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Guy D. Moore

Publications and source records attributed to Guy D. Moore.

At least 73 records · Page 4Linked to original sources
arXiv2016

Axion String Dynamics I: 2+1D

If the axion exists and if the initial axion field value is uncorrelated at causally disconnected points, then it should be possible to predict the efficiency of cosmological axion production, relating the axionic dark matter density to the axion mass. The main obstacle to making this prediction is correctly treating the axion string cores. We develop a new algorithm for treating the axionic string cores correctly in 2+1 dimensions. When the axionic string cores are given their full physical string tension, axion production is about twice as efficient as in previous simulations. We argue that the string network in 2+1 dimensions should behave very differently than in 3+1 dimensions, so this result cannot be simply carried over to the physical case. We outline how to extend our method to 3+1D axion string dynamics.

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arXiv2016

Jet-Medium Interactions at NLO in a Weakly-Coupled Quark-Gluon Plasma

We present an extension to next-to-leading order in the strong coupling constant $g$ of the AMY effective kinetic approach to the energy loss of high momentum particles in the quark-gluon plasma. At leading order, the transport of jet-like particles is determined by elastic scattering with the thermal constituents, and by inelastic collinear splittings induced by the medium. We reorganize this description into collinear splittings, high-momentum-transfer scatterings, drag and diffusion, and particle conversions (momentum-preserving identity-changing processes). We show that this reorganized description remains valid to NLO in $g$, and compute the appropriate modifications of the drag, diffusion, particle conversion, and inelastic splitting coefficients. In addition, a new kinematic regime opens at NLO for wider-angle collinear bremsstrahlung. These semi-collinear emissions smoothly interpolate between the leading order high-momentum-transfer scatterings and collinear splittings. To organize the calculation, we introduce a set of Wilson line operators on the light-cone which determine the diffusion and identity changing coefficients, and we show how to evaluate these operators at NLO.

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arXiv2016

Condensates in Relativistic Scalar Theories

Scalar field theory with large infrared initial occupancy develops very large deep-infrared occupancy, which locally resembles a Bose-Einstein condensate. We study the structure and spatial coherence of this condensate. The O(N) symmetric theory with N>1 is qualitatively different than N=1. We explain the thermodynamical reason why, for N>1, the condensate locally carries nearly maximal conserved charge density. We also show how this property impedes the condensate's decay, and we show that it prevents the condensate from ever becoming fully spatially homogeneous. For N <= 4 the condensate can carry topological defects, but these do not appear to control the large-k tail in its power spectrum, which is the same for N=8 where there are no topological defects.

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arXiv2015

Axion dark matter: strings and their cores

Axions constitute a well-motivated dark matter candidate, and if PQ symmetry breaking occurred after inflation, it should be possible to make a clean prediction for the relation between the axion mass and the axion dark matter density. We show that axion (or other global) string networks in 3D have a network density that depends logarithmically on the string separation-to-core ratio. This logarithm would be about 10 times larger in axion cosmology than what we can achieve in numerical simulations. We simulate axion production in the early Universe, finding that, for the separation-to-core ratios we can achieve, the changing density of the network has little impact on the axion production efficiency.

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arXiv2014

Low Mass Thermal Dilepton Production at NLO in a Weakly Coupled Quark-Gluon Plasma

We present a computation, within weakly-coupled thermal QCD, of the production rate of low invariant mass ($M^2 \sim g^2 T^2$) dileptons, at next-to-leading order (NLO) in the coupling (which is $O(g^3 e^2 T^2)$). This involves extending the NLO calculation of the photon rate which we recently presented to the case of small nonzero photon invariant mass. Numerical results are discussed and tabulated forms and code are provided for inclusion in hydrodynamical models. We find that NLO corrections can increase the dilepton rate by up to 30-40% relative to leading order. We find that the electromagnetic response of the plasma for real photons and for small invariant mass but high energy dilepton pairs (e.g., $M^2 < (300\:\mathrm{MeV})^2$ but $p_T > 1 \: \mathrm{GeV}$) are close enough that dilepton pair measurements really can serve as Ersatz photon measurements. We also present a matching a la Ghisoiu and Laine between our results and results at larger invariant masses.

hep-ph↗
arXiv2014

3.5 keV X-rays as the "21 cm line" of dark atoms, and a link to light sterile neutrinos

