共查询到20条相似文献,搜索用时 31 毫秒
1.
A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as
well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb at an centre-of-mass energy of 183 GeV and about 20 pb equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes
95% confidence level mass limits in the region between 70 GeV/ and 90 GeV/, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio
of the coupling of the excited fermion to its mass () as a function of the mass.
Received: 30 October 1998 / Published online: 1 March 1999 相似文献
2.
Searches for topologies characteristic of Gauge Mediated SUSY Breaking models (GMSB) are performed by analysing 173.6 of data collected at = 188.6 GeV with the ALEPH detector. These topologies include acoplanar photons, non-pointing single photon, acoplanar leptons,
large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and four leptons plus missing
energy final states. No evidence for these new phenomena is observed and limits on production cross sections and sparticle
masses are derived. A scan of a minimal GMSB parameter space is performed and model dependent lower limits of about 45 GeV/ on the next-to-lightest supersymmetric particle (NLSP) mass and of about 9 TeV on the mass scale parameter are derived, independently of the NLSP lifetime.
Received: 30 November 1999 / Published online: 14 April 2000 相似文献
3.
A. Heister et al. 《The European Physical Journal C - Particles and Fields》2002,25(3):339-351
A total of 628 bp-1 of data collected with the ALEPH detector at centre-of-mass energies from 189 to 209 GeV is analysed in the search for gauge
mediated SUSY breaking (GMSB) topologies. These topologies include two acoplanar photons, non-pointing single photons, acoplanar
leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and multi-leptons
plus missing energy final states. No evidence is found for new phenomena, and lower limits on masses of supersymmetric particles
are derived. A scan of a minimal GMSB parameter space is performed and lower limits are set for the next-to-lightest supersymmetric
particle (NLSP) mass at 54 GeV/c2 and for the mass scale parameter at 10 TeV/c2, independently of the NLSP lifetime. Including the results from the neutral Higgs boson searches, a NLSP mass limit of 77
GeV/c2 is obtained and values of up to 16 TeV/c2 are excluded.
Received: 14 March 2002 / Published online: 20 September 2002 相似文献
4.
《Physics letters. [Part B]》1996,380(3-4)
During the last 1995 data acquisition period at LEP, the DELPHI experiment collected an integrated luminosity of 5.9 pb−1 at centre-of-mass energies of 130 GeV and 136 GeV. Radiative leptonic events (e, μ, τ) with high energy photons were studied and compared to Standard Model predictions. The data were used to search for charged excited leptons decaying through an electromagnetic transition. No significant signal was found. From the search for pair produced excited leptons, the limits me* > 62.5 GeV/c2, mμ* > 62.6 GeV/c2 and mτ* > 62.2 GeV/c2 at 95% confidence level were established. For single excited lepton production, upper limits on the ratio λ/ml* of the coupling of the excited charged lepton to its mass were derived. 相似文献
5.
C. Adloff et al. 《The European Physical Journal C - Particles and Fields》1999,11(3):447-471
A search for new bosons possessing couplings to lepton-quark pairs is performed in the H1 experiment at HERA using 1994 to
1997 data corresponding to an integrated luminosity of pb. First generation leptoquarks (LQs) are searched in very high neutral (NC) and charged (CC) current data samples. The measurements are compared to Standard Model (SM) expectations from
deep-inelastic scattering (DIS). A deviation in the spectrum previously observed in the 1994 to 1996 dataset at GeV remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at GeV which is not observed with the 1997 dataset alone. The NC DIS data is used to constrain the Yukawa couplings of first generation scalar and vector LQs in the Buchmüller–Rückl–Wyler effective model. Scalar LQs are excluded for masses
up to 275 GeV for a coupling of electromagnetic strength, . A sensitivity to coupling values is established for masses up to 400 GeV for any LQ type. The NC and CC DIS data are combined to constrain for arbitrary branching ratios of the LQ into eq in a generic model. For a decay branching ratio into pairs as small as 10%, LQ masses up to 260 GeV are ruled out for . LQs possessing couplings to mixed fermion generations, which could lead to signals of lepton flavor violation (LFV), are
searched in events with a high transverse momentum or . No or event candidate is found that is compatible with LQ kinematics. Constraints are set on the Yukawa coupling involving the
and lepton in a yet unexplored mass range.
Received: 2 July 1999 / Published online: 28 September 1999 相似文献
6.
The data collected by the OPAL experiment at GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general
models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated
luminosity of approximately 54 pb. None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background.
This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs
boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard
Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, GeV and GeV are obtained for , no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range is excluded for minimal scalar top mixing and GeV. More general scans of the MSSM parameter space are also considered.
