Search Results (160 total)

We study the consequences of limited recoil sense reconstruction on the number of events required to reject isotropy and detect a WIMP signal using a directional detector. For a constant probability of determining the sense correctly, 3-d readout and zero background, we find that as the probability is decreased from 1.0 to 0.75 the number of events required increases by a factor of a few. As the probability is decreased further the number of events increases sharply, and isotropy can be rejected more easily by discarding the sense information and using axial statistics. This however requires an order of magnitude more events than vectorial data with perfect sense determination. We also consider energy dependent probabilities of correctly measuring the sense. Our main finding is that correctly determining the sense of the abundant, but less anisotropic, low energy recoils is most important.

We investigate the finite temperature magnetothermoelectric response in the vicinity of a superfluid–Mott-insulator quantum phase transition. We focus on the particle-hole symmetric transitions of the Bose-Hubbard model, and combine Lorentz invariance arguments with quantum Boltzmann calculations. By means of an epsilon expansion, we find that a nonvanishing thermoelectric tensor and a finite thermal transport coefficient are supported in this quantum critical regime. We comment on the singular Nernst effect in this problem.

Systems near to quantum critical points show universal scaling in response to external probes. We consider whether this scaling is reflected in their out-of-equilibrium fluctuations. We study current noise in the metallic state at the z=1 quantum critical point between a superconductor and an insulator in two dimensions. Using a Boltzmann-Langevin approach within a 1/N expansion, we show that the current noise obeys a universal scaling form S_j=TΦ[T/T_eff(E)], with T_eff∝sqrt[E]. This treatment recovers Johnson noise in thermal equilibrium and S_j∝sqrt[E] at strong electric fields. The latter differs significantly from both the shot noise in conventional metals (diffusive Fermi liquids) and the free carrier result, due to strong correlations between the critical bosonic excitations. Current-noise measurements could therefore help clarify the physics of the destruction of superconductivity in thin film superconductors.

An extension to the quantum Langevin equation is derived, that is valid in the incoherent hopping regime, and which allows one to incorporate quantum tunneling events. This is achieved by the inclusion of additional stochastic variables in the Langevin equation representing the tunneling events. A systematic derivation of this extension and of its regime of validity is presented. The study is motivated by efforts to determine the error in reading the state of a superconducting quantum bit.

Scaling arguments imply that quantum-critical points exhibit universal nonlinear responses to external probes. We investigate the origins of such nonlinearities in transport, which is especially problematic since the system is necessarily driven far from equilibrium. We argue that for a wide class of systems the new ingredient that enters is the Schwinger mechanism—the production of carriers from the vacuum by the applied field—which is then balanced against a scattering rate that is itself set by the field. We show by explicit computation how this works for the case of the symmetric superfluid-Mott insulator transition of bosons.

The direction dependence of the WIMP direct detection rate provides a powerful tool for distinguishing a WIMP signal from possible backgrounds. We study the number of events required to discriminate a WIMP signal from an isotropic background for a detector with 2-d readout using nonparametric circular statistics. We also examine the number of events needed to (i) detect a deviation from rotational symmetry, due to flattening of the Milky Way halo and (ii) detect a deviation in the mean direction due to a tidal stream. If the senses of the recoils are measured then of order 20–-70 events (depending on the plane of the 2-d readout and the detector location) will be sufficient to reject isotropy of the raw recoil angles at 90% confidence. If the senses can not be measured these number increase by roughly 2 orders of magnitude (compared with an increase of 1 order of magnitude for the case of full 3-d readout). The distributions of the reduced angles, with the (time-dependent) direction of solar motion subtracted, are far more anisotropic, however, and if the isotropy tests are applied to these angles then the numbers of events required are similar to the case of 3-d readout. A deviation from rotational symmetry will only be detectable if the Milky Way halo is significantly flattened. The deviation in the mean direction due to a tidal stream is potentially detectable, however, depending on the density and direction of the stream. The meridian plane (which contains the Earth’s spin axis) is, for all detector locations, the optimum readout plane for rejecting isotropy. However readout in this plane can not be used for detecting flattening of the Milky Way halo or a stream with direction perpendicular to the galactic plane. In these cases the optimum readout plane depends on the detector location.