The recently discovered 3.5 keV X-ray line from extragalactic sources may be evidence of dark matter scatterings or decays. We show that dark atoms can be the source of the emission, through their hyperfine transitions, which would be the analog of 21 cm radiation from a dark sector. We identify two families of dark atom models that match the X-ray observations and are consistent with other constraints. In the first, the hyperfine excited state is long-lived compared to the age of the universe, and the dark atom mass is relatively unconstrained; dark atoms could be strongly self-interacting in this case. In the second, the excited state is short-lived and viable models are parameterized by the value of the dark proton-to-electron mass ratio $R$: for $R = 10^2-10^4$, the dark atom mass is predicted be in the range $350-1300$ GeV, with fine structure constant $α'\cong 0.1-0.6$. In either class of models, the dark photon must be massive with $m_{γ'} \gtrsim$ 1 MeV and decay into $e^+ e^-$. Evidence for the model could come from direct detection of the dark atoms. In a natural extension of this framework, the dark photon could decay predominantly into invisible particles, for example $\sim 0.5$ eV sterile neutrinos, explaining the extra radiation degree of freedom recently suggested by data from BICEP2, while remaining compatible with BBN.

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arXiv2014

nPI Resummation in 3D SU(N) Higgs Theory

We test the utility of the nPI formalism for solving nonperturbative dynamics of gauge theories by applying it to study the phase diagram of SU(N) Higgs theory in 3 Euclidean spacetime dimensions. Solutions reveal standard signatures of a first order phase transition with a critical endpoint leading to a crossover regime, in qualitative agreement with lattice studies. The location of the critical endpoint, x sim 0.14 for SU(2) with a fundamental Higgs, is in rough but not tight quantitative agreement with the lattice. We end by commenting on the overall effectiveness and limitations of an nPI effective action based study. In particular, we have been unable to find an nPI gauge-fixing procedure which can simultaneously display the right phase structure and correctly handle the large-VEV Higgs region. We explain why doing so appears to be a serious challenge.

hep-ph↗
arXiv2014

Renormalization of Null Wilson Lines in EQCD

Radiation and energy loss of a light, high-energy parton in a perturbative Quark-Gluon Plasma is controlled by transverse momentum exchange. The troublesome infrared contributions to transverse momentum exchange can be computed on the lattice using dimensional reduction to EQCD. However a novel extended operator, the Null Wilson Line of EQCD, is involved. We compute the renormalization properties of this object's lattice implementation to next-to-leading order, which should facilitate its efficient calculation on the lattice.

hep-lat↗
arXiv2014

Scattering properties of dark atoms and molecules

There has been renewed interest in the possibility that dark matter exists in the form of atoms, analogous to those of the visible world. An important input for understanding the cosmological consequences of dark atoms is their self-scattering. Making use of results from atomic physics for the potentials between hydrogen atoms, we compute the low-energy elastic scattering cross sections for dark atoms. We find an intricate dependence upon the ratio of the dark proton to electron mass, allowing for the possibility to "design" low-energy features in the cross section. Dependences upon other parameters, namely the gauge coupling and reduced mass, scale out of the problem by using atomic units. We derive constraints on the parameter space of dark atoms by demanding that their scattering cross section does not exceed bounds from dark matter halo shapes. We discuss the formation of molecular dark hydrogen in the universe, and determine the analogous constraints on the model when the dark matter is predominantly in molecular form.

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arXiv2014

UV Cascade in Classical Yang-Mills via Kinetic Theory

We show that classical Yang-Mills theory with statistically homogeneous and isotropic initial conditions has a kinetic description and approaches a scaling solution at late times. We find the scaling solution by explicitly solving the Boltzmann equations, including all dominant processes (elastic and number-changing). Above a scale $p_{max} \propto t^{1/7}$ the occupancy falls exponentially in $p$. For asymptotically late times and sufficiently small momenta the occupancy scales as $f(p)\propto 1/p$, but this behavior sets in only at very late time scales. We find quantitative agreement of our results with lattice simulations, for times and momenta within the range of validity of kinetic theory.

hep-ph↗
arXiv2013

3-loop 3PI effective action for 3D SU(3) QCD

The 3PI method is a technique to resum an infinite class of diagrams, which may be useful in studying nonperturbative thermodynamics and dynamics in quantum field theory. But it has never been successfully applied to gauge theories, where there are serious questions about gauge invariance breaking. We show how to perform the 3PI resummation of QCD in 3 Euclidean spacetime dimensions, a warmup problem to the 4 or 3+1 dimensional case. We present the complete details of the technical problems and how they are overcome. We postpone a comparison of gauge invariant correlation functions with their lattice-determined counterparts to a future publication.

