Search Results (202 total)

We present the results of a search for short-duration gravitational-wave bursts associated with 39 gamma-ray bursts (GRBs) detected by gamma-ray satellite experiments during LIGO’s S2, S3, and S4 science runs. The search involves calculating the crosscorrelation between two interferometer data streams surrounding the GRB trigger time. We search for associated gravitational radiation from single GRBs, and also apply statistical tests to search for a gravitational-wave signature associated with the whole sample. For the sample examined, we find no evidence for the association of gravitational radiation with GRBs, either on a single-GRB basis or on a statistical basis. Simulating gravitational-wave bursts with sine-Gaussian waveforms, we set upper limits on the root-sum-square of the gravitational-wave strain amplitude of such waveforms at the times of the GRB triggers. We also demonstrate how a sample of several GRBs can be used collectively to set constraints on population models. The small number of GRBs and the significant change in sensitivity of the detectors over the three runs, however, limits the usefulness of a population study for the S2, S3, and S4 runs. Finally, we discuss prospects for the search sensitivity for the ongoing S5 run, and beyond for the next generation of detectors.

We report on a search for gravitational waves from the coalescence of compact binaries during the third and fourth LIGO science runs. The search focused on gravitational waves generated during the inspiral phase of the binary evolution. In our analysis, we considered three categories of compact binary systems, ordered by mass: (i) primordial black hole binaries with masses in the range 0.35M_⊙<m₁, m₂<1.0M_⊙, (ii) binary neutron stars with masses in the range 1.0M_⊙<m₁, m₂<3.0M_⊙, and (iii) binary black holes with masses in the range 3.0M_⊙<m₁, m₂<m_max⁡ with the additional constraint m₁+m₂<m_max⁡, where m_max⁡ was set to 40.0M_⊙ and 80.0M_⊙ in the third and fourth science runs, respectively. Although the detectors could probe to distances as far as tens of Mpc, no gravitational-wave signals were identified in the 1364 hours of data we analyzed. Assuming a binary population with a Gaussian distribution around 0.75-0.75M_⊙, 1.4-1.4M_⊙, and 5.0-5.0M_⊙, we derived 90%-confidence upper limit rates of 4.9  yr^-1L₁₀^-1 for primordial black hole binaries, 1.2  yr^-1L₁₀^-1 for binary neutron stars, and 0.5  yr^-1L₁₀^-1 for stellar mass binary black holes, where L₁₀ is 10¹⁰ times the blue-light luminosity of the Sun.

We discuss the properties of a model incorporating both a scalar electroweak Higgs doublet and an electroweak Higgs triplet. We construct the low-energy effective theory for the light Higgs doublet in the limit of small (but nonzero) deviations in the ρ parameter from one, a limit in which the triplet states become heavy. For Δρ>0, perturbative unitarity of WW scattering breaks down at a scale inversely proportional to the renormalized vacuum expectation value of the triplet field (or, equivalently, inversely proportional to the square root of Δρ). This result imposes an upper limit on the mass scale of the heavy triplet bosons in a perturbative theory; we show that this upper bound is consistent with dimensional analysis in the low-energy effective theory. Recent articles have shown that the triplet bosons do not decouple, in the sense that deviations in the ρ parameter from one do not necessarily vanish at one-loop in the limit of large triplet mass. We clarify that, despite the nondecoupling behavior of the Higgs triplet, this model does not violate the decoupling theorem since it incorporates a large dimensionful coupling. Nonetheless, we show that if the triplet-Higgs boson masses are of order the grand unified theory scale, perturbative consistency of the theory requires the (properly renormalized) Higgs-triplet vacuum expectation value to be so small as to be irrelevant for electroweak phenomenology.

We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50–1000 Hz and with the frequency’s time derivative in the range -1×10^-8  Hz s^-1 to zero. Data from the fourth LIGO science run (S4) have been used in this search. Three different semicoherent methods of transforming and summing strain power from short Fourier transforms (SFTs) of the calibrated data have been used. The first, known as StackSlide, averages normalized power from each SFT. A “weighted Hough” scheme is also developed and used, which also allows for a multi-interferometer search. The third method, known as PowerFlux, is a variant of the StackSlide method in which the power is weighted before summing. In both the weighted Hough and PowerFlux methods, the weights are chosen according to the noise and detector antenna-pattern to maximize the signal-to-noise ratio. The respective advantages and disadvantages of these methods are discussed. Observing no evidence of periodic gravitational radiation, we report upper limits; we interpret these as limits on this radiation from isolated rotating neutron stars. The best population-based upper limit with 95% confidence on the gravitational-wave strain amplitude, found for simulated sources distributed isotropically across the sky and with isotropically distributed spin axes, is 4.28×10^-24 (near 140 Hz). Strict upper limits are also obtained for small patches on the sky for best-case and worst-case inclinations of the spin axes.