The electronic structure of the prototypical p-type transparent conducting oxide CuAlO₂ has been studied by O K and Cu L₃ shell x-ray absorption and emission and Al Kα excited x-ray photoemission spectroscopy. The nonresonant O K shell emission is dominated by the O 2p partial density of states, while the Al Kα excited valence photoemission and nonresonant Cu L₃ emission spectra are dominated by the Cu 3d partial density of states. All three techniques reveal mixing between O 2p and Cu 3d states. Cu L₃ emission spectra excited just above the L₃ threshold are dominated by inelastic scattering with 5.5 eV energy loss. This energy is shown to correspond to the separation between the dominant peaks in the filled and empty densities of states.

The possible existence of light sterile neutrinos is of great interest in many areas of particle physics, astrophysics, and cosmology. Furthermore, should the MiniBooNE experiment at Fermilab confirm the Liquid Scintillating Neutrino Detector (LSND) oscillation signal, then new measurements are required to identify the mechanism responsible for these oscillations. Possibilities include sterile neutrinos, CP or CPT violation, variable mass neutrinos, Lorentz violation, and extra dimensions. In this paper, we consider an experiment at a stopped pion neutrino source to determine if active-sterile neutrino oscillations with Δm² greater than 0.1  eV² can account for the signal. By exploiting stopped π⁺ decay to produce a monoenergetic ν_μ source, and measuring the rate of the neutral current reaction ν_x¹²C→ν_x¹²C^*(15.11) as a function of distance from the source, we show that a convincing test for active-sterile neutrino oscillations can be performed.

The bilayer ruthenate Sr₃Ru₂O₇ has been cited as a textbook example of itinerant metamagnetic quantum criticality. However, recent studies of the ultrapure system have revealed striking anomalies in magnetism and transport in the vicinity of the quantum critical point. Drawing on fresh experimental data, we show that the complex phase behavior reported here can be fully accommodated within the framework of a simple Landau theory. We discuss the potential physical mechanisms that underpin the phenomenology, and assess the capacity of the ruthenate system to realize quantum tricritial behavior.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

Band structures and Bloch modes give a generalized description of light in infinite photonic crystals. We show that the band structure and Bloch modes also contain the information necessary to find the amplitude and phase of light reflected and transmitted from interfaces in systems made using finite and semi-infinite photonic crystals. We obtain the equivalent of the Fresnel coefficients for photonic crystals. We use these coefficients to derive the reflection of light from a photonic crystal of finite size and the resonant modes of photonic crystal cavities and line defects. Results are given for ideal two-dimensional crystals, as well as crystals etched in semiconductor slab waveguides.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

The direction dependence of the event rate in WIMP direct detection experiments provides a powerful tool for distinguishing WIMP events from potential backgrounds. We use a variety of (nonparametric) statistical tests to examine the number of events required to distinguish a WIMP signal from an isotropic background when the uncertainty in the reconstruction of the nuclear recoil direction is included in the calculation of the expected signal. We consider a range of models for the Milky Way halo, and also study rotational symmetry tests aimed at detecting nonsphericity/isotropy of the Milky Way halo. Finally we examine ways of detecting tidal streams of WIMPs. We find that if the senses of the recoils are known then of order ten events will be sufficient to distinguish a WIMP signal from an isotropic background for all of the halo models considered, with the uncertainties in reconstructing the recoil direction only mildly increasing the required number of events. If the senses of the recoils are not known the number of events required is an order of magnitude larger, with a large variation between halo models, and the recoil resolution is now an important factor. The rotational symmetry tests require of order a thousand events to distinguish between spherical and significantly triaxial halos, however a deviation of the peak recoil direction from the direction of the solar motion due to a tidal stream could be detected with of order a hundred events, regardless of whether the sense of the recoils is known.

The coupled ηN, πN, γN, ππN system is described by a K-matrix method. The parameters in this model are adjusted to get an optimal fit to πN→πN, πN→ηN, γN→πN, and γN→ηN data in an energy range of about 100 MeV or so each side of the η threshold. Compared with our earlier analysis, we now utilize recent Crystal Ball data. However, the outcome confirms our previous result that the η-nucleon scattering length a is large with a value of 0.91(6)+i  0.27(2)  fm.

We revisit the calculation of the abundance of primordial black holes (PBHs) formed from primordial density perturbations, using a formation criterion derived by Shibata and Sasaki which refers to a metric perturbation variable rather than the usual density contrast. We implement a derivation of the PBH abundance which uses peaks theory, and compare it to the standard calculation based on a Press-Schechter-like approach. We find that the two are in reasonable agreement if the Press-Schechter threshold is in the range Δ_th≃0.3 to 0.5, but advocate use of the peaks theory expression which is based on a sounder theoretical footing.

