共查询到20条相似文献,搜索用时 31 毫秒
1.
Pauline Gagnon 《Pramana》2004,62(3):545-550
Ever since the center-of-mass energy was increased in 1995 above the Zℴ resonance, the four LEP experiments (ALEPH, DELPHI,
OPAL and L3) have renewed their effort to search for the Higgs boson. Data taking ended in the year 2000 with about 130 pb−1 of data collected per experiment above 206 GeV ine
+
e
−1 collisions but the data analysis is still very active. Most recently, the wealth of theoretical models and predictions has
stimulated new analyses and model interpretations which go beyond the standard model and minimal supersymmetric standard model.
These include the searches for charged Higgs bosons, models with two Higgs field doublets, searches for ‘fermiophobic’ Higgs
decay, invisible Higgs boson decays, decay-mode independent searches, and limits on Yukawa and anomalous Higgs couplings.
I review the searches done by the four LEP experiments and present the LEP combined results when they exist. 相似文献
2.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2000,13(4):553-572
Cross-sections and angular distributions for hadronic and lepton pair final states in collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are presented and compared with
the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling
constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle
such as a sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the and final states is also presented.
Received: 9 July 1999 / Published online: 17 February 2000 相似文献
3.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2000,14(3):373-425
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, 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 , significantly below the theoretical error of currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in
the OPAL measurement of , a quantity of basic physical interest which depends crucially on the luminosity measurement.
Received: 28 September 1999 / Published online: 14 April 2000 相似文献
4.
Searches for unstable neutral and charged heavy leptons, N and , and for excited states of neutral and charged leptons, , , , and , have been performed in collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of
about 58 pb at a centre-of-mass energy of 183 GeV, and about 10 pb each at 161 GeV and 172 GeV. No evidence for new particles was found. Lower limits on the masses of unstable heavy and excited
leptons are derived. From the analysis of charged-current, neutral-current, and photonic decays of singly produced excited
leptons, upper limits are determined for the ratio of the coupling to the compositeness scale, , for masses up to the kinematic limit. For excited leptons, the limits are established independently of the relative values
of the coupling constants f and .
Received: 29 November 1999 / Published online: 6 March 2000 相似文献
5.
Production of events with hadronic and leptonic final states has been measured in collisions at centre-of-mass energies of 130–172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward
asymmetries are presented, both including and excluding the dominant production of radiative Z events, and compared to Standard Model expectations. The ratio of the cross-section for production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have
been used to obtain an improved precision on the measurement of -Z interference. The energy dependence of has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described
by effective four-fermion contact interactions, to search for -channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of
the gaugino into a light gluino and a quark pair.
Received: 30 July 1997 / Published online: 10 March 1998 相似文献
6.
Cross-sections for hadronic, and lepton pair final states in collisions at =183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward
asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data
recorded in 1997 at =130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle
such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation.
Received: 3 July 1998 / Published online: 15 October 1998 相似文献
7.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2001,18(3):425-445
Searches for the neutral Higgs bosons and 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 of data collected with the OPAL detector at GeV with previous runs at and GeV. The searches are sensitive to the and 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.
Received: 11 July 2000 / Published online: 21 December 2000 相似文献
8.
The branching ratios of the
and decays have been measured using the 1991–1995 data recorded with the OPAL detector at LEP. These branching ratios are measured
simultaneously using three selection criteria and are found to be where the first error is statistical and the second is systematic.
Received: 4 December 1997 / Published online: 2 June 1998 相似文献
9.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》2006,45(2):307-335
The mass and width of the W boson are measured using e+e– → W+W– events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV.
The mass (mW) and width (ΓW) are determined using direct reconstruction of the kinematics of W+W– →
and W+W– →
events. When combined with previous OPAL measurements using W+W– →
events and the dependence on of the WW production cross-section at threshold, the results are determined to be
where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam
energy. By measuring in the channel using several different determinations of the direction of jets with differing sensitivities
to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects inW+W– →
events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides
an important test of the Standard Model of electroweak interactions.
Arrival of the final proofs: 28 November 2005
This paper is dedicated to the memory of Steve O'Neale 相似文献
10.
The Michel parameters of the leptonic decays are measured using the OPAL detector at LEP. The parameters , , (with ) and are extracted from the energy spectra of the charged decay leptons and from their energy–energy correlations. A new method
involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment
has been applied to the data recorded at center-of-mass energies close to corresponding to an integrated luminosity of 155 pb. If e- universality is assumed and inferring the polarization from neutral current data, the measured Michel parameters are: where the value of has been constrained using the published OPAL measurements of the leptonic branching ratios and the lifetime. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in
agreement with the V–A prediction of the Standard Model.
Received: 25 June 1998 / Published online: 1 March 1999 相似文献
11.
A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57
pb at centre-of-mass energies of 181–184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production
has been 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 (). For large chargino masses the limits have been improved with respect to the previous analyses at lower centre-of-mass energies.
Exclusion regions at 95% confidence level (C.L.) of parameters of the Constrained Minimal Supersymmetric Standard Model are
determined for the case of a large universal scalar mass, , implying heavy scalar fermions, and for the case of a small resulting in light scalar fermions and giving the worst-case limits. Within this framework and for GeV the 95% C.L. lower limits on for GeV are 90.0 and 90.2 GeV for and 35 respectively. These limits for all (the worst-case) are 69.1 and 65.2 GeV for and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95%
C.L. for the mass of the lightest neutralino of 30.1 GeV for GeV (24.2 GeV for all ), with implications for experimental searches for the lightest neutralino as a dark matter candidate.
