arXiv subjects

George T. Fleming

Publications and source records attributed to George T. Fleming.

At least 55 records · Page 3Linked to original sources

arXiv2010

Toward TeV Conformality

We study the chiral condensate $<\barψ ψ>$ for an SU(3) gauge theory with $N_f$ massless Dirac fermions in the fundamental representation when $N_f$ is increased from 2 to 6. For $N_f=2$, our lattice simulations of $<\barψ ψ>/F^3$, where $F$ is the Nambu-Goldstone-boson decay constant, agree with the measured QCD value. For $N_f = 6$, this ratio shows significant enhancement, presaging an even larger enhancement anticipated as $N_f$ increases further, toward the critical value for transition from confinement to infrared conformality.

hep-ph↗

arXiv2009

Lattice Study of Conformal Behavior in SU(3) Yang-Mills Theories

Using lattice simulations, we study the extent of the conformal window for an SU(3) gauge theory with N_f Dirac fermions in the fundamental representation. We extend our recently reported work, describing the general framework and the lattice simulations in more detail. We find that the theory is conformal in the infrared for N_f = 12, governed by an infrared fixed point, whereas the N_f = 8 theory exhibits confinement and chiral symmetry breaking. We therefore conclude that the low end of the conformal window N_f^c lies in the range 8 <= N_f^c <= 12. We discuss open questions and the potential relevance of the present work to physics beyond the standard model.

hep-ph↗

arXiv2009

Excited-State Effective Masses in Lattice QCD

We apply black-box methods, i.e. where the performance of the method does not depend upon initial guesses, to extract excited-state energies from Euclidean-time hadron correlation functions. In particular, we extend the widely used effective-mass method to incorporate multiple correlation functions and produce effective mass estimates for multiple excited states. In general, these excited-state effective masses will be determined by finding the roots of some polynomial. We demonstrate the method using sample lattice data to determine excited-state energies of the nucleon and compare the results to other energy-level finding techniques.

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arXiv2008

Strong Interactions for the LHC

Dynamical electroweak symmetry breaking (DEWSB) has been a viable option for the completion of the standard model for over thirty years. Precision electroweak studies indicate that the new strong interactions that break EW symmetry cannot be a scaled-up copy of QCD. Building viable models of DEWSB is difficult without a detailed understanding of such non-QCD gauge theories which still confine and break chiral symmetry. We review past difficulties of studying these theories using lattice methods and describe recent progress, focusing on the role of approximate infrared conformal symmetry.

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arXiv2008

Lattice super-Yang-Mills using domain wall fermions in the chiral limit

Lattice N=1 super-Yang-Mills theory formulated using Ginsparg-Wilson fermions provides a rigorous non-perturbative definition of the continuum theory that requires no fine-tuning as the lattice spacing is reduced to zero. Domain wall fermions are one explicit scheme for achieving this and using them we have performed large-scale Monte Carlo simulations of the theory for gauge group SU(2). We have measured the gaugino condensate, static potential, Creutz ratios and residual mass for several values of the domain wall separation L_s, four-dimensional lattice volume, and two values of the gauge coupling. With this data we are able to extrapolate the gaugino condensate to the chiral limit, to express it in physical units, and to establish important benchmarks for future studies of super-Yang-Mills on the lattice.

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arXiv2008

Gluinos condensing at the CCNI: 4096 CPUs weigh in

We report preliminary results of lattice super-Yang-Mills computations using domain wall fermions, performed at an actual rate of 1000 Gflop/s, over the course of six months, using two BlueGene/L racks at Rensselaer's CCNI supercomputing center. This has allowed us to compute the gluino condensate and string tension over a wide range of lattice parameters, setting the stage for continuum, chiral extrapolations.

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arXiv2008

Lattice Study of the Conformal Window in QCD-like Theories

Using lattice simulations, we study the extent of the conformal window for an $\text{SU}(3)$ gauge theory with $N_f$ Dirac fermions in the fundamental representation. We present evidence that the infrared behavior is conformal for $12 \leq N_f \leq 16$, governed by an infrared fixed point, while confinement and chiral symmetry breaking are present for $N_f \leq 8$.

hep-ph↗

arXiv2007

The Pion Form Factor at Large Momentum Transfer

We present our calculations of the electromagnetic form factor of pions. We explore the properties of pion form factor at momentum transfer larger than previous studies by including more combinations of source and sink momenta and using more configurations.We fit our results using vector meson dominance (VMD) hypothesis.

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arXiv2006

The Effect of Reduced Spatial Symmetries on Lattice States: Results for Non-zero Linear Momentum

The irreducible representations of the cubic space group are described and used to determine the mapping of continuum states to lattice states with non-zero linear momentum. The Clebsch-Gordan decomposition is calculated from the character table for the cubic space group. These results are used to identify multiparticle states which appear in the hadron spectrum on the lattice.