We present the results of a search for the production of new particles decaying into two jets in p̅ p collisions at sqrt[s]=1.8 TeV, using the DØ 1992–1995 data set corresponding to 109 pb^-1. We exclude at the 95% confidence level the production of excited quarks (q^*) with masses below 775 GeV/c², the most restrictive limit to date. We also exclude standard-model-like W^′ (Z^′) bosons with masses between 300 and 800 GeV/c² (400 and 640 GeV/c²). A W^′ boson with mass <786 GeV/c² has been excluded by previous measurements, and our lower limit is therefore the most stringent to date.

We determine the energies of the excited states of a heavy-light meson Qq̅ , with a static heavy quark and light quark with mass approximately that of the strange quark from both quenched lattices and with dynamical fermions. We are able to explore the energies of orbital excitations up to L=3, the spin-orbit splitting up to L=2 and the first radial excitation. These bs̅ mesons will be very narrow if their mass is less than 5775 MeV—the BK threshold. We investigate this in detail and present evidence that the scalar meson (L=1) will be very narrow and that altogether 6 bs̅ excited states will have energies close to the BK threshold and all will be relatively narrow.

We have measured the circular polarization of light emitted from both atomic H and molecular H₂ after bombarding H₂ with longitudinally polarized electrons. For both atomic and molecular fluorescence near threshold we observe a circular polarization as great as 10% of the electron polarization. This represents the first direct observation of spin transfer in electron-molecule collisions.

A K-matrix formalism is used to relate the amplitudes for the three reactions pd→³Heη, π^{+  3}H→³Heη, and pd→³Hπ⁺. Free parameters are fitted to the available experimental data and an extrapolation below the η³He threshold is made to see the origin of the η³He threshold enhancement. The existence of a virtual—and not a quasibound—state finds support in the data. The K matrix permits a discussion of η-π mixing. A mixing parameter of 0.010(5), i.e., a mixing angle θ=0.6(3)°, is extracted from a best fit to the very recent pd→³Heπ⁰ reaction data.

TRIUMF experiment 497 has measured the parity-violating longitudinal analyzing power A_z in p→p elastic scattering at 221.3 MeV incident proton energy. This comprehensive paper includes details of the corrections, some of magnitude comparable to A_z itself, required to arrive at the final result. The largest correction was for the effects of first moments of transverse polarization. The addition of the result, A_z=[0.84±0.29 (stat.)±0.17 (syst.)]×10^-7, to the p→p parity-violation experimental data base greatly improves the experimental constraints on the weak meson-nucleon coupling constants h_ρ^pp and h_ω^pp, and also has implications for the interpretation of electron parity-violation experiments.

The orbit of the Earth about the Sun produces an annual modulation in the weakly interacting massive particles (WIMP) direct detection rate. If the local WIMP velocity distribution is isotropic then the modulation is roughly sinusoidal with maximum in June; however, if the velocity distribution is anisotropic the phase and shape of the signal can change. Motivated by conflicting claims about the effect of uncertainties in the local velocity distribution on the interpretation of the DAMA annual modulation signal (and the possibility that the form of the modulation could be used to probe the structure of the Milky Way halo), we study the dependence of the annual modulation on various astrophysical inputs. We first examine the approximations used for the Earth’s motion about the Sun and the Sun’s velocity with respect to the Galactic rest frame. We find that overly simplistic assumptions lead to errors of up to ten days in the phase and up to tens of percent in the shape of the signal, even if the velocity distribution is isotropic. Crucially, if the components of the Earth’s velocity perpendicular to the motion of the Sun are neglected, then the change in the phase which occurs for anisotropic velocity distributions is missed. We then examine how the annual modulation signal varies for physically and observationally well-motivated velocity distributions. We find that the phase of the signal changes by up to 20 days and the mean value and amplitude change by up to tens of percent.

We model the effects of cross-phase modulation in frequency (or wavelength) division multiplexed optical communications systems, using a Schrödinger equation with a spatially and temporally random potential. Green’s functions for the propagation of light in this system are calculated using Feynman path-integral and diagrammatic techniques. This propagation leads to a non-Gaussian joint distribution of the input and output optical fields. We use these results to determine the amplitude and timing jitter of a signal pulse and to estimate the system capacity in analog communication.