We experimentally investigate electrical transport properties of graphene, which is a two dimensional (2D) conductor with relativistic energy dispersion relation. By investigating single-and bi-layer graphene devices with different aspect ratios, we confirm experimentally that the minimum conductivity in wide and short graphene strips approaches the universal value of 4e 2 /πh. At low temperatures, quantum interference of multiply-reflected waves of electrons and holes in graphene give rise to periodic conductance oscillations with bias and gate voltages. Thus graphene acts as a quantum billiard, a 2D ballistic, phase coherent electron system with long phase coherence length that exceeds 5 µm. Additional features in differential conductance emerge when graphene is coupled to superconducting electrodes. We observe proximity-induced enhanced conductance at low bias, and conductance dips at energy scales far above the superconducting gap of the electrodes. The latter provides preliminary evidence for a novel superconducting material that consists of graphene coated with metallic atoms. *To whom correspondence should be addressed. Email: lau@physics.ucr.edu Graphene, a two-dimensional (2D) a honey-comb lattice of carbon atoms, exhibits rather unusual energy dispersion relations -the low-lying electrons in single layer graphene behave like massless relativistic Dirac fermions with vanishing density of states at the Dirac point, and bilayer's band structure resembles that of a zero band gap semiconductor (Fig. 1a). Since recent experimental isolation and measurement of graphene [1][2][3], it has attracted tremendous attention, as the special band structures in single and bi-layer graphenes yield novel aspects to the physics of two-dimensional electron systems. The Dirac spectrum in graphene is predicted to give rise to a number of phenomena, such as quantum spin hall effects [4][5][6], enhanced Coulomb interaction [7][8][9][10][11], and suppression of weak localization [12,13]. Technologically, graphene is an attractive material for nanoscale electronics engineering. As a two-dimensional (2D) relative of carbon nanotubes, it manifests high mobility, extraordinary thermal conductivity and atomic perfection; but in contrast to nanotubes, traditional lithographic techniques can potentially be employed for tailoring of transport properties and device synthesis [14].In the past two years, much progress has been made on theoretical understanding of the novel electronic properties that may emerge in graphene. In contrast, experimental measurements of graphene have been relatively scarce.Here we demonstrate experimentally that single layer (SLG) and bi-layer (BLG) graphene can act as a quantum billiard, i.e. a 2D ballistic system where scattering only occur at boundaries, with a phase coherence length that exceeds 5 µm. The ballistic transport for charge carriers, coupled with phase coherent multiple-reflection at the electrodes, give rise to quantum interference of waves of the charge carriers, thus realizing a quant...

A search is presented for physics beyond the standard model (SM) using electron or muon pairs with high invariant mass. A data set of proton-proton collisions collected by the CMS experiment at the LHC at $$ \sqrt{s} $$ s = 13 TeV from 2016 to 2018 corresponding to a total integrated luminosity of up to 140 fb−1 is analyzed. No significant deviation is observed with respect to the SM background expectations. Upper limits are presented on the ratio of the product of the production cross section and the branching fraction to dileptons of a new narrow resonance to that of the Z boson. These provide the most stringent lower limits to date on the masses for various spin-1 particles, spin-2 gravitons in the Randall-Sundrum model, as well as spin-1 mediators between the SM and dark matter particles. Lower limits on the ultraviolet cutoff parameter are set both for four-fermion contact interactions and for the Arkani-Hamed, Dimopoulos, and Dvali model with large extra dimensions. Lepton flavor universality is tested at the TeV scale for the first time by comparing the dimuon and dielectron mass spectra. No significant deviation from the SM expectation of unity is observed.

Results are reported from a search for supersymmetric particles in the final state with multiple jets and large missing transverse momentum. The search uses a sample of proton-proton collisions at √ s = 13 TeV collected with the CMS detector in 2016-2018, corresponding to an integrated luminosity of 137 fb −1 , representing essentially the full LHC Run 2 data sample. The analysis is performed in a four-dimensional search region defined in terms of the number of jets, the number of tagged bottom quark jets, the scalar sum of jet transverse momenta, and the magnitude of the vector sum of jet transverse momenta. No significant excess in the event yield is observed relative to the expected background contributions from standard model processes. Limits on the pair production of gluinos and squarks are obtained in the framework of simplified models for supersymmetric particle production and decay processes. Assuming the lightest supersymmetric particle to be a neutralino, lower limits on the gluino mass as large as 2000 to 2310 GeV are obtained at 95% confidence level, while lower limits on the squark mass as large as 1190 to 1630 GeV are obtained, depending on the production scenario.

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Measurements are presented of the differential cross sections for Z bosons produced in proton-proton collisions at √ s = 13 TeV and decaying to muons and electrons. The data analyzed were collected in 2016 with the CMS detector at the LHC and correspond to an integrated luminosity of 35.9 fb −1. The measured fiducial inclusive product of cross section and branching fraction agrees with next-to-next-to-leading order quantum chromodynamics calculations. Differential cross sections of the transverse momentum p T , the optimized angular variable φ * η , and the rapidity of lepton pairs are measured. The data are corrected for detector effects and compared to theoretical predictions using fixed order, resummed, and parton shower calculations. The uncertainties of the measured normalized cross sections are smaller than 0.5% for φ * η < 0.5 and for p Z T < 50 GeV.