arXiv subjects

Thomas DeGrand

Publications and source records attributed to Thomas DeGrand.

At least 37 records · Page 2Linked to original sources

arXiv2018

Baryon spectrum of SU(4) composite Higgs theory with two distinct fermion representations

We use lattice simulations to compute the baryon spectrum of SU(4) lattice gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations simultaneously. This model is closely related to a composite Higgs model in which the chimera baryon made up of fermions from both representations plays the role of a composite top-quark partner. The dependence of the baryon masses on each underlying fermion mass is found to be generally consistent with a quark-model description and large-Nc scaling. We combine our numerical results with experimental bounds on the scale of the new strong sector to derive a lower bound on the mass of the top partner.

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arXiv2017

Chiral Transition of SU(4) Gauge Theory with Fermions in Multiple Representations

We report preliminary results on the finite temperature behavior of SU(4) gauge theory with dynamical quarks in both the fundamental and two-index antisymmetric representations. This system is a candidate to present scale separation behavior, where fermions in different representations condense at different temperature or coupling scales. Our simulations, however, reveal a single finite-temperature phase transition at which both representations deconfine and exhibit chiral restoration. It appears to be strongly first order. We compare our results to previous single-representation simulations. We also describe a Pisarski-Wilczek stability analysis, which suggests that the transition should be first order.

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arXiv2017

Simple chromatic properties of gradient flow

It has become customary to use a smoothing algorithm called "gradient flow" to fix the lattice spacing in a simulation, through a parameter called $t_0$. It is shown that in order to keep the length $t_0$ fixed with respect to mesonic or gluonic observables as the number of colors $N_c$ is varied, the fiducial point for the flow parameter must be scaled nearly linearly in $N_c$. In simulations with dynamical fermions, the dependence of $t_0$ on the pseudoscalar meson mass flattens as the number of colors rises, in a way which is consistent with large $N_c$ expectations.

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arXiv2017

Lattice study of large $N_c$ QCD

We present a lattice simulation study of large $N_c$ regularities of meson and baryon spectroscopy in $SU(N_c)$ gauge theory with two flavors of dynamical fundamental representation fermions. Systems investigated include $N_c=2$, 3, 4, and 5, over a range of fermion masses parametrized by a squared pseudoscalar to vector meson mass ratio between about 0.2 to 0.7. Good agreement with large $N_c$ scaling is observed in the static potential, in meson masses and decay constants, and in baryon spectroscopy. This is an update of the published version of the paper (Phys. Rev. D94 (2016) 034506).

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arXiv2016

One-loop Chiral Perturbation Theory with two fermion representations

We develop Chiral Perturbation Theory for chirally broken theories with fermions in two different representations of the gauge group. Any such theory has a non-anomalous singlet $U(1)_A$ symmetry, yielding an additional Nambu-Goldstone boson when spontaneously broken. We calculate the next-to-leading order corrections for the pseudoscalar masses and decay constants, which include the singlet Nambu-Goldstone boson, as well as for the two condensates. The results can be generalized to more than two representations.

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arXiv2015

One-loop anomalous dimension of top-partner hyperbaryons in a family of composite Higgs models

An important ingredient of many composite-Higgs scenarios is partial compositeness: a mass for the top quark is generated via mixing with baryonic operators of the same new strong dynamics that produces the composite Higgs particle. These baryonic operators are scale dependent. We construct these operators, and calculate their one-loop anomalous dimension, in a set of ultraviolet completions of the Standard Model proposed by Ferretti and Karateev.

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arXiv2015

Lattice tests of beyond Standard Model dynamics

Over the last few years lattice techniques have been used to investigate candidate theories of new physics beyond the Standard Model. This review gives a survey of results from these studies. Most of these investigations have been of systems of gauge fields and fermions that have slowly-running coupling constants. A major portion of the review is a critical discussion of work in this particular subfield, first describing the methods used, and then giving a compilation of results for specific models.

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arXiv2015

Spectroscopy of SU(4) gauge theory with two flavors of sextet fermions

We present a first look at the spectroscopy of SU(4) gauge theory coupled to two flavors of Dirac fermions in the two-index antisymmetric representation, which is a real representation. We compute meson and diquark masses, the pseudoscalar and vector meson decay constants, and the masses of six-quark baryons. We make comparisons with large-Nc expectations.

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arXiv2014

Spectroscopy of SU(4) lattice gauge theory with fermions in the two index anti-symmetric representation

We present a study of spectroscopy of SU(4) lattice gauge theory coupled to two flavors of Dirac fermions in the anti-symmetric two index representation. The fermion representation is real, and the pattern of chiral symmetry breaking is SU(2Nf) -> SO(2Nf) with Nf flavors of Dirac fermions. It is an interesting generalization of QCD, for several reasons: it allows direct exploration of an alternate large Nc expansion, it can be simulated at non-zero chemical potential with no sign problem, and several UV completions of composite Higgs systems are built on it. We present preliminary results on the baryon and meson spectra of the theory and compare them with SU(3) results and with expectations for large Nc scaling.

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arXiv2014

Results from lattice simulations of N=4 supersymmetric Yang--Mills

We report recent results and developments from our ongoing lattice studies of $\mathcal N = 4$ supersymmetric Yang--Mills theory. These include a proof that only a single fine-tuning needs to be performed, so long as the moduli space is not lifted by nonperturbative effects. We extend our investigations of supersymmetry restoration in the continuum limit by initiating Monte Carlo renormalization group studies. We present additional numerical evidence that the lattice theory does not suffer from a sign problem. Finally we study the static potential, which we find to be Coulombic at both weak and strong coupling. We compare the static potential Coulomb coefficients to perturbation theory, including initial results for N=3 colors in addition to N=2.