Received: 19 August 1998 / Published online: 11 March 1999 相似文献
12.
Hadronic events produced in e+e- collisions by the LEP collider and recorded by the OPAL detector were used to form distributions based on the number of reconstructed
jets. The data were collected between 1995 and 2000 and correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The
jet rates were determined using four different jet-finding algorithms (Cone, JADE, Durham and Cambridge). The differential
two-jet rate and the average jet rate with the Durham and Cambridge algorithms were used to measure in the LEP energy range
by fitting an expression in which
calculations were matched to a NLLA prediction and fitted to the data. Combining the measurements at different centre-of-mass
energies, the value of () was determined to be
αS(MZ) = 0.1177 ± 0.0006 (stat.) ± 0.0012 (expt.) ± 0.0010 (had.) ± 0.0032 (theo.). 相似文献
13.
Using the OPAL detector at LEP, the running of the effective QED coupling α(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost
ideal QED framework, with very favourable experimental conditions, we obtain:
where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty.
This agrees with current evaluations of α(t). The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ. Similarly, our results are inconsistent at the level of 3σ with the hypothesis
that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running
α(t) is probed differentially within the measured t range.
Received: 2 March 2005, Revised: 30 August 2005, Published online: 3 November 2005 相似文献
14.
《The European Physical Journal C - Particles and Fields》2008,53(1):21-39
Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass
of the Z boson with the OPAL detector at LEP. The strong coupling αs is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies
ranging from 24 GeV to 78 GeV, and the energy dependence of αs is studied. Our results are consistent with the running of αs as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and
isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different
event shape observables and energies gives αs(MZ)=0.1182±0.0015(stat.)±0.0101(syst.). 相似文献
15.
The production of charged hadrons and mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and
the pseudorapidity of the charged hadrons and mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading
order (NLO) QCD calculations. The distributions have been measured in the range GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in interactions with -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.
Received: 4 June 1998 / Published online: 6 November 1998 相似文献
16.
A. Gehrmann-De Ridder E.W.N. Glover 《The European Physical Journal C - Particles and Fields》1999,7(1):29-48
We present a detailed study of photon production in hadronic events in electron-positron annihilation at LEP energies. We
show that estimates of the inclusive photon spectrum using the quark-to-photon fragmentation function determined using the
ALEPH ‘photon’ + 1 jet data agree well with the observations of the OPAL collaboration. This agreement shows that the photon
fragmentation function determined in this way can be used for inclusive observables. We also compare next-to-leading order
and beyond leading logarithm predictions obtained using the numerically resummed solutions of the fragmentation function evolution
equation of Bourhis, Fontannaz and Guillet and Glück, Reya and Vogt with the data. Moreover, in order to check the general
behaviour of the fragmentation function, we consider an analytic series expansion in the strong coupling. We find that the
parameterizations are inaccurate at large x values. While the OPAL data is in broad agreement with estimates based on any of these approaches, the ALEPH data prefers
the resummed BFG parameterization. Finally, there is some ambiguity as to whether the fragmentation function is treated as
or . We show that at present this ambiguity affects mainly the prediction for the ‘photon’ + 1 jet rate at large z.
Received: 9 June 1998 / Published online: 21 August 1998 相似文献
17.
C. Adloff et al. 《The European Physical Journal C - Particles and Fields》2000,18(2):293-302
The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum,
rapidity, and Feynman x of the mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below GeV. The measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon
direction down to a value of in the centre-of-mass frame. The transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential
fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo
models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse
momentum values in the photon.
Received: 14 June 2000 / Published online: 27 November 2000 相似文献
18.
G. Abbiendi et al. The OPAL Collaboration 《The European Physical Journal C - Particles and Fields》2001,20(3):445-454
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 meson into . The mesons are reconstructed in the decay channel with . No evidence for a narrow resonance is found. A limit on the production of narrow in hadronic Z decays is derived:
Received: 16 January 2001 / Published online: 28 May 2001 相似文献
19.
《The European Physical Journal C - Particles and Fields》2007,49(2):457-472
This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e+e-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV
corresponding to a total integrated luminosity of 629 pb-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and
decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal
and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state.
For moderate Higgs boson decay widths, these range from about 0.07 pb (MH=60 GeV) to 0.57 pb (MH=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in
general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the
so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario
experimentally accessible at LEP 2. 相似文献
20.
A.N. Aleev V.P. Balandin E.A. Dvalishvili A.N. Gaponenko A.V. Gusev P.Z. Hristov Z.M. Ivanchenko I.M. Ivanchenko M.N. Kapishin N.N. Karpenko 《The European Physical Journal C - Particles and Fields》2000,13(3):427-432
New precise data of the polarization are obtained in the EXCHARM experiment at the Serpukhov accelerator. The s have been produced in nC interactions in the neutron energy interval 40–70 GeV and detected in the kinematic range of and GeV/c. The obtained results are compared with other measurements in the pp and pA interactions.
Received: 25 August 1999 / Published online: 25 February 2000 相似文献