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排序方式: 共有451条查询结果,搜索用时 31 毫秒
51.
We prove that the ultimate fate of a bubble of negative energy density which forms in a metastable universe of zero energy density is gravitational collapse. We improve on previous treatments in that we allow for departures from O(3, 1) symmetry in the initial state, so long as they are not too great. 相似文献
52.
The dynamics of neural networks is influenced strongly by the spectrum of eigenvalues of the matrix describing their synaptic connectivity. In large networks, elements of the synaptic connectivity matrix can be chosen randomly from appropriate distributions, making results from random matrix theory highly relevant. Unfortunately, classic results on the eigenvalue spectra of random matrices do not apply to synaptic connectivity matrices because of the constraint that individual neurons are either excitatory or inhibitory. Therefore, we compute eigenvalue spectra of large random matrices with excitatory and inhibitory columns drawn from distributions with different means and equal or different variances. 相似文献
53.
Abazov VM Abbott B Abolins M Acharya BS Adams M Adams T Agelou M Ahn SH Ahsan M Alexeev GD Alkhazov G Alton A Alverson G Alves GA Anastasoaie M Andeen T Anderson S Andrieu B Anzelc MS Arnoud Y Arov M Askew A Asman B Jesus AC Atramentov O Autermann C Avila C Ay C Badaud F Baden A Bagby L Baldin B Bandurin DV Banerjee P Banerjee S Barberis E Bargassa P Baringer P Barnes C Barreto J Bartlett JF Bassler U Bauer D Bean A Begalli M Begel M Belanger-Champagne C Bellantoni L Bellavance A Benitez JA 《Physical review letters》2006,97(16):161802
We present a search for a neutral particle, pair produced in pp[over] collisions at sqrt[s]=1.96 TeV, which decays into two muons and lives long enough to travel at least 5 cm before decaying. The analysis uses approximately 380 pb(-1) of data recorded with the D0 detector. The background is estimated to be about one event. No candidates are observed, and limits are set on the pair-production cross section times branching fraction into dimuons + X for such particles. For a mass of 10 GeV and lifetime of 4x10(-11) s, we exclude values greater than 0.14 pb (95% C.L.). These results are used to limit the interpretation of NuTeV's excess of dimuon events. 相似文献
54.
DeWitt KM Valadez L Abbott HL Kolasinski KW Harrison I 《The journal of physical chemistry. B》2006,110(13):6714-6720
The dissociative sticking coefficient for C2H6 on Pt(111) has been measured as a function of both gas temperature (Tg) and surface temperature (Ts) using effusive molecular beam and angle-integrated ambient gas dosing methods. A microcanonical unimolecular rate theory (MURT) model of the reactive system is used to extract transition state properties from the data as well as to compare our data directly with supersonic molecular beam and thermal equilibrium sticking measurements. We report for the first time the threshold energy for dissociation, E0 = 26.5 +/- 3 kJ mol(-1). This value is only weakly dependent on the other two parameters of the model. A strong surface temperature dependence in the initial sticking coefficient is observed; however, the relatively weak dependence on gas temperature indicates some combination of the following (i) not all molecular excitations are contributing equally to the enhancement of sticking, (ii) that strong entropic effects in the dissociative transition state are leading to unusually high vibrational frequencies in the transition state, and (iii) energy transfer from gas-phase rovibrational modes to the surface is surprisingly efficient. In other words, it appears that vibrational mode-specific behavior and/or molecular rotations may play stronger roles in the dissociative adsorption of C2H6 than they do for CH4. The MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction, that is, it allows us to predict the dissociative sticking coefficient of C2H6 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another. 相似文献
55.
Application of ionic liquids to the electrodeposition of metals 总被引:3,自引:0,他引:3
The electrodeposition of most of technologically important metals has been shown to be possible from a wide range of room temperature molten salts, more commonly known today as 'ionic liquids'. These liquids are currently under intense scrutiny for a wide variety of applications some of which have already been commercialized. Despite the fact that electrodeposition was the first application studied in these liquids no metal deposition processes have as yet been developed to an industrial scale. This review addresses the practical and theoretical aspects that need to be considered when choosing ionic liquids for metal deposition. It details the current understanding of the physical and chemical properties of these interesting fluids and highlights the areas that need to be considered to develop practical electroplating systems. The effect of composition and temperature on viscosity and conductivity are discussed together with the fundamental approaches required to synthesise new liquids. 相似文献
56.
