arXiv subjects

Guy D. Moore

Publications and source records attributed to Guy D. Moore.

At least 109 records · Page 6Linked to original sources

arXiv2007

Heavy quark diffusion in perturbative QCD at next-to-leading order

We compute the momentum diffusion coefficient of a nonrelativistic heavy quark in a hot QCD plasma, to next-to-leading order in the weak coupling expansion. Corrections arise at O(g); physically they represent interference between overlapping scatterings, as well as soft, electric scale ($p\sim gT$) gauge field physics, which we treat using the hard thermal loop (HTL) effective theory. In 3-color, 3-flavor QCD, the momentum diffusion constant of a fundamental representation heavy quark at NLO is $κ= \frac{16π}{3} α_s^2 T^3 (\ln \frac{1}{g} + 0.07428 + 1.8869 g)$. The convergence of the weak coupling expansion is poor.

hep-ph↗

arXiv2007

Decoherence due to the Horizon after Inflation

The fluctuations in the inflaton field at the end of inflation which seed the density perturbations are prepared in a pure quantum state. It is generally assumed that some physics causes this pure state to decohere so that it should be treated probabilistically. We show that the entanglement entropy between the universe inside our observable horizon and that outside our horizon is sufficient to do this. For the modes which are super-Hubble at the end of inflation, this entanglement entropy grows with volume inside the horizon, rather than with the horizon's area, and is proportional to the number of e-folds since Hubble crossing.

gr-qc↗

arXiv2007

Non-Abelian Plasma Instabilities for Extreme Anisotropy

Thermalization of quark-gluon plasmas in heavy-ion collisions is a difficult theoretical problem. One theoretical goal has been to understand the physics of thermalization in the relatively simplifying limit of arbitrarily high energy collisions, where the running coupling alpha_s is weak. One of the current roadblocks to achieving this goal is lack of knowledge about the behavior of plasma instabilities when particle distributions are highly anisotropic. In particular, it has not been known how the magnetic fields generated by plasma instabilities scale with anisotropy. In this paper, we use numerical simulations in a first attempt to determine this scaling.

hep-ph↗

arXiv2006

Defect Formation with Bulk Fields

It has recently been realized that brane-antibrane annihilation (a possible explanation for ending inflation) may result in defect formation, due to the dynamics of the tachyon field. Studies of this possibility have generally ignored the interaction of the brane fields with fields in the bulk; recently it has been argued [1] that interactions with bulk fields suppress or even eliminate defect formation. To investigate the impact of bulk fields on brane defect formation, we construct a toy model that captures the essential features of the tachyon condensation with bulk fields. We study the structure of defects in this toy model, and simulate their formation and evolution on the lattice. We find that, while the energetics and interactions of defects are influenced by the size of the extra dimension and the bulk-brane coupling, the bulk-brane coupling does not prevent the formation of a defect network.

hep-th↗

arXiv2006

Shear viscosity in weakly coupled ${\cal N} {=} 4$ Super Yang-Mills theory compared to QCD

We compute the shear viscosity of weakly coupled N=4 supersymmetric Yang-Mills (SYM) theory. Our result for [eta/s], the viscosity to entropy-density ratio, is many times smaller than the corresponding weak-coupling result in QCD. This suggests that [eta/s] of QCD near the transition point is several times larger than the viscosity bound, [eta/s] >= 1/4 pi.

hep-ph↗

arXiv2006

The Bulk Viscosity of High-Temperature QCD

We compute the bulk viscosity zeta of high-temperature QCD to leading order in powers of the running coupling alpha_s(T). We find that it is negligible compared to shear viscosity eta for any alpha_s that might reasonably be considered small. The physics of bulk viscosity in QCD is a bit different than in scalar phi^4 theory. In particular, unlike in scalar theory, we find that an old, crude estimate of zeta as 15 ((1/3)-v_s^2)^2 eta gives the correct order of magnitude, where v_s is the speed of sound. We also find that leading-log expansions of our result for zeta are not accurate except at very small coupling.

