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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 721 records · Page 40
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DOE OSTI2011

Emergence of a universal limiting speed

We display several examples of how fields with different limiting velocities (the ''speed of light'') at a high energy scale can nevertheless have a common limiting velocity at low energies due to the effects of interactions. We evaluate the interplay of the velocities through the self-energy diagrams and use the renormalization group to evolve the system to low energy. The differences normally vanish only logarithmically, so that an exponentially large energy trajectory is required in order to satisfy experimental constraints. However, we also display a model in which the running is a power-law type, which could be more phenomenologically useful. The largest velocity difference should be in the system with the weakest interaction, which suggests that the study of the speed of gravitational waves would be the most stringent test of this phenomenon.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Universality in generalized models of inflation

We discuss the cosmological evolution of a scalar field with non standard kinetic term in terms of a Renormalization Group Equation (RGE). In this framework inflation corresponds to the slow evolution in a neighborhood of a fixed point and universality classes for inflationary models naturally arise. Using some examples we show the application of the formalism. The predicted values for the speed of sound c {sub s} {sup 2} and for the amount of non-Gaussianities produced in these models are discussed. In particular, we show that it is possible to introduce models with c {sub s} {sup 2} ≠ 1 that can be in agreement with present cosmological observations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Error estimates and specification parameters for functional renormalization

We present a strategy for estimating the error of truncated functional flow equations. While the basic functional renormalization group equation is exact, approximated solutions by means of truncations do not only depend on the choice of the retained information, but also on the precise definition of the truncation. Therefore, results depend on specification parameters that can be used to quantify the error of a given truncation. We demonstrate this for the BCS–BEC crossover in ultracold atoms. Within a simple truncation the precise definition of the frequency dependence of the truncated propagator affects the results, indicating a shortcoming of the choice of a frequency independent cutoff function.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

The gravitational waves from the first-order phase transition with a dimension-six operator

We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) in the extended standard model of particle physics with a dimension-six operator, which is capable of exhibiting the recently discovered slow first-order PT in addition to the usually studied fast first-order PT. To simplify the discussion, it is sufficient to work with an example of a toy model with the sextic term, and we propose an unified description for both slow and fast first-order PTs. We next study the full one-loop effective potential of the model with fixed/running renormalization-group (RG) scales. Compared to the prediction of GW energy density spectrum from the fixed RG scale, we find that the presence of running RG scale could amplify the peak amplitude by amount of one order of magnitude while shift the peak frequency to the lower frequency regime, and the promising regime of detection within the sensitivity ranges of various space-based GW detectors shrinks down to a lower cut-off value of the sextic term rather than the previous expectation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Quench of non-Markovian coherence in the deep sub-Ohmic spin–boson model: A unitary equilibration scheme

The deep sub-Ohmic spin–boson model shows a longstanding non-Markovian coherence at low temperature. Motivating to quench this robust coherence, the thermal effect is unitarily incorporated into the time evolution of the model, which is calculated by the adaptive time-dependent density matrix renormalization group algorithm combined with the orthogonal polynomials theory. Via introducing a unitary heating operator to the bosonic bath, the bath is heated up so that a majority portion of the bosonic excited states is occupied. It is found in this situation the coherence of the spin is quickly quenched even in the coherent regime, in which the non-Markovian feature dominates. With this finding we come up with a novel way to implement the unitary equilibration, the essential term of the eigenstate-thermalization hypothesis, through a short-time evolution of the model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Atomic matter-wave revivals with definite atom number in an optical lattice

We study the collapse and revival of interference patterns in the momentum distribution of atoms in optical lattices using a projection technique to properly account for the fixed total number of atoms in the system. We consider the common experimental situation in which weakly interacting bosons are loaded into a shallow lattice, which is suddenly made deep. The collapse and revival of peaks in the momentum distribution is then driven by interactions in a lattice with essentially no tunneling. The projection technique allows to us to treat inhomogeneous (trapped) systems exactly in the case that noninteracting bosons are loaded into the system initially, and we use time-dependent density-matrix renormalization group techniques to study the system in the case of finite tunneling in the lattice and finite initial interactions. For systems of more than a few sites and particles, we find good agreement with results calculated via a naive approach, in which the state at each lattice site is described by a coherent state in the particle occupation number. However, for systems on the order of ten lattice sites, we find experimentally measurable discrepancies to the results predicted by this standard approach.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Quantum decay in renormalizable field theories: Quasiparticle formation, Zeno and anti-Zeno effects

