共查询到20条相似文献,搜索用时 15 毫秒
1.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2003,26(3):331-344
The process 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 , 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 is found and limits are placed on the product of the production cross-section and branching ratio.
Received: 22 July 2002 / Published online: 8 November 2002 相似文献
2.
The ALEPH Collaboration A. Heister et al. 《The European Physical Journal C - Particles and Fields》2002,25(1):1-12
A search for single sneutrino production under the assumption that R-parity is violated via a single dominant coupling is presented. This search considers the process and is performed using the data collected by the ALEPH detector at centre-of-mass energies from 189 GeV up to 209 GeV corresponding
to an integrated luminosity of 637.1 pb-1. The numbers of observed candidate events are in agreement with Standard Model expectations and 95% confidence level upper
limits on five of the couplings are given as a function of the assumed sneutrino mass.
Received: 19 December 2001 / Published online: 26 July 2002 相似文献
3.
The OPAL Collaboration G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2003,27(4):467-481
Charged particle momentum distributions are studied in the reaction 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.
Received: 17 July 2002 / Published online: 3 March 2003 相似文献
4.
A study of Z-boson pair production in e + e- annihilation at center-of-mass energies between 190 GeV and 209 GeV is reported. Final states containing only leptons, (
and
), quark and lepton pairs, (
,
) and only hadrons (
) are considered. In all states with at least one Z boson decaying hadronically, lifetime, lepton and event-shape tags are used to separate
pairs from
final states. Limits on anomalous ZZ
and ZZZ couplings are derived from the measured cross sections and from event kinematics using an optimal observable method. Limits on low scale gravity with large extra dimensions are derived from the cross sections and their dependence on polar angle.Received: 14 July 2003, Published online: 18 December 2003 相似文献
5.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》1999,10(4):547-561
Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at ee centre-of-mass energies and 172 GeV. The jets are reconstructed using a cone jet finding algorithm. The angular distributions of direct and double-resolved
processes are measured and compared to the predictions of leading order and next-to-leading order perturbative QCD. The jet
energy profiles are also studied. The inclusive di-jet cross-section is measured as a function of and and compared to next-to-leading order perturbative QCD calculations. The inclusive di-jet cross-section as a function of
is compared to the prediction of the leading order Monte Carlo generators PYTHIA and PHOJET. The Monte Carlo predictions
are calculated with different parametrisations of the parton distributions of the photon. The influence of the ‘underlying
event’ has been studied to reduce the model dependence of the predicted jet cross-sections from the Monte Carlo generators.
Received: 15 July 1999 / Published online: 12 August 1999 相似文献
6.
The production of single- and multi-photon events has been studied in the reaction
. The data collected with the DELPHI detector during the years 1999 and 2000 at centre-of-mass energies between 191 GeV and 209 GeV was combined with earlier data to search for phenomena beyond the Standard Model. The measured number of light neutrino families was consistent with three and the absence of an excess of events beyond that predicted by the Standard Model processes was used to set limits on new physics. Both model-independent searches and searches for new processes predicted by supersymmetric and extra-dimensional models have been made. Limits on new non-standard model interactions between neutrinos and electrons were also determined.Received: 31 March 2004, Revised: 14 October 2004, Published online: 7 December 2004 相似文献
7.
Di-jet production is studied in collisions of quasi-real photons at e + e- centre-of-mass energies
from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive
-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
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,
, for different regions of
. 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
and
is presented, where
is the jet pseudo-rapidity. Different regions of the
-
-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.Received: 20 December 2002, Published online: 17 October 2003 相似文献
8.
《The European Physical Journal C - Particles and Fields》2006,46(2):307-341
Searches were performed for topologies predicted by gauge-mediated supersymmetry breaking models (GMSB). All possible lifetimes of the next-to-lightest SUSY particle (NLSP), either the lightest neutralino or slepton, decaying into the lightest SUSY particle, the gravitino, were considered. No evidence for GMSB signatures was found in the OPAL data sample collected at centre-of-mass energies up to $\sqrt{s}=209 \mathrm{GeV}$ at LEP. Limits on the product of the production cross-sections and branching fractions are presented for all search topologies. To test the impact of the searches, a complete scan over the parameters of the minimal model of GMSB was performed. NLSP masses below $53.5 \mathrm{GeV}/c^2$ in the neutralino NLSP scenario, below $87.4 \mathrm{GeV}/c^2$ in the stau NLSP scenario and below $91.9 \mathrm{GeV}/c^2$ in the slepton co-NLSP scenario are excluded at 95% confidence level for all NLSP lifetimes. The scan determines constraints on the universal SUSY mass scale Λ from the direct SUSY particle searches of Λ>40, 27, 21, 17, $15 \mathrm{GeV}/c^2$ for messenger indices N=1,2,3,4,5 for all NLSP lifetimes. 相似文献
9.
Search for pair-produced leptoquarks in e + e- interactions at $\sqrt{s} \simeq 189\text{--}209$ GeV
A search for pair-produced leptoquarks is performed using
collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z0. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.Received: 6 May 2003, Published online: 24 October 2003 相似文献
10.
Searches for stable, hadronizing scalar quarks and gluinos are performed using the data collected with the ALEPH detector at LEP. Gluon splitting into a gluino or a squark pair is searched for at centre-of-mass energies around the Z resonance, in the e + e-
and
processes. Stable squark pair production, and stop pair production with subsequent decays into a stable gluino,
, are also directly searched for at centre-of-mass energies from 183 to 209 GeV. Altogether, stable hadronizing stop (sbottom) quarks are excluded up to masses of 95 (92)
, and stable hadronizing gluinos are excluded up to 26.9
, at 95% confidence level. In the framework of R-parity-conserving supersymmetric models in which the gluino and the stop quark are the two lightest supersymmetric particles, a 95% C.L. lower limit of 80
is set on the stop quark mass.Received: 23 May 2003, Published online: 24 October 2003 相似文献
11.