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arXiv2014

Parallel software for lattice N=4 supersymmetric Yang--Mills theory

We present new parallel software, SUSY LATTICE, for lattice studies of four-dimensional $\mathcal N = 4$ supersymmetric Yang--Mills theory with gauge group SU(N). The lattice action is constructed to exactly preserve a single supersymmetry charge at non-zero lattice spacing, up to additional potential terms included to stabilize numerical simulations. The software evolved from the MILC code for lattice QCD, and retains a similar large-scale framework despite the different target theory. Many routines are adapted from an existing serial code, which SUSY LATTICE supersedes. This paper provides an overview of the new parallel software, summarizing the lattice system, describing the applications that are currently provided and explaining their basic workflow for non-experts in lattice gauge theory. We discuss the parallel performance of the code, and highlight some notable aspects of the documentation for those interested in contributing to its future development.

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arXiv2014

N=4 Supersymmetry on a Space-Time Lattice

Maximally supersymmetric Yang--Mills theory in four dimensions can be formulated on a space-time lattice while exactly preserving a single supersymmetry. Here we explore in detail this lattice theory, paying particular attention to its strongly coupled regime. Targeting a theory with gauge group SU(N), the lattice formulation is naturally described in terms of gauge group U(N). Although the U(1) degrees of freedom decouple in the continuum limit we show that these degrees of freedom lead to unwanted lattice artifacts at strong coupling. We demonstrate that these lattice artifacts can be removed, leaving behind a lattice formulation based on the SU(N) gauge group with the expected apparently conformal behavior at both weak and strong coupling.

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arXiv2014

Suppressing dislocations in normalized hypercubic smearing

Normalized hypercubic smearing improves the behavior of dynamical Wilson-clover fermions, but has the unwanted side effect that it can occasionally produce spikes in the fermion force. These spikes originate in the chain rule connecting the derivative with respect to the smeared links to the derivative with respect to the dynamical links, and are associated with the presence of dislocations in the dynamical gauge field. We propose and study an action designed to suppress these dislocations. We present evidence for improved performance of the hybrid Monte Carlo algorithm. A side benefit is improvement in the properties of valence chiral fermions.

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arXiv2013

Gauge theories with fermions in two-index representations

After some introductory comments on the peculiar features of slowly running theories, I will report results obtained using the Schrodinger functional technique for two gauge theories that are believed to lie near the bottom of the conformal window: the SU(3) theory with two adjoint Dirac fermions, and the SU(4) theory with six Dirac fermions in the two-index antisymmetric representation. In both cases we find a small beta function in strong coupling, but we cannot confirm or rule out an infrared fixed point. In both theories the mass anomalous dimension levels off, staying well below 0.5, much like the theories with fermions in the two-index symmetric representation investigated earlier.

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arXiv2013

Near the sill of the conformal window: gauge theories with fermions in two-index representations

We apply Schroedinger functional methods to two gauge theories with fermions in two-index representations: the SU(3) theory with Nf=2 adjoint fermions, and the SU(4) theory with Nf=6 fermions in the two-index antisymmetric representation. Each theory is believed to lie near the bottom of the conformal window for its respective representation. In the SU(3) theory we find a small beta function in strong coupling but we cannot confirm or rule out an infrared fixed point. In the SU(4) theory we find a hint of walking - a beta function that approaches the axis and then turns away from it. In both theories the mass anomalous dimension remains small even at the strongest couplings, much like the theories with fermions in the two-index symmetric representation investigated earlier.

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arXiv2013

Lattice calculations of the spectroscopy of baryons with broken flavor SU(3) symmetry and 3, 5, or 7 colors

Lattice Monte Carlo calculations of baryon spectroscopy in gauge groups SU(N), N=3, 5, 7, are presented. The quenched valence fermions come in three flavors, two degenerate mass ones and a third heavier flavor. The data shows striking regularities reminiscent of the real-world case of N=3: higher angular momentum states lie higher in mass, and Sigma-like states lie higher than Lambda-like ones. These simple regularities are reasonably well described by 1/N expansions.

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arXiv2013

Mass anomalous dimension in sextet QCD

We extend our previous study of the SU(3) gauge theory with N_f=2 flavors of fermions in the sextet representation of color. Our tool is the Schroedinger functional method. By changing the lattice action, we push the bulk transition of the lattice theory to stronger couplings and thus reveal the beta function and the mass anomalous dimension gamma_m over a wider range of coupling, out to g^2 ~ 11. Our results are consistent with an infrared fixed point, but walking is not ruled out. Our main result is that gamma_m never exceeds 0.45, making the model unsuitable for walking technicolor. We use a novel method of extrapolation to the large-volume/continuum limit, tailored to near-conformal theories.

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arXiv2012

Lattice baryons in the 1/N expansion

Results are presented for hadron spectroscopy with gauge groups SU(N) with N=3, 5, 7. Calculations use the quenched approximation. Lattice spacings are matched using the static potential. Meson spectra show independence on N and vacuum-to-hadron matrix elements scale as the square root of N. The baryon spectrum shows the excitation levels of a rigid rotor.

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