Roy, 10 J.M. Rubio, 4 H. Sako, 14 S.S. Sambyal, 8 R. Santo, 9 S. Sato, 14 H. Schlagheck, 9 H.-R. Schmidt, 11 Y. Schutz, 10 G. Shabratova, 6 T.H. Shah, 8 I. Sibiriak, 5 T. Siemiarczuk, 17 D. Silvermyr, 13 B.C. Sinha, 3 N. Slavine, 6 K. S¨ derstr¨ m, 13 o o N. Solomey, 4 S.P. Sorensen, 7,20 P. Stankus, 7 G. Stefanek, 17 P. Steinberg, 18 E. Stenlund, 13 D. St¨ ken, 9 u M. Sumbera, 15 T. Svensson, 13 M.D. Trivedi, 3 A. Tsvetkov, 5 L. Tykarski, 17 J. Urbahn, 11 E.C.v.d. Pijll, 12 N.v. Eijndhoven, 12 G.J.v. Nieuwenhuizen, 18 A. Vinogradov, 5 Y.P. Viyogi, 3 A. Vodopianov, 6 S. V¨ r¨ s, 4 o o B. Wyslouch, 18 K. Yagi, 14 Y. Yokota, 14 G.R. Young 7 (WA98 collaboration) University of Panjab, Chandigarh 160014, India University of Rajasthan, Jaipur 302004, Rajasthan, India 3 Variable Energy Cyclotron Centre, Calcutta 700 064, India 4 University of Geneva, CH-1211 Geneva 4,Switzerland 5 RRC Kurchatov Institute, RU-123182 Moscow, Russia 6 Joint Institute for Nuclear Research, RU-141980 Dubna, Russia 7 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6372, USA 8 University of Jammu, Jammu 180001, India 9 University of M¨ nster, D-48149 M¨ nster, Germany u u 10 SUBATECH, Ecole des Mines, Nantes, France

Inclusive production of D*± mesons in photon-photon collisions at √see = 183 and 189 GeV and a first measurement of Fγ2,c

Abstract: The inclusive production of ${{\rm D}^{\ast\pm}}$ mesons in photon-photon collisions has been measured using the OPAL detector at LEP at $\mboxe^+\mboxe^-$ centre-of-mass energies $\sqrt{s_{\rm ee}}$ of 183 and 189 GeV The ${{\rm D}^{\ast+}}$ mesons are reconstructed in their decay to ${\rm D}^0\pi^+$ with the ${\rm D}^0$ observed in the two decay modes ${\rm K}^-\pi^+$ and ${\rm K}^-\pi^+\pi^-\pi^+$ After background subtraction, $1004\pm126 (\rm stat)$ ${\rm D}^{\ast\pm}$ mesons have been selected in events without observed scattered beam electron (“anti-tagged”) and $298 \pm 59 ({\rm stat}){{\rm D}^{\ast\pm}}$ mesons in events where one beam electron is scattered into the detector (“single-tagged”) Direct and single-resolved events are studied separately Differential cross-sections ${\rm d}\sigma/{\rm d}p_{\rm T}^{{\rm D}^{\ast}}$ and ${\rm d} \sigma/{\rm d} \vert \eta^{{\rm D^{\ast}}}\vert$ as functions of the ${{\rm D}^{\ast\pm}}$ transverse momentum $p^{\rm D^{\ast}}_{\rm T}$ and pseudorapidity $\eta^{{\rm D}^{\ast}}$ are presented in the kinematic region $2{\rm GeV} < p^{\rm D^{\ast}}_{\rm T} < 12{\rm GeV}$ and $|\eta^{{\rm D}^{\ast}}|< 15$ They are compared to next-to-leading order (NLO) perturbative QCD calculations The total cross-section for the process $\mboxe^+\mboxe^- \to \mboxe^+\mboxe^- \mbox c\overline{\mbox c}$ where the charm quarks are produced in the collision of two quasi-real photons is measured to be $\sigma({\rm e}^+{\rm e}^- \to{\rm e}^+{\rm e}^-{\rm c}\bar{{\rm c}}) = 963\pm 110 ({\rm stat})\pm 86({\rm sys})\pm 224 ({\rm extrapolation}){\rm pb}$ A first measurement of the charm structure function $F^\gamma _{2,{\rm c}}$ of the photon is performed in the kinematic range $00014 < x < 087$ and $5 {\rm GeV}^2 < Q^2 < 100{\rm GeV}^2$ , and the result is compared to a NLO perturbative QCD calculation