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arXiv2006

Angular Momentum on the Lattice: The Case of Non-Zero Linear Momentum

The irreducible representations (IRs) of the double cover of the Euclidean group with parity in three dimensions are subduced to the corresponding cubic space group. The reduction of these representations gives the mapping of continuum angular momentum states to the lattice in the case of non-zero linear momentum. The continuous states correspond to lattice states with the same momentum and continuum rotational quantum numbers decompose into those of the IRs of the little group of the momentum vector on the lattice. The inverse mapping indicates degeneracies that will appear between levels of different lattice IRs in the continuum limit, recovering the continuum angular momentum multiplets. An example of this inverse mapping is given for the case of the ``moving'' isotropic harmonic oscillator.

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arXiv2006

Multiparticle States and the Hadron Spectrum on the Lattice

The Clebsch-Gordan decomposition is calculated for direct products of the irreducible representations of the cubic space group. These results are used to identify multiparticle states which appear in the hadron spectrum on the lattice. Consideration of the cubic space group indicates how combinations of both zero momentum and non-zero momentum multiparticle states contribute to the spectrum.

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arXiv2005

$B_7$, $B_8$ and chiral Ward identities

We present recent progress in understanding weak matrix elements on the lattice. We use HYP staggered fermions in quenched QCD to study numerically various properties of the $K^+\toπ^+$ amplitudes of the electroweak penguin operators $Q_7$ and $Q_8$. We check chiral Ward identities to probe the validity of using improved staggered fermions in the calculation of weak matrix elements. We address the issue of mixing with unphysical lower dimension operators, which causes a divergent term in the case of the $ΔI = 1/2$ amplitudes. We propose a particular subtraction method as the best choice. We also measure the gold-plated ratio $R$ originally suggested by Becirevic and Villadoro.

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arXiv2005

Combining Quark and Link Smearing to Improve Extended Baryon Operators

The effects of Gaussian quark-field smearing and analytic stout-link smearing on the correlations of gauge-invariant extended baryon operators are studied. Gaussian quark-field smearing substantially reduces contributions from the short wavelength modes of the theory, while stout-link smearing significantly reduces the noise from the stochastic evaluations. The use of gauge-link smearing is shown to be crucial for baryon operators constructed of covariantly-displaced quark fields. Preferred smearing parameters are determined for a lattice spacing a_s ~ 0.1 fm.

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arXiv2005

Clebsch-Gordan Construction of Lattice Interpolating Fields for Excited Baryons

Large sets of baryon interpolating field operators are developed for use in lattice QCD studies of baryons with zero momentum. Operators are classified according to the double-valued irreducible representations of the octahedral group. At first, three-quark smeared, local operators are constructed for each isospin and strangeness and they are classified according to their symmetry with respect to exchange of Dirac indices. Nonlocal baryon operators are formulated in a second step as direct products of the spinor structures of smeared, local operators together with gauge-covariant lattice displacements of one or more of the smeared quark fields. Linear combinations of direct products of spinorial and spatial irreducible representations are then formed with appropriate Clebsch-Gordan coefficients of the octahedral group. The construction attempts to maintain maximal overlap with the continuum SU(2) group in order to provide a physically interpretable basis. Nonlocal operators provide direct couplings to states that have nonzero orbital angular momentum.

hep-lat↗

arXiv2005

Testing improved staggered fermions with $m_s$ and $B_K$

We study the improvement of staggered fermions using hypercubically smeared (HYP) links. We calculate the strange quark mass and the kaon B-parameter, $B_K$, in quenched QCD on a $16^3 \times 64$ lattice at $β=6.0$. We find $m_s(\bar{\rm MS},2 {\rm GeV})=101.2\pm1.3\pm4 $MeV and $B_K(\bar{\rm MS},2 {\rm GeV}) = 0.578 \pm 0.018\pm 0.042$, where the first error is from statistics and fitting, and the second from using one-loop matching factors. The scale ($1/a=1.95$GeV) is set by $M_ρ$, and $m_s$ is determined using the kaon mass. Comparing to quenched results obtained using unimproved staggered fermions and other discretizations, we argue that the size of discretization errors in $B_K$ is substantially reduced by improvement.

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arXiv2005

What can Lattice QCD theorists learn from NMR spectroscopists?

Euclidean-time hadron correlation functions computed in Lattice QCD (LQCD) are modeled by a sum of decaying exponentials, reminiscent of the exponentially damped sinusoid models of free induction decay (FID) in Nuclear Magnetic Resonance (NMR) spectroscopy. We present our initial progress in studying how data modeling techniques commonly used in NMR perform when applied to LQCD data.

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arXiv2005

Looking for pentaquarks in Lattice QCD

Pentaquark states in lattice QCD probably lie close in energy to two particle scattering states. Correctly identifying the resonant state is a challenging, yet tractable, problem given the terascale computing facilities available today. We summarize the initial round of exploratory lattice calculations and discuss what should be accomplished in the next round.

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arXiv2004

Lattice Computations of the Pion Form Factor

We report on a program to compute the electromagnetic form factors of mesons. We discuss the techniques used to compute the pion form factor and present results computed with domain wall valence fermions on MILC asqtad lattices, as well as with Wilson fermions on quenched lattices. The methods can easily be extended to rho-to-gamma-pi transition form factors.

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