We searched for an anisotropic background of gravitational waves usingdata from the LIGO S4 science run and a method that is optimizedfor point sources. This is appropriate if, for example, the gravitationalwave background is dominated by a small number of distinct astrophysical sources.No signal was seen. Upper limit maps were produced assuming two differentpower laws for the source strain power spectrum. For an f^-3 power law and using the50 Hz to 1.8 kHz band the upper limits on the sourcestrain power spectrum vary between 1.2×10^-48  Hz^-1 (100  Hz/f)³ and 1.2×10^-47  Hz^-1 (100  Hz/f)³, depending on the position in the sky. Similarly,in the case of constant strain power spectrum, the upper limits vary between 8.5×10^-49  Hz^-1 and 6.1×10^-48  Hz^-1. As a side product a limiton an isotropic background of gravitational waves was also obtained. All limitsare at the 90% confidence level. Finally, as an application, we focused onthe direction of Sco-X1, the brightest low-mass x-ray binary. We compare theupper limit on strain amplitude obtained by this method to expectations basedon the x-ray flux from Sco-X1.

We carry out two searches for periodic gravitational waves using the most sensitive few hours of data from the second LIGO science run. Both searches exploit fully coherent matched filtering and cover wide areas of parameter space, an innovation over previous analyses which requires considerable algorithm development and computational power. The first search is targeted at isolated, previously unknown neutron stars, covers the entire sky in the frequency band 160–728.8 Hz, and assumes a frequency derivative of less than 4×10^-10  Hz/s. The second search targets the accreting neutron star in the low-mass x-ray binary Scorpius X-1 and covers the frequency bands 464–484 Hz and 604–624 Hz as well as the two relevant binary orbit parameters. Because of the high computational cost of these searches we limit the analyses to the most sensitive 10 hours and 6 hours of data, respectively. Given the limited sensitivity and duration of the analyzed data set, we do not attempt deep follow-up studies. Rather we concentrate on demonstrating the data analysis method on a real data set and present our results as upper limits over large volumes of the parameter space. In order to achieve this, we look for coincidences in parameter space between the Livingston and Hanford 4-km interferometers. For isolated neutron stars our 95% confidence level upper limits on the gravitational wave strain amplitude range from 6.6×10^-23 to 1×10^-21 across the frequency band; for Scorpius X-1 they range from 1.7×10^-22 to 1.3×10^-21 across the two 20-Hz frequency bands. The upper limits presented in this paper are the first broadband wide parameter space upper limits on periodic gravitational waves from coherent search techniques. The methods developed here lay the foundations for upcoming hierarchical searches of more sensitive data which may detect astrophysical signals.

We have searched for gravitational waves (GWs) associated with the SGR 1806-20 hyperflare of 27 December 2004. This event, originating from a Galactic neutron star, displayed exceptional energetics. Recent investigations of the x-ray light curve’s pulsating tail revealed the presence of quasiperiodic oscillations (QPOs) in the 30–2000 Hz frequency range, most of which coincides with the bandwidth of the LIGO detectors. These QPOs, with well-characterized frequencies, can plausibly be attributed to seismic modes of the neutron star which could emit GWs. Our search targeted potential quasimonochromatic GWs lasting for tens of seconds and emitted at the QPO frequencies. We have observed no candidate signals above a predetermined threshold, and our lowest upper limit was set by the 92.5 Hz QPO observed in the interval from 150 s to 260 s after the start of the flare. This bound corresponds to a (90% confidence) root-sum-squared amplitude h_rss-det⁡^90%=4.5×10^-22  strain  Hz^-1/2 on the GW waveform strength in the detectable polarization state reaching our Hanford (WA) 4 km detector. We illustrate the astrophysical significance of the result via an estimated characteristic energy in GW emission that we would expect to be able to detect. The above result corresponds to 7.7×10⁴⁶  erg (=4.3×10^-8  M_⊙c²), which is of the same order as the total (isotropic) energy emitted in the electromagnetic spectrum. This result provides a means to probe the energy reservoir of the source with the best upper limit on the GW waveform strength published and represents the first broadband asteroseismology measurement using a GW detector.