In a renormalizable theory the survival probability of an unstable quantum state features divergences as a consequence of the rapid growth of the density of states with energy. Introducing a high energy cutoff Λ, the transient dynamics during a time scale ≃1∕Λ describes the renormalization of the bare into a “quasiparticle” state which decays on longer time scales. During this early transient the decay law features Zeno behavior e{sup −(t∕t{sub Z}){sup 2}} with the Zeno time-scale t{sub Z}∝1∕Λ{sup 3∕2}. We introduce a dynamical renormalization framework that allows to separate consistently the dynamics of formation of the quasiparticle state and its decay on longer time scales by introducing a renormalization time scale along with alternative “schemes”. The survival probability obeys a renormalization group equation with respect to this scale. We find a transient suppression of the decay law for large Lorentz factor as a consequence of the narrowing of the phase space for decay different from the usual time dilation. In the presence of higher mass thresholds, the energy uncertainty associated with transient dynamics leads to an anti Zeno enhancement with a transient acceleration of the decay into heavier particles. There remains memory of the transient effects in the survival probability even at long time. We discuss possible consequences of these effects in cosmology.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Surface hopping with a manifold of electronic states. II. Application to the many-body Anderson-Holstein model

We investigate a simple surface hopping (SH) approach for modeling a single impurity level coupled to a single phonon and an electronic (metal) bath (i.e., the Anderson-Holstein model). The phonon degree of freedom is treated classically with motion along–and hops between–diabatic potential energy surfaces. The hopping rate is determined by the dynamics of the electronic bath (which are treated implicitly). For the case of one electronic bath, in the limit of small coupling to the bath, SH recovers phonon relaxation to thermal equilibrium and yields the correct impurity electron population (as compared with numerical renormalization group). For the case of out of equilibrium dynamics, SH current-voltage (I-V) curve is compared with the quantum master equation (QME) over a range of parameters, spanning the quantum region to the classical region. In the limit of large temperature, SH and QME agree. Furthermore, we can show that, in the limit of low temperature, the QME agrees with real-time path integral calculations. As such, the simple procedure described here should be useful in many other contexts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Nonlinear matter spectra in coupled quintessence

We consider cosmologies in which a dark-energy scalar field interacts with cold dark matter. The growth of perturbations is followed beyond the linear level by means of the time-renormalization-group method, which is extended to describe a multicomponent matter sector. Even in the absence of the extra interaction, a scale-dependent bias is generated as a consequence of the different initial conditions for baryons and dark matter after decoupling. The effect is enhanced significantly by the extra coupling and can be at the 2%-3% level in the range of scales of baryonic acoustic oscillations. We compare our results with N-body simulations, finding very good agreement.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Numerical Study of Vortex Core in Turbulent Swirling Flow

Swirling flow has attracted a great deal of research interest. An application of swirling flow can be found in many engineering areas; such as gas cyclone and heat transfer enhancement. In nuclear engineering, swirling flow is used as heat transfer enhancement in rod bundle by installing grid spacer and mixing vane. Because of its extensive applications, there has been a great number of experimental and numerical investigations of swirling flow. McClusky et al., experimentally studied mapping of the lateral velocity in rod bundle using Laser Doppler Velocimetry (LDV) and Conner et al. predicted the flow field in rod bundle by CFD. To understand the characteristics of swirling flow such as tangential velocity and vortex core swirling flow, some researchers investigated swirling flow in a pipe instead of in rod bundle. This is due to the similarity between swirling flow in a pipe and rod bundle. In recent study, flow mapping of 2D velocity field of single-phase swirling flow had been conducted experimentally by Particle Image Velocimetry (PIV). However, this method needs calibration and complicated. The new measurement technique for flow mapping in swirling flow was done by phased array Ultrasonic Velocity Profiler (PAUVP). Hamdani et al., successfully plotted velocity field and determined the swirling core region (vortex core). This technique is nonintrusive and no need calibration. However, there was a limitation of the measurement because phased array UVP cannot capture 360 deg. velocity field inside the pipe. In addition, tangential velocity in CFD results was not agree well with experiment data due asymmetric problem. To evaluate this limitation of experimental method, the numerical simulations using Computational Fluid Dynamics (CFD) needs to be performed. The objective of this study is to use the commercial CFD code Fluent to model the developing turbulent swirling flow induced by twisted tape inside a straight pipe, and to gain an understanding of such a flow and the turbulence models involved in the simulation. Straight pipe is used instead of rod bundle for a simplification. In this work, the Renormalized Group (RNG) k-{epsilon} turbulence model and Reynolds Stress Model (RSM) turbulence model, are performed to study the phenomena of vortex core in a pipe with twisted tape inserts. Effects of the Reynolds number and asymmetric of twisted tape (inclination, {alpha} = 5 deg. and {alpha} = 20 deg.) on swirling core position are examined using water as the testing fluid. Furthermore, the Navier-Stokes equations will be solved to obtain results that can be validated with the previous experimental results. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