Received: 27 October 1998 / Published online: 19 February 1999 相似文献
7.
G. Abbiendi et al. 《The European Physical Journal C - Particles and Fields》1999,11(4):619-642
Searches for pair-produced charginos and neutralinos with R-parity violating decays have been performed using a data sample corresponding to an integrated luminosity of 56 pb collected with the OPAL detector at LEP at a centre-of-mass energy of 183 GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle becomes unstable. The searches have been performed under the
assumptions that the lightest supersymmetric particle promptly decays and that only one R-parity violating coupling is dominant for each of the decay modes considered. Such processes would yield multiple leptons,
jets plus leptons, or multiple jets with or without significant missing energy in the final state. No excess of such events
above Standard Model backgrounds has been observed. Limits are presented on the production cross-sections of gauginos in R-parity violating scenarios. Limits are also presented in the framework of the Minimal Supersymmetric Standard Model.
Received: 14 December 1998 / Published online: 14 October 1999 相似文献
8.
C. Adloff et al. 《The European Physical Journal C - Particles and Fields》2001,19(2):269-288
The inclusive single and double differential cross sections for neutral and charged current processes are measured with the H1 detector
at HERA, in the range of four-momentum transfer squared between 150 and 30 000 GeV, and Bjorken x between 0.002 and 0.65. The data were taken in 1998 and 1999 with a centre-of-mass energy of 320 GeV and correspond to an
integrated luminosity of 16.4 pb. The data are compared with recent measurements of the inclusive neutral and charged current cross sections. For clear evidence is observed for an asymmetry between and neutral current scattering and the generalised structure function is extracted for the first time at HERA. A fit to the charged current data is used to extract a value for the W boson propagator mass. The data are found to be in good agreement with Standard Model predictions.
Received: 21 December 2000 / Published online: 15 March 2001 相似文献
9.
J. Breitweg et al. 《The European Physical Journal C - Particles and Fields》2000,16(2):253-267
The -jet invariant mass spectrum produced in the reaction has been studied at a center-of-mass energy of 300 GeV. The data were collected using the ZEUS detector operating at the
HERA collider, and correspond to an integrated luminosity of pb. The observed mass spectrum is in good agreement with Standard Model expectations up to an -jet mass of GeV. Above this mass, some excess is seen. The angular distribution of these events is typical of high- neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state. Limits
are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark
or R-parity-violating squark production. Specific leptoquark types are ruled out at 95% confidence level for coupling strength
for masses between 150 and 280 GeV.
Received: 15 February 2000 / Published online: 18 May 2000 相似文献
10.
A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample
of 237.4 pb at centre-of-mass 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 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 , where is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for GeV and right-handed staus with masses below 81.0 GeV for GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for GeV, within the framework of the Minimal Supersymmetric Standard Model assuming GeV and . Charged Higgs bosons, H, are excluded at 95% CL for masses below 82.8 GeV, assuming a 100% branching ratio for the decay .
Received: 6 September 1999 / Published online: 6 April 2000 相似文献
11.
C. Adloff et al. 《The European Physical Journal C - Particles and Fields》2000,17(4):567-581
A search for excited fermions f
of the first generation in scattering at the collider HERA is presented using H1 data with an integrated luminosity of 37 pb. All electroweak decays of excited fermions, f
f
f
W,f
Z are considered and all possible final states resulting from the Z orW hadronic decays or decays into leptons of the first two generations are taken into account. No evidence for f
production is found. Mass dependent exclusion limits on cross-sections and on the ratio of coupling constants to the compositeness
scale are derived.
Received: 19 July 2000 / Published online: 23 October 2000 相似文献
12.
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
13.
Heavy Majorana neutrinos (N), predicted in various extensions of the standard model, are examined with respect to the present limits on their masses and mixings with ordinary leptons resulting in explicit examples of allowed values of interest for present and planned accelerator energies. The decayN→Zv is added to the previously available formalism and all dominating branching ratios are calculated. The production of Majorana neutrinos through charged current interactions in theep colliders HERA and LEP ⊕ LHC is investigated using Monte Carlo event simulation. Signals in terms of isolated leptons and jets are found and shown to be effective in suppressing the dominating standard model backgrounds. The discovery limits of such Majorana neutrinos are, for a mixing of 1%, about 160 GeV at HERA and 700 GeV at LEP ⊕ LHC. 相似文献
14.