We present an experimental and theoretical study of the role of band filling effects in the hydrostatic pressure dependence of photoluminescence (PL) from InN. The PL peak pressure coefficient dE_PL∕dp is shown to decrease from 27.3±1.1  meV∕GPa  to  20.8±0.8  meV∕GPa when the electron concentration increases from 3.6×10¹⁷  cm⁻³  to  1.1×10¹⁹  cm⁻³. We argue that this decrease is caused by the pressure sensitivity of the Fermi level in InN, which induces a lowering of dE_PL∕dp with respect to the band gap pressure coefficient dE_G∕dp. dE_PL∕dp is shown to depend on the electron concentration in accordance with predictions based on ab initio calculations, taking into account the influence of conduction-band nonparabolicity.

We present upper limits on the gravitational wave emission from 78 radio pulsars based on data from the third and fourth science runs of the LIGO and GEO 600 gravitational wave detectors. The data from both runs have been combined coherently to maximize sensitivity. For the first time, pulsars within binary (or multiple) systems have been included in the search by taking into account the signal modulation due to their orbits. Our upper limits are therefore the first measured for 56 of these pulsars. For the remaining 22, our results improve on previous upper limits by up to a factor of 10. For example, our tightest upper limit on the gravitational strain is 2.6×10^-25 for PSR J1603-7202, and the equatorial ellipticity of PSR J2124–3358 is less than 10^-6. Furthermore, our strain upper limit for the Crab pulsar is only 2.2 times greater than the fiducial spin-down limit.

Data from the LIGO Livingston interferometer and the ALLEGRO resonant-bar detector, taken during LIGO’s fourth science run, were examined for cross correlations indicative of a stochastic gravitational-wave background in the frequency range 850–950 Hz, with most of the sensitivity arising between 905 and 925 Hz. ALLEGRO was operated in three different orientations during the experiment to modulate the relative sign of gravitational-wave and environmental correlations. No statistically significant correlations were seen in any of the orientations, and the results were used to set a Bayesian 90% confidence level upper limit of Ω_gw(f)≤1.02, which corresponds to a gravitational-wave strain at 915 Hz of 1.5×10^-23  Hz^-1/2. In the traditional units of h₁₀₀²Ω_gw(f), this is a limit of 0.53, 2 orders of magnitude better than the previous direct limit at these frequencies. The method was also validated with successful extraction of simulated signals injected in hardware and software.

The properties of excitons in the rocksalt high-pressure phase of AlN are investigated theoretically by means of a first-principles approach based on solution of the Bethe-Salpeter equation. The excitons, in that phase, change nature when the applied pressure is varied, from being very extended in space at low pressures to being significantly more localized at higher pressures. The transition is associated with a sudden increase in the exciton binding energy. The change of the character of the excitons is related to the pressure-induced rearrangement of the energy bands.

Using polarized neutron scattering we establish that the magnetic order in La_1.48Nd_0.4Sr_0.12CuO₄ is either (i) one dimensionally modulated and collinear, consistent with the stripe model or (ii) two dimensionally modulated with a novel noncollinear structure. The measurements rule out a number of alternative models characterized by 2D electronic order or 1D helical spin order. The low-energy spin excitations are found to be primarily transversely polarized relative to the stripe ordered state, consistent with conventional spin waves.

The scale of fermion mass generation can, as shown by Appelquist and Chanowitz, be bounded from above by relating it to the scale of unitarity violation in the helicity nonconserving amplitude for fermion-anti-fermion pairs to scatter into pairs of longitudinally polarized electroweak gauge bosons. In this paper, we examine the process tt̅ →W_L⁺W_L^- in a family of phenomenologically-viable deconstructed Higgsless models and we show that scale of unitarity violation depends on the mass of the additional vectorlike fermion states that occur in these theories (the states that are the deconstructed analogs of Kaluza-Klein partners of the ordinary fermions in a five-dimensional theory). For sufficiently light vector fermions, and for a deconstructed theory with sufficiently many lattice sites (that is, sufficiently close to the continuum limit), the Appelquist-Chanowitz bound can be substantially weakened. More precisely, we find that, as one varies the mass of the vectorlike fermion for fixed top-quark and gauge-boson masses, the bound on the scale of top-quark mass generation interpolates smoothly between the Appelquist-Chanowitz bound and one that can, potentially, be much higher. In these theories, therefore, the bound on the scale of fermion mass generation is independent of the bound on the scale of gauge-boson mass generation. While our analysis focuses on deconstructed Higgsless models, any theory in which top-quark mass generation proceeds via the mixing of chiral and vector fermions will give similar results.

