14965415
Best unbiased estimators for the three point correlators of the cosmic microwave background radiation
Measuring the three-point correlators of the cosmic microwave background (CMB) anisotropies could help to get a handle on the level of non-Gaussianity present in the observational data sets and therefore would strongly constrain models of the early Universe. However, typically, the expected non-Gaussian signal is very small. Therefore, one has to face the problem of extracting it from the noise, in particular from the ``cosmic variance'' noise. For this purpose, one has to construct the best unbiased estimators for the three-point correlators that are needed for concrete detection of non-Gaussian features. In this article, we study this problem for both the CMB third moment and the CMB angular bispectrum. We emphasize that knowledge of the best estimator for the former does not permit one to infer the best estimator for the latter and vice versa. We present the corresponding best unbiased estimators in both cases and compute their corresponding cosmic variances.
210886695
DBI-essence
Models where the dark energy is a scalar field with a non-standard Dirac-Born-Infeld (DBI) kinetic term are investigated. Scaling solutions are studied and proven to be attractors. The corresponding shape of the brane tension and of the potential is also determined and found to be, as in the standard case, either exponentials or power-law of the DBI field. In these scenarios, in contrast to the standard situation, the vacuum expectation value of the field at small redshifts can be small in comparison to the Planck mass which could be an advantage from the model building point of view. This situation arises when the present-day value of the Lorentz factor is large, this property being per se interesting. Serious shortcomings are also present such as the fact that, for simple potentials, the equation of state appears to be too far from the observational favored value −1. Another problem is that, although simple stringy-inspired models precisely lead to the power-law shape that has been shown to possess a tracking behavior, the power index turns out to have the wrong sign. Possible solutions to these issues are discussed.
13505150
The Stress energy tensor for transPlanckian cosmology
This article presents the derivation of the stress-energy tensor of a free scalar field with a general non-linear dispersion relation in curved spacetime. This dispersion relation is used as a phenomelogical description of the short distance structure of spacetime following the conventional approach of trans-Planckian modes in black hole physics and in cosmology. This stress-energy tensor is then used to discuss both the equation of state of trans-Planckian modes in cosmology and the magnitude of their backreaction during inflation. It is shown that gravitational waves of trans-Planckian momenta but subhorizon frequencies cannot account for the form of cosmic vacuum energy density observed at present, contrary to a recent claim. The backreaction effects during inflation are confirmed to be important and generic for those dispersion relations that are liable to induce changes in the power spectrum of metric fluctuations. Finally, it is shown that in pure de Sitter inflation there is no modification of the power spectrum except for a possible magnification of its overall amplitude independently of the dispersion relation.
31268700
Quintessence with two energy scales
We study quintessence models using low energy supergravity inspired from string theory. We consider effective supergravity with two scales m_S, the string scale, and m_PL, the Planck scale and show that quintessence naturally arises from a supersymmetry breaking hidden sector. As long as supersymmetry is broken by the $F$-term of a Polonyi-like field coupled to the quintessence field in the K\"ahler potential we find that the Ratra-Peebles potential and its supergravity version are generic predictions. This requires that the string scale decouples from the Planck scale, m_S << m_PL. In the context of supergravity, the potential possesses a minimum induced by the supergravity corrections to the Ratra-Peebles potential at low redshifts. We study the physical consequences of the presence of this minimum.
119370816
Shift Symmetry and Inflation in Supergravity
We consider models of inflation in supergravity with a shift symmetry. We focus on models with one modulus and one inflaton field. The presence of this symmetry guarantees the existence of a flat direction for the inflaton field. Mildly breaking the shift symmetry using a superpotential which depends not only on the modulus, but also on the inflaton field allows one to lift the inflaton flat direction. Along the inflaton direction, the $\ensuremath{\eta}$ problem is alleviated. Combining the KKLT mechanism for modulus stabilization and a shift symmetry breaking superpotential of the chaotic inflation type, we find models reminiscent of ``mutated hybrid inflation'' where the inflationary trajectory is curved in the modulus-inflaton plane. We analyze the phenomenology of these models and stress their differences with both chaotic and hybrid inflation.
118889842
Superimposed oscillations in the WMAP data
The possibility that the cosmic variance outliers present in the recently released Wilkinson Microwave Anisotropy Probe (WMAP) multipole moments are due to oscillations in the primordial power spectrum is investigated. Since the most important contribution to the WMAP likelihood originates from the outliers at a relatively small angular scale (around the first Doppler peak), special attention is paid to these in contrast with previous studies on the subject which have concentrated on the large scales outliers only (i.e., the quadrupole and octupole). As a physically motivated example, the case where the oscillations are of trans-Planckian origin is considered. It is shown that the presence of the oscillations causes an important drop in the WMAP χ2 of about 15. The F test reveals that such a drop has a probability less than 0.06% to occur by chance and can therefore be considered as statistically significant.
