arXiv subjects

Guy D. Moore

Publications and source records attributed to Guy D. Moore.

At least 127 records · Page 7Linked to original sources

arXiv2004

Limits on Lorentz Violation from the Highest Energy Cosmic Rays

We place several new limits on Lorentz violating effects, which can modify particles' dispersion relations, by considering the highest energy cosmic rays observed. Since these are hadrons, this involves considering the partonic content of such cosmic rays. We get a number of bounds on differences in maximum propagation speeds, which are typically bounded at the 10^{-21} level, and on momentum dependent dispersion corrections of the form v = 1 +- p^2/Lambda^2, which typically bound Lambda > 10^{21} GeV, well above the Planck scale. For (CPT violating) dispersion correction of the form v = 1 + p/Lambda, the bounds are up to 15 orders of magnitude beyond the Planck scale.

hep-ph↗

arXiv2004

The phantom menaced: constraints on low-energy effective ghosts

It has been suggested that a scalar field with negative kinetic energy, or ``ghost,'' could be the source of the observed late-time cosmological acceleration. Naively, such theories should be ruled out by the catastrophic quantum instability of the vacuum. We derive phenomenological bounds on the Lorentz-violating ultraviolet cutoff Lambda which must apply to low-energy effective theories of ghosts, in order to keep the instability at unobservable levels. Assuming only that ghosts interact at least gravitationally, we show that Lambda < 3 MeV for consistency with the cosmic gamma ray background. We also show that theories of ghosts with a Lorentz-conserving cutoff are completely excluded.

hep-ph↗

arXiv2004

Electromagnetic Emission and Energy Loss in the QGP

I discuss why photon production from the Quark Gluon Plasma (QGP) presents an interesting problem, both experimentally and theoretically. I show how the photon emission rate can be computed under the simplifying assumption that the QGP fully thermalizes. The theoretical issues are very similar to those for jet energy loss; so it should be possible to treat them in a common formalism and relate the predictions of one phenomenon to those of the other.

hep-ph↗

arXiv2003

Energy Loss of Leading Partons in a Thermal QCD Medium

We consider bremsstrahlung energy loss for hard partons traversing a quark-gluon plasma. Accounting correctly for the probabilistic nature of the energy loss, and making a leading-order accurate treatment of bremsstrahlung, we find that the suppression of the spectrum is nearly flat, with the most suppression at energies $E \sim 30 T$ ($T$ the QGP temperature), in contrast to previous literature but in agreement with experimental data. This flat pattern should also be observed at the LHC.

hep-ph↗

arXiv2003

Monte Carlo simulation of O(2) phi^4 field theory in three dimensions

Using standard numerical Monte Carlo lattice methods, we study non-universal properties of the phase transition of three-dimensional phi^4 theory of a 2-component real field phi = (phi_1,phi_2) with O(2) symmetry. Specifically, we extract the renormalized values of /u and r/u^2 at the phase transition, where the continuum action of the theory is \int d^3x [ (1/2) |\gradϕ|^2 + \half r ϕ^2 + {u\over4!} ϕ^4 ]. These values have applications to calculating the phase transition temperature of dilute or weakly-interacting Bose gases (both relativistic and non-relativistic). In passing, we also provide perturbative calculations of various O(a) lattice-spacing errors in three-dimensional O(N) scalar field theory, where (a) is the lattice spacing.

cond-mat↗

arXiv2003

Transport coefficients in high temperature gauge theories: (II) Beyond leading log

Results are presented of a full leading-order evaluation of the shear viscosity, flavor diffusion constants, and electrical conductivity in high temperature QCD and QED. The presence of Coulomb logarithms associated with gauge interactions imply that the leading-order results for transport coefficients may themselves be expanded in an infinite series in powers of 1/log(1/g); the utility of this expansion is also examined. A next-to-leading-log approximation is found to approximate the full leading-order result quite well as long as the Debye mass is less than the temperature.

hep-ph↗

arXiv2003

Pressure of Hot QCD at Large N_f

We compute the pressure and entropy of hot QCD in the limit of large number of fermions, N_f >> N_c ~ 1, to next to leading order in N_f. At this order the calculation can be done exactly, up to ambiguities due to the presence of a Landau pole in the theory; the ambiguities are O(T^8/Λ^4_{Landau}) and remain negligible long after the perturbative series (in g^2 N_f) has broken down. Our results can be used to test several proposed resummation schemes for the pressure of full QCD.

hep-ph↗

arXiv2003

Comment on and Erratum to "Pressure of Hot QCD at Large N_f"

We repeat and correct the recent calculation of the thermodynamic potential of hot QCD in the limit of large number N_f of fermions. The new result for the thermal pressure turns out to agree significantly better with results obtained from perturbation theory at small coupling. For large coupling, a nonmonotonic behaviour is reproduced, but the pressure of the strongly coupled theory does not exceed the free pressure as long as the Landau pole ambiguity remains negligible numerically.

hep-ph↗

arXiv2003

Effective Kinetic Theory for High Temperature Gauge Theories

Quasiparticle dynamics in relativistic plasmas associated with hot, weakly-coupled gauge theories (such as QCD at asymptotically high temperature $T$) can be described by an effective kinetic theory, valid on sufficiently large time and distance scales. The appropriate Boltzmann equations depend on effective scattering rates for various types of collisions that can occur in the plasma. The resulting effective kinetic theory may be used to evaluate observables which are dominantly sensitive to the dynamics of typical ultrarelativistic excitations. This includes transport coefficients (viscosities and diffusion constants) and energy loss rates. We show how to formulate effective Boltzmann equations which will be adequate to compute such observables to leading order in the running coupling $g(T)$ of high-temperature gauge theories [and all orders in $1/\log g(T)^{-1}$]. As previously proposed in the literature, a leading-order treatment requires including both $2<->2$ particle scattering processes as well as effective ``$1<->2$'' collinear splitting processes in the Boltzmann equations. The latter account for nearly collinear bremsstrahlung and pair production/annihilation processes which take place in the presence of fluctuations in the background gauge field. Our effective kinetic theory is applicable not only to near-equilibrium systems (relevant for the calculation of transport coefficients), but also to highly non-equilibrium situations, provided some simple conditions on distribution functions are satisfied.