Presented in this paper is the description of a Markov chain Monte Carlo (MCMC) routine for conducting coherent parameter estimation for interferometric gravitational wave observations of an inspiral of binary compact objects using multiple detectors. Data from several interferometers are processed, and all nine parameters (ignoring spin) associated with the binary system are inferred, including the distance to the source, the masses, and the location on the sky. The data is matched with time-domain inspiral templates that are 2.5 post-Newtonian (PN) in phase and 2.0 PN in amplitude. We designed and tuned an MCMC sampler so that it is able to efficiently find the posterior mode(s) in the parameter space and perform the stochastic integration necessary for inference within a Bayesian framework. Our routine could be implemented as part of an inspiral detection pipeline for a world-wide network of detectors. Examples are given for simulated signals and data as seen by the LIGO and Virgo detectors operating at their design sensitivity.

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La_1.895Sr_0.105CuO₄ (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer Q_IC=(0.5,0.5±δ,0),(0.5±δ,0.5,0) exhibits an anomaly at the superconducting T_c which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.

The electronic structures of substitutional rare-earth (RE) impurities in GaAs and cubic GaN are calculated. The total energy is evaluated with the self-interaction corrected local spin density approximation, by which several configurations of the open 4f shell of the rare-earth ion are investigated. The defects are modeled by supercells of type REGa_n−1As_n, for n=4, 8, and 16. The preferred defect is the rare-earth substituting Ga, for which case the rare-earth valency in intrinsic material is found to be trivalent. The 3+→2+ f-level is found above the theoretical conduction band edge in all cases and within the experimental gap only for Eu, Tm, and Yb in GaAs and for Eu in GaN. The exchange interaction of the rare-earth impurity with the states at both the valence band maximum and the conduction band minimum is weak, one to two orders of magnitude smaller than that of Mn impurities. Hence the coupling strength is insufficient to allow for ferromagnetic ordering of dilute impurities, except at very low temperatures.

The optical absorption and excitonic properties of wurtzite AlN are investigated by means of an ab initio approach taking into account electron-hole correlations. This is done by solving the Bethe-Salpeter equation, using the results of density functional theory calculations as a starting point. The main focus is on the calculation of excitonic spectra near the conduction-band edge. The response is dominated by the exciton A formed out of excitations from valence Γ₇ band. The n⁻² quantum-number dependence of the energies in Elliott’s model fits rather well the ab initio calculations whereas the n⁻³ decay of the intensities is less obvious for the calculated oscillator strengths.

We construct extended technicolor (ETC) models that can produce the large splitting between the masses of the t and b quarks without necessarily excessive contributions to the ρ parameter or to neutral flavor-changing processes. These models make use of two different ETC gauge groups, such that left- and right-handed components of charge Q=2/3 quarks transform under the same ETC group, while left- and right-handed components of charge -1/3 quarks and charged leptons transform under different ETC groups. The models thereby suppress the masses m_b and m_τ relative to m_t, and m_s and m_μ relative to m_c because the masses of the Q=-1/3 quarks and charged leptons require mixing between the two ETC groups, while the masses of the Q=2/3 quarks do not. A related source of the differences between these mass splittings is the effect of the two hierarchies of breaking scales of the two ETC groups. We analyze a particular model of this type in some detail. Although we find that this model tends to suppress the masses of the first two generations of down-type quarks and charged leptons too much, it gives useful insights into the properties of theories with more than one ETC group.

Using ab initio calculations of electronic structures and electron-phonon coupling in each phonon state we examine the superconductive properties of Li and Na under pressure. From the Eliashberg equations it is found that a Coulomb pseudopotential parameter (μ^*) of “usual” magnitude (0.13) yields T_c values for fcc-Li close to experiments, and they also agree well with recent similar calculations by Tse [J. Phys.: Condens. Matter 17, S911 (2005)], Maheswari [J. Phys. Soc. Jpn. 74, 3227 (2005)], and Kasinathan [Phys. Rev. Lett. 96, 047004 (2006)]. Consequently, the T_c values for fcc-Li predicted earlier by us [Phys. Rev. Lett. 86, 1816 (2001)] are clearly too high, and reasons for this are discussed. The calculations for sodium suggest that superconductivity should not be observed in the fcc phase, except, maybe very close to the fcc-cI16 transition, but then with T_c at most ∼1 K. Both fcc-Li and fcc-Na become dynamically unstable (soft modes) as the pressure exceeds ∼40 and ∼90 GPa, respectively.

We search for coincident gravitational wave signals from inspiralling neutron star binaries using LIGO and TAMA300 data taken during early 2003. Using a simple trigger exchange method, we perform an intercollaboration coincidence search during times when TAMA300 and only one of the LIGO sites were operational. We find no evidence of any gravitational wave signals. We place an observational upper limit on the rate of binary neutron star coalescence with component masses between 1 and 3M_⊙ of 49 per year per Milky Way equivalent galaxy at a 90% confidence level. The methods developed during this search will find application in future network inspiral analyses.