TURBULENT GENERAL MAGNETIC RECONNECTION

Plasma flows with a magnetohydrodynamic (MHD)-like turbulent inertial range, such as the solar wind, require a generalization of general magnetic reconnection (GMR) theory. We introduce the slip velocity source vector per unit arclength of field line, the ratio of the curl of the non-ideal electric field in the generalized Ohm’s Law and magnetic field strength. It diverges at magnetic nulls, unifying GMR with null-point reconnection. Only under restrictive assumptions is the slip velocity related to the gradient of quasi-potential (which is the integral of parallel electric field along magnetic field lines). In a turbulent inertial range, the non-ideal field becomes tiny while its curl is large, so that line slippage occurs even while ideal MHD becomes accurate. The resolution is that ideal MHD is valid for a turbulent inertial range only in a weak sense that does not imply magnetic line freezing. The notion of weak solution is explained in terms of renormalization group (RG) type theory. The weak validity of the ideal Ohm’s law in the inertial range is shown via rigorous estimates of the terms in the generalized Ohm’s Law. All non-ideal terms are irrelevant in the RG sense and large-scale reconnection is thus governed solely by ideal dynamics. We discuss the implications for heliospheric reconnection, in particular for deviations from the Parker spiral model. Solar wind observations show that reconnection in a turbulence-broadened heliospheric current sheet, which is consistent with Lazarian–Vishniac theory, leads to slip velocities that cause field lines to lag relative to the spiral model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Factorization and resummation of Higgs boson differential distributions in soft-collinear effective theory

We derive a factorization theorem for the Higgs boson transverse momentum (p{sub T}) and rapidity (Y) distributions at hadron colliders, using the soft-collinear effective theory (SCET), for m{sub h}>>p{sub T}>>{Lambda}{sub QCD}, where m{sub h} denotes the Higgs mass. In addition to the factorization of the various scales involved, the perturbative physics at the p{sub T} scale is further factorized into two collinear impact-parameter beam functions (IBFs) and an inverse soft function (ISF). These newly defined functions are of a universal nature for the study of differential distributions at hadron colliders. The additional factorization of the p{sub T}-scale physics simplifies the implementation of higher order radiative corrections in {alpha}{sub s}(p{sub T}). We derive formulas for factorization in both momentum and impact parameter space and discuss the relationship between them. Large logarithms of the relevant scales in the problem are summed using the renormalization group equations of the effective theories. Power corrections to the factorization theorem in p{sub T}/m{sub h} and {Lambda}{sub QCD}/p{sub T} can be systematically derived. We perform multiple consistency checks on our factorization theorem including a comparison with known fixed-order QCD results. We compare the SCET factorization theorem with the Collins-Soper-Sterman approach to low-p{sub T} resummation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Bimetric truncations for quantum Einstein gravity and asymptotic safety

In the average action approach to the quantization of gravity the fundamental requirement of 'background independence' is met by actually introducing a background metric but leaving it completely arbitrary. The associated Wilsonian renormalization group defines a coarse graining flow on a theory space of functionals which, besides the dynamical metric, depend explicitly on the background metric. All solutions to the truncated flow equations known to date have a trivial background field dependence only, namely via the classical gauge fixing term. In this paper, we analyze a number of conceptual issues related to the bimetric character of the gravitational average action and explore a first nontrivial bimetric truncation in the simplified setting of conformally reduced gravity. Possible implications for the Asymptotic Safety program and the cosmological constant problem are discussed in detail.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Computational investigation on tunable optical band gap in armchair polyacenes