A study of spin alignment of $\rho(770)^{\pm}$ and $\omega(782)$ mesons in hadronic Z $^0$ decays

Abstract: The helicity density matrix elements $\rho_{00}$ of $\rho(770)^{\pm}$ and $\omega(782)$ mesons produced in Z $^0$ decays have been measured using the OPAL detector at LEP. Over the measured meson energy range, the values are compatible with 1/3, corresponding to a statistical mix of helicity $-1$ , 0 and +1 states. For the highest accessible scaled energy range 0.3 $<$ $x_E$ $<$ 0.6, the measured $\rho_{00}$ values of the $\rho^{\pm}$ and the $\omega$ are 0.373 $\pm$ 0.052 and 0.142 $\pm$ 0.114, respectively. These results are compared to measurements of other vector mesons.

Search for anomalous production of acoplanar di-lepton events in e+e- collisions at √s = 183 and 189 GeV

Abstract: A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb-1 at e+e- centre-of-rnass energies of approximately 183 GeV and 189 GeV. The observed numbers of events - 78 at 183 GeV and 301 at 189 GeV - are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with both W bosons decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Discrimination techniques are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent. Upper limits on the production cross-section times branching ratio squared for sleptons and for leptonically decaying charginos and charged Higgs are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay l±R → l±χ01, where χ01 is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for mμ- - mχ01 > 3 GeV and right-handed staus with masses below 81.0 GeV for mτ- -mχ01 > 8 GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for mẽ - mχ01 > 5 GeV, within the framework of the Minimal Supersymmetric Standard Model assuming μ, < - 100 GeV and tan β= 1.5. Charged Higgs bosons, H±, are excluded at 95% CL for masses below 82.8 GeV, assuming a 100% branching ratio for the decay H± → τ ± vτ.

Photonic events with missing energy in e(+)e(-) collisions at root s=189 GeV

Abstract: Photonic events with large missing energy have been observed in e(+)e(-) collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process e(+)e(-) --> nu )over bar> + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on sigma (e(+)e(-) --> XY) . BR(X --> Y-gamma) and sigma (e(+)e(-) --> XX) . BR2(X --> Y gamma) are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X = nu*,Y = nu), to neutralino production (X = )over tilde>(0)(2),Y = )over tilde>(0)(1)) and to supersymmetric models in which X = )over tilde>(0)(1) and Y = (G) over tilde is a Light gravitino. The case of macroscopic decay lengths of particle X is considered for e(+)e(-) --> XX, X --> Y gamma, when M-Y approximate to 0. The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

A study of parton fragmentation in hadronic Z° decays using ΛΛ̄ correlations

Abstract: The correlated production of \(\Lambda\) and \(\bar\Lambda\) baryons has been studied using 4.3 million multihadronic \({\rm Z}^0\) decays recorded with the Opal detector at Lep. Lambda pairs were investigated in the full data sample and for the first time also in 2-jet and 3-jet events selected with the \(k_\perp\) algorithm. The distributions of rapidity differences from correlated \(\Lambda\bar\Lambda\) pairs exhibit short-range, local correlations and prove to be a sensitive tool to test models, particularly for 2-jet events. The Jetset model describes the data best but some extra parameter tuning is needed to improve agreement with the experimental results in the rates and the rapidity spectra simultaneously. The recently developed modification of Jetset, the MOdified Popcorn Scenarium (Mops), and also Herwig do not give satisfactory results. This study of di-lambda production in 2- and 3-jet events supports the short-range compensation of quantum numbers.

