arXiv subjects

Guy D. Moore

Publications and source records attributed to Guy D. Moore.

At least 55 records · Page 3Linked to original sources

arXiv2020

Global cosmic string networks as a function of tension

We investigate the properties of global cosmic string networks as a function of the ratio of string tension to Goldstone-field coupling, and as a function of the Hubble damping strength. Our results show unambiguously that the string density is sensitive to this ratio. We also find that existing semi-analytical (one-scale) models must be missing some important aspect of the network dynamics. Our results point the way towards improving such models.

hep-ph↗

arXiv2020

Transverse momentum broadening from the lattice

We present a calculation of the transverse momentum broadening of a high-energy fundamental-representation particle, as calculated nonperturbatively within EQCD using the technique proposed by Caron-Huot and pioneered by Panero, Schäfer, and Rummukainen. Our results are continuum extrapolated and provided at four temperatures: 250 MeV, 500 MeV, 1 GeV, and 100 GeV.

hep-lat↗

arXiv2020

Improved Reweighting for QCD Topology at High Temperature

In a previous paper \cite{Jahn:2018dke} we presented a methodology for computing the topological susceptibility of QCD at temperatures where it is small and standard methods fail. Here we improve on this methodology by removing two barriers to the reweighting method's moving between topological sectors. We present high-statistics, continuum-extrapolated results for the susceptibility of pure-glue QCD up to $7 T_c$. We show that the susceptibility varies with temperature as $T^{-6.7\pm 0.3}$ between $T=2.5 T_c$ and $T=7 T_c$, in good agreement with expectations based on the dilute instanton gas approximation.

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arXiv2019

Full $\mathcal{O}(a)$ improvement of EQCD

EQCD is a 3D bosonic theory containing SU(3) and an adjoint scalar, which efficiently describes the infrared, nonperturbative sector of hot QCD and which is highly amenable to lattice study. We improve the matching between lattice and continuum EQCD by determining the final unknown coefficient in the $\mathcal{O}(a)$ matching, an additive scalar mass renormalization. We do this numerically by using the symmetry-breaking phase transition line of EQCD as a line of constant physics. This prepares the ground for a precision study of the transverse momentum diffusion coefficient $C(q_\perp)$ within this theory.

hep-lat↗

arXiv2019

Full O(a) improvement in EQCD

EQCD is a 3D bosonic theory containing SU(3) and an adjoint scalar, which efficiently describes the infrared, nonperturbative sector of hot QCD and which is highly amenable to lattice study. We improve the matching between lattice and continuum EQCD by determining the final unknown coefficient in the O(a) matching, an additive scalar mass renormalization. We do this numerically by using the symmetry-breaking phase transition point of the theory as a line of constant physics. This prepares the ground for a precision study of the transverse momentum diffusion coefficient C(qperp) within this theory. As a byproduct, we provide an updated version of the EQCD phase diagram.

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arXiv2019

Estimating $χ_\mathrm{top}$ Lattice Artifacts from Flowed SU(2) Calorons

Lattice computations of the high-temperature topological susceptibility of QCD receive lattice-spacing corrections and suffer from systematics arising from the type and depth of gradient flow. We study the lattice spacing corrections to $χ_\mathrm{top}$ semi-analytically by exploring the behavior of discretized Harrington-Shepard calorons under the action of different forms of gradient flow. From our study we conclude that $N_τ= 6$ is definitely too small of a time extent to study the theory at temperatures of order $4~T_\mathrm{c}$ and we explore how the amount of gradient flow influences the continuum extrapolation.

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arXiv2018

Topological Susceptibility to High Temperatures via Reweighting

We measure the topological susceptibility of quenched QCD on the lattice at two high temperatures. For this, we define topology with the help of gradient flow and mitigate the statistical problem of topology at high temperatures using a reweighting technique. This allows us to enhance tunneling events between topological sectors and alleviate topological freezing. We quote continuum extrapolated results for the susceptibility at $2.5$ and $4.1~T_\mathrm c$ that agree well with the existing literature. We conclude that the method is feasible and can be extended to unquenched QCD with no conceptual problems.

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arXiv2018

$χ_{\textrm{top}}(T \gg T_{\textrm{c}})$ in pure-glue QCD through reweighting

We calculate the topological susceptibility at 2.5 Tc and 4.1 Tc in SU(3) pure Yang-Mills theory. We define topology with the help of gradient flow and we largely overcome the problem of poor statistics at high temperatures by applying a reweighting technique in terms of the topological charge, measured after a specific small amount of gradient flow. This allows us to obtain a sample of configurations which compares topological sectors with good statistics, with enhanced tunneling between topologies. We quote continuum extrapolated results at these two temperatures and conclude that our method is viable and can be extended without new conceptual problems to the case of full QCD with fermions.

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arXiv2018

Second-order Hydrodynamics in Next-to-Leading-Order QCD

We compute the hydrodynamic relaxation times $τ_π$ and $τ_j$ for hot QCD at next-to-leading order in the coupling with kinetic theory. We show that certain dimensionless ratios of second-order to first-order transport coefficients obey bounds which apply whenever a kinetic theory description is possible; the computed values lie somewhat above these bounds. Strongly coupled theories with holographic duals strongly violate these bounds, highlighting their distance from a quasiparticle description.

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arXiv2018

Stress-stress correlator in lambda phi^4 theory: Poles or a Cut?