《Zeitschrift fur Physik C Particles and Fields》1997,73(4):613-628
Using the ZEUS detector at the HERA electronproton collider, we have searched for lepton flavor violation in ep collisions at a center—of—mass energy ($sqrt S$) of 300 GeV. Events of the type e + p → ? + X with a final—state lepton of high transverse momentum, ? = μ or τ, were sought. No evidence was found for lepton flavor violation in the combined 1993 and 1994 data samples, for which the integrated luminosities were 0.84 pb{?1}for e{~p collisions and 2.94 pb{?1} for e+p collisions. Limits on coupling vs. mass are provided for leptoquarks and R—parity violating squarks. For flavor violating couplings of electromagnetic strength, we set 95% confidence level lower limits on leptoquark masses between 207 GeV and 272 GeV, depending on the leptoquark species and final—state lepton. For leptoquark masses larger than 300 GeV, limits on flavor—changing couplings are determined, many of which supersede prior limits from rare decay processes. 相似文献
15.
Yu.B. Bushnin A.F. Dunaytzev S.V. Golovkin V.P. Kubarovsky L.G. Landsberg V.M. Leontyev T.I. Petrunina V.A. Senko A.M. Zaytzev 《Nuclear Physics B》1973,58(2):476-493
Negative results of a search for heavy quasistable charged leptons at the IHEP accelerator are reported. For the selection of heavy leptons a system of scintillation and gas ?erenkov counters was used. The upper limit estimations of the differential cross sections for heavy lepton production in pN collisions at Ep = 70 GeV are, for example, d2σ/dpω (p = 30 GeV/c, θ = 2 mrad, 1 ? Mλ ? 4.8 GeV) = 4.10?38 cm2/sr · GeV. The results of the present work, together with the data on muon pair production in nucleon-nucleon interactions, show that there are no heavy charged leptons with masses from 0.55 GeV (τλ > 7 · 10?10 sec) up to 4.5 GeV (τλ > 3 · 10?8 sec). 相似文献
16.
C. Adloff et al. 《The European Physical Journal C - Particles and Fields》2001,20(4):639-657
A search for squarks in R-parity violating supersymmetry is performed in collisions at HERA at a centre of mass energy of 300 GeV, using H1 data corresponding to an integrated luminosity of 37 pb. The direct production of single squarks of any generation in positron-quark fusion via a Yukawa coupling is considered, taking into account R-parity violating and conserving decays of the squarks. No significant deviation from the Standard Model expectation is found.
The results are interpreted in terms of constraints within the Minimal Supersymmetric Standard Model (MSSM), the constrained
MSSM and the minimal Supergravity model, and their sensitivity to the model parameters is studied in detail. For a Yukawa
coupling of electromagnetic strength, squark masses below 260 GeV are excluded at 95% confidence level in a large part of
the parameter space. For a 100 times smaller coupling strength masses up to 182 GeV are excluded.
Received: 28 February 2001 / Published online: 29 June 2001 相似文献
17.
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
18.
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
19.
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 at a centre-of-mass energy of 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.
Received: 16 March 1999 / Published online: 14 October 1999 相似文献
20.
Abulencia A Adelman J Affolder T Akimoto T Albrow MG Ambrose D Amerio S Amidei D Anastassov A Anikeev K Annovi A Antos J Aoki M Apollinari G Arguin JF Arisawa T Artikov A Ashmanskas W Attal A Azfar F Azzi-Bacchetta P Azzurri P Bacchetta N Badgett W Barbaro-Galtieri A Barnes VE Barnett BA Baroiant S Bartsch V Bauer G Bedeschi F Behari S Belforte S Bellettini G Bellinger J Belloni A Benjamin D Beretvas A Beringer J Berry T Bhatti A Binkley M Bisello D Blair RE Blocker C Blumenfeld B Bocci A 《Physical review letters》2007,98(13):131804
We report a search for the anomalous production of events with multiple charged leptons in pp[over] collisions at sqrt[s]=1.96 TeV using a data sample corresponding to an integrated luminosity of 346 pb(-1) collected by the CDF II detector at the Fermilab Tevatron. The search is divided into three-lepton and four-or-more-lepton data samples. We observe six events in the three-lepton sample and zero events in the > or =4-lepton sample. Both numbers of events are consistent with standard model background expectations. Within the framework of an R-parity-violating supergravity model, the results are interpreted as mass limits on the lightest neutralino (chi[over](1)(0)) and chargino (chi[over](1+/-) particles. For one particular choice of model parameters, the limits are M(chi[over](1)(0)>110 GeV/c2 and M(chi[over](1+/-)>203 GeV/c2 at 95% confidence level; the variation of these mass limits with model parameters is presented. 相似文献