24877696
A Variable Field Magnetic Extraction Channel for ORIC
An improved magnetic beam extraction channel for the Oak Ridge Isochronous Cyclotron (ORIC) has been designed to significantly reduce the external field disturbance and provide uniform in-channel field. This will make beam extraction near ¿r = 1 more predictable. The new channel consists of an iron tube of constant cross section with independently adjustable windings, both inside and outside the iron. The windings have a cos¿ current density distribution. The iron tube is 1 meter long with a bore of 6 cm; aperture for beam is 4 cm. The external field is negligible except for small perturbations in the field arising from the geometry modifications required at the ends so that the beam can enter and leave. The field reduction inside the channel is variable from 0.4 to 0.6 Tesla without significant change in either the internal field uniformity or the external field level.
122793815
The 4-MeV Separated-Orbit Cyclotron
The Separated-Orbit Cyclotron Experiment (SOCE) will extend and complement earlier theoretical and experimental studies and will provide a unique facility for the evaluation of an operating SOC system. The six-sector, fourturn accelerator will provide maximum energies of 4 MeV for protons and deuterons and 8 MeV for 3He++ and 4He++ ions. The output energy is variable over a 2:1 range by adjustment of the magnetic field and acceleration of the ions at the appropriate harmonic of the ion frequency. Ions are injected into the SOC at one quarter the final energy. The injection system consists of a duoplasmatron ion source, a 500-kV dc accelerator, and a three-cavity linear accelerator. Proton currents in the 10-to 20-mA range are predicted. The principal characteristics of the accelerator are given in Table I. All of the major components have been fabricated and delivered on site except the injector's linac cavities, which are expected shortly. The photograph in Fig. 1 shows the accelerator as seen from the control area on a mezzanine about 40 feet away. The SOC sector magnets and rf cavities are in the approximate location, with the dc injector components in the background. The tops of the magnet yokes have been temporarily removed to permit completion of pole tip alignment. An rf power amplifier will be mounted on the outer wall of each cavity. One of the PA units can be seen in the background (Fig. 1) on a test stand with a water-cooled dummy load.
120424709
The histrap proposal: Heavy-ion storage ring for atomic physics
Abstract HISTRAP, Heavy-Ion Storage Ring for Atomic Physics, is a proposed 46.8-m-circumference synchrotron-cooling-storage ring optimized to accelerate, decelerate, and store beams of highly charged very-heavy ions at energies appropriate for advanced atomic physics research. The ring is designed to allow studies of electron-ion, photon-ion, ion-atom, and ion-ion interactions. An electron cooling system will provide ion beams with small angular divergence and energy spread for precision spectroscopic studies and also is necessary to allow the deceleration of heavy ions to low energies. HISTRAP will have a maximum bending power of 2.0 T m and will be injected with ions from either the existing Holifield Heavy Ion Research Facility 25-MV tandem accelerator or from a dedicated ECR source and 250 keV/nucleon RFQ linac.
107691388
Preliminary study of possible ORELA replacement options
Based on two conceptual design studies performed by the LANL Accelerator Technology Division, the possibilities in terms of accelerator systems for replacing ORELA with a more intense Maxwellian-type continuous-energy neutron source are summarized and discussed. The neutron intensities from ORELA are compared with those from existing or potential accelerator systems used for cross-section and condensed-matter studies. The best replacement options seem to involve a spallation source from 200- to 400-MeV protons on an ORELA-like target. Pulsing and intensity desiderata with such a source are discussed which correspond to a spectrum-averaged 100-fold improved figure of merit over ORELA for TOF measurements with only a tenfold increased source strength. Existing accelerator designs seem to be inadequate for such a source. Consequently, two conceptual designs were developed for this study by the LANL Accelerator Technology Division. The first conceptual design is for a 200-MeV large linac capable of accelerating 1.3 A during a macropulse; this linac standing alone could serve as an ORELA replacement source. The second conceptual design is for a much smaller 250-MeV PIGMI linac with a 28-mA macropulse current which feeds a proton accumulator ring and bunch-compressor transport line. This linac-ring-compressor (LIRIC) option would give a more cost-effective neutron sourcemore » for cross-section measurements, whereas the large linac, or a modified version of it, would give a much simpler system more suitable for expansion. In particular, both conceptual designs would incorporate the present ORELA building and would provide approximately 100-fold improved neutron sources over ORELA for cross-section measurements. The total estimated cost of the LIRIC system would be $43M (1984), whereas the cost of the large linac would be about a factor of two more. 55 references, 11 figures, 19 tables.« less
115477044
Magnet Model Studies for Separated-Sector Heavy Ion Cyclotrons
A four-sector model of a 2400 ton (2200 metric ton), K=440, separated-sector cyclotron magnet has been built to 0.15 scale. The sector angle of the magnet is 52°. Magnetic field measurements have been made using a single Hall-effect element that was positioned by a high-precision numerically controlled table. Magnetic field data and the ion focusing characteristics are presented.