We explore the analytical properties of the traceless stress tensor 2-point function at zero momentum and small frequency (relevant for shear viscosity and hydrodynamic response) in hot, weakly coupled lambda phi^4 theory. We show that, rather than one or a small number of poles, the correlator has a cut along the negative imaginary frequency axis. We briefly discuss this result's relevance for constructing 2'nd order hydrodynamic models of hot relativistic field theories.

hep-ph↗

arXiv2018

QCD Shear Viscosity at (almost) NLO

We compute the shear viscosity of QCD with matter, including almost all next-to-leading order corrections -- that is, corrections suppressed by one power of $g$ relative to leading order. We argue that the still missing terms are small. The next-to-leading order corrections are large and bring $η/s$ down by more than a factor of 3 at physically relevant couplings. The perturbative expansion is problematic even at $T \simeq 100$ GeV. The largest next-to-leading order correction to $η/s$ arises from modifications to the qhat parameter, which determines the rate of transverse momentum diffusion. We also explore quark number diffusion, and shear viscosity in pure-glue QCD and in QED.

hep-ph↗

arXiv2018

Gradient-flowed thermal correlators: how much flow is too much?

Gradient flow has been proposed in the lattice community as a tool to reduce the sensitivity of operator correlation functions to noisy UV fluctuations. We test perturbatively under what conditions doing so may contaminate the results. To do so, we compute gradient-flowed electric field two-point correlators and stress tensor one- and two-point correlators at finite temperature in QCD. Gradient flow has almost no influence on the value of correlators until a (temperature- and separation-dependent) level of flow is reached, after which the correlator is rapidly compromised. We provide a prescription for how much flow is "safe."

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arXiv2017

Axion dark matter and the Lattice

First I will review the QCD theta problem and the Peccei-Quinn solution, with its new particle, the axion. I will review the possibility of the axion as dark matter. If PQ symmetry was restored at some point in the hot early Universe, it should be possible to make a definite prediction for the axion mass if it constitutes the Dark Matter. I will describe progress on one issue needed to make this prediction -- the dynamics of axionic string-wall networks and how they produce axions. Then I will discuss the sensitivity of the calculation to the high temperature QCD topological susceptibility. My emphasis is on what temperature range is important, and what level of precision is needed.

hep-ph↗

arXiv2017

The dark-matter axion mass

We evaluate the efficiency of axion production from spatially random initial conditions in the axion field, so a network of axionic strings is present. For the first time, we perform numerical simulations which fully account for the large short-distance contributions to the axionic string tension, and the resulting dense network of high-tension axionic strings. We find nevertheless that the total axion production is somewhat less efficient than in the angle-averaged misalignment case. Combining our results with a recent determination of the hot QCD topological susceptibility (Borsanyi et al 2016), we find that if the axion makes up all of the dark matter, then the axion mass is m_a = 26.2 +-3.4 micro-electron volts.

hep-ph↗

arXiv2017

How to simulate global cosmic strings with large string tension

Global string networks may be relevant in axion production in the early Universe, as well as other cosmological scenarios. Such networks contain a large hierarchy of scales between the string core scale and the Hubble scale, log(f/H) around 70, which influences the network dynamics by giving the strings large tensions T = pi f^2 log(f/H). We present a new numerical approach to simulate such global string networks, capturing the tension without an exponentially large lattice.

hep-ph↗

arXiv2017

Electroweak Bubble Wall Speed Limit

In extensions of the Standard Model with extra scalars, the electroweak phase transition can be very strong, and the bubble walls can be highly relativistic. We revisit our previous argument that electroweak bubble walls can "run away," that is, achieve extreme ultrarelativistic velocities $γ\sim 10^{14}$. We show that, when particles cross the bubble wall, they can emit transition radiation. Wall-frame soft processes, though suppressed by a power of the coupling $α$, have a significance enhanced by the $γ$-factor of the wall, limiting wall velocities to $γ\sim 1/α$. Though the bubble walls can move at almost the speed of light, they carry an infinitesimal share of the plasma's energy.

hep-ph↗

arXiv2016

Challenges for models with composite states

Composite states of electrically charged and QCD-colored hyperquarks (HQs) in a confining SU(N_HC) hypercolor gauge sector are a plausible extension of the standard model at the TeV scale, and have been widely considered as an explanation for the tentative LHC diphoton excess. Additional new physics is required to avoid a stable charged hyperbaryon in such theories. We classify renormalizable models allowing the decay of this unwanted relic directly into standard model states, showing that they are significantly restricted if the new scalar states needed for UV completion are at the TeV scale. Alternatively, if hyperbaryon number is conserved, the charged relic can decay into a neutral hyperbaryon. Such theories are strongly constrained by direct detection, if the neutral constituent hyperquark carries color or weak isospin, and by LHC searches for leptoquarks if it is a color singlet. We show that the neutral hyperbaryon can have the observed relic abundance if the confinement scale and the hyperquark mass are above TeV scale, even in the absence of any hyperbaryon asymmetry.

hep-ph↗

arXiv2016

Intercommutation of U(1) global cosmic strings

Global strings (those which couple to Goldstone modes) may play a role in cosmology. In particular, if the QCD axion exists, axionic strings may control the efficiency of axionic dark matter abundance. The string network dynamics depend on the string intercommutation efficiency (whether strings re-connect when they cross). We point out that the velocity and angle in a collision between global strings "renormalize" between the network scale and the microscopic scale, and that this plays a significant role in their intercommutation dynamics. We also point out a subtlety in treating intercommutation of very nearly antiparallel strings numerically. We find that the global strings of a O(2)-breaking scalar theory do intercommute for all physically relevant angles and velocities.

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