Search

We extend to the periodic Anderson model (PAM) the diagrammatic expansion in cumulants that was employed by Hubbard to study his model of a narrow band of strongly correlated electrons. The PAM is a lattice of localized and strongly correlated electrons with spin one-half and without orbital degeneracy, hybridized with a wide band of uncorrelated conduction electrons. We have extended the model by considering localized electronic states with an arbitrary scheme of energy levels: this extension would be useful to study intermediate valence compounds of Eu or Tm with the present formalism. We give the rules for the diagrammatic calculation of the grand canonical potential and of the Green’s functions for the general model: only connected diagrams appear in those calculations and the lattice sums are unrestricted. To generate the cumulant averages it was necessary to employ external fields ξ that are Grassman variables. We have found a simple way to extend the diagrammatic rules to the ξ≠0 case. The absence of excluded site restrictions, that leads to complicated excluded volume problems in other treatments, and the existence of linked cluster expansions, are features of the cumulant expansion. As an application of the present method, we have calculated the occupation numbers of localized and conduction electrons for the PAM in the limit of infinite Coulomb repulsion (U→∞).

This paper reports the multiple band gaps in the two-dimensional semiconductor-dielectric photonic crystals of several compositions: semiconductor cylinders in the dielectric background. We consider both square lattice and hexagonal lattice arrangements and compute extensive band structures using a plane-wave method within the framework of an efficient standard eigenvalue problem for both $E$ and $H$ polarizations. The whole range of filling fraction has been explored to claim the existence of the lowest (the so-called acoustic) band gap and multiple higher-energy band gaps within the first 30 to 40 bands for various compositions. Such semiconductor-dielectric photonic crystals which are shown to possess huge lowest band gaps below a threshold frequency (the plasma frequency ${\omega }_p$) have an advantage over the dielectric photonic crystals in the emerging technology based on the photonic crystals.

The invariant differential cross section for inclusive electron production in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range $0.4\le p_T\le 5.0\mathrm{GeV}/c$ in the central rapidity region ($|\eta |\le 0.35$). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, i.e., charm quarks or, at high $p_T$, bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy-flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined to be ${\sigma }_{c\overline{c}}=0.92\pm 0.15(\mathrm{stat})\pm 0.54(\mathrm{syst})\mathrm{mb}$.

Experimental and theoretical studies of the energy diagram of magneto-optical transitions observed in both luminescence and luminescence excitation spectra of an n-type modulation-doped quantum well are presented. The transitions related to the lowest populated electronic subband are well understood in terms of interband transitions with many-body corrections included within the density-functional approach. The excited subbands are differently renormalized and show a two-particle excitonic correlation.

The Raman line shape of GaP has been measured under hydrostatic pressure up to 106 kbar at room temperature. The results are interpreted with a model involving a "Fano-type" resonance between the single TO ($\Gamma$) phonon mode and several double-phonon densities of states. This model allow us to determine for the first time the sign of the anharmonic interaction. In contradiction to previous interpretations, no anomalous behavior of the parameters used in this model is found as a function of the pressure.

We report on the first measurement in heavy-ion collisions of hard photons of extremely high energies that extend the photon spectrum to 5 times the beam energy per nucleon. The photon spectrum was measured for the systems ${}^{86}\mathrm{Kr}{+}^{\mathrm{nat}}\mathrm{Ni}$ at $60A\mathrm{MeV}$ and ${}^{181}\mathrm{Ta}{+}^{197}\mathrm{Au}$ at $40A$ MeV. The data are interpreted using the Dubna cascade model. The radiative channel of pion-nucleon interaction $\pi +N\to N+\gamma$, involving subthreshold pions produced in the nuclear medium, dominates all other processes considered for the production of the very energetic photons. This channel was never considered so far at beam energies below the pion threshold.

