Showing papers by "D. Axen published in 2003"
TL;DR: In this paper, the authors describe topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process at centre-of-mass energies of 91 GeV and 183-209 GeV.
Abstract: This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process \(e^+e^- \to{\rm S}^0{}{\rm Z}^0\) at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of \({\rm Z}^0 \to e^+e^-\) and \(\mu^+ \mu^-\) events and on a search for \({\rm S}^0{\rm Z}^0\) with \({\rm Z}^0 \to
u\bar{
u}\) and S\(^0 \to e^+ e^-\) or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable.
131 citations
TL;DR: In this article, a study of b quark hadronisation using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP is presented.
Abstract: A study of b quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be [xE] = 0.7193 +/- 0.0016(stat)(-0.0033)(+0.0038)(syst).
86 citations
TL;DR: In this article, a search for stable and long-lived massive particles of electric charge |Q/e|=1 or 2/3, pair-produced in e+e− collisions at center-of-mass energies from 130 to 183 GeV, was reported by the OPAL collaboration at LEP.
81 citations
TL;DR: In this article, a search was performed for charginos with masses close to the mass of the lightest neutralino in e+e− collisions at center-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP.
Abstract: A search was performed for charginos with masses close to the mass of the lightest neutralino in e+e− collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570 pb-1. Upper limits were derived on the chargino pair-production cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity- and anomaly-mediated Supersymmetry breaking scenarios.
67 citations
TL;DR: In this paper, the authors studied the process of the OPAL detector at LEP between the years 1997 and 2000 and determined the total and differential cross-sections in good agreement with the predictions of QED.
Abstract: The process \(\mathrm{e}^+\mathrm{e}^- \to\gamma\gamma(\gamma)\) is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type \(\mathrm{e}^+\mathrm{e}^- \gamma\gamma\), gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance \(X\to\gamma\gamma\) is found and limits are placed on the product of the \(\rm X \gamma\) production cross-section and branching ratio.
57 citations
TL;DR: In this paper, the search for the Standard Model Higgs boson in e+e- collisions at center-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP is summarized.
Abstract: This paper summarises the search for the Standard Model Higgs boson in e+e- collisions at centre-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP. The consistency of the data with the background hypothesis and various Higgs boson mass hypotheses is examined. No indication of a signal is found in the data and a lower bound of 112.7 Gev/c2 is obtained on the mass of the Standard Model Higgs boson at the 95% CL.
54 citations
TL;DR: In this paper, the authors analyzed the data collected by OPAL at center-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process of the decay of the Higgs particle to the CP-odd Higgs particles.
Abstract: We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process $\mathrm {e^+e^-} \to \mathrm {h^0}\mathrm {Z}^0$
followed by the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$
where $\mathrm {A^0}$
is the CP-odd Higgs boson. The search is done in the region where the $\mathrm {A^0}$
mass, $m_{\mathrm{A}}$
, is below the production threshold for $\mathrm {b \bar b}$
, and the CP-even Higgs boson mass $m_{\mathrm{h}}$
is within the range 45-86 GeV/c
2
. In this kinematic range, the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$
may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process $\mathrm {e^+e^-}\rightarrow \mathrm {h^0} \mathrm {Z}^0$
are derived assuming 100% decays of the $\mathrm {h^0}$
into $\mathrm {A^0} \mathrm {A^0}$
and 100% decays of the $\mathrm {A^0} \mathrm {A^0}$
into each of the following final states: $\mathrm {c \bar c} \mathrm {c \bar c}$
, $\mathrm {gggg}$
, $\tau^+ \tau^- \tau^+ \tau^-$
, $\mathrm {c \bar c} \mathrm {gg}$
, $\mathrm {gg} \tau^+ \tau^-$
and $\mathrm {c \bar c} \tau^+ \tau^-$
. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region $45 \le m_{\mathrm{h}} \le 85 {\mathrm{GeV}}/c^2$
and $2\le m_{\mathrm{A}} \le 9.5 {\mathrm{GeV}}/c^2$
is excluded.
42 citations
TL;DR: In this article, a search for doubly-charged Higgs bosons is performed using e+e- collision data collected by the OPAL expt. at center-of-mass energies between 189 GeV and 209 GeV.
34 citations
TL;DR: In this article, the results of a tree level calcn. for the process e+e-->gg are evaluated for the specific orientation of the OPAL detector and compared to the measurements, and the results show that non-commutative QED leads to deviations from the Std. Model depending on a new energy scale LNC and a unique direction in space defined by two angles h and x.
31 citations
TL;DR: In this paper, the OPAL data was used to study the acceleration and energy dependence of the strong interaction and to test QCD as the theory describing it, and a good agreement was found between the measurements and the corresponding QCD predictions.
Abstract: Charged particle momentum distributions are studied in the reaction $e^+e^-\rightarrow$
hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of-mass energy of 201.7 GeV and a total integrated luminosity of 433 pb-1. The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions.
29 citations
TL;DR: In this article, the authors studied di-jet production in collisions of quasi-real photons at e+ e- centre-of-mass energies from 189 to 209 GeV at LEP.
