Do interactions increase or reduce the conductance of disordered electrons? It depends!

We investigate the influence of electron-electron interactions on the conductance of two-dimensional disordered spinless electrons. By using an efficient numerical method which is based on exact diagonalization in a truncated basis of Hartree-Fock states we are able to determine the exact low-energy properties of comparatively large systems in the diffusive as well as in the localized regimes. We find that weak interactions increase the d.c. conductance in the localized regime while they decrease the d.c. conductance in the diffusive regime. Strong interactions always decrease the conductance. We also study the localization of single-particle excitations close to the Fermi energy which turns out to be only weakly influenced by the interactions.Comment: final version as publsihed, 4 pages REVTEX, 6 EPS figures include

Interacting particles at a metal-insulator transition

We study the influence of many-particle interaction in a system which, in the single particle case, exhibits a metal-insulator transition induced by a finite amount of onsite pontential fluctuations. Thereby, we consider the problem of interacting particles in the one-dimensional quasiperiodic Aubry-Andre chain. We employ the density-matrix renormalization scheme to investigate the finite particle density situation. In the case of incommensurate densities, the expected transition from the single-particle analysis is reproduced. Generally speaking, interaction does not alter the incommensurate transition. For commensurate densities, we map out the entire phase diagram and find that the transition into a metallic state occurs for attractive interactions and infinite small fluctuations -- in contrast to the case of incommensurate densities. Our results for commensurate densities also show agreement with a recent analytic renormalization group approach.Comment: 8 pages, 8 figures The original paper was splitted and rewritten. This is the published version of the DMRG part of the original pape

Search for New Physics with a Monojet and Missing Transverse Energy in pp Collisions at √s= 7 TeV

A study of events with missing transverse energy and an energetic jet is performed using pp collision data at a center-of-mass energy of 7 TeV. The data were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 36 pb-1. An excess of these events over standard model contributions is a signature of new physics such as large extra dimensions and unparticles. The number of observed events is in good agreement with the prediction of the standard model, and significant extension of the current limits on parameters of new physics benchmark models is achieved

Search for New Physics with a Monojet and Missing Transverse Energy in pp Collisions at √s= 7 TeV

A study of events with missing transverse energy and an energetic jet is performed using pp collision data at a center-of-mass energy of 7 TeV. The data were collected by the CMS detector at the LHC, and correspond to an integrated luminosity of 36 pb-1. An excess of these events over standard model contributions is a signature of new physics such as large extra dimensions and unparticles. The number of observed events is in good agreement with the prediction of the standard model, and significant extension of the current limits on parameters of new physics benchmark models is achieved

Measurement of the t-Channel Single Top Quark Production Cross Section in pp̅ Collisions at √s= 7 TeV

Electroweak production of the top quark is measured for the first time in pp collisions at √s= 7 TeV, using a data set collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 36 pb-1. With an event selection optimized for t-channel production, two complementary analyses are performed. The first one exploits the special angular properties of the signal, together with background estimates from the data. The second approach uses a multivariate analysis technique to probe the compatibility with signal topology expected from electroweak top-quark production. The combined measurement of the cross section is 83.6±29.8 (stat + syst) ± 3.3 (lumi) pb, consistent with the standard model expectation

Search for charge-asymmetric production of W\u27 bosons in tt + jet events from pp collisions at √s = 7 TeV

A search is presented for charge-asymmetric production of a W\u27 boson that has been proposed to accommodate the forward–backward asymmetry observed in the production of top–antitop quark pairs at the Tevatron. The new heavy W\u27 boson would be produced in association with a top quark and would decay into top and down quarks. The data correspond to an integrated luminosity of 5.0 fb−1 in pp collisions at a center-of-mass energy of 7 TeV, recorded by the CMS detector at the LHC. No significant excess above the standard model expectations is observed, and, from a combination of the electron-plusjets and muon-plus-jets channels, a 95% confidence level lower limit of 840 GeV/c2 is set on the W\u27 boson mass for a W\u27 boson model with values for coupling constants to top and down quarks gL = 0 and gR = 2. In addition, a kinematic reconstruction of the W[1] resonance mass using the inherent charge asymmetry of this model finds no indication of the presence of W\u27 events in the data

Measurement of the B0s Production Cross Section with B0s → J/ Decays in pp Collisions at √s = 7 TeV

The B0s differential production cross section is measured as functions of the transverse momentum and rapidity in pp collisions at √s= 7 TeV, using the B0s → J/UO decay, and compared with predictions based on perturbative QCD calculations at next-to-leading order. The data sample, collected by the CMS experiment at the LHC, corresponds to an integrated luminosity of 40 pb-1. The B0s is reconstructed from the decays J/U → µ+µ- and O→ K+K- The integrated B0s cross section times B0s→ J/UO branching fraction in the range 8BT\u3c 50 GeV/c and │yB \u3c 2.4 is measured to be 6.9 ± 0.6 ± 0.6 nb, where the first uncertainty is statistical and the second is systematic

Study of W boson production in PbPb and pp collisions at √sNN = 2.76 TeV

A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon–nucleon (NN) centre-of-mass energy √sNN of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 μb−1, the number of W → μνμ decays is extracted in the region of muon pseudorapidity |ημ| \u3c 2.1 and transverse momentum pμT \u3e 25 GeV/c. Yields of muons found per unit of pseudorapidity correspond to (159 ± 10(stat.) ± 12(syst.)) × 10−8 W+ and (154 ± 10(stat.) ± 12(syst.)) × 10−8 W− bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at √s = 2.76 TeV corresponding to an integrated luminosity of 231 nb−1. The individual W+ and W− yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations

Search for physics beyond the standard model in events with a Z boson, jets, and missing transverse energy in pp collisions at √s = 7 TeV

A search is presented for physics beyond the standard model (BSM) in events with a Z boson, jets, and missing transverse energy (EmissT ). This signature is motivated by BSM physics scenarios, including supersymmetry. The study is performed using a sample of proton–proton collision data collected at √s = 7 TeV with the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb−1. The contributions from the dominant standard model backgrounds are estimated from data using two complementary strategies, the jet-Z balance technique and a method based on modeling EmissT with data control samples. In the absence of evidence for BSM physics, we set limits on the non-standard-model contributions to event yields in the signal regions and interpret the results in the context of simplified model spectra. Additional information is provided to facilitate tests of other BSM physics models

Search for the Standard Model Higgs Boson in the Decay Channel H → ZZ → 4l in pp Collisions at √s = 7 TeV

A search for a Higgs boson in the four-lepton decay channel H→ZZ, with each Z boson decaying to an electron or muon pair, is reported. The search covers Higgs boson mass hypotheses in the range of 110 \u3c mH \u3c 600 GeV. The analysis uses data corresponding to an integrated luminosity of 4.7 fb-1 recorded by the CMS detector in pp collisions at √s = 7 TeV from the LHC. Seventy- two events are observed with four-lepton invariant mass m4ℓ \u3e 100 GeV (with 13 below 160 GeV), while 67.1 ± 6.0 (9.5 ± 1.3) events are expected from background. The four-lepton mass distribution is consistent with the expectation of standard model background production of ZZ pairs. Upper limits at 95% confidence level exclude the standard model Higgs boson in the ranges of 134–158 GeV, 180–305 GeV, and 340–465 GeV. Small excesses of events are observed around masses of 119, 126, and 320 GeV, making the observed limits weaker than expected in the absence of a signal