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arXiv2013

Next-to-leading order thermal photon production in a weakly coupled quark-gluon plasma

We compute the next-to-leading order O(g) correction to the thermal photon production rate in a QCD plasma. The NLO contributions can be expressed in terms of gauge invariant condensates on the light cone, which are amenable to novel sum rules and Euclidean techniques. We expect these technologies to be generalizable to other NLO calculations. For the phenomenologically interesting value of alpha_s=0.3, the NLO correction represents a 20% increase and has a functional form similar to the LO result.

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arXiv2012

Thermodynamical second-order hydrodynamic coefficients

Transport coefficients in non-conformal second-order hydrodynamics can be classified as either dynamical or thermodynamical. We derive Kubo formuale for the thermodynamical coefficients and compute them at leading perturbative order in a theory with general matter content. We also discuss how to approach their evaluation on the lattice.

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arXiv2012

Composite magnetic dark matter and the 130 GeV line

We propose an economical model to explain the apparent 130 GeV gamma ray peak, found in the Fermi/LAT data, in terms of dark matter annihilation through a dipole moment interaction. The annihilating dark matter particles represent a subdominant component, with mass density 7-17% of the total DM density; and they only annihilate into gamma gamma, gamma Z, and ZZ, through a magnetic (or electric) dipole moment. Annihilation into other standard model particles is suppressed, due to a mass splitting in the magnetic dipole case, or to p-wave scattering in the electric dipole case. In either case, the observed signal requires a dipole moment of strength mu ~ 2/TeV. We argue that composite models are the preferred means of generating such a large dipole moment, and that the magnetic case is more natural than the electric one. We present a simple model involving a scalar and fermionic techniquark of a confining SU(2) gauge symmetry. We point out some generic challenges for getting such a model to work. The new physics leading to a sufficiently large dipole moment is below the TeV scale, indicating that the magnetic moment is not a valid effective operator for LHC physics, and that production of the strongly interacting constituents, followed by techni-hadronization, is a more likely signature than monophoton events. In particular, 4-photon events from the decays of bound state pairs are predicted.

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arXiv2012

New algorithm for classical gauge theory simulations in an expanding box

We propose a new algorithm for classical statistical simulations in scalar and gauge theories undergoing a one dimensional expansion, which allows simulations to study boxes of larger transverse extent and to continue for longer times, without losing lattice resolution in the expanding direction.

hep-lat↗
arXiv2012

UV Cascade in Classical Yang-Mills Theory

We study the real-time behavior of classical Yang-Mills theory under initial conditions with nonperturbatively large, infrared field amplitudes. Our lattice study confirms the cascade of energy towards higher momenta and lower occupancy, which occurs via a scaling solution $f[p,t_1] = (t_0/t_1)^{4/7}\, f[p (t_0/t_1)^{1/7},t_0]$. Above a characteristic scale p_{max}, f falls exponentially; below p_{max}, $f[p] \propto p^{-4/3}$. We find no evidence for different infrared exponents or for infrared occupancies in excess of those described by this scaling solution. We also investigate what the fate of large occupancies would be, both in the electric and the magnetic sector.

hep-ph↗
arXiv2011

Bjorken Flow, Plasma Instabilities, and Thermalization

At asymptotically high energies, thermalization in heavy ion collisions can be described via weak-coupling QCD. We present a complete treatment of how thermalization proceeds, at the parametric weak-coupling level. We show that plasma instabilities dominate the dynamics, from immediately after the collision until well after the plasma becomes nearly in equilibrium. Initially they drive the system close to isotropy, but Bjorken expansion and increasing diluteness makes the system again become more anisotropic. At time τ~ α^(-12/5) Q^(-1) the dynamics become dominated by a nearly-thermal bath; and at time τ~ α^(-5/2) Q^(-1)$ the bath comes to dominate the energy density, completing thermalization. After this time there is a nearly isotropic and thermal Quark-Gluon Plasma.

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