hep-ph↗

arXiv2006

Dileptons, spectral weights, and conductivity in the Quark-Gluon Plasma

We re-examine soft dilepton emission from a weakly coupled Quark-Gluon Plasma. We show that Braaten, Pisarski, and Yuan's result that the dilepton rate rises as E^-4 (and the spectral weight scales as 1/E) at small energy E<<gT is correct, but that the coefficient they found for this behavior is not correct, because their analysis was incomplete. At still smaller scales, the behavior moderates to ~1/E^2 for E <= g^4 T, consistent with a finite electrical conductivity. We evaluate the spectral weight in the E~g^4 T region by kinetic theory techniques and show that it satisfies a sum rule, which makes the determination of electrical conductivity from the Euclidean correlation function very challenging.

hep-ph↗

arXiv2005

Numerical Studies of QGP Instabilities and Implications

Because the initial shape of the QGP in a heavy ion collision is anisotropic, the momentum distribution becomes anisotropic after a short time. This leads to plasma instabilities, which may help explain how the plasma isotropizes. We explain the physics of instabilities and give the latest results of numerical simulations into their evolution. Nonabelian interactions cut off the size to which the soft unstable fields grow, and energy in the soft fields subsequently cascades towards more ultraviolet scales. We present first results for the power spectrum of this cascade.

hep-ph↗

arXiv2005

Three Dimensional N=2 Supersymmetry on the Lattice

We show how 3-dimensional, N=2 supersymmetric theories, including super QCD with matter fields, can be put on the lattice with existing techniques, in a way which will recover supersymmetry in the small lattice spacing limit. Residual supersymmetry breaking effects are suppressed in the small lattice spacing limit by at least one power of the lattice spacing a.

hep-lat↗

arXiv2005

The Fate of Non-Abelian Plasma Instabilities in 3+1 Dimensions

Plasma instabilities can play a fundamental role in plasma equilibration. There are similarities and differences between plasma instabilities in abelian and non-abelian gauge theories. In particular, it has been an open question whether non-abelian self-interactions are the limiting factor in the growth of non-abelian plasma instabilities. We study this problem with 3+1 dimensional numerical simulations. We find that non-abelian plasma instabilities behave very differently from abelian ones once they grow to be non-perturbatively large, in contrast with earlier results of 1+1 dimensional simulations. In particular, they grow more slowly at late times, with linear rather than exponential dependence of magnetic energy on time.

hep-ph↗

arXiv2005

The turbulent spectrum created by non-Abelian plasma instabilities

Recent numerical work on the fate of plasma instabilities in weakly-coupled non-Abelian gauge theory has shown the development of a cascade of energy from long to short wavelengths. This cascade has a steady-state spectrum, analogous to the Kolmogorov spectrum for turbulence in hydrodynamics or for energy cascades in other systems. In this paper, we theoretically analyze processes responsible for this cascade and find a steady-state spectrum f_k ~ k^-2, where f_k is the phase-space density of particles with momentum k. The exponent -2 is consistent with results from numerical simulations. We also discuss implications of the emerging picture of instability development on the "bottom-up" thermalization scenario for (extremely high energy) heavy ion collisions, emphasizing fundamental questions that remain to be answered.

hep-ph↗

arXiv2005

QCD Plasma Instabilities: The Nonabelian Cascade

Magnetic plasma instabilities appear to play an important role in the early stages of quark-gluon plasma equilibration in the high energy (weak coupling) limit. Numerical studies of the growth of such instabilities from small seed fluctuations have found initial exponential growth in their energy, followed by linear growth once the associated color magnetic fields become so large that their non-abelian interaction are non-perturbative. In this paper, we use simulations to determine the nature of this linear energy growth. We find that the long-wavelength modes associated with the instability have ceased to grow, but that they cascade energy towards the ultraviolet in the form of plasmon excitations of ever increasing energy. We find a quasi-steady-state power-law distribution $f_k \propto k^{-ν}$ for this cascade, with spectral index $ν\simeq 2$.

hep-ph↗

arXiv2005

Constraining the New Aether: Gravitational Cherenkov Radiation

We study the simplest concrete theory for spontaneous Lorentz violation, the ``New Aether Theory'' of Jacobson and Mattingly, which is a vector-tensor gravitational theory with a fixed-modulus condition on the vector field. We show that the observation of ultra-high energy cosmic rays (which implies the absence of energy loss via various Cherenkov type processes) places constraints on the parameters of this theory, which are much stronger than those previously found in the literature and are also stronger than the constraints generically arising when gravity displays sub-luminal propagation.