hep-ph↗

arXiv2002

Transport Coefficients in Large $N_f$ Gauge Theory: Testing Hard Thermal Loops

We compute shear viscosity and flavor diffusion coefficients for ultra-relativistic gauge theory with many fermionic species, Nf >> 1, to leading order in 1/Nf. The calculation is performed both at leading order in the effective coupling strength g^2 Nf, using the Hard Thermal Loop (HTL) approximation, and completely to all orders in g^2 Nf. This constitutes a nontrivial test of how well the HTL approximation works. We find that in this context, the HTL approximation works well wherever the renormalization point sensitivity of the leading order HTL result is small.

hep-ph↗

arXiv2002

Transport Coefficients at Leading Order: Kinetic Theory versus Diagrams

I review what is required to compute transport coefficients in ultra-relativistic, weakly coupled gauge theories, at leading order in $g$, using kinetic theory. Then I discuss how the calculation would look in alternative approaches: the 2PI method, and direct diagrammatic analysis. I argue that the 2PI method may be a good way to derive the kinetic theory, but is not very useful directly (in a gauge theory). The diagrammatic approach is almost hopeless.

hep-ph↗

arXiv2002

Photon and Gluon Emission in Relativistic Plasmas

We recently derived, using diagrammatic methods, the leading-order hard photon emission rate in ultra-relativistic plasmas. This requires a correct treatment of multiple scattering effects which limit the coherence length of emitted radiation (the Landau-Pomeranchuk-Migdal effect). In this paper, we provide a more physical derivation of this result, and extend the treatment to the case of gluon radiation.

hep-ph↗

arXiv2001

Photon Emission from Ultrarelativistic Plasmas

The emission rate of photons from a hot, weakly coupled ultrarelativistic plasma is analyzed. Leading-log results, reflecting the sensitivity of the emission rate to scattering events with momentum transfers from $gT$ to $T$, have previously been obtained. But a complete leading-order treatment requires including collinearly enhanced, inelastic processes such as bremsstrahlung. These inelastic processes receive O(1) modifications from multiple scattering during the photon emission process, which limits the coherence length of the emitted radiation (the Landau-Pomeranchuk-Migdal effect). We perform a diagrammatic analysis to identify, and sum, all leading-order contributions. We find that the leading-order photon emission rate is not sensitive to non-perturbative $g^2 T$ scale dynamics. We derive an integral equation for the photon emission rate which is very similar to the result of Migdal in his original discussion of the LPM effect. The accurate solution of this integral equation for specific theories of interest will be reported in a companion paper.

hep-ph↗

arXiv2001

Photon Emission from Quark-Gluon Plasma: Complete Leading Order Results

We compute the photon emission rate of an equilibrated, hot QCD plasma at zero chemical potential, to leading order in both alpha_{EM} and the QCD coupling g_s(T). This requires inclusion of near-collinear bremsstrahlung and inelastic pair annihilation contributions, and correct incorporation of Landau-Pomeranchuk-Migdal suppression effects for these processes. Analogous results for a QED plasma are also included.

hep-ph↗

arXiv2001

Problems with lattice methods for electroweak preheating

Recently Garcia Bellido et. al. have proposed that electroweak baryogenesis may occur at the end of inflation, in a scenario where the reheat temperature is too low for electroweak symmetry restoration. I show why the scenario is difficult to test reliably by classical field techniques on the lattice.

hep-ph↗

arXiv2001

Atomic Bose Condensation and the Lattice

I show how interaction corrections to the Bose condensation temperature of an atomic gas can be computed using a combination of perturbative effective field theory and lattice techniques.

hep-lat↗

arXiv2001

Lower Bound on the Propagation Speed of Gravity from Gravitational Cherenkov Radiation

Recently, interesting 4-D Lorentz violating models have been proposed, in which all particles have a common maximum velocity $c$, but gravity propagates (in the preferred frame) with a different maximum velocity $c_g \neq c$. We show that the case $c_g < c$ is very tightly constrained by the observation of the highest energy cosmic rays. Assuming a galactic origin for the cosmic rays gives a conservative bound of $c-c_g < 2 \times 10^{-15} c$; if the cosmic rays have an extragalactic origin the bound is orders of magnitude tighter, of order $c-c_g < 2 \times 10^{-19} c$.

hep-ph↗

arXiv2001

T_c for homogeneous dilute Bose gases: a second-order result

The transition temperature for a dilute, homogeneous, three-dimensional Bose gas has the expansion T_c = T_0 {1 + c_1 a n^(1/3) + [c_2' ln(a n^(1/3)) + c_2''] a^2 n^(2/3) + O(a^3 n)}, where a is the scattering length, n the number density, and T_0 the ideal gas result. The first-order coefficient c_1 depends on non-perturbative physics. In this paper, we show that the coefficient c_2' can be computed perturbatively. We also show that the remaining second-order coefficient c_2'' depends on non-perturbative physics but can be related, by a perturbative calculation, to quantities that have previously been measured using lattice simulations of three-dimensional O(2) scalar field theory. Making use of those simulation results, we find T_c = T_0 {1 + (1.32+-0.02) a n^(1/3) + [19.7518 ln(a n^(1/3)) + (75.7+-0.4)] a^2 n^(2/3) + O(a^3 n)}.

cond-mat.stat-mech↗