The DELPHI Collaboration J. Abdallah et al. 《The European Physical Journal C - Particles and Fields》2003,28(1):15-26
Searches for resonant production in e
+
e
-
collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is or used data recorded by DELPHI in 1997 to 2000 at centre-of-mass energies of 183 to 208 GeV. No deviation from the Standard
Model was observed. Upper limits are given for the and couplings as a function of the sneutrino mass and total width. The limits are especially stringent for sneutrino masses equal
to the centre-of-mass energies with the highest integrated luminosities recorded.
Received: 14 January 2003 / Published online: 14 March 2003 相似文献
12.
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
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
GeV and pseudorapidity
is determined to be
. Differential cross-sections are compared to the predictions of a next-to-leading-order (NLO) calculation.Received: 6 May 2003, Published online: 4 November 2003 相似文献
13.
Approximately 438 pb-1 of e + e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production,
, or neutralino associated production,
. Limits are set at the 95% confidence level on the product of the cross-section for the process
and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for
and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the
and
, and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on
with the assumption that each chargino decays via a W boson, and on
with the
assumed to decay via a Z0.Received: 15 December 2003, Revised: 16 February 2004, Published online: 23 April 2004 相似文献
14.
Ian Johnson 《Acta Physica Hungarica A》2002,15(3-4):247-255
We report raw photon, neutral pion and eta measurements at RHIC. Photons in the energy range from 100MeV ? 4GeV were detected by reconstructing e + e ? pair production, γ+Z→e ++e ?+Z, with the STAR Time Projection Chamber (TPC). Along with the photon detection technique we discuss the purity of the photon candidates and measurements of hadronic decays via their electromagnetic decay channels. The π0→γ and η→γγ decay channels are addressed. 相似文献
15.
J. Abdallah et al. 《The European Physical Journal C - Particles and Fields》2006,45(2):273-289
The production of single charged and neutral intermediate vector bosons in e + e- collisions has been studied in the data collected by the DELPHI experiment at LEP at centre-of-mass energies between 183
and 209 GeV, corresponding to an integrated luminosity of about 640 pb-1. The measured cross-sections for the reactions, determined in limited kinematic regions, are in agreement with the Standard
Model predictions.
Arrival of the final proofs: 21 November 2005 相似文献
16.
P. Abreu et al. 《The European Physical Journal C - Particles and Fields》2001,19(2):201-212
A search for pair-production of neutralinos at a LEP centre-of-mass energy of 189 GeV gave no evidence for a signal. This
limits the neutralino production cross-section and excludes regions in the parameter space of the Minimal Supersymmetric Standard
Model (MSSM).
Received: 22 November 2000 / Revised version: 11 January 2001 / Published online: 15 March 2001 相似文献
17.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2000,14(2):187-198
A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production
is found. Upper limits on chargino and neutralino pair production (, ) cross-sections are obtained as a function of the chargino mass (), the lightest neutralino mass () and the second lightest neutralino mass (). Within the Constrained Minimal Supersymmetric Standard Model framework, and for GeV, the 95% confidence level lower limits on are 93.6 GeV for and 94.1 GeV for . These limits are obtained assuming a universal scalar mass 500 GeV. The corresponding limits for all are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of are 32.8 GeV for GeV and 31.6 GeV for all .
Received: 14 October 1999 / Published online: 6 March 2000 相似文献
18.
J. Manninen E. L. Bratkovskaya W. Cassing O. Linnyk 《The European Physical Journal C - Particles and Fields》2011,71(4):1615
We study dilepton production in proton–proton, Cu+Cu as well as in Au+Au collisions at the center-of-mass energy \(\sqrt{s} =200\) GeV per participating nucleon pair within an extended statistical hadronization model. In extension to earlier studies we incorporate transport calculations for an estimate of uncorrelated e + e ? -pairs from semileptonic D meson decays. While the invariant mass spectrum of dielectrons is well understood in the p+p collisions, severe discrepancies among different model scenarios based on hadronic degrees of freedom and recent data from the PHENIX Collaboration are found in heavy-ion collisions in the low-mass region from 0.15 to 0.6 GeV as well as in the intermediate mass regime from 1.1 to 3 GeV when employing the standard dilepton sources. We investigate, furthermore, the background from correlated dileptons that are not emitted as a pair from a parent hadron but emerge from semileptonic decays of two correlated daughter hadrons. Our calculations suggest a sizeable contribution of such sources in central heavy-ion collisions in the low-mass region. However, even the upper limits of our calculations are found to be far below the dilepton mass spectra of the PHENIX Collaboration. 相似文献
19.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2000,18(2):253-272
Photonic events with large missing energy have been observed in 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 + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on and are derived for the case of stable and invisible . These limits apply to single and pair production of excited neutrinos (), to neutralino production () and to supersymmetric models in which and is a light gravitino. The case of macroscopic decay lengths of particle X is considered for , , when . 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.
Received: 4 April 2000 / Published online: 27 November 2000 相似文献
20.
A search for excited lepton production in e+e- collisions was performed using the data collected by the DELPHI detector at LEP at centre-of-mass energies ranging from 189 GeV
to 209 GeV, corresponding to an integrated luminosity of approximately 600 pb-1. No evidence for excited lepton production was found. In searches for pair-produced excited leptons, lower mass limits were
established in the range 94–103 GeV/c2, depending on the channel and model assumptions. In searches for singly-produced excited leptons, upper limits on the parameter
f/Λ were established as a function of the mass. 相似文献