hep-ph↗

arXiv2005

Energy Loss of Leading Hadrons and Direct Photon production in Evolving Quark-Gluon Plasma

We calculate the nuclear modification factor of neutral pions and the photon yield at high p_T in central Au-Au collisions at RHIC (\sqrt{s}=200 GeV) and Pb-Pb collisions at the LHC (\sqrt{s}=5500 GeV). A leading-order accurate treatment of jet energy loss in the medium has been convolved with a physical description of the initial spatial distribution of jets and a (1+1) dimensional expansion. We reproduce the nuclear modification factor of pion R_{AA} at RHIC, assuming an initial temperature T_i=370 MeV and a formation time τ_i=0.26 fm/c, corresponding to dN/dy=1260. The resulting suppression depends on the particle rapidity density dN/dy but weakly on the initial temperature. The jet energy loss treatment is also included in the calculation of high p_T photons. Photons coming from primordial hard N-N scattering are the dominant contribution at RHIC for p_T > 5 GeV, while at the LHC, the range 8<p_T<14 GeV is dominated by jet-photon conversion in the plasma.

hep-ph↗

arXiv2005

How Much do Heavy Quarks Thermalize in a Heavy Ion Collision?

We investigate the thermalization of charm quarks in high energy heavy ion collisions. To this end, we calculate the diffusion coefficient in the perturbative Quark Gluon Plasma and relate it to collisional energy loss and momentum broadening. We then use these transport properties to formulate a Langevin model for the evolution of the heavy quark spectrum in the hot medium. The model is strictly valid in the non-relativistic limit and for all velocities $γv < \alphas^{-1/2}$ to leading logarithm in $T/m_D$. The corresponding Fokker-Planck equation can be solved analytically for a Bjorken expansion and the solution gives a simple estimate for the medium modifications of the heavy quark spectrum as a function of the diffusion coefficient. Finally we solve the Langevin equations numerically in a hydrodynamic simulation of the heavy ion reaction. The results of this simulation are the medium modifications of the charm spectrum $R_{AA}$ and the expected elliptic flow $v_2(p_T)$ as a function of the diffusion coefficient.

hep-ph↗

arXiv2005

Apparent Thermalization due to Plasma Instabilities in Quark-Gluon Plasma

Hydrodynamical modeling of heavy ion collisions at RHIC suggests that the quark-gluon plasma (QGP) "thermalizes" in a remarkably short time scale, about 0.6 fm/c. We argue that this should be viewed as indicating fast isotropization, but not necessarily complete thermalization, of the non-equilibrium QGP. Non-Abelian plasma instabilities can drive local isotropization of an anisotropic QGP on a time scale which is faster than ordinary perturbative scattering processes. As a result, we argue that theoretical expectations based on weak coupling analysis are not necessarily in conflict with hydrodynamic modeling of the early part of RHIC collisions, provided one recognizes the key role of non-Abelian plasma instabilities.

nucl-th↗

arXiv2004

QCD Plasma Instabilities and Bottom-Up Thermalization

We study the role of QCD plasma instabilities in non-equilibrium quark-gluon plasmas. First, we argue that such instabilities must drastically modify the "bottom-up" thermalization scenario for heavy-ion collisions. Second, we discuss conditions for the existence of instabilities in a more general context than previously treated in the QCD literature. We also give a thorough qualitative review of the origin of instabilities. We discuss some mechanisms whereby the growth of plasma instabilities saturates. Finally, we solve explicitly for instabilities and their growth rates for two extreme cases of anisotropic non-equilibrium plasmas that can be treated relatively simply and analytically: $f(\p) = F(p_\perp) δ(p_z)$ and $f(\p) = F(p_z) δ^{(2)}(p_\perp)$, where $f(\p)$ is the distribution of particles in momentum space.

hep-ph↗

arXiv2004

Transport Coefficients in Hot QCD

I give a physical explanation of what shear viscosity is, and what physics determines its value. Then I explain why determining the shear viscosity of the Quark-Gluon Plasma is interesting. I outline the leading-order calculation of the QGP shear viscosity (and baryon number diffusion constant), explaining why the quite complicated physics of parton splitting and Landau-Pomeranchuk-Migdal interference effects are required for its calculation. Then I briefly explore the range of applicability, emphasizing the importance of plasma instabilities.

hep-ph↗