Coherent Bremsstrahlung in the alpha+p system at 50 MeV/nucleon

Abstract: Photons originating from coherent bremsstrahlung have been measured over a large dynamic range for the reaction of 200 MeV alpha particles with protons. At low photon energies the bremsstrahlung spectrum exhibits the classical behavior with an approximate 1/E-gamma shape. At higher photon energies there is a pronounced contribution from capture into the unbound ground state and first excited state of Li-5. These results allow one, for the first time, to test theoretical models for a consistent description of bremsstrahlung and radiative capture in a complex system. Calculations predict both features qualitatively but fail to account for their relative importance.

Measurements of \(R_{\rm b}\), \(A_{\rm FB}^{\rm b}\), and \(A_{\rm FB}^{\rm c}\) in \({\rm e^+ e^-}\) collisions at 130 – 189 GeV

Abstract: The cross-section ratio \(R_{\mathrm{b}} = \sigma( \rm e^+ e^- \to{\rm b\overline{\rm b}} )/\sigma(\rm e^+ e^-\to{\mathrm q\bar{\mathrm q}} )\) and the bottom and charm forward-backward asymmetries \(A_{\mathrm{FB}}^{\mathrm{b}}\) and \(A_{\mathrm{FB}}^{\mathrm c}\) are measured using event samples collected by the OPAL detector at centre-of-mass energies between 130 and 189 GeV. Events with bottom quark production are selected with a secondary vertex tag, and a hemisphere charge algorithm is used to extract \(A_{\mathrm{FB}}^{\mathrm{b}}\). In addition, the bottom and charm asymmetries are measured using leptons from semileptonic decays of heavy hadrons and pions from \(\mathrm{D}^{*+}\to\rm D^0{\pi^+}\) decays. The results are in agreement with the Standard Model predictions.

Search for acoplanar lepton pair events in e+e- collisions at √s = 161, 172 and 183 GeV

Abstract: A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 77.0 pb $^{-1}$ at e $^+$ e $^-$ centre-of-mass energies of 161 GeV, 172 GeV and 183 GeV. The observed numbers of events: four at 161 GeV, nine at 172 GeV, and 78 at 183 GeV, are consistent with the numbers expected from Standard Model processes, which arise predominantly from ${\rm W^+W^-}$ production with each W decaying leptonically. This topology is an experimental signature also for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent and limits on the production cross-section times branching ratio squared for various new physics processes are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay $\tilde\ell^\pm_R \rightarrow \ell^\pm \tilde\chi^0_1$ , we exclude at 95% CL: right-handed smuons with masses below 65 GeV for $m_{\tilde\mu^-} - m_{\tilde\chi^0_1} > 2$ GeV and right-handed staus with masses below 64 GeV for $m_{\tilde\tau^-} - m_{\tilde\chi^0_1} > 10$ GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for $m_{\tilde{\rm e}^-} - m_{\tilde\chi^0_1} > 5$ GeV within the framework of the Minimal Supersymmetric Standard Model assuming $\mu < -100$ GeV and $\tan{\beta} = 1.5$ .

Two large-area anode-pad MICROMEGAS chambers as the basic elements of a pre-shower detector

Abstract: The design of a detector based on MICROMEGAS (MICRO MEsh GAseous Structure) technology is presented. Our detector is characterized by a large active area of 398(\times)281 mm(^{2}), a pad read-out with 20(\times)22 mm(^{2}) segmentation, and an uniform amplification gap obtained by insulating spacers (100 (\mu)m high and 200 (\mu)m in diameter). The performances of several prototypes have been evaluated under irradiation with secondary beams of 2 GeV/c momentum charged pions and electrons. We consider such a detector as the basic element for a pre-shower detector to equip the PHOton Spectrometer (PHOS) of the ALICE experiment. Its assets are modularity, small amount of material, robustness and low cost.