Abstract: Di-jet production is studied in collisions of quasi-real photons at e+ e- centre-of-mass energies √see from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k⊥-clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy E¯jetT of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, xγ, for different regions of E¯jetT. Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of ηjet and Δηjet is presented, where ηjetcntr is the jet pseudo-rapidity. Different regions of the x+γ - x−γ -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.
TL;DR: In this article, a measurement of the forward-backward asymmetries of e + e − → b b and e − e− → c c events using electrons and muons produced in semileptonic decays of bottom and charm hadrons is presented.
TL;DR: In this paper, the exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at the OPAL detector at LEP.
Abstract: The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at \(\sqrt{s_{ee}} = 183 \) GeV and 189 GeV with the OPAL detector at LEP. Results are presented for \(p\bar{p}\) invariant masses, W, in the range 2.15
TL;DR: In this article, a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP was collected and the branching ratio was measured to be (2.64 +- 0.79(stat)+-0.39(syst)) x 10-3 for the JP=1+ (D01) state.
Abstract: The decay chain b -> .hivin.B -> D**0l-nX, D**0 -> D*+p-, D0p+, D0 -> (Kp or K3p) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR(b -> .hivin.B) x BR (.hivin.B -> D01l-nX) x BR(D01 -> D*+p-) is measured to be (2.64 +- 0.79(stat)+-0.39(syst)) x 10-3 for the JP=1+ (D01) state. For decays into the JP=2+ (D*01) state, an upper limit of 1.4 x 10-3 is placed on the branching ratio at the 95% confidence level. [on SciFinder (R)]
TL;DR: In this article, Bose-Einstein correlations in π0 pairs were observed using back-to-back two jet hadronic events from Z0 decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995.
TL;DR: The study indicated that random modulator errors have a strong tendency to reduce minimum target dose and homogeneity and random perturbation of both MLCs and backup diaphragms in the order of sigma = 1 mm can lead to 5% errors in prescribed dose.
Abstract: This paper reports on the dosimetric effects of random and systematic modulator errors in delivery of dynamic intensity modulated beams. A sliding-widow type delivery that utilizes a combination of multileaf collimators (MLCs) and backup diaphragms was examined. Gaussian functions with standard deviations ranging from 0.5 to 1.5 mm were used to simulate random positioning errors. A clinical example involving a clival meningioma was chosen with optic chiasm and brain stem as limiting critical structures in the vicinity of the tumour. Dose calculations for different modulator fluctuations were performed, and a quantitative analysis was carried out based on cumulative and differential dose volume histograms for the gross target volume and surrounding critical structures. The study indicated that random modulator errors have a strong tendency to reduce minimum target dose and homogeneity. Furthermore, it was shown that random perturbation of both MLCs and backup diaphragms in the order of σ = 1 mm can lead to 5% errors in prescribed dose. In comparison, when MLCs or backup diaphragms alone was perturbed, the system was more robust and modulator errors of at least σ = 1.5 mm were required to cause dose discrepancies greater than 5%. For systematic perturbation, even errors in the order of ±0.5 mm were shown to result in significant dosimetric deviations.
TL;DR: The τ − →μ − ν μ ν τ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider as mentioned in this paper.
TL;DR: In this paper, a novel method of determining the mass of the W boson in the channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP.
Abstract: A novel method of determining the mass of the W boson in the channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.
TL;DR: In this article, the production of prompt photons in the collisions of quasi-real photons using OPAL data taken at e(+)e(-) center-of-mass energies between 183 GeV and 209 GeV was studied.
Abstract: For the first time at LEP the production of prompt photons is studied in the collisions of quasi-real photons using the OPAL data taken at e(+)e(-) centre-of-mass energies between 183 GeV and 209 GeV. The total inclusive production cross-section for isolated prompt photons in the kinematic range of transverse momentum p(T)(gamma)>3.0 GeV and pseudorapidity \eta(gamma)\<1 is determined to be sigma(tot)=0.32 +/- 0.04(stat)+/- 0.04 (sys) pb. Differential cross-sections are compared to the predictions of a next-to-leading-order (NLO) calculation.
Journal Article•
TL;DR: In this paper, the process e(+)e(-) --> gammagamma(gamma) was studied using data collected by the OPAL detector at LEP between the years 1997 and 2000.
Abstract: The process e(+)e(-) --> gammagamma(gamma) is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb(-1) at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type e(+)e(-) gammagamma, gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance X --> gammagamma is found and limits are placed on the product of the Xgamma production cross-section and branching ratio.
Journal Article•
TL;DR: In this article, a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP was used to identify the decay chain b > 0.
Abstract: The decay chain b --> (B) over bar --> D-**0 l(-) (nu) over bar X , D-**0 --> D(*+)pi(-), D*+ --> D(0)pi(+), D-0 --> (Kpi or K3pi) is identified in a sample of 3.9 million hadronic Z decays collected with the OPAL detector at LEP. The branching ratio BR (b --> (B) over bar) x BR ((B) over bar --> D-1(0) l(-) (nu) over bar X) x BR (D-1(0) --> D(*+)pi(-)) is measured to be (2.64 +/- 0.79 (stat) +/- 0.39 (syst)) x 10(-3) for the J(P)=1(+) (D-1(0)) state. For decays into the J(P)=2(+) (D-2(*0)) state, an upper limit of 1.4 x 10(-3) is placed on the branching ratio at the 95% confidence level.