Search for long-lived particles decaying into electron or photon pairs with the D0 detector

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.     

Superconducting energy gap and c-axis plasma frequency of (Nd,Sm)FeAsO0.82F0.18 superconductors from infrared ellipsometry

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor RFeAsO0.82F0.18 (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar to the cuprate high-temperature superconductors. We deduce an upper limit of the c-axis superconducting plasma frequency of omega pl,c(SC)< or =260 cm(-1) corresponding to a lower limit of the c-axis magnetic penetration depth of lambda c > or =6 microm and lambda c/lambda ab > or =30 as compared to lambda ab=185 nm from muon spin rotation [A. Drew, arXiv:0805.1042 [Phys. Rev. Lett. (to be published)]]. We also observe a gaplike suppression of the conductivity in the superconducting state with a shoulderlike feature at omegaSC* approximately 300 cm(-1) and spectral shape which is consistent with an unconventional order parameter with 2Delta approximately omegaSC* approximately 37 meV.     

Observation of the Bc meson in the exclusive decay Bc-->J/psipi

A fully reconstructed Bc-->J/psipi signal is observed with the D0 detector at the Fermilab Tevatron pp[over] collider using 1.3 fb(-1) of integrated luminosity. The signal consists of 54+/-12 candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the Bc meson is 6300+/-14(stat)+/-5(syst) MeV/c2.     

Measurement of the forward-backward charge asymmetry in top-quark pair production

We present the first measurement of the integrated forward-backward charge asymmetry in top-quark-top-antiquark pair (tt) production in proton-antiproton (pp) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming tt + X production and decay, a sample of 0.9 fb(-1) of data, collected by the D0 experiment at the Fermilab Tevatron Collider, is used to measure the asymmetry for different jet multiplicities. The result is also used to set upper limits on tt+X production via a Z' resonance.     

Search for Randall-Sundrum gravitons in dilepton and diphoton final states

We report the first direct search for the Kaluza-Klein (KK) modes of Randall-Sundrum gravitons using dielectron, dimuon, and diphoton events observed with the D0 detector operating at the Fermilab Tevatron pp(-) Collider at sqrt[s]=1.96 TeV. No evidence for resonant production of gravitons has been found in the data corresponding to an integrated luminosity of approximately equal to 260 pb(-1). Lower limits on the mass of the first KK mode at the 95% C.L. have been set between 250 and 785 GeV, depending on its coupling to standard model particles.     

A broadband grating-coupled silicon nitride waveguide for the mid-IR: characterization and sensitive measurements using an external cavity quantum cascade laser

We present the design and fabrication of a single-mode slab waveguide structure for mid-infrared spectroscopy optimized for broadband coupling. The sensor uses grating couplers for robust off-axis coupling and a silicon nitride guiding layer for mechanical robustness. An external cavity quantum cascade laser-based transmission method is introduced for characterizing the structure’s broadband coupling behavior. Light from an external cavity quantum cascade laser with a spectral range of 0.5 μm around 6 μm was coupled into the waveguide without the need for moving parts. First spectra taken with this sensor are presented.     

The Munich Compact Light Source: initial performance measures

While large‐scale synchrotron sources provide a highly brilliant monochromatic X‐ray beam, these X‐ray sources are expensive in terms of installation and maintenance, and require large amounts of space due to the size of storage rings for GeV electrons. On the other hand, laboratory X‐ray tube sources can easily be implemented in laboratories or hospitals with comparatively little cost, but their performance features a lower brilliance and a polychromatic spectrum creates problems with beam hardening artifacts for imaging experiments. Over the last decade, compact synchrotron sources based on inverse Compton scattering have evolved as one of the most promising types of laboratory‐scale X‐ray sources: they provide a performance and brilliance that lie in between those of large‐scale synchrotron sources and X‐ray tube sources, with significantly reduced financial and spatial requirements. These sources produce X‐rays through the collision of relativistic electrons with infrared laser photons. In this study, an analysis of the performance, such as X‐ray flux, source size and spectra, of the first commercially sold compact light source, the Munich Compact Light Source, is presented.     

Observation and properties of the orbitally excited B_(s2)(*) meson

We report the direct observation of the excited L=1 state B_(s2)(*) in fully reconstructed decays to B+K-. The mass of the B_(s2)(*) meson is measured to be 5839.6+/-1.1(stat)+/-0.7(syst) MeV/c(2), and its production rate relative to the B+ meson is measured to be [1.15+/-0.23(stat)+/-0.13(syst)]%.     

Efficient unbiased variance reduction techniques for Monte Carlo simulations of radiative transfer in cloudy atmospheres: The solution

We present five new variance reduction techniques applicable to Monte Carlo simulations of radiative transfer in the atmosphere: detector directional importance sampling, n-tuple local estimate, prediction-based splitting and Russian roulette, and circum-solar virtual importance sampling. With this set of methods it is possible to simulate remote sensing instruments accurately and quickly. In contrast to all other known techniques used to accelerate Monte Carlo simulations in cloudy atmospheres - except for two methods limited to narrow angle lidars - the presented methods do not make any approximations, and hence do not bias the result. Nevertheless, these methods converge as quickly as any of the biasing acceleration techniques, and the probability distribution of the simulation results is almost perfectly normal. The presented variance reduction techniques have been implemented into the Monte Carlo code MYSTIC (“Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres”) in order to validate the techniques.     

Signature of a type-a glass transition and intrinsic confinement effects in a binary glass-forming system

We study dynamically highly asymmetric binary mixtures comprised of small methyl tetrahydrofuran (MTHF) molecules and polystyrene. Combined use of dielectric spectroscopy, ^{2}H nuclear magnetic resonance, incoherent quasielastic neutron scattering, and depolarized dynamic light scattering allows us to selectively probe the dynamics of the components in a broad dynamic range. It turns out that the mixtures exhibit two glass transitions in a wide concentration range although being fully miscible on a macroscopic scale. In between both glass transition temperatures, the dynamics of the small molecules show strong confinement effects, e.g., a crossover from Vogel-Fulcher to Arrhenius behavior of the time constants. Moreover, the dynamical behavior of small molecules close to the slow matrix is consistent with mode coupling theory predictions for a type-A glass transition, which was expected from recent theoretical and simulation studies in comparable systems.