Photoluminescence experiments on various selectively silicon ‘‘δ-doped’’ GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As multiple quantum wells, grown by molecular-beam epitaxy, have been performed. For comparison, photoluminescence of a similar undoped structure and of silicon ‘‘δ-doped’’ bulk GaAs were studied. The assignment of the luminescence structures was obtained through a careful analysis of the temperature, excitation power, and magnetic-field dependence of the spectra. As expected, although most of the silicon atoms behave as donors, acceptor states related to this impurity have been detected. Due to the small doping level of the samples, it turns out that the amount of Si acceptor states is comparable to that of the residual carbon acceptor states, allowing a direct comparison between the related densities. It is found that some of the silicon dopants are located where they were intended to be placed during the doping procedure. However, a significant number of Si impurities are found to be shifted in the direction of growth with respect to their intended locations, in agreement with magnetotransmission measurements performed on the same samples. These investigations provide interesting information about the self-compensation and segregation of impurities during the molecular-beam epitaxy of the samples.

The electron-bound hole luminescence of a GaAs-based heterojunction has been investigated as a function of magnetic fields at low temperatures. The study of the intensities originating from different Landau levels allows one to obtain precious information about the relaxation rates within these levels and provides values far above those predicted by standard theories. A new mechanism involving impurity-assisted phonon emission is proposed which gives good agreement with the experimental results.

Results are reported of a search for a deviation in the jet production cross section from the prediction of perturbative quantum chromodynamics at next-to-leading order. The search is conducted using a 7 TeV proton-proton data sample corresponding to an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$, collected with the Compact Muon Solenoid detector at the Large Hadron Collider. A deviation could arise from interactions characterized by a mass scale $\Lambda$ too high to be probed directly at the LHC. Such phenomena can be modeled as contact interactions. No evidence of a deviation is found. Using the ${\mathrm{CL}}_{\mathrm{s}}$ criterion, lower limits are set on $\Lambda$ of 9.9 TeV and 14.3 TeV at 95% confidence level for models with destructive and constructive interference, respectively. Limits obtained with a Bayesian method are also reported.

The first search at the LHC for the extinction of QCD jet production is presented, using data collected with the CMS detector corresponding to an integrated luminosity of $10.7{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. The extinction model studied in this analysis is motivated by the search for signatures of strong gravity at the TeV scale (terascale gravity) and assumes the existence of string couplings in the strong-coupling limit. In this limit, the string model predicts the suppression of all high-transverse-momentum standard model processes, including jet production, beyond a certain energy scale. To test this prediction, the measured transverse-momentum spectrum is compared to the theoretical prediction of the standard model. No significant deficit of events is found at high transverse momentum. A 95% confidence level lower limit of 3.3 TeV is set on the extinction mass scale.

Results are reported from a search for new particles that decay into a photon and two gluons, in events with jets. Novel jet substructure techniques are developed that allow photons to be identified in an environment densely populated with hadrons. The analyzed proton-proton collision data were collected by the CMS experiment at the LHC, in 2016 at $\sqrt{s}=13\mathrm{TeV}$, and correspond to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The spectra of total transverse hadronic energy of candidate events are examined for deviations from the standard model predictions. No statistically significant excess is observed over the expected background. The first cross section limits on new physics processes resulting in such events are set. The results are interpreted as upper limits on the rate of gluino pair production, utilizing a simplified stealth supersymmetry model. The excluded gluino masses extend up to 1.7 TeV, for a neutralino mass of 200 GeV and exceed previous mass constraints set by analyses targeting events with isolated photons.

Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single lepton, multiple jets, including at least one $b$-tagged jet, and large missing transverse momentum. The search uses a sample of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. The observed event yields in the signal regions are consistent with those expected from standard model backgrounds. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production, with gluino decay into either on- or off-mass-shell top squarks. Assuming that the top squarks decay into a top quark plus a stable, weakly interacting neutralino, scenarios with gluino masses up to about 1.9 TeV are excluded at 95% confidence level for neutralino masses up to about 1 TeV.

When an intense radio-frequency field is applied to a suspension of dielectric particles, the suspended phase condenses under the effect of dipolar interactions between particles. We study the interaction energy when losses are present and address the problem of the formation of linear structures, using a Monte Carlo simulation. We find features of a cooperative phenomenon, with a threshold of field intensity. The dependence of the process with concentration, field intensity, and frequency is studied.