Polyacenes in their armchair geometry (phenacenes) have recently been found to possess appealing electronic and optical properties with higher chemical stability and comparatively larger band gap as compared to linear polyacenes. They also behave as high-temperature superconductors upon alkali metal doping. Moreover, the optical properties of crystalline picene can be finely tuned by applying external pressure. We investigated the variation of optical gap as a function of altering the interplanar distances between parallel cofacial phenacene dimers. We employed both time-dependent density functional theory and density matrix renormalization group (DMRG) technique to investigate the lowest singlet excitations in phenacene dimer. Our study showed that the lowest singlet excitation in these systems evolved as a function of interplanar separation. The optical excitation energy gap decreases as a function of inverse interplanar separation of the phenacene dimer. The distant dependent variation of optical absorption at the dimer level may be comparable with experimental observation in picene crystal under pressure. DMRG study also demonstrates that besides picene, electronic properties of higher phenacenes can also be tunable by altering interplanar separation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

J{sub 1}-J{sub 2} Heisenberg model at and close to its z=4 quantum critical point

We study the frustrated J{sub 1}-J{sub 2} Heisenberg model with ferromagnetic nearest-neighbor coupling J{sub 1}<0 and antiferromagnetic next-nearest-neighbor coupling J{sub 2}>0 at and close to the z=4 quantum critical point (QCP) at J{sub 1}/J{sub 2}=-4. The J{sub 1}-J{sub 2} model plays an important role for recently synthesized chain cuprates as well as in supersymmetric Yang-Mills theories. We study the thermodynamic properties using field theory, a modified spin-wave theory, as well as numerical density-matrix renormalization group calculations. Furthermore, we compare with results for the classical model obtained by analytical methods and Monte Carlo simulations. As one of our main results, we present numerical evidence that the susceptibility at the QCP seems to diverge with temperature T as {chi}{approx}T{sup -1.2} in the quantum case, in contrast to the classical model where {chi}{approx}T{sup -4/3}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Three-dimensional imaging of pion using lattice QCD: generalized parton distributions

In this work, we report a lattice calculation of x-dependent valence pion generalized parton distributions (GPDs) at zero skewness with multiple values of the momentum transfer −t. The calculations are based on an N f = 2 + 1 gauge ensemble of highly improved staggered quarks with Wilson-Clover valence fermion. The lattice spacing is 0.04 fm, and the pion valence mass is tuned to be 300 MeV. We determine the Lorentz-invariant amplitudes of the quasi-GPD matrix elements for both symmetric and asymmetric momenta transfers with similar values and show the equivalence of both frames. Then, focusing on the asymmetric frame, we utilize a hybrid scheme to renormalize the quasi-GPD matrix elements obtained from the lattice calculations. After the Fourier transforms, the quasi-GPDs are then matched to the light-cone GPDs within the framework of large momentum effective theory with improved matching, including the next-to-next-to-leading order perturbative corrections, and leading renormalon and renormalization group resummations. We also present the 3-dimensional image of the pion in impact-parameter space through the Fourier transform of the momentum transfer −t.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Lower bounds on the smallest lepton mixing angle

We give minimal values for the smallest lepton mixing parameter |U{sub e3}|, applying 2-loop renormalization group equations in an effective theory approach. This is relevant in scenarios that predict an inverted neutrino mass spectrum with the smallest mass and |U{sub e3}| being zero at tree level, a situation known to be preserved at 1-loop order. At 2-loop, |U{sub e3}| is generated at a level of 10{sup -12}-10{sup -14}. Such small values are of interest in supernova physics. Corresponding limits for the normal mass ordering are several orders of magnitude larger. Our results show that |U{sub e3}| can in general be expected to be nonzero.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Energy Level Distribution of Perturbed Conformal Field Theories

We study the energy level spacing of perturbed conformal minimal models in finite volume, considering perturbations of such models that are massive but not necessarily integrable. We compute their spectrum using a renormalization group improved truncated conformal spectrum approach. With this method we are able to study systems where more than 40,000 states are kept and where we determine the energies of the lowest several thousand eigenstates with high accuracy. We find, as expected, that the level spacing statistics of integrable perturbed minimal models are Poissonian while the statistics of non-integrable perturbations are GOE-like. However, by varying the system size (and so controlling the positioning of the theory between its IR and UV limits) one can induce crossovers between the two statistical distributions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