Abazov VM Abbott B Abolins M Acharya BS Adams M Adams T Alexeev GD Alkhazov G Alton A Alverson G Alves GA Ancu LS Aoki M Arnoud Y Arov M Askew A Asman B Atramentov O Avila C BackusMayes J Badaud F Bagby L Baldin B Bandurin DV Banerjee S Barberis E Baringer P Barreto J Bartlett JF Bassler U Beale S Bean A Begalli M Begel M Belanger-Champagne C Bellantoni L Benitez JA Beri SB Bernardi G Bernhard R Bertram I Besançon M Beuselinck R Bezzubov VA Bhat PC Bhatnagar V Blazey G Blessing S Bloom K 《Physical review letters》2010,105(25):251801
We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb(-1) of pp collisions, collected with the D0 detector at the Fermilab Tevatron at sqrt[s] =1 .96 TeV. Selected events contain one reconstructed Z → e+ e- or Z → μ+ μ- candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M(H) = 115 GeV is a factor of 5.9 larger than the standard model prediction. 相似文献
57.
Aad G Abbott B Abdallah J Abdelalim AA Abdesselam A Abdinov O Abi B Abolins M Abramowicz H Abreu H Acerbi E Acharya BS Ackers M Adams DL Addy TN Adelman J Aderholz M Adomeit S Adragna P Adye T Aefsky S Aguilar-Saavedra JA Aharrouche M Ahlen SP Ahles F Ahmad A Ahsan M Aielli G Akdogan T Akesson TP Akimoto G Akimov AV Alam MS Alam MA Albrand S Aleksa M Aleksandrov IN Aleppo M Alessandria F Alexa C Alexander G Alexandre G Alexopoulos T Alhroob M Aliev M Alimonti G Alison J Aliyev M Allport PP 《Physical review letters》2010,105(25):252303
By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium. 相似文献
58.
Mark T. Sims Laurence C. Abbott Robert M. Richardson John W. Goodby 《Liquid crystals》2019,46(1):11-24
An assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques. 相似文献
59.
Abazov VM Abbott B Abolins M Acharya BS Adams M Adams T Aguilo E Ahsan M Alexeev GD Alkhazov G Alton A Alverson G Alves GA Anastasoaie M Ancu LS Andeen T Andrieu B Anzelc MS Aoki M Arnoud Y Arov M Arthaud M Askew A Asman B Assis Jesus AC Atramentov O Avila C Badaud F Bagby L Baldin B Bandurin DV Banerjee P Banerjee S Barberis E Barfuss AF Bargassa P Baringer P Barreto J Bartlett JF Bassler U Bauer D Beale S Bean A Begalli M Begel M Belanger-Champagne C Bellantoni L Bellavance A Benitez JA 《Physical review letters》2008,101(11):111802
In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, such as hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark. 相似文献
60.
Abazov VM Abbott B Abolins M Acharya BS Adams M Adams T Aguilo E Ahn SH Ahsan M Alexeev GD Alkhazov G Alton A Alverson G Alves GA Anastasoaie M Ancu LS Andeen T Anderson S Andrieu B Anzelc MS Arnoud Y Arov M Arthaud M Askew A Asman B Assis Jesus AC Atramentov O Autermann C Avila C Ay C Badaud F Baden A Bagby L Baldin B Bandurin DV Banerjee S Banerjee P Barberis E Barfuss AF Bargassa P Baringer P Barreto J Bartlett JF Bassler U Bauer D Beale S Bean A Begalli M Begel M Belanger-Champagne C 《Physical review letters》2008,100(19):192003
We present the first simultaneous measurement of the ratio of branching fractions, R=B(t --> Wb)/B(t --> Wq), with q being a d, s, or b quark, and the top-quark pair production cross section sigma(tt[over]) in the lepton plus jets channel using 0.9 fb(-1) of pp[over] collision data at sqrt(s)=1.96 TeV collected with the D0 detector. We extract R and sigma(tt[over]) by analyzing samples of events with 0, 1, and > or =2 identified b jets. We measure R=0.97(+0.09)/(-0.08)(stat+syst) and sigma(tt[over])=8.18(+0.09)(-0.84)(stat+syst) +/- 0.50(lumi) pb, in agreement with the standard model prediction. 相似文献