The per-event yield of the highest transverse momentum charged particle and charged-particle jet, integrated above a given $p_{\mathrm{T}}^{\min }$ threshold starting at $p_{\mathrm{T}}^{\min }=0.8$ and 1 GeV, respectively, is studied in $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$. The particles and the jets are measured in the pseudorapidity ranges $|\eta |<2.4$ and 1.9, respectively. The data are sensitive to the momentum scale at which parton densities saturate in the proton, to multiple partonic interactions, and to other key aspects of the transition between the soft and hard QCD regimes in hadronic collisions.

A search for new long-lived particles decaying to leptons is presented using proton-proton collisions produced by the LHC at $\sqrt{s}=8\mathrm{TeV}$. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$. Events are selected with an electron and muon with opposite charges that both have transverse impact parameter values between 0.02 and 2 cm. The search has been designed to be sensitive to a wide range of models with nonprompt $e\text{−}\mu$ final states. Limits are set on the “displaced supersymmetry” model, with pair production of top squarks decaying into an $e\text{−}\mu$ final state via $R$-parity-violating interactions. The results are the most restrictive to date on this model, with the most stringent limit being obtained for a top squark lifetime corresponding to $c\tau =2\mathrm{cm}$, excluding masses below 790 GeV at 95% confidence level.

Searches for supersymmetry (SUSY) are presented based on the electroweak pair production of neutralinos and charginos, leading to decay channels with Higgs, $Z$, and $W$ bosons and undetected lightest SUSY particles (LSPs). The data sample corresponds to an integrated luminosity of about $19.5{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV collected in 2012 with the CMS detector at the LHC. The main emphasis is neutralino pair production in which each neutralino decays either to a Higgs boson ($h$) and an LSP or to a $Z$ boson and an LSP, leading to $hh$, $hZ$, and $ZZ$ states with missing transverse energy ($E_{\mathrm{T}}^{\text{miss}}$). A second aspect is chargino-neutralino pair production, leading to $hW$ states with $E_{\mathrm{T}}^{\text{miss}}$. The decays of a Higgs boson to a bottom-quark pair, to a photon pair, and to final states with leptons are considered in conjunction with hadronic and leptonic decay modes of the $Z$ and $W$ bosons. No evidence is found for supersymmetric particles, and 95% confidence level upper limits are evaluated for the respective pair production cross sections and for neutralino and chargino mass values.

The results of searches for supersymmetry by the CMS experiment are interpreted in the framework of simplified models. The results are based on data corresponding to an integrated luminosity of 4.73 to $4.98{\mathrm{fb}}^{-1}$. The data were collected at the LHC in proton–proton collisions at a center-of-mass energy of 7 TeV. This paper describes the method of interpretation and provides upper limits on the product of the production cross section and branching fraction as a function of new particle masses for a number of simplified models. These limits and the corresponding experimental acceptance calculations can be used to constrain other theoretical models and to compare different supersymmetry-inspired analyses.

Nuclear magnetic resonance (NMR) and transport measurements have been performed at high magnetic fields and low temperatures in a series of $n$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals. In low-density samples, a complete spin polarization of the electronic system is achieved, as observed from the saturation of the isotropic component of the ${}^{209}\mathrm{Bi}$ NMR shift above a certain magnetic field. The corresponding spin splitting, defined in the phenomenological approach of a 3D electron gas with a large (spin-orbit-induced) effective $g$ factor, scales as expected with the Fermi energy independently determined by simultaneous transport measurements. Both the effective electronic $g$ factor and the “contact” hyperfine coupling constant are precisely determined. The magnitude of this latter reveals a nonnegligible $s$ character of the electronic wave function at the bottom of the conduction band. Our results show that the bulk electronic spin polarization can be directly probed via NMR and pave the way for future NMR investigations of the electronic states in Bi-based topological insulators.

The first search for the $Z$ boson decay to $\tau \tau \mu \mu$ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the $Z\to \tau \tau \mu \mu$ to $Z\to 4\mu$ branching fractions. Limits are also placed on the six flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators.